Three point bed adjustment

I am working on my third printer (cartesian). For bed height adjustment, I have always used four points ( in the corners of the bed). I’ve been thinking maybe I should try a three point system...maybe two adjusters in the back corners and one in the middle in front. I think getting a good flat plain is more difficult with four points but would be easy with three. Has anyone tried this? I would think inertia wouldn’t be an issue with a good spring adjustment system.

Wadaya think.
Charlie.

This is a topic that's been one of my favorites for years.

If you have a rigid, flat bed plate- cast aluminum, for example- 3 point leveling is the best way to go.

There are different ways to implement a 3 point leveling system. My most recent design, used in Ultra MegaMax Dominator, is a kinematic mount that supports the bed with 3 screws and allows thermal expansion without creating any forces that will cause anything to bend. There are only two adjustments to make, and they are made when the bed is first set up and only rarely does it require any more adjustment. In this system, springs are used to hold the bed plate down on the leveling screws instead of pushing the plate up against the screw heads.

Here's a blog post on 3 point vs 4 point leveling.

Here's one on the kinematic mount I used in my printer.

A few months ago I switched from a Smoothieboard to a Duet controller and found the manual bed leveling assist built into the firmware made leveling the bed very accurate and very easy.

I am now in the process of rebuilding the Y axis in my second printer, Son of MegaMax. That machine has the bed moving in Y (I'll never do that again!). I am converting the bed support to kinematic type and managed to get some of the work done at the makerspace yesterday. The photo shows the Y axis base plate with two linear guides. There are 3 bearing blocks under the cross-shaped carriage plate which is made of 2.5 mm aluminum. The bed supports are made from 2" square aluminum tubing and each has a teflon block for a leveling screw. The two ball head levelers for reference and pitch adjustment are installed in the teflon, the roll adjuster, not installed yet, will just be a screw that touches the bottom of the bed plate. There will be springs at each leveler to hold the bed plate down onto the leveling screws.



This axis was driven by a ball screw, but I am converting it to belt drive. The belt will probably run inside the reference and pitch support tubes.

Edited 3 time(s). Last edit at 11/05/2018 09:54PM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

In my previous printer (#2), I used a 12” square of 1/8” aluminum with a Kapton stick on heater sheet on the bottom of the plate and a glass plate over that. The corners of the glass were clipped t allow space for the spring loaded adjusters.

I have 1/4” sheet that I can substitute for the 1/8”. Obviously the 1/4” is twice the weight of the 1/8”, there by increasing the inertia. I suspect I am better to stay with an 1/8”.

Also, to avoid having to completely refab, I was thinking of using the back right adjuster as the “fixed point”, the front left as the leveler, and the front right as the pitch adjustment.

Alternately, I could shorten the glass sheet to 11” front to back to allow a center front pitch adjuster and one of the back as a fixed point and the other as a leveler.

Sorry, I am thinking out loud in the presence of the people who can call me on bad ideas.

Your blog presents several ideas that will have to chew on for awhile.

Think about what the bed does when you adjust one of the levelers- the bed plate will pivot on a line between the other two. If a line between two of the levelers is parallel to one of the axes, you want to use one of those as the reference and one as the pitch adjuster. The third adjuster adjusts roll. If you adjust pitch first, you can then adjust roll without it having any effect on the pitch. If you try to adjust roll first or use the roll adjuster as the reference, each time you tweak either of the other two screws you'll cause pitch and roll to change and you'll have to mess with it a bunch to get the bed into tram.

If you use 1/4" aluminum with a thin piece of PEI, the mass probably won't be much greater than if you use a thinner aluminum sheet and a piece of glass. You'll have fewer first layer problems with the aluminum/PEI combo because the print surface will be evenly heated. If you use something like cast tooling plate, the surface will be flat and evenly heated. With thin aluminum that isn't flat, air gaps between the glass and aluminum will result in cool spots on the glass.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Ok, so here’s the sandwich plan... I have a bottom mounting plate of 1/8th aluminum that rides on v-slot rails and wheels. The 1/4” aluminum plate will be mounted to the mounting plate with a semi fixed point in the left rear and a spring loaded leveling point in the right rear. I will have a spring loaded pitch adjusting point in the middle of the front. The heating element will be stuck to the bottom of this plate. On top of that, I will have a PEI sheet adhered to the 1/4 aluminum with 3M 468MP adhesive . The surface will be 12x12”.

