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Old 05-17-2017, 06:30 PM
cekonecky cekonecky is offline
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Join Date: Sep 2016
Location: Brooklyn, NY
Posts: 3
Default X servo tripped - bit broken

I am a new owner of a 5x10 Cobra. I have been using the machine successfully for a few weeks now to quickly cut parts from 3/4" birch ply and 3/4" MDF using a 3/8" compression bit run at 18,000 rpm and 400 ipm, cutting through 3/4" material in a single pass.
Today we started a project using 3/4" void free Baltic birch ply and I ran into some trouble. About two thirds into the first sheet my machine stopped abruptly with a loud grinding noise (sounded to me as if the tool had slipped and dug into the table). But upon inspection the tool and table were fine and WinCNC had given me an "X SERVO TRIPPED" message. A quick look on the forum said that maybe I was cutting too deep in one pass, or that perhaps it was just a fluke and wouldn't happen again. As I was nearly done with the sheet and had never experienced this before I restarted the cut from where it left off only to get the same message from WinCNC and the machine stopped again - this time however the bottom half of my 3/8" compression bit rolled away from the cutpath - having broken just above the top of the sheet of ply.
I'm assuming that the density of the Baltic birch is just too much for the bit/machine to handle in one pass and when my new bit comes tomorrow I'll try continuing with two passes, but I'd like to be sure and would love any input anyone has to offer. I am also relatively new to feeds and speeds so am not sure if I'm perhaps pushing the bit too hard? It's never seemed to have a problem though with MDF and shop grade birch ply. Also if I do run my parts in two passes do you think I can then speed up the ipm any or should I not push it?
One last thing worth noting that may have influenced what happened is that the chips left behind in the cutpath are extremely dense - I normally have quite a bit of chips left behind even with my dust collector running but a quick pass with a shop vac afterwords clears it right up. With the Baltic birch I can hardly shove a screw driver into the cutpath to loosen the chips enough to vacuum them up they're packed so dense. Could this have also contributed to the problem? If so what might solve the chip issue?

Thanks so much in advance for any feedback/input you might have to offer,
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Old 05-17-2017, 09:54 PM
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Gary Campbell Gary Campbell is offline
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Join Date: Dec 2012
Location: Marquette, MI USA
Posts: 2,760

First off, Baltic Birch (the real stuff) has hardwood core ply's, while the shop grades usually have a Poplar or soft Pine core. These plywoods are made for the CNC industry to cut fast.

My guess is that the extra density of the hardwood cores are slowing the spindle, which in turn increases the chipload dramatically. Due to thinner plys, there is also much more glue in the BB. This glue has the tendency to remelt and clog the chip flow away from the bit. Once that happens heat builds, clogging increases, the bit dulls and increases cutting loads beyond what the servos can push.

Your original feeds/speeds are good, a nice amount under what your machine could handle. That said, you will need to test with the new bit to see first where chip evacuation starts to deteriorate with depth, then see if speed increases reduce that. I would start at half depth (~.375) in the hardwood around 300 ipm and work to increase back to 400.

When testing for a good feed/speed combo, always start conservative and increase feeds (chipload) until cut quality or chip evacuation deteriorates.
Gary Campbell
CNC Technology and Training
The Ultimate Woodworking Machine

"There are 10 kinds of people in the world. Those that understand binary logic, and those who don't"
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Old 05-18-2017, 10:31 AM
Ger21 Ger21 is offline
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Join Date: Apr 2014
Location: Shelby Township, MI
Posts: 352

If you have an ATC, here's something you might want to try. Make the first pass with a downcut spiral, or better yet, a downcut roughing spiral. Leave an onion skin, and maybe .02 for a finish pass, then do your second, full depth pass with a compression bit. You can get a lot more use out of the downcut, even when it starts to get dull, and the compression bit will last a lot longer.
If you use a roughing spiral, you can increase the feedrate quite a bit, as they cut through stuff like butter.

I have a much bigger machine, but I rough cut baltic birch as 600-700ipm with a standard downcut spiral, and finish with the compressin at 500-600ipm, and about 17,000.

JointCAM - CNC Dovetails & Box Joints

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baltic birch, chips clogging, cobra, compression bit, x servo tripped

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