Messages

1

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« on: February 17, 2012, 02:01:29 PM »

Got the new couplers installed this morning, x backlash is down to 0.0012", y down to 0.0024". Definitely an improvement. With BC on, I see no variation in position from Mach3 to DRO. I'll hopefully get to cut some curves this evening, and we'll see what happens.

2

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« on: February 16, 2012, 08:11:27 PM »

What material coupler did you use? Aluminum helical couplers are total crap, and wear out rather quickly if used out of spec. The ones I have on the way are stainless, and rated far beyond what I'll need for torque. I worked in a manufacturing plant where we used these for a number of different operations - some with good results, and others with horrid results. The stainless couplers are indeed not cheap, but it's a lot easier to spend the money once than to worry about grinding your bearings to bits over misalignment that you can't even effectively measure. They're plenty stiff, and enjoy a fatigue limit that aluminum can't achieve.

3

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« on: February 16, 2012, 06:47:23 PM »

http://www.ebay.com/itm/CNC-Stepper-Motor-Lovejoy-Coupling-3-8-X-1-4-spider-/330548952567

Basically two hubs that squeeze a rubber spider, which allows some misalignment while transmitting torque. The squeeze of the spider and any space between it and the coupler ends results in backlash.

4

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« 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.

5

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« 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.   

6

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« 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.

7

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« 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:

g0	x	y
0.010	0.0096	0.0100
0.020	0.0196	0.0200
0.030	0.0294	0.0300
0.040	0.0394	0.0400
0.050	0.0496	0.0500
0.060	0.0600	0.0600
0.070	0.0706	0.0700
0.080	0.0802	0.0800
0.090	0.0898	0.0900
0.100	0.0992	0.1000
0.110	0.1088	0.1100
0.120	0.1186	0.1198
0.130	0.1286	0.1300
0.140	0.1386	0.1398
0.150	0.1494	0.1498

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.

8

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« 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.

9

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« on: February 15, 2012, 10:53:02 AM »

Doesn't look like backlash to me. I would, however, guess that it is a mechanical problem in one (or both) of the axes. If you have belt transmission, for instance, a run-out of the pulley could cause something like this.

My motors are direct drive, and the Lovejoy couplers are nice and snug. Plus, if it were a problem such as that, I'd also see ridges (or at least feed changes) on long, straight sections, and I'm not seeing that at all. Per part inspection, the feed on the straight sections is remarkably uniform.

By the way, the transition points you specified in the above drawing are not all correct. The 2 diagonal lines should be removed as there is no direction change in either axis.

Ah, good point. I was thinking in velocity and not displacement.

10

G-Code, CAD, and CAM discussions / Re: Arcs not cutting smoothly

« on: February 15, 2012, 08:49:33 AM »

As I said, I tried CV with BC on and off, and there was no difference. I can give exact stop a try if you think it would help.