Belts too tight? (video) Pockets cutting too small, even with a spring pass

Hello everyone! I’m finding that when I’m cutting small pockets, they cut a fraction too small, even with a spring pass to remove any leftover material. The pockets I’ve cut so far are circular and range from 6MM to 30MM in diameter and all are approximately 0.2-0.5MM too small (I find the same thing with squares too!). They’re intended for screw holes, so this makes things difficult. I could manually increase the diameter to accommodate for this, but I’d prefer to figure it out properly.

Interestingly, when doing contour cuts around my object, the object will measure slightly larger than the desired size; inside cuts measure smaller, outside cuts measure bigger. This made me think it might be the programmed size of the cutting tool, but when measured (the stock 6.35MM flat end mill from Carbide 3D) it measured exactly the size that was programmed.

I’ve tried adjusting steps per MM by calibrating long distance moves with a ruler, which results in very accurate and repeatable results along the stretch of the ruler, but when cutting results in an even greater disparity between what I’m telling the cutter to do, and what the cutter is actually doing. Something of note is that when I measured movements of 1MM in opposite directions with a dial indicator, I was getting about 0.1MM of backlash on both the X and Y axes, with the belt tightened fully. Out of curiosity, I loosened the belt a fraction and did the same test, finding far less backlash (0.03MM).

I am wondering if I’m losing steps due to an over-tightened belt. I’ve attached a video showing the tension on the belt ( with both X and Y axes in the middle and the Z axis about 0.5CM off the bottom) - is this too tight? I know I tightened them slightly more than suggested in the guide - I’ve seen posts on this forum saying that can help.

The circles cut look perfectly round and measure so too - from different angles they measure equal sizes.

Could I be missing anything else?



Have you very accurately measured you mills. Slight discrepancies in width of the mill can make large differneces in small cuts that you may not notice over a very large span. A caliper will get you close but you may want to purchase a micrometer to accurately measure you bits if cutting smaller parts.


I have had the same issue when CC changed the diameter of my end mill from 4mm to 3.96mm or so. It was impossible to change it to 4mm again so I ended up deleting the tool and adding it again. Your belts look much tighter than mine, that might be an issue also.

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Thanks both for your replies. I’ve measured my cutting diameter by plunging into stock and measuring the diameter of the hole - it comes out perfectly at 6.35MM!

So that’s one for belts too tight… I thought they might be. Does anyone have them as tight as I do/could anyone show how tight they have them?

Thanks all!

Did you calibrate for belt stretch per:

also, as noted there, it’s best to cut at least a slot and measure that to determine effective endmill cutting diameter.

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Hi Will, I adjusted my steps per MM using a ruler, but I did not follow this guide, so thank you! Will - would you say my belts are too tight?


To me this sounds like a classic bit diameter error.

Having belt tension too tight or too loose will cause issues with cut quality. With size errors the concern is whether the machine is calibrated for belt stretch.

If the machine is not calibrated correctly then both inside and outside dimensions will be either too big or too small. Longer dimensions will have more error than shorter dimensions.

If the outside dimensions are too large, and the inside dimensions are too small. Then the problem is not calibration, but cutter diameter. If we have the same error on both small and large holes, then that is another indication that the bit is not cutting the same dimension the machine thinks it will.

I would decrease the bit diameter by 0.3mm and run again.

Although the plunge cut test says one thing, many other things say another. In my life not everything makes sense. I just try to adjust things so that the important one make sense and try to ignore the others…

The tricky thing is when we have belt calibration issues and diameter error. Then you have one error masking another and it is hard to separate them. This sounds like classic diameter error.

If you are skipping steps then the outside perimeter will not meet up, it will appear that the image has shifted. I don’t hear you mentioning these issues.

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Thank you pdx - I think you may well be right. The cuts are clean and crisp and visually look perfect. However, I swear I ran a project a few weeks ago with the belts looser and found the pockets only just short of their programmed dimensions. I don’t want to go through the trouble of getting the Y belts equally tensioned if it’s not necessary, so I am asking here if people think my belts are too tight.

Here’s a video showing the backlash I’m getting on the Y axis (same result for X). When the dial indicator moves exactly one turn, I’m moving in the same direction. When it fails to move the entire turn (approx 0.05 - 0.1MM out since it’s a 1MM/turn dial indicator) I’m going back on myself.

Note: I have a wooden board being held manually against the end plates so this is why there is some variance when the machine moves back to its original position!

For backlash, please check:

Thanks - pulley set screws were all good, but I’ve loosened up the V wheels a tiny bit and now I’m getting about 0.02MM of backlash on a 1MM move… think that’s pretty good, so now I’ll run a test file and see what happens!

Out of curiosity, could the additional drag from the tighter belt create the same effect?

So loosening my V wheels has indeed helped with the size of the circles - the 30MM circles now come out to around 29.85 - 29.95, which seems pretty good to me. Thanks for the link Will :slight_smile:

Be a little carful with measuring a plunged hole - it’s really easy to get a lot of runout and get a hole a lot bigger than the endmill. The best way to do this is measure the mill directly with a pair of calipers.

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Measured a slot which came out just under at 6.33, possibly accounting for the slight discrepancy remaining.

I have noticed however that my moves less far for a given programmed distance, when cutting, versus when just moving in open air. Is there a way to account for this?

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Movement when cutting / not cutting should not be different — please send a note to along w/ your description of this and your testing methodology and we’ll work this out with you.

Just curious, my belts are not as tight as that. Is that too much or just right?

The belt tension thing is weird. Gates (the folks who make the belts) recommend numbers much lower than will work for most people — it may have something to do w/ plunge and feed rates and which materials folks cut — so long as you’re not skipping teeth, they’re tight enough.