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On the Shopbot (PRSalpha 96-48) at my makerspace, I'm getting holes that are not round. The picture below is about 1.5" diameter. I cut it with a 3/8" end mill. Any suggestions as to what's causing this?
http://www.talkshopbot.com/forum/att...id=34268&stc=1
My first step when looking for cutting problems is to power up the tool and then grab each moving part...both ends of the gantry, the YZ car, the nose of the spindle... and shake it to see if you feel any slop. Since it's in a makerspace my first guess is worn out pinion gears, but they could also be loose on the shaft or not tight in the rack.
I found when I was running my router with a climb cut on small holes at high feed rate, my holes looked like yours. I had a little slop in the machine like bill.young just talked about and the climb cut pulled the router a little before the drive motors could compensate.
I switched to conventional cut for the holes and took out some of the slop in the machine. Plus I slowed down the feed rate some. Not so slow that I burned the spoilboard, but not superfast.
Did you make the hole using the circle tool or is it an imported file? Lots of times, with imported files, circles are chopped up into lots of straight line vectors that mimic a circle. If that's not the issue, make sure you V-tracks are really clean and there are no nicks in the rail. Second, like mentioned above, check to make sure your machine is nice and tight in all directions.
Another thing is to check you feeds and speeds. If you speeds are to slow for the feed, you will flex the bit more. If the feed is to slow for the speed, you will heat up the bit and shorten its life span.
I did grab the spindle and gantry and try to move them to see if there was any play, I didn't notice anything, but I'll do it again. I have noticed a difference between climb and conv cutting. Basically for the circle, the odd shape remains, but it's mirrored. I also tried to cut some square slots (1" x 3/8") with a 1/4" bit. The short sides were angled, so it looked like a parallelogram. This was with a climb cut. I tried again with conv cut and it was much butter but not perfect.
I'm cutting with 0.001" to 0.002" chip load
I took a closer look at the shopbot and it turns out the pinon gear on the y-motor was a little loose. I tightening the set screws and it's working MUCH better now.
You might volunteer to replace the pinions as that is the surest, simplest and cheapest way to restore the tolerances to a SB. The pinions probably last no more than two or three hundred hours before wearing out. They are the soft part of the equation and meant to wear.
Thanks, I'll do that