Some weird cutting problems

[knight_toolworks]

I need help with a weird screwup. on a prt alpha using the second newest version of the control software. I forgot to download the pic but here is the drawing of what I was cutting out of ¾” ply using a ¼” compression bit cutting a single pass 6 ips.
This bit has only cut two other sheets of ply so I don’t think it was dull.
Anyway as I was cutting the first piece out (after the slots were all cut) the bit wandered off as the black line shows. Well I was glad I was watching it and stopped it before it cut the sheet in half. Well I was like HUH?? I was thinking all it could be was the bit was dull or something. I rezeroed it and saved the toolpaths again with just the outside cuts and I slowed it down to 300 ipm. It finished the first side ok and at the bottom of the second it wandered again. I had thought that it was because the vacuum failed but I put a regular ¼” endmill in rezeroed after I turned the machine off as it derailed a bit when the ply came loose) cut the parts out and it was ok. Well till I went to assembled it. I figured to just put some ply over the cuts to support them. These are just utility shelves for my shop.
Well all the slots were .05 too small and the sides were .5 too narrow.
I checked the toolpaths (vcarve) and checked the drawing and all was on.
I have had a few problems where the part was cut a bit off then it was cut before. So I am wondering what is going on?
I did some engraving on this bronze piece the day before. I made a round pocket in some ply to hold it then I zeroed on the pocket and cut the toolpath to see how centered it was. Go to lunch put the bronze in run the toolpath again ( saved it again to make sure I did not miss anything) and it was off centered.
So is it some communication issue between the bot and computer? I have a loss of communication once in awhile but I just have to click on the retry button and it is back. I used to have that a lot till I ran a ground wire between the bot and the computer.

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[Brady Watson]

I doubt it has anything to do with communication. It is either connected to the PC & the tool moves or it doesn't. Aside from static issues, which it sounds like you have covered already.

The 1st thing that comes to mind here is inadequate hold down and running the tool too hard. There is no reason why it has to be run @ 6 IPS, is there? Unless you have a seriously powerful vacuum, those holes in the center of the parts are making your vacuum leak like a sieve. I think if you run things a bit more conservatively like multiple passes and slower move speeds, you will not be dealing with part movement or bit deflection. Both of these things can contribute to what you are experiencing. If you are running a 1:1 Alpha (not PRS 7.2:1s) I'd put money on the fact that you are asking the motors to deliver more torque at that speed than they were designed to deliver.

The surest way to verify your CAM is to set the bit up high & air cut it. If all is well air cutting, then you are most likely pushing it a bit too hard and need to tone it down a bit.

See what you come up with...keep us posted!

-B

[knight_toolworks]

yes it may be that. I know I have cut 3/4" ply in two passes at that speed with good results. but that was with a straight bit i think. I have been cutting such oddball things I have not worked much with sheetgoods yet.
I am sure learning how to cut tropical woods (G) seems most things are easy compared to purpleheart.
cutting angles in it is tricky I have mirror imaged pieces and getting them to match is very tricky. even cutting two square shapes out at 1.5" deep and they don't always match in size. the angled cuts are the worst. the left side always cuts larger then the right. that both with conventional cut. plus the left is always rough and the right is always smooth. changing the left to climb helps. but I think it is the bit rotation that causes the problem. it is really only a problem on harder tropicals and I have been battling it with a hand held router and before that with a saw and chisel. one side cuts clean the other size has tearout and is a different size.
right now I cut them out at 2.5ipm max .5" depth of cot with a 5/16" rougher. I want to go to a 3/8" rougher when I can afford to invest in the tooling again.



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[knight_toolworks]

I forgot though I did cut the shelves out with the same speed without a problem. but the wedges were a bit small but that could have been that the tabs were not big enough for the speed. well live and learn (G)