DiffBot - Differential Screw-Driven XY Gantry Hey folks, After spending many years on my Tripteron and Delteron projects, I recently came up with a new and, I believe completely unique, parallel XY gantry design. I've been developing it for about a month now, and figured I'd post about it here. I call it DiffBot, and there's an in-depth writeup with pictures and animations [url=https://github.com/Apsu/DiffBot/blob/master/README.md]here[/url]. The general idea is to use screws (leadscrews in the prototype) to transfer motion along X and Y axes, with two differential motors like you'd have in an H-Bot/CoreXY setup. The unique part is that I am driving nuts on the motor-connected screws differentially. They can operate in 3 modes: [list] [1] Kept from rotating, moving along the screw (the usual way to use screw nuts) [2] Kept from moving along the screw, rotating in-place with it [3] Both rotating with the screw and moving along it in some ratio [/list] The next unique thing is I have each screw nut attached to a bevel gear in a gearbox, and each of those are connected to mating bevel gears attached to another screw, that goes across the gantry between the two gearboxes, which serves as the X axis. Altogether it looks like this: [img]https://github.com/Apsu/DiffBot/raw/master/Kinematics.png[/img] It's important to note that the Y geared-nuts are free to rotate with the screw or travel along it, and that the X gears are rigidly attached to the X screw. Furthermore, the X gears mate with the Y gears on the "inside" edges; this means if the Y gears try to spin the same way, the X gears will try to spin oppositely, and lock. This is a key part of how the mechanism is able to function and change between the 3 modes listed above. I have an animation illustrating some of this idea [url=https://www.youtube.com/watch?v=kXukfDHG4Io]here[/url]. The next key thing to understand is that putting a gear on a screw nut gives a torque advantage to mode 2 vs mode 1 based on the screw's 'lead' -- the distance the threads travel linearly with one revolution of the screw. This can be a little tricky to explain, because there's almost no discussion of this concept when learning about screw physics, since the nut is almost always assumed to be fixed rigidly in place, so all the torque it sees converts to friction and linear force. But if you take a screw and nut threaded on it but otherwise free, and spin the screw, you can easily see the nut spins with it. The torque vs linear force is in a ratio based on the helix angle. You can see this too with a simple thought experiment. If you imagine a screw with a very fine thread, it's "fine" because the threads meet at a very small angle, which provides tremendous torque multiplication, and the nut would want to travel down the screw more easily than rotate with it, because very little thread contact will push it in that direction. On the opposite extreme, imagine a shaft that's splined lengthwise, or with a very long slow helix of thread. A nut on such a shaft would have a very large helix angle (180º for the spline), and almost all the force would be transmitted as torque, rotating the nut with the shaft, and almost none pushing along it. So we can take from these extremes that there's a ratio of "wants to spin in place (with the screw)" and "wants to travel down the screw (and not spin)", based on the screw lead. --- The screws I'm using for prototyping are Tr8x8, or trapezoidal-threaded 8mm diameter 8mm/rev lead. While screws with a longer lead allow for faster motion transfer and may be beneficial, Tr8x8's are common and cheap. In addition, the helix angle, via the formula angle = arctan(lead/(pi*effective_diameter)), is ~18º, which is enough to provide good mode 2 torque with very small gantry cogging. The rest of the discussion here is based on using screws of this lead or longer. --- Now, with the background theory out of the way, the actual differential function of this gantry comes into play based on the gear arrangement and the fact the gear-nuts can switch modes based on where the resistance comes from. First, an example gearbox layout with fixed spacing just to get an idea of the gantry setup: [img]https://i.imgur.com/CbezN6r.png[/img] It only takes a slight amount of resistance on the nut to switch the gear-nut into mode 2 (spinning with the screw), or a slight amount of resistance on the gear teeth to switch to mode 1. The gantry and gear arrangement makes this easy, since the X gears mesh on opposite sides of the Y gears. If you spin both Y screws the same way and speed, they will try to turn the X gears opposite, lock, and the Y nuts will travel down the screws, resulting in equal Y motion (mode 1). If you spin both Y screws the opposite way and speed, they will try to turn the X gears the same, and X will rotate, resulting in X motion (mode 2). In addition, since each Y screw is spinning oppositely, any amount of the torque that attempts to move the nuts along the screws will be in opposite directions. The rigid connection across the gantry via the X screw/rail will resist this, reinforcing the mode 2 behavior. As mentioned above, the screw lead will make the resistance required for mode 2 much smaller than mode 1 as the screw lead increases. This means that *unlike* an H-Bot, there is not a large gantry cogging torque to deal with. --- Mode 3 requires special explanation, as it isn't entirely intuitive how it works given the screw/nut ratios and gear differential behavior, so I will try to make it clear. For diagonal gantry motion (X and Y at the same time, in some ratio), the motor inputs are at different speeds. For the simple case of 45 degree motion, which is equal speed in X and Y, one motor will be held still while the other moves twice as fast. This is the same as H-Bot/CoreXY, of course. Let's call the held motor side A, and the spinning side B. As the B screw spins, it tries to spin the Yb gear nut, Xb gear, which turns Xa, and tries to turn Ya. Since A is held still, Ya will rotate around the A screw, moving the gantry in Y. When A moves along Y, the rigid X rail/screw tries to move the B side, which pushes the Yb gear-nut along the B screw, which changes the speed it rotates with the screw. Since physics is effectively instantaneous, the result of this chain of kinematic relationships is that the Y gear-nuts will rotate X at a ratio to how fast they move along Y based on the speed difference between the A and B screws, or a mix of modes 1 and 2, which is mode 3. --- Hopefully this is a clear enough description and explanation of the mechanisms and kinematic relationships of DiffBot, and I'm happy to clarify further if something doesn't make sense. As is tradition, I have built some prototype gearboxes and they are able to easily switch between modes with minor resistance, so I am not considering this mechanism to require proof, just design refinement and building full prototypes. I'm in the process of constructing working prototypes and will update with pics/video of it moving soon. If you made it this far through that word soup, thanks for reading, and if you're going to be at ERRF this year, I am working hard to have a working DiffBot to show off there! https://reprap.org/forum/read.php?166,859694,859694#msg-859694 Wed, 07 Oct 2026 02:56:57 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?166,859694,878966#msg-878966 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,878966#msg-878966
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MKSA
@boggen79