What do you think of that arrangement. At this point, I am moving into unknown areas for me (no glass, PEI, 3 point leveling, etc) so I am open to suggestion.

My existing arrangement has served me well in that I have no adheasion issues, only need to reapply hairspray every 5 to 10 prints, and clean the glass plate with acetone every year or so. Having never done anything else, I thought it time to explore other options. I obviously don’t subscribe to the “if it aint broke...” mind set.

Edited 1 time(s). Last edit at 11/07/2018 08:50AM by charlieRC.

It sounds like it should work, but the right rear leveler will be the pitch and the front leveler will be the roll adjuster. Adjust pitch first, then roll.

The 1/4" plate should be cast tooling/jig plate. Regular plate is extruded or rolled and usually isn't as flat as we need in a 3D printer. Cast tooling plate comes milled flat on both sides. Look for MIC6 or ALCA5, or other cast tooling/jig plate. Cast plate expands evenly when heated because of the random orientation of the metal crystals. Extruded or rolled plate might not behave that way.

If the bed is being thrown back and forth in the Y axis at print speed, you have to have a very solid coupling between the plate and the belt to prevent print defects. That means the leveling screws have to be anchored solidly so they don't allow the bed to wobble when it changes direction. I like to use teflon blocks and allow the leveling screws to roll their own threads into them. Teflon doesn't melt when the screws threaded into it heat up to 100C.

The bed plate also has to be solidly coupled to the leveling screws. That means you need close fitting holes and strong springs to push the plate up against the screw heads (if you use that type of leveler). If you use countersunk flat head screws for the levelers, the bed will be constrained laterally by the screw head in the countersunk hole, and nothing will stand above the bed surface to bang into the extruder nozzle. If you use a kinematic mount, the springs will hold the bed plate down on the levelers and the reference and pitch screws will sit in holes in the bed which prevents the plate from shifting on the screw heads.



The photo above is from an instructable I wrote on upgrading/replacing a printer's bed.

I came up with the kinematic mount because the bed gets hot and the carriage plate, on which the levelers mount, does not. That means that when the plate heats up it pushes the levelers sideways which makes things (usually the carriage plate) bend. The kinematic mount allows the bed plate to expand without creating any bending forces. The plate can expand in X or Y at the pitch adjuster by just sliding over the pitch adjuster's head, and can expand in X and Y at the roll adjuster because it can just slide on top of the roll adjuster screw.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I am building this printer to be all lead screw. You said that you were moving from ball screw to belts. May I ask why? I would think that ball screw would provide the best accuracy of the three (lead, ball, belt). Of course this is coming from someone that until I started this project, has never used anything but belts and threaded rod.

I will check out the cast/tooling plate.

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charlieRC
I am building this printer...
I will check out the cast/tooling plate.

If you're building a printer and want to use a rigid 1/4 bed, a printer with the bed only traveling in the Z is the better option I think. For Prusa style printers with a bed moving laterally, bed weight should be light as you mentioned. A 1/4 aluminum bed on a 1/8 mounting sheet is close to 2.5kg which is a lot of moving mass. Your current 1/8 aluminum and glass arrangement is probably still 1.5kg, but an extra 1kg is still not inconsequential.

That said, I really like a thick bed with three mounting points if the design can accommodate it. Adjustment is a snap, the bed stays flat, and heat transfer is even.

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the_digital_dentist
I came up with the kinematic mount because the bed gets hot and the carriage plate, on which the levelers mount, does not. That means that when the plate heats up it pushes the levelers sideways which makes things (usually the carriage plate) bend. The kinematic mount allows the bed plate to expand without creating any bending forces. The plate can expand in X or Y at the pitch adjuster by just sliding over the pitch adjuster's head, and can expand in X and Y at the roll adjuster because it can just slide on top of the roll adjuster screw.

That's a clean implementation to accommodate bed expansion. Though for a smallish 30cm x 30cm build plate, and printing PLA, the thermal expansion is less than 0.3mm lengthwise, which would easily be taken up in the bolt play. More suited for a larger bed and ABS such as your large printer.

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charlieRC
I am building this printer to be all lead screw. You said that you were moving from ball screw to belts. May I ask why? I would think that ball screw would provide the best accuracy of the three (lead, ball, belt). Of course this is coming from someone that until I started this project, has never used anything but belts and threaded rod.

