broken bit

[lutchman]

hi guys, my bit broke about mid way inside the collet, any ideas why? i was cutting alu at .05 pass depth, .5ips, at 18000rpm with a .25 single flute onsrud 63-620

[dlcw]

Was it hot? Next would be improper program driving the machine? Everything set correctly? z-zero, safe z, proper x,y home, etc. The occasional Shopbot control software brain farts cause bit breaks.

It's tough to diagnose without more info....

[Brady Watson]

It probably got too hot and you are cutting too deep. Try pulling RPM down to 13,000 and reducing your stepdown to .02" deep per pass. You can increase move speed to somewhere in the .7 to 1 IPS range on XY depending on the shape of your design. Be sure to always ramp into the cut with a smooth or spiral ramp.

-B

[lutchman]

i wasnt hot, and i did 20000rmp .125 pass depth at 1ips with good results with that bit...i saw some jagged edges after i posted this question the material moved and pulled up and the bit jammed into it.

[MogulTx]

Brady's advice is good.
Shallower cut. Slower RPM's
And be SURE TO RAMP.

I have done a lot of aluminum. I have broken a few bits... I don't normally have any trouble any more- mostly because I follow Brady's advice on this...

One other thing: What is the alloy you are working with??? I mostly cut 6061 and 5052....


MGM

[ssflyer]

One other thing to check - if you broke the bit halfway into the collet, I'd be checking my collet. It should hold the bit tight enough not to break inside the collet...

[gundog]

Increase your feed or slow down your RPM that bit according to Onsrud should have .003"-.006" chipload which at that RPM should be .9 - 1.8 IPS @ 18000.

Here is the page for reference. Use the posted formulas.

http://www.onsrud.com/files/pdf/2012%20LMT%20Onsrud%20Production%20Cutting%20Tools %20Aluminum.pdf

If your feed is too slow you can cause chip thinning or rubbing.

You were only making a .001" chip load. The depth of cut seems reasonable. I don't know what machine you are using.

[Burkhardt]

One other thing to check - if you broke the bit halfway into the collet, I'd be checking my collet. It should hold the bit tight enough not to break inside the collet...
Agree. that does not seem right. The flute section of this bit is so much weaker if anything it should break there. I would suspect maybe a chip is stuck in the collet or spindle taper, that makes it contract unsymmetrically?

Not sure if that is relevant, but Onsrud sold a while ago surplus specials of such a bit for half price on eBay that had a flat in the shank section. These must not be clamped with the flat inside the collet to avoid breakage.