This thread has been dead for more than a year !
Let bad ideas, unsubstantiated claims rest in peace for fear that like vampires they pop up again :)

You should consider talking to someone about your feelings. Carrying a weird unsubstantiated grudge for over a year isn't healthy. Best of luck in your healing!]]> Apsu Look what I made! Fri, 20 Nov 2020 14:29:16 -0500 https://reprap.org/forum/read.php?166,859694,878941#msg-878941 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,878941#msg-878941
This thread has been dead for more than a year !
Let bad ideas, unsubstantiated claims rest in peace for fear that like vampires they pop up again :)]]> MKSA Look what I made! Fri, 20 Nov 2020 04:35:22 -0500 https://reprap.org/forum/read.php?166,859694,878926#msg-878926 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,878926#msg-878926
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cozmicray
Perhaps a sprial bevel gear

[www.thingiverse.com]

they are very smooth and used in Hi torque applications (differentials)

?What is the best (smoothest) right angle gear configuration?

Have to also consider the locking of the gear?

:S
plus 1

i no longer remember details. i know there is a better gear setup out there.

at moment both Apsu, and what you link. the gears will either want to jump "up" or push "up. one spun one way, and then reversed they will want to push "down" right were the teeth mesh together. this causing wear on teeth and putting vibration down the screws rods.

*bah* life of me i do not remember names of the joints.
1. differential on a car/truck for the drive shaft that feeds power to both rear tires. i know there is some extra "gear and then gear sets" to allow for rear wheels to spin more freely of each other. but there was some different types of differential setups that had a more better setup for this perhaps?
2. or CV joints for front drive wheels. but most likely not.
3. 2 additional gears toss in (current set of 2 gears then other side of each gear parallel them) but that just combines additional vibrations, backlashing, jamming.
4. there are few other "joints" that do not use gears. but with the main lead screw going all the way through, and the other rod going 90 degrees. those joints can not be used. without cutting the main lead screw in half.

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JoergS5
This a very interesting idea.

If you suffer from backlash in the gears, you can try a friction drive (with 90 degree pivoted), or the idea of [www.youtube.com]

Looking forward your further development!

Apsu you could do what JoergS5 shows. it has merit. with ability to pull the 2 top pulleys "upwards" for a belt tension device.

what cozimycray, shows there are some good amount of math for "teeth profile" that can get really nasty. some teeth profiles better for certain directions, while others for reverse direction. while others forward and backwards. at moment your gears will have major wear at the "sharp / 90 degree" like corners. what cozimycray shows is more of the tooth will touch and mesh together resulting in longer term use with less wear.

there is a wave pattern for the teeth vs 90 degrees or 45 degrees offset. but i no longer remember the details and might be placing the wave pattern teeth profile for some other gear setup.

