Breaking bits

[Michael Arruda]

Hello all,

Any ideas why I keep breaking bits? No, "you're an idiot" is not an acceptable answer. One I've considered, but not acceptable nonetheless.

Anyway, I keep breaking the tips off of my V bits, and snapping end mills. I'm using whitesite solid carbide 60 degree V bits, 1/4" shank, for my V carving. The tip breaks off every time on the first job I do with it. Baltic birch, corian, maple, MDF- doesn't matter.

I just snapped a 1/8" upcut spiral carbide bit. I've used it for, oh, about 5 small projects, about 3 hours cut time in all. I cut .075" depth with a 100IPM cut speed in Corian at 28K RPM, and 150IPM at 24K in ply, MDF and hardwood. I was cutting some Corian just now and the bit snapped off cleanly about 1/2" up the flutes, cutting .075" deep. The flutes are 1/2" long on it.

I'm using my home made CNC, based on the FLA kit. K2 mount with a Makita RF1101 router.

Is my feed too high? I rapid at 350IPM on the machine and have cut with a 1/4" bit at 300IPM into soft maple. I know the machine will handle more, but the kernel speed of Mach3 is the limiting factor; I can't do much better until I get a smoothstepper. I thought I was already cutting pretty slow for what I've seen the machine do- I just decided to go slower, because with the small things I cut, the speed difference isn't too noticable due to all the changes in direction.

Thanks,
Michael

[Scott Shepherd]

If you are breaking off bit after bit, all different sorts, I'd start looking at the collet being the problem as a possibility.

[Paul Phillips]

Michael,
You could try cutting back on your feed rate a little. Also, if you are only making shallow cuts you don't need to use a bit with a 1/2" cutting length, try switching to a 1/4" cutting length because the longer the bit, the more it's likely snap under high shear loads, and as a general rule, keep the very top of the flute no more than 1/8" out of the collet. You can also use a torque wrench to tighten your collet nut in order to make sure you have the proper holding force on your bit because like Scott said, if your bit is loose in any way that can be a problem. Check out this link for info on collets.

http://www.techniksusa.com/metal/pwrctnt.htm

Paul

[Michael Arruda]

I'll order a new 1/4" collet. I really want to get a chinese spindle, so I can use ER20 collets, but it's out of the budget for now. I use the 1/2" flutes so I can through cut with it- I take multiple passes, but on some cuts eventually get down to 1/2" deep.

Thanks,
Michael

[Gary Hair]

I can almost guarantee that your collett is the problem, too much runout. I have an inlay job in ebony that I use a .0156 bit and it there is ANY runout they will break. Feed speed is also critical for these small bits, but not so much with yours.

Gary

[Michael Arruda]

I just took some measurements-

Runout just under the face of the collet, with the 1/8" bit that broke chucked up, is .003. Diameter of the cutter at the flutes, is .123; width of a straight line cut in the corian is .126.

.003 runout shouldn't be causing broken bits, should it? This is a new router with about 20 hours on it- I know that normal routers after some use can get up to .005-.008 runout, or higher.

Thanks,
Michael

[Dan Hintz]

Runout just under the face of the collet, with the 1/8" bit that broke chucked up, is .003. Diameter of the cutter at the flutes, is .123; width of a straight line cut in the corian is .126.

At 1-2" away from the face of the collet (end of your bit), that runout can be pretty large.

This is not my field, so I'm taking a huge stab in the dark, but I imagine that would be large enough to cause some serious stress on the bit...

[James Leonard]

Hi,

.003 runout is unacceptable. It will cause big problems.

-James

[james mcgrew]

28000 rpm spindle sounds high for corian, is it making chips or dust?

[Gerry Grzadzinski]

@28K rpm, 150-175ipm would be better. Also, for cutting Corian, never cut anything else with the same bit. Use it for Corian only, and it'll stay sharp for a lot longer.

You should never break a bit cutting .075 deep. It's either runout, or cutting too slow.

[Joey Jarrard]

You also can over tighten your collets and have problems. If a new collet does not fix the problem look at the collet nut.

[Steve knight]

yes I agree your cut speeds and depth are low. I can cut 3 ips with a 1/8" bit 1/2" deep in mdf. I just cut acrylic at 1ips 1/4" passes without a problem. been eyars since I broke a tip off a v vit so it sounds like runout or machine slop.

[Bryan Cowan]

You also can over tighten your collets and have problems. If a new collet does not fix the problem look at the collet nut.

I snapped 4 bits in one day because I kept overtightening the collet...lesson quickly learned. Purchased a collet set from PreciseBits and haven't had the problem since.

[jack Halley]

@28K rpm, 150-175ipm would be better. Also, for cutting Corian, never cut anything else with the same bit. Use it for Corian only, and it'll stay sharp for a lot longer.

You should never break a bit cutting .075 deep. It's either runout, or cutting too slow.

Just trying to learn something here.
You can break a bit cutting too slowly?
Anyone care to tell me how this can happen?

[Joey Jarrard]

Bits are made of carbide and cobalt steel when you have a slow rpm on your spindle or router you run the risk of smoking your bit. The bit gets that black burnt look to it. What you have done is get the bit so hot from an improper chip load that you have compromised the make up of the steel and this will make it dull prematurely. If this is gone on to long it can break a bit. Carbide is a brittle metal and without the good edge it is just a mater of time until it breaks.