[Zipdrive]

I have been cutting some circular aluminum parts on my mill and have been noticing that I get a noticable transisition at the +/- X and Y axis points on the part. It is very small (in the few thousands of an inch range) but very noticable as well.

What is the major cause of these marks and what can be done to get rid of them? Better tuning of the servos, preload on the ball screws, any ideas?

[HuFlungDung]

All forms of backlash need to be tracked down and eliminated. This includes both ballnut backlash and ballscrew thrust bearing support and adjustments, as well as drive coupling problems, or incorrect adjustment of software backlash settings.

To test your motor tuning, you can set up a straightedge, at an 45 degree angle to X and Y axis. Then program an interpolated table move along that edge, and true up the position of the straightedge until you see no progressive indicator movement during the entire traverse of that edge.

Then use the same dial indicator to track movement along that edge, but watching in particular, the indicator needle during the very beginning and ending of the movement. If you see a small pulse of movement (like a few thousandths) at the beginning of the move, but when it ends, there is an opposite direction pulse which brings the indicator back to zero, then motor tuning can help with that. This test would demonstrate that one motor is starting late and ending late. This could be because of softer motor tuning settings, or stiffer frictional resistance on one particular slideway.

[NC Cams]

The prime cause of - to + transition error is a combination of ball screw clearance in concert wtih a lack of rigidity in the ball screw bearings (axial clearance and/or give from theoretical "solid" yet still rotates).

You can have the ball screw loaded with slightly oversize (as in +0.0001" or so) balls to reduce backlash potential here. Any good ball screw service should be able to do this.

Increasing bearing rigidity is achieved by increasing the preload on same. The way to do this depends on how much money you want to spend.

To check, simpy mount a tenths indicator in the spindle and zero it on a part bolted to the table. THen start shoving the table in X and Y direction (back and forth, to and fro). Don't be gentle, LEAN ON IT!!! WE saw over 0.002" and we had OEM preloaded bearings on our mill and they were like new.

To fix the problem, on a Bridgeport mill, you can install 20TAC47 true, abec 7 ball screw bearings. These have 480lb preload and absolutely NO axial play - they turn like silk and yield something like 0.0002 backlash comp on our machine.

Unfortunately, they cost nearly $500/set. BUT, we see 0.0001 or less transition error (flats due to dwell, measrued on our cam measuring machine) if we do a slow transition cut thru the + to - section of a travel

For a bit less cash (still a bit pricey though) you can install some 7204A5TYDUH's. Not as stiff as the TAC's above but much more so than the OEM ball bearings.

The lowest cost option is to install some 7204BYG's. These are angular contacts for motors but not preloaded out of the box. These can be preloaded by shimming or send them to KAF Mfg in Stamford Ct (I think) for a 150-200lb preload grind.

Servo tuning will adress the 45 deg issue mentioned earlier.

Backlash comp will take care of SOME of the residual slop but you have to eliminate any mechanical slop if you want deadly accuracy. The computer can't deal with back and forth slop - you have to eliminate the mechnanical slop to fix that....

You can shim the OEM bearings to reduce axial endplay but you'll find that the force to turn the screw goes up pretty dramatically as you hack together shims. The issue of ball screw versus acme lead and accuracy and backlash are discussed elsewheres on this site. I vote for preloaded ball screws - check out ANY high end mill, VMC, etc should you choose to disagree...

You MUST have the gibs properly adjusted. Too loose and you'll have clearance slop and too tite and you'll have drag induced resistance at the + to - transition which causes the same "flats" due to friction induced lost motion.

We went thru all of the above gyrations fixing our BPT mill (we sprung for the TAC bearings). We can now mill cam masters to nearly the accuracy of a CNC ground part. Exactly the same no, but you can NOT see the difference between the ground and cut master in a finish ground cam.

[Zipdrive]

Thanks guys. I think I will have to mess around with the table and see how much movement I can get out of it.

[B_Bueno]

I prefer to bore a hole than interpolate. The focus on backlash, etc. is correct, but if your'e splitting hairs, bore it.

[ViperTX]

B Bueno....well you are correct if the hole if bore-able.....but then if you have any other arc that's not a hole....you need the circular interpolation to be within limits....

[NC Cams]

Someting else to look at: Does the machine return to the X,Y coordinate after you move the table???

If you bore the hole in one setting and get it all done at one shot, you don't have problems. HOWEVER if you bore it and move on and then OOPS, got to go back, table slop, backlash error, aftore mentioned issses re: table position accuracy can lead to non-concentric holes (unless you recenter on the hole with dial indicators which can be a PITA as this costs time and $$$).

We got tired of the screwing around and fixed the slop via servo tuning, expensive ball screw bearings, reloaded ball screws, etc. You would be AMAZED at the time savings and reduced frustration. THe accuracy improvement is NOTICEABLE.

YES the ball screw bearings at nearly $1000/full set of 2 for X & Y was pricey - the ball screw reload fix was a few hundred. But, what does 20-30 minutes here and there, time after time add up to over the period of a year with regard to setting up and re-tramming???

What do scrapped parts cost???

What is your frustration worth???

How important to you is it NOT to have to deal with the + to - transition "issue"??? (try milling an ellipse without the transition flats which screw up the aesthetics on putter heads big time. Couldn't before, can do now - game, set, match).

Besides, when someones stops by and sees you do something that they struggle with REPEATEDLY (IE: the ellipse thing) , the pride factor is priceless when they ask, "...How'd you do that????".

After you explain it, they inevitably say "I can't afford that). We couldn't either but did it anyway - now we get the business we couldn't get before. More importantly, our services are now a step ABOVE those of the "me too - price sensitive shops".

Life doesn't get much better when that happens....