cutting disc

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phred
Posts: 9
Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

cutting disc

Post by phred »

1968SH4.dxf
(1.22 MiB) Downloaded 208 times
Profile 2 sh4.txt
(33.61 KiB) Downloaded 173 times
hello
i have a problem
i am trying to cut a 500mm dia disc from chipboard, but it comes out oval (sort of) by about 2 mm
i have had three tries with different settings and adjustments but still the same error
the dxf file and g-code list are attached in the hope that someone would be kind enough to look and see
if they can find where the error lies. the relevant section is n6330 to n6580. as far as i can tell the circle is cut in 24 segments
some of which appear to be straight lines !!
my machine is a chinese 1000x600 gantry mill cutting speed is 150mm/min, depth of cut 3mm ,tool is 1/4" router running at 20000rpm
i reduced the feed rate because i thought there might be some deflection in the gantry as it seems a bit flimsy.
i am using cut2d version 1.1 dated march 13 2008 and mach3
any help will be much appreciated
thankyou
phred

bravesoul
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Re: cutting disc

Post by bravesoul »

I can not see anything wrong with the circle, when I open it with Aspire v8 and check the number of nodes, it only has 4 which is normal. I even created another circle with the same dimensions as the 500 (largest one) and it is an exact replica so the only possible answer that I can give you would be something wrong with your post processor.

Joe

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FixitMike
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Re: cutting disc

Post by FixitMike »

What happens when you cut a square? If it is the same problem, I would suspect the controller program.

My guess would be that your controller calibration is off. X or Y steps per inch.
Good judgement comes from experience.
Experience comes from bad judgement.

phred
Posts: 9
Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

Re: cutting disc

Post by phred »

hi fixitmike
squares are fine also individual holes e.g. a square with
a hole at each corner is ok it just seems to be circles
phred

hi bravesoul
did you create the circle from the dxf file or the list?
phred

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Adrian
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Re: cutting disc

Post by Adrian »

Why not attach the actual Cut2D file rather than a DXF?

bravesoul
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Re: cutting disc

Post by bravesoul »

@Phred - I imported the DXF, then drew a circle the exact same size and placed it on top of yours and there were no differences at all.

Joe

phred
Posts: 9
Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

Re: cutting disc

Post by phred »

1968sh4.crv
(1.34 MiB) Downloaded 218 times
hello bravesoul
that checks the dxf but does it check the code?
sorry if thats a silly question but i am a complete novice at cnc
although i have had the machine for some years i had so much trouble with
it originally that i gave up on it
i have only started again this year since i retired
phred

hello adrian
i sent the dxf because i felt that was the starting point .
an error on the dxf could have been the problem
i will attach the crv file
phred
.

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Leo
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Re: cutting disc

Post by Leo »

I looked at the properties of the nodes on the 500 mm circle and it is in fact a 500 mm circle

There are some duplicate circles in the middle - north south east and west around the small middle circle. But that will not make the 500mm oval.

I believe the problem is in the machine not in the program.

Perhaps Mach3 and your machine need to be calibrated.
Imagine the Possibilities of a Creative mind, combined with the functionality of CNC

toymaker
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Re: cutting disc

Post by toymaker »

As an owner of a chineese machine I´d guess that the machine is not square, did you cross measure your squares?

Christer
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bravesoul
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Re: cutting disc

Post by bravesoul »

@phred, I created a ShopBot.sbp cut file from your crv file and did a preview test and it looks great and would cut just like the preview. I've attached a snapshot of the preview for you. At this point I would suspect your machine, you might was to do a simple test of moving your machine in a specified X position Like 15" then Y similar distance and see if you end up where you should. For my ShopBot, I would issue a j2,0,0 to get me home, then jx,15 which would move it 15" and then I would manually measure it to be sure I moved 15", same for Y. This will at least tell you if your movement is ok. Not sure how you would G Code Mach3 to cut a circle from the command line but with shopbot I would simply issue a CC, 7.5 to cut a 15" diameter circle, with other parameters of course....

Joe
Attachments
circles.JPG

phred
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Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

Re: cutting disc

Post by phred »

hello toymaker
no problem with squares i have cut squares of 500mm and the diagonals are as near
equal as i can measure

hello leo and bravesoul

i have checked the calibration over 500mm on each axis and they are as near as i can measure with a steel rule
i do suspect the machine as i said in an earlier post i thought the gantry was a bit flimsy i therefore reduced
the feedrate but it has made no difference .

bravesoul, the circle is a good idea although programming circles in g-code seem rather more complicated than
with your shopbot. i will however draw and produce a dxf fortunatly chipboard is cheap

thankyou all for your input

phred

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metalworkz
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Model of CNC Machine: SX3 CNC, DIY 24x20 & 48x60 routers
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Re: cutting disc

Post by metalworkz »

Hello,
I have experienced the somewhat elliptical or egg shaped cutting of circles with my SX3 CNC mill shortly after getting it. After a lot of posts and the help of others it was suggested to look into the method used to couple the motors to the ballscrews and in fact my problem was due to a loose set screw on either the X or the Y coupler. I would check the machine to see if the leadscrew/ballscrews are attached with set screw couplers and check to see if perhaps one could be loose I would also recommend filling a flat on the shaft for the set screws to seat in if they do not already have flats. This problem is kind of obvious in that if is noticeable at the 90 degree direction changes of the cut circle.
Best regards,
Wes
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Lathe, Horiz. Mill, Shaper, Leaf Brake

phred
Posts: 9
Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

Re: cutting disc

Post by phred »

hello metalworkz
yes,thats it
i have tried various checks as you can see from previous posts
following a suggestion from bravesoul i created a single circle in cut2d
this translated as four arcs in mach3
when i cut the circle it was correct on the axes but oversize on one diagonal
and undersize on the other
i transpires the the front of the gantry is about 20thou in front of the back
i am now trying to realign the bridge that links the ends of the gantry to the
ballnut by trial and error,which seems the only way.

phred

rbraeking
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Re: cutting disc

Post by rbraeking »

The reason you get good squares and oval circles is most likely due to backlash in one axis. Check the following:
1. pulley mount to motor and rack shaft
2. belt tension
3. pinion engagement of rack

It is hard to get a steel gantry mill out of square. You really have to slam into the stops at full speed which should be prevented with soft limits (Mach3 OS)

Bob
sscncrouter.com

phred
Posts: 9
Joined: Wed Sep 16, 2015 2:04 pm
Model of CNC Machine: gantry mill (chinese)

Re: cutting disc

Post by phred »

hello all
ah. bob were it that easy
the gantry is driven by a single ball screw which is coupled to the gantry by a bridge piece
bolted to the base of each side frame
when adjusting you have loosen everything ,move one side frame and then re-tighten.
i have however managed ,after several attempts ,to reduce the error to about 8 to10 thou
which is as near as i can measure with a steel rule.
i can't understand why the squares came out so well unless something moved in between
cutting the squares and the discs. when i have it right i shall put dowel pins in to prevent
any future movement.
in my first post i mentioned that the circle,when imported into mach3, was comprised of 24
segments . it transpires that this is due to inserting holding tabs in cut 2d, the tabs break up the
arc by inserting two short straight lines. i don't see why this occurs ,perhaps someone from
vectric would like to comment

thanks to you all for your help
phred

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