[Hauntfreaks]

I noticed my cnc router is cutting undersized.... the machine is only a month old... i thought it was software related, but ive ruled that out... i was told to look for machine flex... I went over ever screw and they were all tight... but found that my top Y axis bearing seem to slightly rattle on the shaft if i pulled lightly on the spindle (router)... I decided to pull them out.... first thing i want to know should there be grease in them?.... and when you shake the bearings should the balls rattle in the bearing?.... any thought on this would be great.... thanks

[ger21]

What kind of bearings are they?

[Hauntfreaks]

lme16uu (16mm)

[fmederos]

How much undersized are your parts?
If you are getting parts 0.5mm undersized then you should look somewhere else... The play you feel by hand on the linear bearing will certainly be less than 0.2mm.

Could you have a mis calibrated axis?

Regards,
Fernando

[Hauntfreaks]

I did a test cut.... a 0.1" square hole... it came out 0.085" (which is unacceptable for my projects)

all the axis have been calibrated and are spot on...

[Joe S.]

I did a test cut.... a 0.1" square hole... it came out 0.085" (which is unacceptable for my projects)

all the axis have been calibrated and are spot on...

Tooling Flex? Does it cut more acurately with carbide?

[RomanLini]

Put a dial gauge sideways, touching on the collet or end of the spindle (turned OFF!) and then push it in all directions and see how much slop or flex there is (how much the dial gauge deflects).

That should give you a good starting point to see if there is slop and/or flex.

[fmederos]

Dial gauge is best because you can debug a lot on your machine. But if you don't have one you may try making your part in several shallow passes, flexions should be significantly reduced.

Regards,
Fernando

[Hauntfreaks]

thanks for the input guys... here is how i have tested so far....
I'm cutting 1/16" G-10 (kinda like fiberglass)
I was using a 1/16" carbide end mill (1/8" flutes)
Ive tried lowing my feed rate as low as possible
Ive tried making 2 and 3 passes

I'll put a gauge on the collet later today and see what it says... I know when I lightly push on the Z axis stepper i can see the collet/end mill move... so I will see how much

[Hauntfreaks]

ok I put a gauge on the collet.... pushed on the Z axis stepper with one finger... the movement was about .005" - .006" ..... so double that its around the .015" that its off...

I went ahead and ordered new bearings.... I guess ill see if that was the answer

[Joe S.]

Looks like you found it . . . be careful of that G10 dust, like asbestos, bad for the lungs.
BTW all the linear bearings like yours I ever saw had grease in them, try to get one that does not polymerize (solidify). The MSDS sheet should tell you.

[keebler303]

Also make sure that your cutter is actually the right size. It may be a simple matter of adjusting the tool size in CAM. Cutter compensation is important and your cutter is not exactly the labeled size.

Matt

[Joe S.]

Also make sure that your cutter is actually the right size. It may be a simple matter of adjusting the tool size in CAM. Cutter compensation is important and your cutter is not exactly the labeled size.

Matt

Oh YA, that one bit me last week, a new attrax .125 carbide cutter measured at .122.

[Hauntfreaks]

ok new bearing in... they did tighten up most of the unwanted movement... but didnt cure the undersized cutting 100%... Matt you are correct about the cutters being undersized...so I've gone into my tool selection and added some cutter comp. its cutting WAY better now... not perfect but damn close... thanks again guys.... I'll post updates as i play more with the cutter comp.

[RomanLini]

If you want to spend a couple minutes cutting out a little shape in plastic, I designed a shape that lets you measure the actual backlash of X and Y axes;
http://www.cnczone.com/forums/mechan...ash_gurus.html

The way the shape works it gives double the backlash so it is very easy to measure using a vernier caliper or micrometer on the shape after it is cut.

It is also not affected by tool diameter, it JUST measure your actual cutting backlash.