started designing a new mostly-plastic reprap SCARA with fishing line motion transmission and unorthodox winch for Z

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theothermike
Go ahead with controllers you´re feeling most comfortable. ;-)

Funny you say that considering that I didn't even see a real 3d printer when I started discussing Wally on Nicholas Seward's forum, planning to have it built by someone with a 3d printer in my area. But then, those ideas became another bot altogether. Then I learned about SCARA, then I learned some CAD, then I faced engineering challenges, now software and electronics. I'm a noob at everything here, am quite surprise I can still swim through all this

The sanguinolulu is a "spare" a friend has, so I am most confortable with what I can get my hands on right now and will have to learn everything in the process. This is why this built might take a while.

Please let me know if there is any success with the teacup SCARA. I am mostly interested in that.

Thanks.

Edited 1 time(s). Last edit at 06/28/2015 08:08PM by realthor.

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Hi guys, I need to take this off my chest. Although the most of the design is settled, the spur-screw winch is something I am not totally happy with. Although I consider the idea awesome and would like to prototype it at some point, I am not confident I understand everything about how it would affect the print. Nicholas was interested in the idea and offered to prototype it some time but also expressed some doubts regarding artifacts it might create on the print and ultimate accuracy. I didn't understand then and I don't now what he was talking about nor do I understand how a left-handed spiral (suggested by him) would help.

I am not that confident in my CAD skills either, to create the correct worm, the correct spur, the correct helical thread for the screw and the correct nuts profile for a smooth engagement.

So I am thinking of modifying the design, by getting rid of the worm part and replacing it with a spectra-drive solution. My beef with the worm is the backlash it would introduce if engaged both directions but the upside would have been the self locking when the stepper was off tension. There would have been no chances that the screw would have been driven back through the nuts and that the spur teeth would have in any way affected the worm. The stepper could have been off and I think Z wouldn't have moved. I liked this very much about that solution. That would not be the case if I drive the spool with a spectra loop because the stepper's shaft would be axially aligned with the spool, making it harder for the motor to keep Z constant - it would need a continuous effort of the stepper.

Before presenting any sketch for discussion or prototyping the solution I need to know if it makes sense at all from the point of view of the continuous brake post movement. The stepper will raise the Z for the next layer and then keep the same tension/torque to prevent skipping steps. With lead screw driven Cartesian bots it's a non issue. I am thinking that the CoreXZ would have a similar issue? With Deltas the effector is too light and the carriages are constantly moving so I don't think this issue applies there. I think I might need a stepper with a brake that is normally engaged and will release wile the motor is moving to the next position. Is a "normally-braked" stepper even a real thing? Or I must rely on other type of mechanical/electromechanical brakes? Most solutions I can find on google (direct acting brake for example) do not justify the additional costs.





Thank you.

Edited 5 time(s). Last edit at 06/29/2015 07:58AM by realthor.

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There is a parameter in the firmware that sets the timer for motor hold current.
My WALLY dropped the bed often until I set the hold timer to a high setting.
The downside holding current heats up the Z motor.

Very interesting to hear that this is your first printer.
Designing the moon buggy before even seeing or using a golf cart????
But sounds like a mechanical engineering fun exercise.
the thrill of design / development and the frustration / agony that "It don't work"

Find a kit -- build it --- improve it --- get to know hardware/firmware/ software
Then work on producing the ultimate Rube Goldberg 3D printing Machine!

Do not be disencouraged.
Designing, creating, constructing,....that´s not a given Talent, more or less a skill one obtaines by thinking, trying, reflecting, rethinking, retrying, .... and chosing the right compromises.
Go ahead if you´re having fun, you´re doing it the right way.

It´s all about stepper current and torque. Some steppers can operate with a surface temperature between -40°C up to 100°C. Special ones up to 120°C. Take a look into the datasheet, but if you stay below lets say 50-60°C stepper surface temperature you should be safe.
Check your torque vs. Speed Chart, calculate needed torque for your bed,.... perhaps you can reduce the Diameter to match all together....

