Breaking bits

[jack Halley]

Thanks Joey that makes sense. So when asked if they were getting sawdust or chips,
chips should be the right answer?

[John M Torrez]

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.

Did it break while plunging or cutting?

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

Typically MAX TIR a good tool can take is 10% of the diameter. If you have .003" at the collet face that could be enough at the tip.

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.

Quite right. Also, in carbide tooling you have components that hold the carbide together that will be leached out at high temps. Needless to say that's bad.

Thanks Joey that makes sense. So when asked if they were getting sawdust or chips,
chips should be the right answer?

Yes, if you are getting powder/sawdust then you need to either increase your feed or decrease your RPM.

[Matt Clara]

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.

Hey Joey (or anyone, for that matter), I'm still trying to get my head around all this, too. Is there a resource describing how to come up with the correct feed rate and spindle speeds for different materials, or does everyone just learn from doing it and asking questions (and breaking bits)? I do web design as my day job, and I'm imagining a website where one selects what material one is wanting to cut, and then inputs a few other variables, and viola! out pops idealized feeds and speeds! Yes, I'm that lazy. I suppose a book would work, too. Thanks.

[Joey Jarrard]

Matt here is a site from onsrud http://www.onsrud.com/xdoc/FeedSpeeds.

If you make a page I would love to add it to our site. The info at this link is what I tell new ones to go by. It is a starting point other factors come in to play like altitude and moisture.

[james mcgrew]

i have the excell calc based on the onsrud data, i will find it in a bit as it is on camheads somewhere

[Dan Hintz]

Jack,

The biggest thing you can learn is that heat ruins bits... it removes any tempering of the cutting edge for steel and embrittles the cutting edge for carbide. The second biggest thing you can learn is the chips you cut are the major heat removers (a bit of a toughie to get a grip on).

[Dennis OBrien]

Anyone use Robb Jack's feed and speed calulator?

http://www.gensa.com/rjsf/EM.asp

You may have to go here and click on the bullseye at the top right and create an account (it's free and I've never gotten any spam from it.)

http://robbjack.com/

PS I'm not related to these guys and haven't bought anything from them so I have no idea what their reputation is - but the feed and speed calculator seems pretty useful.

PSS No idea where the Gensa.com part comes in. Went to their site and it looks like they may be an application development company, so they may have created the spped and feed calculator for Robb Jack??

Dennis

Dennis

[james mcgrew]

based on onsrud

http://www.camheads.org/showthread.php?t=287

[Gary Beckwith]

Michael, per your opening post of snapping tips of v-bits, and breaking cutters. Mostly, continued broken bit scenario's are related to bad collects, but as your breaking tips on v-bits I would suggest you check your acceleration rate, it may be set a little high.
Gary

[Joey Jarrard]

Michael, per your opening post of snapping tips of v-bits, and breaking cutters. Mostly, continued broken bit scenario's are related to bad collects, but as your breaking tips on v-bits I would suggest you check your acceleration rate, it may be set a little high.
Gary

This is a good point what is the feed rate on the z

[Guy Mathews]

Mike,

Please email me or post as a file for download your .tap, .nc or .txt file that you are cutting. If you email me make sure the file is not over 15MB. guy@thecncguy.com is a 15 MB mailbox.

A few things don't add up in my eyes. One of which is 28K rpms on a V-bit coupled with an egage feedrate of anything over 80 ipm. All this seems way too fast for me.

Guy

[Michael Arruda]

Sorry- long weekend. Not the good kind either.

I'll post up a gcode txt tomorrow. I design in VCP and output to mach3.

BTW- I was wrong on the spindle speed; top speed of the router is 24K- where I have said 28K, insert 24K, when I said 24K, insert 20K.

-Michael

[Gerald Wubs]

Pretty sure your're feeding too fast for the cutter at your tool overhang length and depth of cut. As a machinist I have often seen cutters (end mills) accept a too heavy feed for a period of time and then break. Problem - metal fatigue combined with dulling of the cutter. The cutter is in continuous rotating bending, and at a point developes a crack which quickly progresses and it breaks. You want to feed heavy enough to make chip of course. Calculate your feed per tooth and see if it is in line with the recommended value for the material. Feed per tooth = Feed (IPM) / (RPM X number of teeth on cutter)