Breaking small bits

[Ed_R]

I'm going through .023, .0313, and very small v-carve bit ( 8thinch wide, .004 tp ) like they were made of plastic. I'm cutting wood!
My machine is a Leadwell V25, from 1997, with a Fanuc 21m controller. My pass depth is set to half a diameter in GCode. With the V-carve bit, I'm making passes at .0313 depth. Spingging at the max rpm my spndle can go, which is 8000 rpm, I'm breaking bits at speeds as low as 6 IPM!

I know you're going to say I should be running at around 35 ipm but the bits disintegrate upon contact at that feed rate. anyone got any ideas on why this is happening?

[awerby]

to deal with these tiny bits. It sounds like you're trying to use a vertical mill to cut wood, but that's not really what it was designed for. There are air spindles that you might be able to hold in a collet, which will turn at the 30,000 to 70,000 RPM that those little tools need to cut effectively. (but don't turn on the mill spindle when you run it...) Here's a link to one currently on Ebay:

Air Turbine High Speed Spindle 602JS - Milling / Metal | eBay

Good Luck!

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[Ed_R]

I've been looking into those. Hope I can find one that I can hang in my took changer. For cutting fret slots I don't need a lot of torque.
Ron at Precise Bits thinks that 8K rpm is fast enough to push the bits at 35 IPM. I have tried to tell him what happens but....
So for bits this small what rpm should I be using?

[awerby]

since you're dealing with a range of sizes there. I definitely wouldn't go that deep with the V bit; even with the 1/32" tool you shouldn't go deeper than 1/64". As fast as you can spin them is good for these little tools - if they can't get the material out of the way in time, they'll break in a heartbeat.

If you're using a tool-changer, that could be part of your problem; they can have less concentricity than a standard collet, and runout is a major cause of broken bits, and more so if they're small. I wouldn't suggest trying to hold onto one of those air-spindles with it either.

Cutting slots across the grain is going to be hard on any endmill. If what you're trying to do is cut fret slots, have you thought about using a slitting saw? You can probably get a right-angle milling attachment that would hold the arbor - just a thought...

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[TDA]

Hey Ed. Sorry to hear you are having problems. I thought I might be able to help but will need some more info.
What kind of woods are you cutting?
What kind of spindle or router?
What is your plunge feed?
Have you measured your spindle runout?
Where are they breaking in the cut (plunge, direction change, initial forward movement, ect)?


If you can get me that info I might be able to help.

[Ed_R]

I'm cutting ebony, rosewood, maple, mahogany, and oak, mostly. The smaller bits are used on fretboards, ebony and rosewood for the most part. No idea who built the spindle, but it's not a router , its top speed is 8K rpm and runout is minimal, under 3 thousandths.It's a cross-grain cut, from off one edge to off the other edge- I am not plunging into the wood vertically but moving into it horizontally. The bits seem to handle that fairly well no matter what feed rate I've used. They're breaking in different places in teh cut. I'm going .065 deep over four .015625 passes at a time. Sometimes they break on the second pass, sometimes the third, once or twice it's been on the first but rarely.

I talked to Ron at Precisebits ( where I've been getting teh bits ) and he tolds me the bots would like to have a feed rate of 24ipm at a minimum at *8K rpm, as that would be the low range of the 'sweet spot' at this rpm. I tried it, no go. He told me that at 24K rom I could run it at 100 IPM and that if I'm going 33 percent of 24K I should be able to go 33 percent of 100 rpm, so I tried 33 IPM. Snap.
I don't have a lot of wits to begin with but I have more than reached their middle

[Ed_R]

John! I have been meaning to send you some wood so Ron could ry running the bits at 8K rpm to see if he gets breakage. I will have to do that this week if possible.

[Ed_R]

sorry: runout is under 3 TEN thousandths

[TDA]

Sorry for the delay we have been quite busy.

My data in ebony converted to 8k puts the sweet spot between 25-40 IPM. In one test I intentionally broke 10 tools to see where they would break (all test done plunging one diameter deep, cross grain). The lowest breaking feed was 130 IPM at 24k RPM (43 IPM at 8k converted). The other woods should be much easier to cut.

You could try take shallower passes. But to be breaking them at all at those feeds and plunge depths makes me think there is a larger issue. When you say 3 ten thousandths runout is that at the spindle taper, toolholder, or everything together? I would say check your acceleration but you are plunging outside the material and moving into it. There is little that acceleration would change in that scenario that I can think of. Is there any other slop in your system? Backlash?

The only other thing I could suggest is switching to shell cutters. In the same test and same board of ebony I was never able to break them at one diameter plunges (test maxed out at 180 IPM).

If you would like us to test your material at 8k RPM we can do that. We will have to fit it into our schedule but we will get it done.

[Ed_R]

I don't think it's backlash; the cuts are straight lines and the breaks happen in the middle of them. I'm also making them in one direction only- that is, instead of cutting from one edge to teh other, then dropping z and going back through the channel, I'm lifting over the workpiece and returning to the first edge before going deeper.
I'm not sure ye what the accelleration on the machine is.
I will order some more bits and make some tests.

[BKGUY]

Is the run-out you measured in the spindle or at the tip of the tool while it's in the holder? You definitely need a precision holder for tools that small.
As far as an air spindle that you can run through your ATC, check out

Air Driven Spindle Speeders- BIG Kaiser

They have two models of air spindles. One goes from 30,000 to 50,000 RPM and one goes from 60,000 to 80,000 RPM. Both models will work with your ATC and they have less than 0.0001" Dynamic run-out (thats with the tool spinning).

For the end mills/router bits, I would check with Robb Jack Corp
RobbJack: Manufacturers of Premium Rotary Cutting Tools - Carbide End Mills, Saws, and Router Bits They make fantastic miniature tools with extremely tight tolerances.