parts with arcs grow

Having problems with or questions about SheetCam? Post them here.
balbs
Posts: 13
Joined: Wed Jan 02, 2019 11:07 pm

Re: parts with arcs grow

Post by balbs » Tue Jan 08, 2019 5:45 pm

Its direct drive. No reduction

User avatar
Les Newell
Site Admin
Posts: 2206
Joined: Thu May 11, 2006 8:12 pm

Re: parts with arcs grow

Post by Les Newell » Tue Jan 08, 2019 7:24 pm

OK, a 1 module rack will move 3.14159mm per tooth. Convert to inches 3.14159 /25.4 = 0.12368". Multiply by 16 teeth 16 * 0.12368 = 1.97896" per rev. Most steppers are 200 steps/rev so you should get 200/1.97896 = 101.06338 steps per inch. If you are using 10 microsteps your steps/unit should be 1010.6338

Changing from 10 microsteps to 16 will not make a significant difference to accuracy. It will just change your steps/inch.

Did you do the tape measure test? As I said before measuring your part is not ideal as it can introduce other errors.

balbs
Posts: 13
Joined: Wed Jan 02, 2019 11:07 pm

Re: parts with arcs grow

Post by balbs » Tue Jan 08, 2019 8:23 pm

Thanks, So I was very close for the steps per in and even then the part grew by over .125" I ran it with a tape measure over 40 inches and it was within .0625 which is likely due to the end clasp being loose. I can send you the sheetcam save file to take a look at if you think that will offer any insight. I confirmed that after the job was cut that it returned to the same start point.

User avatar
Les Newell
Site Admin
Posts: 2206
Joined: Thu May 11, 2006 8:12 pm

Re: parts with arcs grow

Post by Les Newell » Tue Jan 08, 2019 8:45 pm

If you give me a copy of the job file (File->save job) I can check it over for you.

balbs
Posts: 13
Joined: Wed Jan 02, 2019 11:07 pm

Re: parts with arcs grow

Post by balbs » Wed Jan 09, 2019 12:58 am

I just sent you a copy of the job file as well as 2 of the gcode files I was using. I really appreciate the help.

User avatar
Les Newell
Site Admin
Posts: 2206
Joined: Thu May 11, 2006 8:12 pm

Re: parts with arcs grow

Post by Les Newell » Wed Jan 09, 2019 12:58 pm

I can't see anything obviously wrong with your job. I measured the part at 12.8" high which looks correct. The kerf also looks reasonable at 0.045".
Try cutting some squares, 5", 10" 1nd 15" square. Do they come out the correct size? If not are they out by the same amount or do the bigger squzres ahev more error?

balbs
Posts: 13
Joined: Wed Jan 02, 2019 11:07 pm

Re: parts with arcs grow

Post by balbs » Wed Jan 09, 2019 5:51 pm

When i cut squares (6" 8" 10" and 11") they have all come out correct in size.

User avatar
Les Newell
Site Admin
Posts: 2206
Joined: Thu May 11, 2006 8:12 pm

Re: parts with arcs grow

Post by Les Newell » Wed Jan 09, 2019 6:20 pm

I have to admit I am puzzled by this one. The g-code is definitely correct. I loaded the g-code into SheetCam over the top of your job and the paths match exactly.

It is very unlikely your controller is doing anything odd on arcs so it must be the motors losing position or a mechanical issue such as the gears slipping on the motor shafts.

mancavedweller
Posts: 89
Joined: Tue Feb 25, 2014 6:53 am

Re: parts with arcs grow

Post by mancavedweller » Thu Jan 10, 2019 10:47 pm

Les,

he may have found what is causing this issue. It was certainly a good eye opener for me.

Check the second page of this thread:
https://www.forum.cncdrive.com/viewtopic.php?f=3&t=1726

Keith.

User avatar
Les Newell
Site Admin
Posts: 2206
Joined: Thu May 11, 2006 8:12 pm

Re: parts with arcs grow

Post by Les Newell » Thu Jan 10, 2019 11:35 pm

You can get problems with spring loaded racks but I would be surprised if it was enough to cause the problem you are experiencing. You say your part grew by 0.125". That's near enough one tooth on the rack. How sharp are the corners on the squares you cut? If you are experiencing problems with the pinions climbing out of the rack you should see significant distortion on the corners, where the machine suddenly changes direction. If you watch the pinions while cutting you should see them lifting if this is the problem.

How accurately aligned are the racks? Could you get away with having fixed pinion to rack spacing rather than spring preload? You will get a bit more backlash but a lot lower wear and no chance of the pinion climbing out of the rack.

balbs
Posts: 13
Joined: Wed Jan 02, 2019 11:07 pm

Re: parts with arcs grow

Post by balbs » Sun Jan 13, 2019 9:11 pm

I went through and hard mounted the steppers, removed the springs and bolted in place so they cant pivot anymore. I also applied lubricant to the gear racks and halved my accel value. With those changes nothing changed in the growing of the parts. They are still coming out oversized. I also went through and scaled the steps to 999.xxx based on the scale factor of the part and moved gantry with a tape measure and it was .25" short over 30" which is what I sort of expected. I havent cut any just squares yet with these new changes but I expect them to come out just fine as before. I just installed the gear rack on either side and then went through and squared up the gantry from there.

Post Reply