Dear Openbuilds forum community, I have an issue. Pockets always seem to be .2 mm too small, while outer diameters are almost spot on. Please see attached pictures below. I have also added the gcode used for this tiny test. I know of another Openbuilds controls with the exact same issue. I have the 16 bit controller and he has the 32 bit controller. I have had the issue as long as I have had the machine +2 years now. Even with two different controllers. Do you guys have the same issue or know what the problem is? Cheers Martin
If outside diameters are correct,and inner are not, it could be endmill diameter being smaller than spec (say a 6.35mm endmill ground to 6.25mm - 2x0.1mm differences (inside and outside cut) = 0.2mm difference in the end) - check endmills with calipers and use the correct value in CAM workflow Also double check calibration (wizards and tools > calibration) - over the longest distance possible (can enter move length in wizard)
Hi Peter, As always, thank you for your quick reply. I have the same issue with all my bits. I have just confirmed the bit used for the pictures above. It is a 3.175mm, which I measure to 3.17 (the most precise my caliber can measure). It is the same for e.g 6mm as well. 0.2 mm every time. I have calibrated as well. I also think that if it was calibration, it would be an issue for both inner and outer.
Correct, though if you undershoot on the calibration, the outside might be correct (incorrect calibration + error in endmill cancelling out) and then double on the inside (calibration error + endmill error compounded)
Is the error 0.2mm on both the X and Y axes of the pocket shape above? If it is, then calibration and bit size as Peter says is a very likely culprit, but if it is different, then it could be flex somewhere in the machine. Machine flex errors can change their behaviour if you cut climb or conventional, and will usually be worse in harder materials (with the exception of bolts moving in holes that only go as far as the hole 'slop' will let them). Also materials can need a higher cutting force in one direction than the other (e.g. wood grain) that can lead to different machine forces causing different cutting offsets. I notice from the gcode you seemed to be cutting 3mm thickness at 3000mm/minute for the outer slotting operation, and then cutting with a smaller stepover for the inner pocket. In the outer slotting operation, the 3.175mm bit should deflect by less than 0.02mm, which would be unnoticeable on the plywood, and the cutting force from the motors to drive the bit through the wood would be about 15N. The pocket operation where there is not 100% stepover would see lower deflection, and also lower machine forces. Is it possible that there is something loose on the Z carriage that is flexing? A 15N (1.5kg, 3.3lbs) cutting force is pretty low, but not out of the question for causing movement if there is a loose bolt somewhere. One test (that is good to try anyway) would be to set up a dial indicator against the collet, and then push against the collet with your hand to see if the dial moves at all. Then move the indicator to the other side/ axis orientation and push again etc. Flex in the structure will result in a dial indication that gets bigger as you push more hard on the collet, and a loose bolt often gives a sudden 'jump' as the bolt slips and then only changes again when you push the other way. The other way to test is to mill the pocket in something really soft such as foam, and then repeat it with something harder such as oak. If there is flex problem, then any errors are likely to be different in the two different materials. It is also worth doing the test where you do not cut full thickness through the material as you have with the plywood. It is scary sometimes just how much internal tension is in some materials and when cut all of the way through, the materials can flex into a lower stress state, changing the size: I once cut an intricate pattern half-thickness through a block of HDPE and ended up with a shallow bowl when the cut surface contracted! Evan
When you cut an item to fit into a pocket is it too tight? The reason I ask is that you are using two different parts of the caliper to measure the two different cuts. I have a cheap Amazon caliper that when I close it all the way and hold it up to the light it only closes at the tip. There is a gap the rest of the way. I figured out the error in the caliper because I was getting inconsistent measurements. If the parts are too tight to fit in the pocket, then it is probably not the caliper. But, if things fit as they should, maybe the caliper is adding to the other items that may be showing up in the cuts such as bit deflection.
