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Offline Dan13

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Re: Arcs not cutting smoothly
« Reply #10 on: February 15, 2012, 11:09:43 AM »
Is the arc segmented all around? And segments are absolutely equal in length (both on same arc and different radius arcs)? Can't be seen on the photo.

Mach3 sometimes does do strange things. Have you tried restarting the PC?

Have you successfully cut arcs before?

Dan
Re: Arcs not cutting smoothly
« Reply #11 on: February 15, 2012, 11:27:42 AM »
Arcs are indeed segmented all around. As I said before, the measurements are different on different radius arcs, but the size change isn't proportional to radius. I've restared mach3, restarted the computer, tried a couple of different things. Same results.

Offline Dan13

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Re: Arcs not cutting smoothly
« Reply #12 on: February 15, 2012, 11:42:27 AM »
Sorry I missed this:

The length of the flats are .160" on the outside of the large diameter (0.75") and 0.120" on the outside of the smaller diameter. They're also not flat, now that I take a close look. They're definitely arcs in and of themselves.

Outside of the backlash, the axes are very consistent - example: I make a g0 move from x0 to x-3.7500, then from x-3.7500 to x-3.5000. I now zero the glass DRO, and I can make any move in the positive direction and be pretty much dead on (within a couple of ten thousandths, anyway).

Can you extend this test. Zero Mach3 and glass scales (after taking out any backlash) and then do short moves of 0.01" up to a total of about 0.15" and compare the numbers in Mach3 and the glass scale at each point and see if they correlate. Repeat on both axes.

Dan
« Last Edit: February 15, 2012, 11:44:19 AM by Dan13 »
Re: Arcs not cutting smoothly
« Reply #13 on: February 15, 2012, 02:30:00 PM »
OK, I did exactly as you asked - I started at 0, then I did g0 x0.010, then g0 x0.020, etc, and on the y axis as well. Here are the results:

g0xy
0.0100.00960.0100
0.0200.01960.0200
0.0300.02940.0300
0.0400.03940.0400
0.0500.04960.0500
0.0600.06000.0600
0.0700.07060.0700
0.0800.08020.0800
0.0900.08980.0900
0.1000.09920.1000
0.1100.10880.1100
0.1200.11860.1198
0.1300.12860.1300
0.1400.13860.1398
0.1500.14940.1498

By the way, and I'll put this in bold, I WOULD PAY GOOD MONEY FOR AN EXPLANATION OF HOW BALLSCREW MAPPING WORKS. Seriously, if someone would explain it (even just the basics) to me, I'd be happy to put together a nice article detailing its use and publish it for everyone. Seriously.

Offline RICH

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Re: Arcs not cutting smoothly
« Reply #14 on: February 15, 2012, 05:02:10 PM »
EXPLANATION OF HOW BALLSCREW MAPPING WORKS

I don't recall anyone posting about it . Just looking at the screen it would seem that  one could find some deviation and then have Mach account for it.
I have profiled ball screws and in doing that you find and graph the deviation of the axis movement as to perfect movement ( your comparing to a know calibrated standard).
Sort of what you did in reply #13 but you do it over the total range of the screw. As a example the screw may gain , deviate, or lose and not necessarily in a linear fashion and that includes both travel directions of the screw.

RICH
Re: Arcs not cutting smoothly
« Reply #15 on: February 15, 2012, 05:24:26 PM »
Oh, I get that much - how it works in Mach3, however, is a mystery.  When you enter the correction points, do you enter the correction you want to apply, or the actual coordinate read on the external DRO?  I try to enter correction points, but nothing happens. Perhaps this is a bug in that it only works in metric? It defaults the screw lengths to 1300, and the graph of corrections is in increments of 0.1. If this is a small bug, it would be a great help if it could be corrected, as I do everything in inches.
Re: Arcs not cutting smoothly
« Reply #16 on: February 15, 2012, 06:03:44 PM »
Poking around on my own, the graph is simply poorly scaled for English units. I went through and entered a series of points along the x-axis by doing incremental g0 moves, then entering what the point reads on the DRO, clicking add correction, then save curves. You can't see any changes on the graph, but they're definitely stored in there. I entered about 40 points along the x-axis, then went back and did some checks - it seems that the map made accuracy decidedly worse. I'm guessing I'm using it wrong, but I dunno. You can definitely hear the motor speed changing based on location along the screw when you rapid jog back and forth.

In any case, this doesn't change anything about the segmented arcs I'm getting on this part. I tried exact stop this afternoon, and it didn't make a difference. Still getting the same behavior.   ???

Offline Dan13

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Re: Arcs not cutting smoothly
« Reply #17 on: February 16, 2012, 04:36:20 AM »
In any case, this doesn't change anything about the segmented arcs I'm getting on this part. I tried exact stop this afternoon, and it didn't make a difference. Still getting the same behavior.   ???

I think the answer lies right there, in the numbers of your test. X axis is pretty much obvious, heaving an absolute deviation of 0.002", which is enough for those segments to be seen. I would guess that if you did the test over a longer distance you would see a distinct repetitive nature of the numbers. Entering the numbers in Excel would make it easier to visualize.

Since you're not using a belt drive, you have to start looking elsewhere in the driver mechanism. For instance, what is the motor? Could be a stepper motor with poor accuracy - unevenly spaced poles or magnets on the rotor. Since your Y axis is much better than the X, you could try swapping the motors.

Dan
Re: Arcs not cutting smoothly
« Reply #18 on: February 16, 2012, 08:56:48 AM »
I'll give that a shot. I'm not sure I agree, given that, for the most part, absolute deviation in the x is under 0.0005" (with several spots up to 0.0014" max), and incremental deviation is even less. Also, recall that the steps that I'm seeing are regular regardless of orientation in x and y, and are dependent on radius of the arc.

In my continuing quest to eliminate backlash, I just placed an order for some stainless helical couplings from McMaster - should be here tomorrow. I'm currently using the lovejoy spider couplings that came with the cncfusion kit, which are far from perfect.

Offline RICH

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Re: Arcs not cutting smoothly
« Reply #19 on: February 16, 2012, 04:51:32 PM »
Quote
lovejoy spider couplings


Have a pic of these as the name is not familar?

RICH