A VERY expensive lesson learned!

[ntraub01]

After months of design, prototyping and nail biting, yesterday was the day we started the milling process. I had been double checking and triple checking everything before we started and I was confident that all was going to go well.

This single piece of material, a 4'x8' piece of 1" cast plex, was the most expensive material I have ever milled so I was a bit nervous.

All was going well, but for some reason a bit broke while doing a profile cut. NBD So I slowed the feed rate on 1/4" bit down to 3"/sec and that went well.

Then I had 20-5/16" holes to pocket down through the material. The first two holes went ok but I was a bit concerned about the plunge rate. I interrupted the cut, insert command, VS, and changed the move and jog speed of the z axis, restarted the machine and within seconds the day went from great to bad.

In the two seconds after restart the machine plunged back into the hole it had just finished and didn't wait long enough for the bit to return to a safe height before moving and took about a 1" long gash out of the material, AND broke the bit!!

SOOO, it was a very expensive day yesterday with 2 bits being broken at $50 each and a ruined $600 piece of plex. Between the broken bits, the new bits I had to order and the ruined piece of plex, I'm at -$800 for the day!!

Needless to say I turned off the machine, closed the computer, turned off the light and went home...


After a few hour and a couple of adult beverages I came to the conclusion that changing the speed of the z axis, mid-cut using an Insert Command, VS, Restart process was probably NOT the smartest thing.

I'm guessing that the VS change really didn't make all of the changes necessary to the script to keep the error from happening?!?! Like it didn't change the pause timing for the bit to be pulled safely from the hole before moving again?!?

I'm also guessing that when we change speeds on say the XY move and jog that it doesn't really have to change anything else to keep an error from occurring?!?

Am I correct in thinking that changing the z axis speeds is not a good idea especially if the bit has a long distance to travel before it is at a safe z height before moving?!?

No I'm not going to trash the $600 piece of plex. I'm going to continue milling the piece and because it is part of a sandwich with another piece of 3/8" plex on top of the 1" piece, I'm going to pray that it isn't noticeable. However, if it is I may also attempt to route a larger circle and plug it with a plug and weld it into place hoping for a flawless or near flawless joint.

There you have it ladies and gentlemen, be VERY careful about changing z axis move and jog rates using the stop, Insert Command, VS method as you may not get what you expect!!

[knight_toolworks]

yes if it is expensive material it is best to just stop it and start again. I have had such things happen too. one thing that can save stress is get a piece of 1" mdf and do the same cuts take it off and run the toolpaths again.

[jTr]

Nicholas,

I feel your pain, (but gratefully, not to the tune of $800).

I've learned to NOT stop any files mid process since the latest major release of the control software, roughly a year ago. Crash and burn every time- absolutely unpredictable what the machine will do upon resuming the cut. Exception is if you're on a long straight line cut. If in the middle of a short duration line of code, such as a carving or plunge move, rest assured, it will crash terribly every time.

I am convinced the issue is not in your insertion of a new command, but an inherent defect in the control software since the latest release. It has rendered stopping/ re-starting functions unusable.

I wish you the very best of luck in completing the task at hand, and regret I have nothing more constructive to interject.

Please, post an image of the completed project if you're able - I am intrigued.

jeff

[paul_z]

I might suggest that you break your project into as many separate cut files as practical. Once you get things working right you might combine them into a single large file.

Breaking them into smaller files also lets you look at the bit before making the next cut. Often melted acrylic (or polycarbonate) will build up on the bit and you will get a chance to clean it before the next cut. If you are melting the material onto the bit, you need to make changes to your cutting. Others on this forum can give you good advice on what to do including speed changes, misters, air cooling, etc. I don't have enough experience.

I have had luck in the past making acrylic "goo" by dissolving acrylic pieces in the welding agent it may take a couple of days to make the goo depending on the size of the pieces you melt. A few applications of the goo can build up the unplanned cutout. Now for the bad news - you should wait a day between each application of the goo to let the welding agent degas from the material before applying the next layer of goo.

As someone mentioned before, I'd cut in "scrap" before committing to the $$$ stuff. 4x8 by 1" MDF is scrap compared to acrylic ($$$) and is dirt cheap scrap compared to polycarbonate ($$$$$$$$$$$).

Paul Z

[ntraub01]

Thank you all for the advise and I think I'm going to tackle the remaining pieces and parts of this project on Monday.

The new bits have arrived and I've make some major changes in the remaining milling setups. Entering from stage left...Mr. 3/8" bit who will hopefully improve the overall finish cut on the 1" plex. I believe I'm asking to much of a 1/4" bit to do the final run on the outside profile without chattering against the 1" edge.

Just to alleviate many of your concerns, we did do quite a bit of prototyping and we even test milled on 1" plex and tried many combinations of bit speeds, feed speeds, climb vs. conventional etc. etc. Thank you though for your help and suggestions!!

I can't wait to post photos of this huge project. There is soooo much of this project that is only possibly because of the CNC and we are making a lot of very cool one-off pieces that should result in an awesome finished project.

Thank you all again!! After the milling comes flame polishing with the big torch and even more opportunities to start all over again!! LoL!!

[David Iannone]

Can't wait to see The finished product. Sounds cool