=================
since you do not have "weight". you have noted you have added some springs inside to help reduce backlash. by keeping a force on things.

i really could not see detail of springs you videos and/or animation videos.

if you went bearing -> washer/s -> spring -> washer/s -> gear..... then screwed the main shaft all the way through. the single spring may not do much. thing of a "sew saw for kids" the were teeth mesh, the spring diameter, and then the lead screw. each of them are acting as a point on a simple lever / fulcrum. and you have a very big kid on very long length of lever. while the little spring still a knee high baby. trying to lift the the big kid on the short length of lever not really going anywhere. "yes yes" things are spinning and turning. but you are still obtaining a lever / fulcrum points, and example was out of proportion. but hopefully idea gets across.

i don't even remember name. i want to say an old style "clutch pack". instead of single spring in center. there is smaller springs around the shaft. the springs set in a "disc" and were the springs are placed the disc is partially drilled out to hold the spring in place. same thing is done for another disc that holds other side of springs. then there some screws between the 2 disc's that put some tension on the springs and keep the 2 discs from coming part. both disc's with springs between them then looks like a huge ugly washer. with bearings as well. Goal is more springs = not needing as heavy duty of single spring. but each spring adds up over all. and it moves the fulcrum point towards the "big kid" above and gives the knee high baby some room to move on a simple lever doing.

what others are saying for different tooth profiles, or belt pulley's type setup. is trying to reduce the "back lash". the spring / "so called clutch pack" is more of a band-aid, vs going more at the direct issue of causing the back lash in the gears.

remember 1 major ugly factor for your system. moving x,y at same time. means one motor spinning faster than other motor and making high speed sudden reverse of current direction. those with belts will see it in belt vibration along with belt expanding contracting. you will see it on other hand as teeth wear and vibrations going through the lead screws. and at moment to me, your gear profiles are highest point of "failure" and/or cause of a worst 3D print that you could improve and have good effect on your system vs other things.]]> boggen79 Look what I made! Thu, 19 Nov 2020 17:52:27 -0500 https://reprap.org/forum/read.php?166,859694,860096#msg-860096 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860096#msg-860096
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JoergS5
I offered the ideas only because in your second video [youtu.be] at second 18, when you change from X motion to Y motion, there is a jerk in the gears which may produce X direction errors. I thought this is due to backlash, but this may have another reason.

I agree that using flexible material will create other problems.

Yeah, I saw it as well. Some of it was definitely backlash between the gear teeth. I have since changed them out for a tighter spacing, and that effect is gone. However, there is some twisting in the rail clamps underneath the gearboxes that I'm investigating. There's no longer a jerk as backlash is taken up, but there are some small inconsistencies on direction changes and differential transfer because the X rail is rotating a little in its clamps. I have some ideas to minimize or fix it, so we'll see what I can come up with.]]> Apsu Look what I made! Sun, 29 Sep 2019 19:32:53 -0400 https://reprap.org/forum/read.php?166,859694,860086#msg-860086 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860086#msg-860086 youtu.be] at second 18, when you change from X motion to Y motion, there is a jerk in the gears which may produce X direction errors. I thought this is due to backlash, but this may have another reason.

I agree that using flexible material will create other problems.]]> JoergS5 Look what I made! Sun, 29 Sep 2019 15:05:01 -0400 https://reprap.org/forum/read.php?166,859694,860081#msg-860081 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860081#msg-860081
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JoergS5
This a very interesing idea.

If you suffer from backlash in the gears, you can try a friction drive (with 90 degree pivoted), or the idea of [www.youtube.com]

Looking forward your further development!

Thanks! I'm really trying to stay away from flexible materials entirely, that's one of the goals here (as well as simplicity and low part count/high temp materials), so I'm not looking at 90deg belt drives and similar.

Friction drives are possible, as well as some more complex gear tooth profiles, but I don't actually think the gear backlash part is much of a real concern. Here's my reasoning:

---

Any tooth-tooth backlash doesn't translate directly to linear motion in X or Y, because only the nuts move the axis. That means any backlash in the gears is divided into the screw/nut resolution, which is backlash/360 * lead.