I will check out the cast/tooling plate.

The printer I am working on originally used a belt drive, but I switched to a precision ground ball screw because it was available for free, and because I thought it might solve some ringing problems I was seeing in the prints. I'm going back to belt drive because the ball screw/motor had a large resonance at about 50 mm/sec that caused it to skip steps. I could print at 150mm/sec with it, but whenever the speed dropped into that 50 mm/sec range, such as when printing large round objects, it would start skipping (and making a lot of noise). I had to keep print speed down to 40 mm/sec to avoid hitting that resonance, and it worked fine but was very slow and relatively noisy. Ball screws are very efficient at transferring vibrations from the motor to the bed plate.

I did a lot of experimenting with microstepping (up to 256:1), dampers, speed and acceleration combos, etc. to try to control that resonance but never could get it under control.

The noise level was such that my wife complained about it a lot and I finally took the machine to the makerspace to get a little peace. That's when I decided to build UMMD, so that I could have a quiet printer that I could use at home without complaints from the wife. UMMD is at the makerspace right now while I work on SoM. My desire to get UMMD back home is what motivates me to finish the work on SoM as quickly as possible.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

When you are looking for suppliers, be aware that ATP-5 is the same alloy as ALCA5 (5083), cast in a different foundry. They weigh about 5% less than MIC6 for the same dimensions and are reportedly easier to machine. MIC6 has inconsequentially higher thermal conductivity.

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gmedlicott
If you're building a printer and want to use a rigid 1/4 bed, a printer with the bed only traveling in the Z is the better option I think..

I've looked at the core X Y configurations and it's intriguing, but I was already too for along with this design. If this does not work, that is probably the first thing I will consider. I also like that it makes it easier to enclose the printer than with the Cartesian design.

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the_digital_dentist
The printer I am working on originally used a belt drive, but I switched to a precision ground ball screw because it was available for free, and because I thought it might solve some ringing problems I was seeing in the prints. I'm going back to belt drive because the ball screw/motor had a large resonance at about 50 mm/sec that caused it to skip steps. I could print at 150mm/sec with it, but whenever the speed dropped into that 50 mm/sec range, such as when printing large round objects, it would start skipping (and making a lot of noise). I had to keep print speed down to 40 mm/sec to avoid hitting that resonance, and it worked fine but was very slow and relatively noisy. Ball screws are very efficient at transferring vibrations from the motor to the bed plate.

I've read about resonance but have yet to experience it (as far as I know). I guess Nema 17s and 23s are equally susceptible to it, right? When I read about it, I wondered if throwing more money at the steppers would help

Edited 1 time(s). Last edit at 11/08/2018 05:51PM by charlieRC.

NEMA-17 motors vibrate less than NEMA-23 motors, and AFAIK, resonance seems to be less of a problem with the smaller motors. Throwing more money at the motors won't help, but throwing it at the drivers might. Advanced drivers (see [www.geckodrive.com]) sense resonance and do tricks to reduce its effect.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
NEMA-17 motors vibrate less than NEMA-23 motors, and AFAIK, resonance seems to be less of a problem with the smaller motors. Throwing more money at the motors won't help, but throwing it at the drivers might. Advanced drivers (see [www.geckodrive.com]) sense resonance and do tricks to reduce its effect.

Hummmmm.... I’m using the Duet WiFi board....that is, I will once I get it back from their repair. It died after only three days of trying to get it set up. It uses TMC2660 drivers. Not sure how that stacks up next to other drivers. I use Gecko G540 for my CNC router, but I would not be eager to plunk down another 300 knowing that I have already committed on the Duet trail. That’s actually a future problem so I’ll see what happens as I move forward.

I’ve been thinking a lot about the Core XY configuration that was mentioned above. Being mid stream on a project and then finding there is another route that should have been explored more,......sheesh. I will continue on for now.

Edited 1 time(s). Last edit at 11/09/2018 02:38PM by charlieRC.

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charlieRC
Ok, so here’s the sandwich plan... I have a bottom mounting plate of 1/8th aluminum that rides on v-slot rails and wheels. The 1/4” aluminum plate will be mounted to the mounting plate with a semi fixed point in the left rear and a spring loaded leveling point in the right rear. I will have a spring loaded pitch adjusting point in the middle of the front. The heating element will be stuck to the bottom of this plate. On top of that, I will have a PEI sheet adhered to the 1/4 aluminum with 3M 468MP adhesive . The surface will be 12x12”.