@Cozmicray: knowing too well the golf cart made the moon buggy look like a higher-tech copy of it. Habit hampers innovation. It takes 10times the amount of time spent when one does it the other way but I've found so many times that being naive and unexperienced was better for the solution than well-honed habits. And I wanted to experiment on myself this finding. Also money must not be a goal for true innovation to be worth it . This might not work in the end and I could be frustrated as you say, but this industry will evolve with or without me. But the fun, skills, knowledge and maybe a novel 3d printer configuration will be what I'll be left with.

@theothermike: I am designing this bot to be able to bot lift a bed with the Z stepper but also lift itself up a wall while printing on the floor, so being able to lift its own weight is a side-goal. I am thinking that even without an electromagnetic brake it will be difficult for a Nema17 (which I have already ordered) to lift this weight. Keeping this weight at a still will be even more challenging. I am thinking of having a cantilever of some sorts that would render the weight of the bot to a positive non-zero value. I remember about reading that cantiliver-ing is not a good idea. Is this true? And if so, why is that?

Edited 2 time(s). Last edit at 06/30/2015 05:42AM by realthor.

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Quote
realthor
I'm a noob at everything here, am quite surprise I can still swim through all this

That's a good attitude. As one who has reasonable experience with both, designing printers and designing controlling software, it's quite obvious to me that designing the hardware ist the easy part. Only few people see this, they're used to make a few adjustments in config(uration).h and be done. Not an option with unusual kinematics.

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realthor
Please let me know if there is any success with the teacup SCARA. I am mostly interested in that.

Even if it doesn't, it can be made to work. The basics are there, compilation issues are trivial. I happily help people who try, who report error messages. But you have to at least try and tell the results. Ideally in an issue report on Github (which notifies all Teacup developers).

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I am sure that mechanical designers say the other way around ... it's always an individual's problem that is the worst, no matter what others have on their plate. Anyway, I was daring (naive) enough to set some goals that prevent me from going the paved way ... which starts to bite my ass right now . The SCARA part was quite easy, although that one is a new approach in itself, the redirect pulley/drum makes this design resemble Morgan from the software pov, with two motors symmetrically rotating drums for the shoulder and for the elbow. The Z on the other side... this is where I am at a loss right now, I tried to avoid linear rails or lead screws or even threaded rod but that's tough. Spectra line can not be treated as a timing belt so any movement is done via friction in the spooled line, which introduces triangulation errors that have to be eliminated in design rather than in software. Software will have to account for enough errors anyway.

So at this point the mechanical challenges are the greatest ) ... when i"ll try teacup I am sure those will be the greatest. That's just how we humans are.

Edited 1 time(s). Last edit at 06/30/2015 09:26AM by realthor.

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Have you considered the technique used on the shapeko x axis, but use it for the z axis. Use a geared stepper motor pulling a timing belt to raise the platform up and down. This would get you away from the complexity of the toothed helical gear.

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I thought about this even before I decided to move on with the project. I like this approach. There was another video on youtube about a guy that used the same belt to extend his deltabot and prolonged it with spectra line on the portion that didn't get to the toothed pulley.

Since my goals for the bot are: no belts, until we can print them, I will still focus on finding a way to use spectra line for Z movement. I am now contemplating on a spectra-drive based solution like Deltabots use but with no errors from the spooled pulleys but it's a bit complicated to explain and I want to prototype it first before making a full of myself presenting here something that can't work and my logic didn't clicked fast enough.

It's also more complicated than the Delta approach, which just uses a motor and two short spools with grooves but I am moving quite a load and will use a worm gear to help.

In the meantime, if necessary I would even put a threaded rod there, just to have it done and moving, so I can see where it needs modification.

Edited 2 time(s). Last edit at 07/01/2015 10:38AM by realthor.

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I thought I won't have to dive in the maths and modifying firmware but nevertheless, for the sake of knowledge I will do it.