For example, 1º of backlash would be: 1/360 rev * 8mm/rev = 0.022mm linear backlash, or 22 micron The worst backlash possible on my 20T (tooth) gears is 360/20 = 18º, and that's like, missing a whole tooth. So practically speaking, let's say it's about half, or 10º for simplicity. That would be 0.2mm linear (200micron), which is certainly not great, but I just want to illustrate that even with extremely poor meshing, the gear -> screw -> nut relationship makes it far better than it seems at a glance.

In comparison, 20T belt driven XY is often quoted at the calculated 25micron resolution that gets you, but as we all know when it comes to belt systems, there's often variations in tension, alignment, concentricity of idlers/pulleys/shafts, and a variety of other factors that reduce the resolution. So it's not like my design is wildly worse in comparison, even in a worst-case tooth mesh scenario.

Of course, there's considerations and concerns in mine as well, such as linear guide alignment, stiffness, straightness, deflection tolerance, screw straightness, etc. But those are present in a belted design as well, in terms of the mechanical gantry.

---

Now all *that* said, my videos had some gear mesh issues and also some linear rail/frame squareness issues. I saw and have improved them since then; I replaced the gear sets and adjusted spacing and can see/feel no discernible tooth-tooth backlash, and I replaced one of the linear rails as the old one was relatively rough and loose, and I printed an X carriage to further connect and stiffen the X screw/X rail portion. I'll have new video soon, but the behavior seems to have improved a lot.

Sorry for the lengthy response, thanks for your thoughts!]]> Apsu Look what I made! Sun, 29 Sep 2019 13:03:40 -0400 https://reprap.org/forum/read.php?166,859694,860068#msg-860068 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860068#msg-860068
If you suffer from backlash in the gears, you can try a friction drive (with 90 degree pivoted), or the idea of [www.youtube.com]

Looking forward your further development!]]> JoergS5 Look what I made! Sun, 29 Sep 2019 08:02:36 -0400 https://reprap.org/forum/read.php?166,859694,860043#msg-860043 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860043#msg-860043
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the_digital_dentist
Cool! Sound?

It's coming, the TV was on when I took those videos so I muted it. I will make a better video with explanations/commentary of the parts and design and operation very soon, and get the X carriage mount in place so it's easier to see both axes.]]> Apsu Look what I made! Sat, 28 Sep 2019 11:10:39 -0400 https://reprap.org/forum/read.php?166,859694,860035#msg-860035 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860035#msg-860035 the_digital_dentist Look what I made! Sat, 28 Sep 2019 08:00:25 -0400 https://reprap.org/forum/read.php?166,859694,860015#msg-860015 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,860015#msg-860015
It works!! :D

First moves
X only then Y only

The left gearbox has some spacing issues and extra tooth-tooth backlash as a result, but that's fixable. I'm just super excited that it works on stock CoreXY kinematics and demonstrates all three differential behavior modes correctly.

I'm going to get the X carriage connected to the X screw with a nut and work on the gear mesh, then I'll record a more comprehensive video with clear demonstration of function and commentary.]]> Apsu Look what I made! Fri, 27 Sep 2019 22:49:11 -0400 https://reprap.org/forum/read.php?166,859694,859887#msg-859887 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859887#msg-859887
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Trakyan
I get you on not wanting flexibility from cables, but what I'm talking about the cables are very short, they are only enough to wrap around the two drums that are almost touching in a figure 8 type arrangement, they serve to take out backlash/slop more than transmit force. It was just a suggestion for some ghetto low backlash gears.

Why would the worm need to back drive the pinion? All the motive force comes from the Y screws being driven by the motor, the worm wheel shouldn't ever be driving the screws. You're right that you wont get a 1:1 gear ratio. Perhaps using a higher lead on the X axis screw to make up for the gear reduction from the worm? Or maybe just say its a feature not a bug :P

Upon some closer thinking, what would be needed for the worm-wormwheel version I think is not different handed-ness of screws, but have one leadscrew connected at the top of a wormwheel, the second at the bottom. That way when both Y screws are trying to advance forward or backward, they try to turn the X screw in opposite directions so it doesn't rotate, and instead the worm wheels advance along the leadscrew giving Y axis movement. Similarly, when the Y screws turn in different directions they will both turn the worm wheels in the same direction, so the X screw spins giving X axis movement. This sort of combines the lead-nut and bevel gears into one using the worm wheel which shortens the component list and reduces sources of backlash. The asymmetry of this design (one leadscrew contacting on top, one on bottom) bugs me though, your version looks more pleasing to the eye.

