Reducing stepper motor marks

[philbur]

I think you guys need to look into entry and exit strategies. ramp, helical, interpolate are a few key words. A direct plunge in and/or out will almost always produce the type of artifact as shown, especially on a roughing cut.

Phil

[Gerry Sweetland]

I think you guys need to look into entry and exit strategies. ramp, helical, interpolate are a few key words. A direct plunge in and/or out will almost always produce the type of artifact as shown, especially on a roughing cut.

Phil

Thanks Phil,
Those are certainly strategies that I need to learn and understand. I have seen those key words mentioned here frequently and am very interested in gaining knowledge on those techniques. I'll keep plugging away as it is all part of the fun figuring this puzzle out :-)
Gerry

[twocik]

+1 Phil ! Tangent in and out

[beanbag]

My marks aren't entry/exit marks. I'm pretty sure it is stepper related due to how the spacing of the lines increases and then decreases away from the center of the pattern. Like some kind of aliasing, or whatever you call it.

[philbur]

Beanbag, the neat increase and decrease in amplitude has the appearance of resonance, possibly due to the cut frequency matching the stepping frequency, or some whole multiple thereof.

Is it just a surface finish or are there detectable steps in the part.

Chatter patterns take many forms.

I think to find it you are going to have to make some guesses and then devise tests to check your guesses.

Phil

[beanbag]

Is it just a surface finish or are there detectable steps in the part.

It really is small steps in the part from the steppers. I have tried higher and lower SFM, feeds, etc and similar marks are always there.

The most recent case of this was when I was doing an interpolated inner diameter circle in a plastic part. I got the entry and exit moves as an arc coming in, so there is no discontinuous change in x or y. No matter the speed or feed, I always got little marks on the surface. I was able to alleviate the problem somewhat by using a bigger end mill, because the arc of the end mill's diameter more closely matched the circle.

[Steve Seebold]

I think you guys need to look into entry and exit strategies. ramp, helical, interpolate are a few key words. A direct plunge in and/or out will almost always produce the type of artifact as shown, especially on a roughing cut.

Phil

Another thing to try is a spring pass. I always use spring passes if I want to not have start and stop marks. On my finish pass, I will also circle in, and circle out. Sometines I'll use 2 or 3 spring passes. I have also learned that if I get vertical marks around the outside or inside of a part, I will increase the feed rate to smooth out the steppers.

[philbur]

Why are you sure it is stepper marks. A full step is only 0.001" and a microstep is only 0.0001".

Phil

It really is small steps in the part from the steppers. I have tried higher and lower SFM, feeds, etc and similar marks are always there.

The most recent case of this was when I was doing an interpolated inner diameter circle in a plastic part. I got the entry and exit moves as an arc coming in, so there is no discontinuous change in x or y. No matter the speed or feed, I always got little marks on the surface. I was able to alleviate the problem somewhat by using a bigger end mill, because the arc of the end mill's diameter more closely matched the circle.

[beanbag]

Why are you sure it is stepper marks. A full step is only 0.001" and a microstep is only 0.0001".

Phil

possibly due to the cut frequency matching the stepping frequency, or some whole multiple thereof.

Could be "resonance marks". I get what you are saying that if it is steps only they should be very finely spaced. This would still count as "marks due to stepping motion, which I am trying to reduce". Marks are still there even at different speeds and feeds.

Basically the machine buzzes too dam much.

[philbur]

But if they are stepper marks only then they should appear symmetrical about the 3, 6, 9, and 12:00 o'clock positions.

My proposal is that they may be a result of resonance caused by a combination of a number of potential sources that may or may not include stepper movement. Cutter vibration/flexing/chatter, axis backlash, lack of spindle preload , climb or conventional milling are some that come to mind, as well as of course, stepper frequency.

I guess many of us have seen similar marks from time to time. A finish pass often reduces it to an acceptable level.

Phil

What other kind of surface feature would change in frequency as you go around a curve or circle?

[beanbag]

I edited my post just as you were posting. But if you go back to the very first picture, the marks or aliasing occur at the same angle of the circle, so to speak, even though I am milling out a conical section. Thus the marks occur at a particular "slope" of x/y, so it must be resonating with some step frequency. When I have some free time, I will do more testing as you suggested.

[philbur]

Do you have the new 3 phase steppers or the original 2 phase. The original steppers/drive have a documented resonance at around the 10 ipm mark.

Phil