What would the steps be once the mechanical design is (almost) ready? My thoughts are:

1) know what is particular to my robot with respect to other similar concepts (2-arm scara, 1-arm scara with stepper on shoulder, etc);
- My design has 2 steppers behind the SCARA arm: one driving the shoulder joint and the other driving the elbow joint via a redirect pulley. Where should I take the line of thought from here to define how my equations are different from the scara variation where there is no redirect pulley and the stepper is right on the shoulder joint?

2) define the equations that describe the motion of my robot:
- inverse kinematics: used for printing (G-code will tell the robot where to go in space and the controller needs to back-calculate to get a number for each stepper motor rotations and in which direction)
- forward kinematics: used for homing (in Inverse Kinematics there are many ways/positions to define a certain point in space while for homing there must be a certain configuration of the arms, thus a single kinematic chain that defines that home position) - at least this is how I understand why we can't use inverse kinematics for homing too.

3) select one SCARA implementation flavor from the several available (Morgan Marlin, Vitaminrad's, Teacup and maybe one or two more) and find where exactly in the code should the equations be inputted. Then define the equations as per the specific programming language of the firmware (C for Teacup, etc).

Am I correct with this approach? I really need some mentoring/guiding here as this is quite new to me.

Thanks for your input.

Edited 1 time(s). Last edit at 07/16/2015 12:28PM by realthor.

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Maybe just build an EVA

Probably NOT complex enough?

Automata's low cost 3D printed Eva arm hopes to bring robotics to the masses



[www.3ders.org]

Are you kidding... $4500? That's very ...complex I like simpler will stick with Lander for a while, the bot is ready, waiting for some coding wizard to put that into firmware .... or guide me through it.

Want simpler? Maybe you's like this then: Polar 3d Printer demo. I'll wait until they get a rotary Z too , still not liking vitamins.

Edited 2 time(s). Last edit at 07/23/2015 03:29PM by realthor.

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Quote
realthor
waiting for some coding wizard to put that into firmware

Why would a developer do this? You created the trivial part, the mechanics, now it's time to approach the challenge. Regarding Teacup I'd start with turning on debugging messages in dda_create() to see which movements come in, to how many steps this is translated and what the timings are (dda->c, dda->c_min = time distance in CPU clocks between two steps of the fastest axis).

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Quote
Traumflug

Quote
realthor
waiting for some coding wizard to put that into firmware

Why would a developer do this? You created the trivial part, the mechanics, now it's time to approach the challenge. Regarding Teacup I'd start with turning on debugging messages in dda_create() to see which movements come in, to how many steps this is translated and what the timings are (dda->c, dda->c_min = time distance in CPU clocks between two steps of the fastest axis).

Because the "trivial" mechanical design is pretty much where my knowledge/skills stops, if at that. I don't know much about coding, that is what a developer knows right? If mechanical design should be trivial for me, coding should be trivial for developers. If everybody would know everything we would not need these forums or any contact for that matter.

Regarding teacup I found that Robert Kulhmann's scara implementation never worked and he can't continue developing it for the time being. And he's a developer, he knows things around C or C++. I, on the other hand, don't and it will be kind of pointless to start learning coding or electronics right now. I should have done it during University, but I didn't.

I believe that eventually coders will implement SCARA in the master branches of the mainstream open source firmwares while mechanical designers will design and model the parts and offer the design files.

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realthor
Because the "trivial" mechanical design is pretty much where my knowledge/skills stops, if at that.

There is a procedure called "learning". Worked well for me.

Regarding me, I'm a mechanical engineer, without code writing education. I'm in the same boat as you, much better at mechanical stuff.

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Does your boat have enough time for learning coding? cause I can barely scratch a few minutes here and there for the mechanical side of this 3d printing interest.

I might do it afterall, but it will take forever to add up enough time to matter.

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realthor
Does your boat have enough time for learning coding?

It simply has to. Else machines don't move.

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In the meantime I am having some mechanical design or fabrication issues I should say. The free wheeling drum on the shoulder is wobbly - I guess the beargins didn't go down straight in the whole and got me thinking how should it be designed to prevent this kind of behavior.