A side effect is that this setup could be cantilevered (I personally wouldn't want to, but ya'know), Two parallel lead screws (your driven lead screws) with a worm wheel sandwiched between and another leadscrew attached to that worm wheel acts as your other axis. I'm sort of rambling here.

But wow, your setup is very easy to back drive, maybe I'm overthinking the parasitic friction.

There's benefits to keeping both axes in the same plane, too, chief of which is keeping the torque moments from being more complex and multi-axis. Given that, I think the bevel setup is a solid choice to continue to pursue for now. It's good you're thinking about robustness and future assembly ideas though!

And yeah what I have now is quite low friction in both directions because of the carefully measured spacing of parts to keep the gear-nut loaded firmly on the thrust bearing behind it, so either direction it's driven it doesn't change the gear mesh depth.]]> Apsu Look what I made! Wed, 25 Sep 2019 19:24:58 -0400 https://reprap.org/forum/read.php?166,859694,859886#msg-859886 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859886#msg-859886
Why would the worm need to back drive the pinion? All the motive force comes from the Y screws being driven by the motor, the worm wheel shouldn't ever be driving the screws. You're right that you wont get a 1:1 gear ratio. Perhaps using a higher lead on the X axis screw to make up for the gear reduction from the worm? Or maybe just say its a feature not a bug :P

Upon some closer thinking, what would be needed for the worm-wormwheel version I think is not different handed-ness of screws, but have one leadscrew connected at the top of a wormwheel, the second at the bottom. That way when both Y screws are trying to advance forward or backward, they try to turn the X screw in opposite directions so it doesn't rotate, and instead the worm wheels advance along the leadscrew giving Y axis movement. Similarly, when the Y screws turn in different directions they will both turn the worm wheels in the same direction, so the X screw spins giving X axis movement. This sort of combines the lead-nut and bevel gears into one using the worm wheel which shortens the component list and reduces sources of backlash. The asymmetry of this design (one leadscrew contacting on top, one on bottom) bugs me though, your version looks more pleasing to the eye.

A side effect is that this setup could be cantilevered (I personally wouldn't want to, but ya'know), Two parallel lead screws (your driven lead screws) with a worm wheel sandwiched between and another leadscrew attached to that worm wheel acts as your other axis. I'm sort of rambling here.

But wow, your setup is very easy to back drive, maybe I'm overthinking the parasitic friction.]]> Trakyan Look what I made! Wed, 25 Sep 2019 18:54:37 -0400 https://reprap.org/forum/read.php?166,859694,859850#msg-859850 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859850#msg-859850
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Trakyan
Well someone came out from under the bridge.

Anyways, the "parasitic" effect I was meaning mostly is if friction between the nut and screw causing the nut to spin with the screw (thus not moving axially as it should) rather than stay stationary when you want it, or vice versa friction in the bevel gears stopping the nut from rotating freely with the lead screw (and thus causing it to move axially when not intended). These would possibly cause racking, which would make these sources of friction worse and so on. I'm planning on finally getting to build a CNC either late this year or early next year and I'd be happy to help with the prototyping and R'n'D.

As for backlash, I just remembered this type of anti backlash nut I saw a while ago, might be useful.
[eu.makerdise.com]
The spring force is applied radially via a collet type system. They means that axial forces in the direction of the screw (say cutting forces on a router/mill) can't compress the spring and reintroduce the backlash.

Also a suggestion for the bevel gears if backlash is an issue, you could use two angle drums coupled by a cable. These sorts of mechanisms are very low backlash and very cheap/easy to make compared to bevel and spiral bevel gears.

Also I think there's a way to simplify the mechanics. If you used two different Y axis screws, one left handed one right handed. Then added a worm wheel to either end of the X axis screw which mates to the Y axis leadscrews. I think this would give the same mechanical response with fewer parts. I could be wrong, haven't had too much time to think this through. The two different Y screws might not be necessary now that I think about it, maybe they can be the same handed-ness.

Yeah, I follow you on the friction. It's a concern but the static setup I'm achieving currently with a fixed spacer instead of spring system is resulting in really low friction on both gears and nut.

I'm hoping to avoid belts/cables/anything flexible, if possible. Part of why I'm pursuing screws and gears in the first place :P

As for worms, I considered it as well, and different handedness screws! Two problems I see are that the driven worm can't backdrive the pinion, so each pinion would see a locked X axis kinematically at all times, and second problem is that you can't get a 1:1 worm gear ratio.

