Choosing Material for 3D Printing CMM Inspection Fixtures

[gkeaton 0]

I am looking for the best material to print inspection fixtures for our coordinate measuring machine (CMM). The parts that we will be measuring are small and the fixtures will need to bend slightly to clamp the part and hold it. The dimensional accuracy is only relatively important but the most important is the extended lifespan of the fixture through the bending. We have a Ultimaker 3 and are open to trying any materials. Thanks!

Please let me know if you have any questions about the application or requirements and I will answer them.

[geert_2 557]

For snap-fit locking mechanisms, or for carabiner hooks, I use PET. This is flexible enough to survive multiple slight bendings, and it does not easily deform permanently. PET is less susceptible to creep deformation due to permanent loads than PLA, although just like any plastics, there is creep. PET is still relatively easy to print.

 

PLA is not suitable: at first it may survive such snap-fits. But after a while it gets harder and more brittle, and then it will start to crack, or it might just break. And it has too much permanent creep deformation under load.

 

If you would like to use PLA because of the ease of printing, an option would be to make the fixture into a rock-solid model that does not have to flex at all. And then provide the locking by a thumbwheel screw. See the pictures below for the concept.

 

Carabiner hooks: cream ones are PLA, green is PET. PLA deforms and cracks after some time.

 

 

Easy locking mechanisms with M4-size nuts and thumbwheel nylon screws. The nut is clamped by the cage where it is sitting in, so no tools are required to keep the nut in place when rotating the thumbwheel. If the cage is a tight fit, the nut won't fall out, even not if the screw is totally removed. These are standard M4 nylon screws, cheap and easily available.

 

Variation on the same concept, to clamp the slider.

 

 

[gkeaton 0]

Thanks @geert_2, I will order a roll of PET and give it a try.  Have you had any experience printing similar parts with Nylon?  I have been reading that it is difficult to print but has great strength and ability to flex compared to other materials.

[gr5 2,167]

Nylon definitely is more flexible.  Which makes it tough.  Very tough.  You can drive a truck over most nylon prints without damaging them.  They just bend/flex.  And there are a variety of different amounts of elasticity.  Materials towards the left are more flexible:

http://gr5.org/mat/

 

It's also harder to print.  The higher the softening temp of a material - the more it shrinks after it becomes solid.  A heated bed helps a lot.  80C is hot enough for nylon.  100C even better.  The other problem with higher temp materials is that new layers don't bond as well to the layer below so the part seems fine until you stress it a bit and it splits along layer lines.  This is solved by lowering the fan speed to almost zero and enclosing the printer to heat up the air.

 

Basically, the higher the softening temperature, the harder a material is to print for these 2 reasons above.  They won't stick as well, they will be more likely to have the corners warp up off the bed and layer adhesion issues might mean the part is very weak if you aren't careful.

[geert_2 557]

I haven't printed with nylon yet, thus no personal experience.

 

So I can't say if the improved flexibility and toughness of nylon would outweight the disadvantages of poorer bed-adhesion, warping, and layer-bonding? This might greatly depend on the model and application. You will have to try.

 

If it was for myself, I think I would start with PET and see if that works well enough. Or I might even start with PLA to get the model and fit right, and then switch to PET for the "production version" that has to survive.

 

[gr5 2,167]

I also like PETG.  It's in between nylon and pla.  Almost as easy as PLA.  More flexible (tougher) than PLA.  Note that if you are building a bridge across a gap with more flexible plastics, the bridge can hold more weight before breaking.  Like snapping a pencil in half.  But if you make a carabiner and the force is mostly tension (pulling), then you won't get much extra strength from a flexible material (typically it will be weaker, not stronger).

 

I believe Ultimaker calls PETG "CPE" and I believe colorfabb sells it as "nGen".   The 3 materials may have differences but I think the differences are minor (different additives?).

 

The other problem with Nylon is it absorbs water like crazy.  It's a pain in the neck to keep dry.  Don't leave it out for more than a few hours.  You have to store it with LARGE packs of rechargable dessicant and occasionally I have to dry my filament on a heated bed overnight.  PETG also seems to absorb water.  I printed some a few days ago that was sizzling and popping a bit but the quality was good enough.  PLA does not have a problem with moisture.  You don't need to keep it dry.

[jkcz 12]

We use mostly Ultimaker Tough PLA and in some cases Ultimaker CPE for most of our printed CMM fixturing. No problems yet, no complaints of cracking or anything.