Question on backlash and if it's possible to compensate

Hey all,

I'm designing in TurboCAD, then setting up my cuts in CamBam, and then machining via Mach3. I'm typically working with plastic (abs) or wood. My cuts have always seemed to be a little off from what I designed, and I'm wondering if it can be attributed to backlash and if it can be compensated for?

Example: I setup a profile operation cutting bassword with a 1/8" 2 flute mill bit. The desired width of the cut was 0.75" and it machined out 0.735". My typical routine is to go back to TurboCAD and redesign everything making my cuts a little larger to compensate. This takes a tremendous amount of time and it seems like there ought to be a better way.

I'm looking for your expertise on if this can be attributed to backlash and how to compensate for it.

LMS HiTorque Mini Mill
CNC Fusion Conversion

Thanks for your help,

-Chris

Yep Mach3 has backlash comp

Mach 3 Backlash Compensation - YouTube

You may also want to check out the mach3 forums as well, but........if you use an external motion controller, as opposed to the Parallel Port, you'll have to look at the documentation for your particular controller.

I'd also be looking at the diameter of your bit (unlikely) and whether the steps per is correct....0.015 is a lot of backlash if you're using ballscrews. Also make sure all your mounts and nuts to lead screws are tight.

Try plunging a hole with your bit and measure it check the diameter hole its actually cutting. I have found that my 1/4 wood cutting bit/router bit measures about. 2410 and my 3 flute 1/4" end mill for aluminum measures 0.247.. Also, make sure your steps per are correct. Measure, it, measures it again, and just to be sure, measure if again. If you call 1" movement and you get 1" of movement, you're good to go there. Make sure all the adjustment bolts are snug and then check backlash. Then just use the backlash compensation in mach3.

While backlash compensation may seem to be the magic bullet, it's one flaw is that it assums that the position of the cutter does not change while it is being applied, but in practice with big cutters the cutter itself will push the work around so in addition to the points about actual cutter size, there is no gaurantee that backlash compensation will actually help, and since you see to get good results by changing the tool size I would suspect that backlash is you real problem.

^^^^true, if taking heavy cuts you'll probably get some movement somewhere to cause accuracy to be affect, but he is cutting wood, not aluminum or steel, he should be cutting it fairly easy. Are your cuts 2" deep or something big like that?"

Backlash compensation should also be used at the end, when you've made sure there are no other sources of error, then you will just be adjusting for the backlash in the nut/screw, not because everything is sloppy, loose, and there is play everywhere.

Quote:

---

Originally Posted by Alax7

^^^^true, if taking heavy cuts you'll probably get some movement somewhere to cause accuracy to be affect, but he is cutting wood, not aluminum or steel, he should be cutting it fairly easy. Are your cuts 2" deep or something big like that?"

---

Wood has 'grain' which makes it more of a problem when the cutter is catching that grain. Plastic can be as problematic with 'hard spots' which cause problems.