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Inaccurate routing
« on: January 11, 2009, 10:37:27 PM »
Man, is there anybody who has as much trouble as I do? I spend more time on the forum that I do cutting. Here's my latest issue: I ran a test cut on 2 identical ovals, doing a profile on one and a pocket on the other. I wanted to see how closely the profile piece would fit into the pocket. The answer was, not very well. The profile fit into the pocket with about 1/8th inch of gap on one end, and the gap was not symmetrical. In other words, both cuts were not perfect ovals, even though they were designed in VCarvePro, not some design I imported into VCP, and one was an exact copy of the other. The ovals were only 3" x 4", so an 1/8th inch gap is huge for that size job.
My machine is tight and there are no loose screws, bearings, etc. I have anti-backlash lead nuts, and I was running very slow and cutting not very deep into soft material. To retest, I cut a 2" square and got similar poor results. Anybody have any ideas of what it could be?
Thanks again, Bob

Offline docltf

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Re: Inaccurate routing
« Reply #1 on: January 12, 2009, 12:28:22 AM »
Hi,Bob

run this program i sent you.use a .250 end mill or router bit,and a scrap piece of wood.it will cut a 2.0 inch circle.if it don't, work on you setup till it does.
do the routine.check motor steps first,then motor tuning,then etc,etc.

bill
Re: Inaccurate routing
« Reply #2 on: January 12, 2009, 10:19:52 AM »
Thanks, Bill, I will do it. However, I may have figured out what the problem is -- my gantry has some flex to it along the x axis. I'm going to weld some 3/8 plate to it and see if that fixes it. Then I will retest.
Thanks again, bob

vmax549

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Re: Inaccurate routing
« Reply #3 on: January 12, 2009, 10:28:02 AM »
ONE thing to always remember. The machine may FEEL tight to you but actually be loose during machining. THe pulleys could be snug but still move if not keyed or locked down. THere may be play in the screw mounting bearings?  Steps could be off, servo tuning off(if Used).

Start by running some simple test. IF you command it to move 12" does it do so EXACTLY. THe longer movement you can use will amplify the error if it is a setup issue. But only move what you can accurately measure(;-).

Missed steps?? can cause inaccuracy double checl your Accel and vel settings. Maybe TOO fast accel?? A lot of people THINK their machine should run faster than the actual design will let it run accurately. NOT to put down you or your machine(;-) But it is hard to beat the physics of the design.

We can help you get to the bottom of the problem if you have the time and patience and the ability to measure accurately.

Just a thought , (;-) TP



One could write a BOOK on this subject.

Offline RICH

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Re: Inaccurate routing
« Reply #4 on: January 12, 2009, 05:29:00 PM »
BOB,
Like Vmax549 said, make sure your motor tuning and settings are correct.
Then do an accuracy check. A simple 1" dia circle will do as you can measure each quadrant thus checking
diameter and out of round. You don't even need to cut it and can just have a pencil in the Z axis.
Now try using your software to do an outside and inside profile. Maybe all is actualy working well and you have
some incorrect inputs when using the software.
RICH
Re: Inaccurate routing
« Reply #5 on: January 16, 2009, 03:00:48 PM »
OK, I've spent the last several days trouble shooting this, and I think I found the problem: My gantry had flex to it. This explains why all my measurement tests came out good, and I was always pretty sure my pulleys, bearings, etc. were tight. I bolted a 3/8" x 8" steel plate to it, and it's doing a lot better. I do think there might be a software problem with the profile/pocket application: When I profile a shape and then try to fit it into a pocket made from the same shape, it's always too big. At least now it's too big the same amount all the way around the shape, and I'm playing with the offset tool in order to dial that difference in.
At any rate, again, thanks to everyone who helped me with this. For me, doing cnc would be impossible without this forum.
All the best, Bob

vmax549

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Re: Inaccurate routing
« Reply #6 on: January 16, 2009, 03:16:31 PM »
hIYA BOB, IF you think it is software please post a small file and I will run it on the mill to verify the code. But I think you will find it is more getting used to how acurrately your machine cuts and how to properly appy the offsets.

Hope that helps, (;-) TP

Offline docltf

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Re: Inaccurate routing
« Reply #7 on: January 16, 2009, 05:22:33 PM »
Bob
did you run that simple 2.0 circle i gave you.if you did how did your machine do.in your post you mentioned doing offsets so you can do a inlay,which i can apreciate the effort.but you got to whip that
simple code circle first.when you get it to cut well then you know your machine is on a solid setup.

bill

Offline RICH

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Re: Inaccurate routing
« Reply #8 on: January 18, 2009, 08:09:22 PM »
rabphxaz,
Someting that would be a good test and frankly an easy way of tweaking in an inlay to fit the opening is to use
compensation. You would just generate  the gcode for the profile. With compensation on you cut first the
inlay and measure, then you cut the opening leaving some extra and rerun the program decreasing the compensation until you get the inlay to fit in the cutout. No need to do separate profiles.
I personaly need to fool with the new compensation some to get a better feel of it. We did something like this at the seminar at Cabin Fever. Certainly saves time.

Thinking about this more, if your cutting is off for the inlay it should also be off for the cutout. How much depends on the machine.
Thats whats nice about CNC, maybe cut wrong but you got to admit, they are all cut wrong. ;)
RICH
 
Re: Inaccurate routing
« Reply #9 on: January 18, 2009, 09:16:59 PM »
OK, I've been working hard on this, following everyone's advice and ideas, and here's what I have learned: First, bolting a 3/8 steel plate to the gantry has helped immensely. You can't get any flex out of it now with a crowbar. Secondly, I cut circles (as per Bill's suggestion), squares, pockets and profiles, and I tried all kinds of different offsets. What I discovered is this -- when you do an offset, you are left with the original vector and the new one, and unless you magnify the area, you can't see that there are two vectors there. You need to delete the outside one. This, of course, doesn't explain why the profile is still coming out bigger than the pocket, but I have used all the slightly oversized veneers I made by making them smaller on the spindle sander. Tomorrow I will generate a new design and, starting from ground zero, see if the same thing happens. BTW, all the circles and squares measured out exactly as they should have. I am sure that the machine is cutting accurately.
Rich, I am not aware of the compensation technique you mentioned -- how do I do it?
Thanks again, Bob