Right now the drum is printed in one piece (left side of the image) and the bearings were cold-pressed in their place. If I take them out and put in again the plastic recess is going to maybe suffer deformation. So I must design it in a way that once the bearing goes in it goes in straight and there will be no taking out ... But how to insert them in a straight fashion? No 8mm bolt or rod is actually 8mm, all have play. I've seen methods of forcefully inserting the bearings with a screw and some washers and a nut being screwed on to get the washers together.

I was wondering if for such large drums isn't a Lazy Susan solution better than only relying on 608 bearings at the center of the drums? I was thinking of a channel around the rim with delrin balls so that the contact between the independently rotating drums is transferred to these balls but I am fearing that where the drum wobbles the wear on the plastic will be greater and the delrin balls might deform or they might deform the channel they sit in. I also suggested a separate piece that receives the bearings but again it increases the complexity of the design.



To summarize, my questions are:
1) making a separate piece to receive the bearing good idea? maybe even cut the small piece in half to hold the bearing and then insert it in its place...
2) delrin balls in a channel around the rim good idea?
3) how to insert the bearings in a controlled and straight fashion?

Thanks.

Edited 2 time(s). Last edit at 07/31/2015 11:54AM by realthor.

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A bolt thru the entire drum with bearings washers and nut
chucked up in lathe. Tightened down and straightened while tightening.


Make up jig with plate and perpendicular threaded shaft
Stack up components --- press them in by threading down nut

you can heat bearings to get them in exactly straight.

when it is straight in all axes glue in bearings outer surface -- avoid gluing bearing so the don't rotate

Bearing with spherical outside may work better.

But this may not get angstrom accuracy you require.
???Will wobble ruin the printer???
??? Perhaps laser tracker to sense and compensate for wobble ???

If the shaft doesn't fit tightly on bearings --- bearing / shaft worthless.

If you can't find a bolt to fit tightly in bearings then nothing will work.

The problem with stacking everything up is that bearings will never go perfectly vertically in the hole, they will go into an angle with the increasing pressure and that might crack the plastic. Having it from the freezer might help.

Wobble will not ruin the printer ... I believe it will have a small toll on accuracy (whichever that is).

Heating bearings?!!

For my setup I need a shaft/rod/bolt/etc that is threaded at the ends so I can clamp down the stack. M8 standard is actually less than 8mm by quite a bit. But I don't thik that will pose major problems because as the tensioning spectra will pull the drum towards the bolt on the motor, the bearing will sit against the same side of the shaft/rod and that will only decrease my arm length by 0.2mm maybe. The tension will always keep it there.

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Freeze the bearings.

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Funny I just put 4 bearings in the freezer 10minutes ago and measured them before ... want to measure them after a couple of hours in the freezer ...how much do you think they will shrink? Anyway i'll post results but I think my caliper will not be able to feel the difference

I am still baffled about the M8 which isn't quite M8. I've seen people that put electric tape or teflon plumbing tape around the shaft to get to the bearing's size but that is a very temporary and poor solution. I actually like an idea of a friend that suggested wrapping some high strength wire that is the right diameter. I actually think this is a stellar idea and will try it. In this other situation I guess heating the bearings will do the trick.

Do you have other suggestions?

Edited 1 time(s). Last edit at 07/31/2015 05:17PM by realthor.

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realthor
how much do you think they will shrink?

Less than if you'd warm the plastic by just 10 deg. Steel has a 5 times smaller shrink coefficient than typical polymers.

The advice in the Sells Mendel era was to actually warm the steel part, e.g. with a soldering iron. This way the printed part melts a bit on the surface and everything aligns nicely.

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Confusing .... The parts are pla I don't dare to warm that up it will just become gelatine. Warming the bearing housing makes sense too ...

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realthor
I don't dare to warm that up it will just become gelatine.

