What went wrong?

[sugarbeatco]

So I am cutting a shopbot provided sign file (the business sign from the demo cd) which I have carved & cut out before, beautifully might I add. So I had my buddy send me a really nice mahogany blank to do one for him. Well everything was going fine until about 1/2 way through the "3D tools" toolpath in the center, I noticed the edge of the circular (dished) shape started getting progressively deeper. I finally hit the button on this last pass when it dove in 1/8". I quit the file and checked my zero, and it appeared to be way off, by like 2 inches! What the heck happened there? We are having some snow off and on here, but the power hasnt flickered or anything like that. I had a licensed electrician connect everything and ground it as per the manual. The shopbot is on its own clean circuit. I'm running a brand new prs alpha 96-48 with PC router. The toolpath used 1/8" ball nose (onsrud). The collet is precision brand.. it hadn't slipped at all (which was my suspect). The router is secure. 2" of visible zero error is kinda hard to explain right? I checked my blank, hadn't moved, hold downs were solid. I was cutting at 18,000, 3ips 1.5 plunge. I got a picture of the part, cant attach copy of the part file, too big, but I even double checked it to make sure I didn't screw something up.. Not sure what more information I could provide. I was progressively gaining confidence as a new shopbot'er, but 15" pieces of 5/4 mahogany aren't exactly cheap!
http://i1188.photobucket.com/albums/z420/foilhowbowzer/DSCN0328_zps73a4d409.jpg (http://s1188.photobucket.com/user/foilhowbowzer/media/DSCN0328_zps73a4d409.jpg.html)
http://i1188.photobucket.com/albums/z420/foilhowbowzer/DSCN0327_zps7ddf441f.jpg (http://s1188.photobucket.com/user/foilhowbowzer/media/DSCN0327_zps7ddf441f.jpg.html)
Anyone care to speculate as to what I am missing? I would appreciate any help. Thanks for your time.

[kurt_rose]

Could it be your collet? Maybe that the bit is vibrating in the collet and slipping out. Just a thought.

PS. Sorry I reread you post and miss that you checked that.

[sugarbeatco]

I was really hoping it was. I read a thread about that not long ago and so that was the first thing I went to. Sure as anything, its tight and clean when I disassembled it. I can get pictures of the bit and collet if anyone wants to see but doesnt seem right that my zero was so far off, heck the bit is only 2" or so. So, if my zero was off by 2 inches I would have bigger problems than a ruined blank right? I just dont know... going to try the file again on a scrap and see what happens.

[ron_moorehead]

I was wondering, did you do a roughing tool pass before you started to use the ball nose. Looks like a lot of material to remove with a ball nose bit?

[kurt_rose]

I'm with you Ron. With the lack of a ruffing pass I'm picking that the bit vibrated out of the collet due to the depth and stress it was under. I'm surprised the bit didn't break. Don't ask me how I know this. LOL!

[sugarbeatco]

Yes there was a roughing path that cut .25 material away using 1/4 end mill before making the 3d path. Like I said this has cut beautifully in the past. I refuse to believe that the bit slipped so consistantly that it created a nice clean and perfect arch over 3" of toolpath, 1/8th in deeper at its final resting place. Its a perfectly plunged ramp. Should look like this
http://i1188.photobucket.com/albums/z420/foilhowbowzer/DSCN5058_zpsf187e0d5.jpg (http://s1188.photobucket.com/user/foilhowbowzer/media/DSCN5058_zpsf187e0d5.jpg.html)

[sugarbeatco]

p.s where you see it so deep, remember this is much too deep. Normally at its deepest cut this takes away 1/8" inch. With steps at .06125 as you can see there are no signs of any chatter. This is a shopbot provided file. I only told the machine to cut slower, the original feedrate was 6ips I changed to 3 and plunge rate was 6 I changed to 1.5. This is straight off the demo disk, written by shopbot. I was pretty sure they would have included of anything else they deemed necessary right?

