Part Tolerance

I am making prototype sprockets for a customer using the Tough 1500 material, and I noticed that the teeth around the circle are around 10 thousands off. I also notice that due to the thickness of the part, the material “pulls” up and slightly warps. Any ideas on how to manage this? I can pressure cast the final part, but I need an acceptable template. Is there a slow shrinkage SLA material I should be using instead? Any thoughts on this is appreciated.

I think slight deviations are unavoidable. I am not certain how to get machinist’s tolerances, other than machining the part. But to get “reasonable” tolerances I tend to slightly oversize them and sand them down by hand, after printing.

That is what I thought, but I figured I would ask. I was hoping the shrinkage on printing would be less than it is on SLA. The thicker the part, the more it seems to pull back from the touchpoints. Its not as bad as ABS on FDM printing, but its still at least 100 to 200 thousands.

I have long since given up on getting tight tolerances from printed parts. Where a printed part needs a tight tolerance, I print the dimension in question slightly oversized and then I hone it down on my CNC. I mostly do this with FDM prints. For Form prints, I mostly open up my tolerances a bit and live with some slop. Though I’ve machined a few of these prints, too.

I could run it on my Fortus 250mc, but I figured the layer lines would be an issue. When you CNC the part to tolerance, how does the surface finish look? Are the layer lines still visible?

If you surface a FDM print, the layers are still evident in the machined surface, but to a much lower degree. The layer lines visible on a SLA print are all surface-layer only, you machine them off and the result looks like you fly cut a chunk of injection molded plastic. But the SLA prints can be brittle, you can’t push the cutter very hard without risking the print chipping.

Thanks for the info. I can’t speak to every SLA material, but tough 1500 machines beautifully.

Hi Tgatliff,

I am a fan of the TOUGH 1500 as a close simulant of PP.

With respect to precision, I have found that the MODEL DENTAL resin the best Formlabs material for precision work. It is my go to for precision work. I often use it to make masters for making silicone moulds. It prints at 25 micron layers so v fine finish.

Precision is very difficult to quantify and is dependent on the number of supports and wall thickness of the part and orientation.

We have a component that is a thin walled cylinder with fine structures. OD =4mm wall thickness 0.25 to 0.3mm. We found the only way to print this in MODEL DENTAL was to print it vertically with no supports directly onto the platform. We designed in sacrificial supports that were machined off. We tried many orientations up to 45 degree angle with supports but found that the structure sagged between supports. As the wall thickness was so thin we designed in vent mouse holes in the sacrificial supports so that the suction did not distort the cylinder.

We also found that temperature excursions of the part had to be managed to reduce the risk of warping.

Off the machine we quote a generic +/- 0.1mm. precision. Holes are drilled or reamed and all threads are cut or inserts used. This morning I cut a male and female M25 thread on a TOUGH 1500 part before final cure. Experimenting with cutting these threads post final cure as I write.

Hope that helps.

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