Not if you stay below 60 °C. If your room temperature is 20 °C and you warm them by 30 deg to 50 °C you have the same growth as if you'd shrink the steel parts in the freezer by 5 x 30 = 150 deg, from 20 °C to -130 °C. Assuming you have a freezer which can do -130 °C.

Looking from the other side, if you have a typical freezer, you can shrink-cool the steel by 30 deg, from 20 °C to -10 °C. To get the equivalent growth with PLA, you have to warm the PLA part by just (30 / 5 = ) 6 degrees.

A factor of 5 is a lot.

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realthor
Warming the bearing housing makes sense too ...

You warm the steel part only, not the PLA. This way the PLA part stays cool, except where the bearing slides in. As soon as the part is in, you stop warming, the steel will cool and you have a nice fit.

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Huh?

"will never go perfectly vertically in the hole"

Then your shaft fit on bearing is not good enough
find a different shaft, or use a sleve

Really inner race should have zero clearance to shaft for bearing to work.

I had no problem pressing in bearings in Wally arms and drums
but
I wasn't worried about sub arc-second alignment

Your axial surface that bearing rests on is wacky
When bearing all the way in -- outer race should be resting on bottom surface of bearing holder.

Perhaps you should use a bearing holder that uses screws to hold outer race
so you can put it in your laser collimator, get your sub arc-second alignment
then lock down bolts

You can re-do the alignment weekly to keep alignment below sub arc-second tolerance

"For my setup I need a shaft/rod/bolt/etc that is threaded at the ends so I can clamp down the stack"

Hey you are going to press in the bearings then take out mounting rig
-- then use your running spindle


Stop fretting about this assembly accuracy!

Work on the code

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realthor
The problem with stacking everything up is that bearings will never go perfectly vertically in the hole, they will go into an angle with the increasing pressure and that might crack the plastic. Having it from the freezer might help.

Wobble will not ruin the printer ... I believe it will have a small toll on accuracy (whichever that is).

Heating bearings?!!

For my setup I need a shaft/rod/bolt/etc that is threaded at the ends so I can clamp down the stack. M8 standard is actually less than 8mm by quite a bit. But I don't thik that will pose major problems because as the tensioning spectra will pull the drum towards the bolt on the motor, the bearing will sit against the same side of the shaft/rod and that will only decrease my arm length by 0.2mm maybe. The tension will always keep it there.

Quote
cozmicray
Work on the code

I've been told that many times so far but it's not on my agenda yet. Will have to learn openscad first (which should get me in the coding mindset as it resembles C afaik) and then will try my hand at maths and whatever coding these firmwares use (I'm a total noob with real coding, can do some bash scripting though but that is housekeeping stuff compared to the engineering behind real code).

For coding the Morgan firmware will do fine, I'll only need new homing routines. I hope

Cheers.

Edited 1 time(s). Last edit at 08/01/2015 01:22PM by realthor.

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Talking of Morgan firmware, just stumbled upon this gentleman that is building a small printer based on Morgan double-arm scara. His github is very current and he's working on version number two already. Waiting might not be that long (or a bad thing) afterall .

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Mr. Gryb (Pavlo Gryb´s Printer) has modified the arm geometry. The extruder sits outside the outer joint, so math is slightly different and cannot be used in this form for "typical" SCARA.
Perhaps he has done parametisation, so that it can be easily switched by defining the offset?

Hi Mike, yes I have seen that the end effector is out of the joint, which I thought was a very good idea as it gets rid of the large inner diameter bearing. I am sure that, as a developer that does ios and android apps, he must have implemented it parametrically, especially as his earlier prototype, as seen on his blog, was more Morgan-like in effector edsign. Will try to peak at the code maybe I can understand where and how the end effector position is defined.

For now having a working and actively developed scara code is the better news. I'll be away from my prototype for the next couple of months, have already been away from it for 1mo so by the time i'll be able to physically play with it again maybe his v2 will be in an advanced state and we'll be able to reuse the code.

Edited 1 time(s). Last edit at 08/02/2015 05:29AM by realthor.

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