[freeman656]

Hi,

I have a small DIY CNC and I'm facing some issue when try to machine holes.
Let me explain a little, when I make some square parts, there is no problem, the part is exactly the right size in both axis.
When I try to make a hole, the diameter is always 1mm less than the expected value.
Has someone already had this kind of problem ? I'm pretty sure it's not linked to backlash because I mesured the table displacement when making the hole and it's ok.

For example, I try to make a hole of 8mm with a tool of 3.17mm.
The X and Y are moving correctly (around 4.8mm), but the resulting hole is only 7mm instead of the 8.

One more precision, I'm machining MDF.

Thanks for your help, if any

[pippin88]

Likely flex. The tool will deflect towards the centre of the hole, as material has been removed.

Grab the tool and push. If you see any movement then there is significant flex.

[freeman656]

Ok, thank you for your response.
I checked by hand and there is some flex, maybe around 1-2mm in each direction when pushing hard on it.
So you think that can be the cause of some precision issue ?

[ger21]

Even the smallest amount of flex will cause precision issues. Cutting small holes will magnify those issues. Try cutting the holes slower, and in the opposite direction.

[wizard]

For a minute here I had to think about your 4.8 value.

Ok, thank you for your response.
I checked by hand and there is some flex, maybe around 1-2mm in each direction when pushing hard on it.

That would be absolutely terrible for accuracy. You really don't want much more than a few tens of microns if you want to hold tight tolerances. Well tight for wood. In terms many around here are familiar with you don't want more than a few thousands of an inch deflection for wood working. In the end the stiffer the better.

So you think that can be the cause of some precision issue ?

A sloppy machine will almost certainly play a role here. One thing you might try is a final finishing cut that is far lighter than previous cuts to minimize deflection. I'm wondering about how precisely you are measuring that error though in your parts. It just seems strange that you would be getting a 1mm error.

If you can play around with feed rates and final finish cut parameters. If your problem is deflection you should be able to vary the size of the error by changing federate or otherwise changing the load on the cutter. If you slow the machine down or reverse cut direction and the size of the hole is still off then we need to consider other issues.

[pupetxls]

Maybe you have a different issue than I had, but maybe it will help:

Measure the tool.
At work we ofthen had issues with shady mills. The 6mm was either 5.9 either 6.1 on so on. The smaller the mills, the bigger the effect.
Just cut a straight line in the material with the bit you are using. Then measure the width of the cut. If you find any difference. just change the size in the CAM.

And yes, when measuring something bigger and square, you tend not to notice it. It's hard to measure MDF or plastics. And you might believe everything;s ok, afterwords, you mill a hole, and buff, voila.

[Devastator]

Do a finish pass removing just a few thou. If that fixes it, either your machine or your bit is deflecting.