Hi people.

I'm currently conducting research for a project at a workshop wherein the output of CNC grinding + lapping line is analyzed. Myself not being too well versed within the intricacies of mechanical engineering or the workings of CNC-machines in general I thought I'd ask some wiser people what they think.

I'm seeing a long-term change in the output of surface roughness and diameter. Now the surface roughness to me sounds pretty logical, I assume there's something in the machine that's being worn down and it's reflected in the surfaces. I would just like to confirm that this is normal, and possibly if someone could tell me what exactly is being worn down here. My statistics tell me the current pace is something like 0,014 Ra annually.

I could perhaps derive a better method of expressing the level of change, i.e. x / product manufactured. Off the top of my head without looking at the numbers (which I currently don't have, as I am sitting at home getting intoxicated) I would approximate the annual volume to 20 000 units, of which the products I'm scrutinizing represent about half.

The other one is weirder, I'm looking at the output of two different, cylindrical products and seeing a definite long-term change in one of them but not the other. The diameter of the smaller product is growing larger at quite a significant pace, clocked at something like 6 ?m annually which in this case corresponds to a good portion of the tolerance range. The larger product exhibits a very mild slope in comparison, in the same direction though, which does not seem to be statistically significant.

The products are being worked at the same production line and going through pretty much the same process. The only differences I can come to think of are the differing diameters and that the product undergoing change is produced in larger batches.

It would be greatly appreciated if someone with some experience in this kind of machining could shed some light on the matter.

As a bonus, if someone can tell me a good way to reduce the short-time variation in diameter, the beer's on me.

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