Great thoughts though, and help prototyping with someone with a CNC would be awesome, sounds fun.

---

Edit: Added a video of just the Y gear-nut operating in some of the modes in different directions with only minor fingertip force on the screw. It's here. And yes, I know one of the thrust bearings has some eccentricity, and neither of them are greased so they're noisy. That'll come later ;P]]> Apsu Look what I made! Wed, 25 Sep 2019 03:52:00 -0400 https://reprap.org/forum/read.php?166,859694,859845#msg-859845 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859845#msg-859845
Anyways, the "parasitic" effect I was meaning mostly is if friction between the nut and screw causing the nut to spin with the screw (thus not moving axially as it should) rather than stay stationary when you want it, or vice versa friction in the bevel gears stopping the nut from rotating freely with the lead screw (and thus causing it to move axially when not intended). These would possibly cause racking, which would make these sources of friction worse and so on. I'm planning on finally getting to build a CNC either late this year or early next year and I'd be happy to help with the prototyping and R'n'D.

As for backlash, I just remembered this type of anti backlash nut I saw a while ago, might be useful.
[eu.makerdise.com]
The spring force is applied radially via a collet type system. They means that axial forces in the direction of the screw (say cutting forces on a router/mill) can't compress the spring and reintroduce the backlash.

Also a suggestion for the bevel gears if backlash is an issue, you could use two angle drums coupled by a cable. These sorts of mechanisms are very low backlash and very cheap/easy to make compared to bevel and spiral bevel gears.

Also I think there's a way to simplify the mechanics. If you used two different Y axis screws, one left handed one right handed. Then added a worm wheel to either end of the X axis screw which mates to the Y axis leadscrews. I think this would give the same mechanical response with fewer parts. I could be wrong, haven't had too much time to think this through. The two different Y screws might not be necessary now that I think about it, maybe they can be the same handed-ness.]]> Trakyan Look what I made! Wed, 25 Sep 2019 01:34:04 -0400 https://reprap.org/forum/read.php?166,859694,859824#msg-859824 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859824#msg-859824
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leadinglights
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MKSA
.............................................................................

So no need for "conversation", just build the prototypes and prove your points.

.......................................

Hmmm, and I always thought that a forum was a place you come to discuss ideas, to have a conversation.
I never realised that it is just a place to present a glorious and finished design, unsullied by the input from anybody else.

Mike

Oh, don't mind MKSA, he always gets up on the wrong side of the bed. I personally like to read about interesting new ideas. New ideas move all of us forward, just not always in the direction we think we're moving... ;)

DLC]]> dlc60 Look what I made! Tue, 24 Sep 2019 15:15:27 -0400 https://reprap.org/forum/read.php?166,859694,859777#msg-859777 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859777#msg-859777
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leadinglights
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MKSA
.............................................................................

So no need for "conversation", just build the prototypes and prove your points.

.......................................

Hmmm, and I always thought that a forum was a place you come to discuss ideas, to have a conversation.
I never realised that it is just a place to present a glorious and finished design, unsullied by the input from anybody else.

Mike

Computer animation are indeed great "discussion piece".

How about resuming the conversation when there is a prototype ?

Will wait.]]> MKSA Look what I made! Tue, 24 Sep 2019 00:16:24 -0400 https://reprap.org/forum/read.php?166,859694,859772#msg-859772 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859772#msg-859772 www.supermagnete.de]

Could be a single magnet with an iron counterpart too ...]]> VDX Look what I made! Mon, 23 Sep 2019 18:52:57 -0400 https://reprap.org/forum/read.php?166,859694,859767#msg-859767 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859767#msg-859767
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VDX
... an interesting "slip-free" alternative to gears are two strong ring-magnets with beveled perimeters ;)

Interesting idea! Not sure how well that'd actually work or how easy to obtain for testing, but interesting.]]> Apsu Look what I made! Mon, 23 Sep 2019 18:03:07 -0400 https://reprap.org/forum/read.php?166,859694,859766#msg-859766 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859766#msg-859766
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leadinglights
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MKSA
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So no need for "conversation", just build the prototypes and prove your points.

.......................................

Hmmm, and I always thought that a forum was a place you come to discuss ideas, to have a conversation.
I never realised that it is just a place to present a glorious and finished design, unsullied by the input from anybody else.