[myxpykalix]

That looks too straight of an edge for bit slippage. What it looks like is that it just stopped cutting your toolpath or lost the balance of the cutting code.

How that happened i'm stumped on...:confused:

check to see if you had any vectors in there that might have acted like a boundry?

[bob_dodd]

Kevin , I've seen this before , I'd check your rack and pinion gears on the Z axis , as there might be dirt or wood chips in the gears and the Z is loosing position a little at a time , because of binding , use wire brush or a sharp pick to clean out the gear .

[Brady Watson]

I can see where it crept down deeper, maybe another 1/8" - but I am not seeing the 2" distance you mentioned. I see that you have this held down with only 4 screws? It is highly possible that it warped up while cutting as you relieved some stress in the wood, making it appear to cut deeper.

If you are running a Standard, it is not impossible to have lost steps, so I would not rule that out either. You said your stepover was 0.06125", which I hope is a typo - I think you meant 10% of your .125 bit (.0125). If I was running that file on a Standard, I would be inclined to run it @ 2,1 and I would also have a better hold down method to make sure that the wood didn't cup in the center.

How many times did you stop/pause the file after the 3D toolpath started? Only after it appeared 'off', or did you stop it a few times to adjust speeds before it started going off?

-B

[sugarbeatco]

Kevin , I've seen this before , I'd check your rack and pinion gears on the Z axis , as there might be dirt or wood chips in the gears and the Z is loosing position a little at a time , because of binding , use wire brush or a sharp pick to clean out the gear .

will check this, thank you.

[sugarbeatco]

I can see where it crept down deeper, maybe another 1/8" - but I am not seeing the 2" distance you mentioned. I see that you have this held down with only 4 screws? It is highly possible that it warped up while cutting as you relieved some stress in the wood, making it appear to cut deeper.

If you are running a Standard, it is not impossible to have lost steps, so I would not rule that out either. You said your stepover was 0.06125", which I hope is a typo - I think you meant 10% of your .125 bit (.0125). If I was running that file on a Standard, I would be inclined to run it @ 2,1 and I would also have a better hold down method to make sure that the wood didn't cup in the center.

How many times did you stop/pause the file after the 3D toolpath started? Only after it appeared 'off', or did you stop it a few times to adjust speeds before it started going off?

-B
The 2" was my zero height on the machine when I stopped it, thats why I say, I should have had bigger problems like a broken bit or worse if that was my correct zero height. The material was 1". Cuts like butter usually. You are correct with 10% stepover. I thought 1.5 seemed kinda fast for such a tiny bit, but it cuts really well, perhaps as I grow in confidence I will give 2/1 a shot lol. I didnt stop the process until I noticed it was starting to progress to dangerous,and the screws as dinky as they may seem, have held up to much worse. I cut cabinet parts all day with a small handful of 2" deck screw and it hasnt been a problem at all. I dont know any better way thats within my budget at the moment, if you have any suggestions to hold downs that would be better suited for such a job I would be greatful to know them. I did test to see if any flex was happening just in case could not cause any significant bowing or flex (prying upward on each side). In fact after your post I decided to try and recreate the problem (unsuccessfully). So I planed it down 1/4 and re ran the same exact toolpath, came out perfect.. Upon closer inspection of the bit and collet I see no signs of slippage or spinning, no grooves, I am very confident that it is not slipping, the bit looks brand new can still see the writing perfectly. If perhaps our weather did cause the power to flicker could something like this happen? We've had snow and 40mph winds so although I didn't notice it, it could have happened. The lights were flickering a touch last night.. Anyhow I did manage to save the piece after sitting around scratching my head long enough.. http://i1188.photobucket.com/albums/z420/foilhowbowzer/DSCN5118_zpsf2e1d890.jpg (http://s1188.photobucket.com/user/foilhowbowzer/media/DSCN5118_zpsf2e1d890.jpg.html) I do like to learn from my (and your) mistakes though, so i do greatly appreciate the support.