What is the B-comp factor?
Hello,
I am currently trying to print a part with very tight tolerances. After printing it once and checking, there was a deviation of 0.2-0.3 mm. I then adjusted the XY scaling factor. However, I then had the problem that another spot did not fit. Now I would like to play around with the b-comp factor. Only I haven't quite understood what this does and I don't have a reference on how to set it.
Would be nice if someone could detailed explain the b-comp factor and how to use it.
If someone has any other suggestions what I could do, let me know :D
Regards
Cem
Comments
5 comments
Negative a-compensation will remove material from inside edges like holes. It is in units of mm. If you have measured 9.85mm diameter hole and want to use a-compensation to correct it, then apply negative 0.15mm/2 (divide by 2 because it applied to the radius and not diameter.
Negative b-compensation will remove material from outer edges.
Cem,
Thanks for your question. It comes up a few times so I will do my best to explain.
First let me explain how compensation works. b compensation is an exterior edge offset in mm applied on the sliced image. a - compensation affects only the inner edges. Using both if you have an extruded profile of a hole and a dowel you can compensate each of them differently to improve the fit. The a/b-compensation is best used to correct for overcure. It can also be applied just to the base layers which have XY overcure sometimes called an elephant's foot, and this way the base layer overcure can be compensated to remove the elephant's foot.
Next let's talk about getting dimensionally accurate parts. Post-processing process including wait and dry times are critical if you care about accuracy in the sub 0.1mm range. The best dimensional accuracy is typically in XY. Scaling factors are primarily used to achieve dimensional accuracy, but be careful to take measurements on dimensionally stable parts. Under-cured parts or parts which have absorbed humidity or not fully off-gassed washing solvents will continue to change in dimension. It usually takes a week after post cure for some materials to become dimensionally stable. Running a second heated cure cycle or storing in a dehydration unit can accelerate the process. Washing with low flash point solvents like IPA will also reduce the time to achieve stability. For context with xABS3843 (a material with moderate water absorption and very opaque so difficult to post cure below the 1-2mm skin of the part) I've observed dimensional changes of 0.05-0.10mm on 20mm cubes measured after post-cure and after a 2nd post cure and 3 day aging.
Hey Brent,
thanks for your answer! Is there a formula to calculate the needed compensation factor? For example if I have a hole that should be 10.00 mm and actually it is 9.85 mm what compensation factor should I use to achieve my desired fit? If there isn't a formula than what effect has a compensation factor of 0.001? Does it make the perimeter (similar to the scaling factor) 0.1 % bigger/smaller?
Regards
Cem
Hi Brent and Cem,
I'd like to tag along on the topic of A/B Comp. Given the explanation of how this setting works, I would like to know if A-Comp is only applied if the slice contains a fully-enclosed interior perimeter? My example is a thru-hole feature in flat, rectangular "block", with the hole-axis oriented in the Z-direction. If the block is printed flat, with the hole-axis perpendiculer to the build-plate, then every slice will see an interior hole-perimeter. However, if you angle the part (and depending on block thickness), the hole may appear as an outer-perimeter feature as it is developed over several slices. I'm guessing A/B Comp is a slice-level feature, and doesn't holistically discern interior perimeters based on a part's geometry. We've had mixed results when applying it to parts of different thicknesses and build orientations, so I presumed it may work best when feature perimeters are fully-defined within a slice.
Regards,
Tony
a/b compensation applies to sliced imaged, so yes - a compensation requires a closed interior contour. In your example the angled block slices on the top and bottom that don't fully enclose the oval will apply the b-factor and not the a-factor. You could apply the same value for a and b to avoid this. Note also when the block is not normal to the LCD, there will be some amount of cure-through on all surfaces facing the build plate, which would have about 1 layer thickness impact on the measured dimensions, so the compensation factors you choose may change with orientation.
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