What do you think of that arrangement. At this point, I am moving into unknown areas for me (no glass, PEI, 3 point leveling, etc) so I am open to suggestion.

My existing arrangement has served me well in that I have no adheasion issues, only need to reapply hairspray every 5 to 10 prints, and clean the glass plate with acetone every year or so. Having never done anything else, I thought it time to explore other options. I obviously don’t subscribe to the “if it aint broke...” mind set.

I have a similar setup to what you are planning. Duet WIFI with a custom CNCed 3mm alu plate running on V-slot (c-beam), 270x230x6.35mm ATP-5 cast aluminium plate with 3 point mounting, 24V kapton heater underneath and a PEI sheet stuck on top.

I'm using The Digital Dentists Teflon Mounting Technique. Teflon sheet from ebay cut on CNC to make the mounts. I've had this setup for over a year now and have only leveled it once (when I installed it!).

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sdavi
I have a similar setup to what you are planning. Duet WIFI with a custom CNCed 3mm alu plate running on V-slot (c-beam), 270x230x6.35mm ATP-5 cast aluminium plate with 3 point mounting, 24V kapton heater underneath and a PEI sheet stuck on top.

I'm using The Digital Dentists Teflon Mounting Technique. Teflon sheet from ebay cut on CNC to make the mounts. I've had this setup for over a year now and have only leveled it once (when I installed it!).

[attachment 107953 Bed.jpg]

very encouraging. I am at the point that I have to sneak all expendatures past my wife, so I was going to try and get by with the 1/4" `12x12 plate instead of the cast aluminum one (shipping and all is probobly about $60) but I put a staight edge ont the 1/4" plate and realize I need the casts plate. Oh well...."Hey honey... Look over there...what is that" while I pull out the credit card.

Another random question.

I was looking into getting some teflon and it's not a big deal, but it occured to me that I have a lot of bits and pieces of "starboard" or HDPE. It's the white material used in boat building. It's extreemly machinable and I pullled out the heat gun and it took quite a while with the gun on high and very close to the material before it got hot enouigh to be deformed. I'm sure the temp was higher than extruder temps so bed temps would seem to be ok. It aslo does not transmit heat very miuch. When I finally got it hot enough to be deformed, I could still hold it comfortably an inch away from the heated end.

So the question is: Is there some special property of teflon that would make it a much better choice than HDPE? (the questions is only based in curiosity and availability, not cheap-atude, which I also have in abundance)

Edited 1 time(s). Last edit at 11/11/2018 09:49AM by charlieRC.

The leveling screws will be in contact with the bed and their temperature will be close to the bed temperature. It looks like the melting temperature of HDPE is 115C, so it probably softens before it gets that hot. That's pretty close to bed temperature if you print ABS. Teflon melts at 327C, but starts decomposing at 260C or so, both much higher than the bed will ever see.

In the interest of cheap-a-tude, try it and see if it works. You can always replace the parts with teflon later if the leveling proves to be unstable because the plastic deforms with heat.

Another material that works fine is Torlon, but it is a b**ch to machine, hard to find for most people, and expensive when you do. One of the makerspace members works at a place that uses a lot of it so he brings cut-offs to the space for people to use for things that require high temperature resistance. I've tried to use it for mounting bed leveling screws, but found it so hard to work with I went back teflon.

Teflon is super easy to machine, is easy to get, and is actually pretty cheap for a high temperature plastic.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yah, I think you've convinced me to go with Teflon. I may make some test parts out of HDPE along the way, but I don't really want to waste the effort to make three of these and a spare only to decide I need to change to Tefflon.

Thanks for talking me away from the edge.

You know, while I was doing some other stuff, I put a piece of HDPE about the thinkness I would use for these tabs on the head bead and cranked it up to 110 deg. I certainly got hot, but I wasn't able to deform it. Next I will tap in a 4 or 5mm screw and tighten it down against a spring that has more tension than I would use for the bed and see if there is any problem with it holding.

I still think I will probably end up with Teflon, but I have this thing about dismissing ideas without trying them. (Can be a very expensive thing sometimes).