Mike

Exactly why I presented my work in progress, theory, and plans, to start a conversation here instead of just on the discords and chats I'm in lately. Guess we both missed the mark ;P]]> Apsu Look what I made! Mon, 23 Sep 2019 18:02:06 -0400 https://reprap.org/forum/read.php?166,859694,859765#msg-859765 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859765#msg-859765
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MKSA
.............................................................................

So no need for "conversation", just build the prototypes and prove your points.

.......................................

Hmmm, and I always thought that a forum was a place you come to discuss ideas, to have a conversation.
I never realised that it is just a place to present a glorious and finished design, unsullied by the input from anybody else.

Mike]]> leadinglights Look what I made! Mon, 23 Sep 2019 18:00:50 -0400 https://reprap.org/forum/read.php?166,859694,859763#msg-859763 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859763#msg-859763 VDX Look what I made! Mon, 23 Sep 2019 17:23:40 -0400 https://reprap.org/forum/read.php?166,859694,859759#msg-859759 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859759#msg-859759
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cozmicray
Perhaps a sprial bevel gear

[www.thingiverse.com]

they are very smooth and used in Hi torque applications (differentials)

?What is the best (smoothest) right angle gear configuration?

Have to also consider the locking of the gear?

:S

Yes indeed, I am actually using spiral bevel gears right now. They are excellent for this application. I'm not sure what you mean by "locking" though. Do you mean fastening to the X screw and Y nuts? For that I'm using a set screw on X geasr and 4 screws in the bottom of the Y gears to lock the screw nuts inside them.]]> Apsu Look what I made! Mon, 23 Sep 2019 15:30:40 -0400 https://reprap.org/forum/read.php?166,859694,859758#msg-859758 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859758#msg-859758
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Apsu

Nah. I tried to engage you in a discussion but since it's such an obvious mechanism you didn't read what I wrote, I will return the favor. Best of luck to you :)

Listen, I don't need to read your explanation to see how it works. I can even tell you that in theory it works BUT I question your claims.

So no need for "conversation", just build the prototypes and prove your points.

Don't count on luck though.

"Never repeat a successful experiment" :)]]> MKSA Look what I made! Mon, 23 Sep 2019 15:26:14 -0400 https://reprap.org/forum/read.php?166,859694,859757#msg-859757 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859757#msg-859757 sprial bevel gear

[www.thingiverse.com]

they are very smooth and used in Hi torque applications (differentials)

?What is the best (smoothest) right angle gear configuration?

Have to also consider the locking of the gear?

:S]]> cozmicray Look what I made! Mon, 23 Sep 2019 15:17:21 -0400 https://reprap.org/forum/read.php?166,859694,859754#msg-859754 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859754#msg-859754
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MKSA
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Apsu
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MKSA

What kind of test did you carry to back your claims ?

I'm not sure you're following what I'm presenting here. The items I listed as potential benefits over a belted system are observations of facts based on assuming a well-executed build in terms of materials and mechanical construction. There's no testing required to note the advantage of a screw over a belt in the aspects I mentioned. Or that the part count is lower than a CoreXY, or that you don't need to worry about the small bearings used in 5mm bore pulleys. And so on.

Doesn't mean it's an automatic win, there's a lot of engineering going on here and ways to fail. Plus there's factual disadvantages that need to be overcome, like requiring a higher motor RPM for lower screw leads to achieve equivalent gantry speeds to belts, something that small frame stepper motors struggle with.

If there's something about this list of obvious observations you think needs to be tested, please elaborate.

Don't worry, I followed, found this mechanism "obvious" and didn't need to read you "explanation" but I consider irrelevant your list of obvious advantages from obvious observations

OK on a computer simulation not in real life.

So do a prototype THEN do a list of REAL pro and cons.

Please avoid listing the build shortcomings of other machines for ex. the small bearings used in CoreXY.

Nah. I tried to engage you in a discussion but since it's such an obvious mechanism you didn't read what I wrote, I will return the favor. Best of luck to you :)]]> Apsu Look what I made! Mon, 23 Sep 2019 14:41:00 -0400 https://reprap.org/forum/read.php?166,859694,859753#msg-859753 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859753#msg-859753
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Apsu
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MKSA

What kind of test did you carry to back your claims ?

I'm not sure you're following what I'm presenting here. The items I listed as potential benefits over a belted system are observations of facts based on assuming a well-executed build in terms of materials and mechanical construction. There's no testing required to note the advantage of a screw over a belt in the aspects I mentioned. Or that the part count is lower than a CoreXY, or that you don't need to worry about the small bearings used in 5mm bore pulleys. And so on.

