Linear bearing selection for 3D printer application... dealing with parallelism of rails. 1

[dubc4]

I recently read an article about the 5 common mistakes when selecting linear guides... one of them was related to selecting linear rails/carriages with tighter internal tolerances than the mounting surface/points.

So I have a general application question about 3D printers and the best approach to linear rail selection... Oftentimes, many of our printers are made up of an assembly of aluminum extrusions to form a cube or similar shape. On one of the axis (usually Y), some people use a parallel linear rail system which carries the X gantry for the print head. Since the two parallel linear rails are mounted to separate extrusions that are connected using perpendicular extrusions and corner gussets, the parallelism of the mounting surface certainly won't meet the specs outlined for most linear rails in parallel. Obviously some sort of adjustment must be made to try and make them as parallel as possible. What is the best approach for this sort of adjustment on typical t-slot style extrusion (ie. 8020)? I usually don't see reference shoulders or pins, or fine adjustment screws... it looks like most people just mount them directly to the extrusion slot with t-nuts or something? Not sure how they are ensuring any sort of parallelism?

Based on the article I read it would seem that it would be recommended to select linear rails with some clearance in this very not precise mounting application to prevent binding and unfavorable wear conditions... However, if there is too much clearance it seems possible that some amount of "slop" will show artifacts in the printed products due to the play in the bearings. Would it be advised to externally preload the X gantry by introducing a small but constant force on the X gantry perpendicular to the Y axis linear rail to pull the ganrty to one side (of the Y axis) and take up that play? Or introduce a small moment on the gantry to take up the play in the Y axis? Would doing this end up causing more bearing wear? Or would it be more ideal to remove the parallel rails altogether and use a larger single rail with bearing carriages farther apart (to handle the moments on a single rail and eliminate the parallelism problem) and the opposite side of the gantry just have it rolling on flat surface unconstrained? Would love to know your thoughts. Thank you!

 

[itsmoked]

I can't answer your specific questions as I've just used these things successfully, not designed them and wrestled with extreme accuracy issues. Geees with extrusion printing there are automatically errors starting at the nozzle that will always be larger than than most bed motion issues.

At any rate check these guys out. I own a Mendel Max 3 (MM3) with rubber V-wheels that they've recently discontinued in deference to their newer linear rail versions. I can update my MM3 to linear rails but frankly the MM3 prints are so nice I can't see any reason to drop-the-change on a linear rails update.

I see some of what you're suggesting in their current offerings, like single rail units.

makerstoolworks.com

They seem to engineer all their products rather than scrabbling things together.

They have a very unique support channel. Click on "Live Chat" and you land in a chat room with about 3 guys who are MakersToolWorks employees that will discuss damn near anything related to 3D printing, some of them even run printer farms. I believe they're the designers. I'm betting they would have some applied insights into your questions.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

Paralellism of linear rails has been a problem for a long time.
The most elegant solution I've seen is to make the supports of one rail rigid, and the other rail's supports are provided with intentional lateral compliance, 'soft' enough so that the carriages are not overloaded by minor misalignments.
'Compliance' does not imply 'clearance'; the second rail supports are relatively flexible in just the lateral direction. This is a simple matter to address at design time, not so much if someone has already screwed up the design.




Mike Halloran
Corinth, NY, USA

 

[IRstuff]

Seems to me that you'd want bearings as tight as needed for proper operation and then use the carriage to set the spacing. Once the spacing is trued, you can either epoxy the bearings in place or stake them.

TTFN (ta ta for now)
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[BrianPetersen]

A structure built up from bolted-together aluminium extrusion is never going to be particularly precise and rigid anyhow. The inherent wobbliness of the structure means you probably don't have to worry about precise parallelism because it's going to deflect whichever way it wants anyhow.

If precision is important then you use a single machined base with one bearing rail installed against a precision-machined datum lip in the structure and have the other one bolted down on a machined-flat surface of the same machined base but without the lip and with intentional clearance in the bolt holes. In the carriage, same idea. The datum on the carriage should be to the same set of bearing blocks that run on the datum rail on the base. Upon assembly, you tighten down the rail and bearing blocks that are against their datum surfaces and leave the other bolts loose, then finger-tighten them and run the carriage back and forth so that the rail and blocks find the path that they want to be in, then snug them and do that again, then torque to specs.

Ain't happenin' to that precision using a structure bolted together from extrusions.

 

[m_martin]

I would look into a guide that is not a profiled rail. There are guides that use rollers rather than ball bearings that move within raceways of the rail that are designed to handle the misalignment. The carriage has eccentric rollers that provide preload and can be adjusted so you don't feel slop. You can still get repeatability of +/- 50 microns. Maybe that is not precise enough for your application. The carriage is also usually a bit longer than a carriage for profiled rail so achieving the stroke you want might be tricky if you have space constraints.

 

[itsmoked]

I doubt the OP is even paying attention but m_martin my MM3 appears to be just what you describe except the the rollers are elastimeric. They're on eccentrics.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[GregLocock]

Mike's solution is the usual one in machine design. The rigid rail is the reference, the other has some form of compliance or relaxation of one degree of freedom.

Cheers

Greg Locock


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