Doesn't mean it's an automatic win, there's a lot of engineering going on here and ways to fail. Plus there's factual disadvantages that need to be overcome, like requiring a higher motor RPM for lower screw leads to achieve equivalent gantry speeds to belts, something that small frame stepper motors struggle with.

If there's something about this list of obvious observations you think needs to be tested, please elaborate.

Don't worry, I followed, found this mechanism "obvious" and didn't need to read you "explanation" but I consider irrelevant your list of obvious advantages from obvious observations

OK on a computer simulation not in real life.

So do a prototype THEN do a list of REAL pro and cons.

Please avoid listing the build shortcomings of other machines for ex. the small bearings used in CoreXY.]]> MKSA Look what I made! Mon, 23 Sep 2019 14:37:19 -0400 https://reprap.org/forum/read.php?166,859694,859748#msg-859748 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859748#msg-859748
Quote
MKSA

What kind of test did you carry to back your claims ?

I'm not sure you're following what I'm presenting here. The items I listed as potential benefits over a belted system are observations of facts based on assuming a well-executed build in terms of materials and mechanical construction. There's no testing required to note the advantage of a screw over a belt in the aspects I mentioned. Or that the part count is lower than a CoreXY, or that you don't need to worry about the small bearings used in 5mm bore pulleys. And so on.

Doesn't mean it's an automatic win, there's a lot of engineering going on here and ways to fail. Plus there's factual disadvantages that need to be overcome, like requiring a higher motor RPM for lower screw leads to achieve equivalent gantry speeds to belts, something that small frame stepper motors struggle with.

If there's something about this list of obvious observations you think needs to be tested, please elaborate.]]> Apsu Look what I made! Mon, 23 Sep 2019 12:16:30 -0400 https://reprap.org/forum/read.php?166,859694,859747#msg-859747 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859747#msg-859747
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Apsu
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MKSA
.
A CoreXY is also a "differential" based system.

I fail to see the advantages, as for issues, you noticed a few already.

There's quite a few potential advantages if executed well. I mention a bunch at the bottom of the github document, but to recap:

No long complex belt paths means less issues with tensioning/alignment/pulleys/tiny bearings/hysteresis/stretch/temperature.
Easier material choices to fully enclose the gantry in a high-temp enclosure for the most challenging materials, like PEEK.
Simple and relatively cheap assembly with low part count.
Easier to change mechanical ratios without dealing with pulley sizes increasing.

What kind of test did you carry to back your claims ?]]> MKSA Look what I made! Mon, 23 Sep 2019 12:07:06 -0400 https://reprap.org/forum/read.php?166,859694,859740#msg-859740 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859740#msg-859740
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MKSA
.
A CoreXY is also a "differential" based system.

I fail to see the advantages, as for issues, you noticed a few already.

There's quite a few potential advantages if executed well. I mention a bunch at the bottom of the github document, but to recap:

No long complex belt paths means less issues with tensioning/alignment/pulleys/tiny bearings/hysteresis/stretch/temperature.
Easier material choices to fully enclose the gantry in a high-temp enclosure for the most challenging materials, like PEEK.
Simple and relatively cheap assembly with low part count.
Easier to change mechanical ratios without dealing with pulley sizes increasing.]]> Apsu Look what I made! Mon, 23 Sep 2019 10:12:50 -0400 https://reprap.org/forum/read.php?166,859694,859738#msg-859738 Re: DiffBot - Differential Screw-Driven XY Gantry https://reprap.org/forum/read.php?166,859694,859738#msg-859738
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the_digital_dentist
Interesting, but I suspect the hardware to make it work is going to be pretty expensive. My experience with using cheap gears to make a belt lifted Z axis tells me that any flaws in the gears are going to show up as repeating defects in the prints each time each of the gears makes one rev. You're going to need very high quality (read expensive) gears to make it work well. What happens to print quality as the gears wear?

I don't think it has to be that expensive. My current printed gears are based on CAD models from off the shelf metal ones from places like McMaster and KHK, where the prices are in the $20-30 range, and can work for at least half the gears I'm using as-is. I'd also say that Z is a bit more sensitive to backlash or irregularity (in terms of visual result) than X and Y, which is why belted Z can be challenging.

As for wear, I'm not too worried about it at the moment. The loads involved are relatively small and not constant (unlike Z lifting a heavy bed assembly) compared to the material choices, especially with metal.]]> Apsu Look what I made! Mon, 23 Sep 2019 10:06:50 -0400