Carbide endmill deflection?

Got into a discussion on a woodworking cnc forum about cutter deflection. Machine in question was clearly lacking in rigidity, but some were convinced that the cutters themselves flexed also. Hard to dissuade them...

Just to quantify things, how much can a rigidly held 3/8" x 2" spiral 2 flute cutter deflect before it snaps?

richard newman said:

Got into a discussion on a woodworking cnc forum about cutter deflection. Machine in question was clearly lacking in rigidity, but some were convinced that the cutters themselves flexed also. Hard to dissuade them...

Just to quantify things, how much can a rigidly held 3/8" x 2" spiral 2 flute cutter deflect before it snaps?

Click to expand...

Does carbide flex ?

richard newman said:

Got into a discussion on a woodworking cnc forum about cutter deflection. Machine in question was clearly lacking in rigidity, but some were convinced that the cutters themselves flexed also. Hard to dissuade them...

Just to quantify things, how much can a rigidly held 3/8" x 2" spiral 2 flute cutter deflect before it snaps?

Click to expand...

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i have seen carbide drill bit that dia and length, drill over sized holes over .040" oversize cause drill bit end was damage on one flute.
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there is deflection then there is vibration resonance. i have seen stable bending for short periods of time, but once it starts vibrating i have seen tools break in seconds

Everything flexes. No material is perfectly rigid. Glass flexes. Ceramic flexes. And even carbide flexes. There is no getting away from it.

As for how much, with carbide, which also is quite brittle, that is going to be a small amount when compared to a tool steel. Exactly how much is difficult to say with the limited data given in your post. There are many formulations for carbide and they are deliberately made with different physical properties. Also, just giving the length. diameter, and number of flutes does not really determine the complete geometry of the end mill. Many details remain, even the contribution of surface finish to the ultimate breaking point. My point is that if you really want definitive data on this, you are going to have to test several such cutters to the actual breaking point. That means actually breaking them. And when you do this, you need to take the flexing of the spindle or other method of holding them into account because it will probably flex as much or more than the cutter.

On the other hand, you did say a woodworking forum. Wood is a very fuzzy material when compared to even a mild steel or soft aluminum alloy. Put a micrometer or caliper on steel or aluminum and the readings will be fairly independent of the amount of pressure you use (what change you do see will probably mostly be the micrometer or caliper flexing). But put that mike or caliper on a block of wood, even hard wood, and the pressure used will make a fairly large difference because the wood will flex a lot more.

My point is, when working with wood, a carbide cutter is, for all intents and purposes, rigid beyond the point of being able to ever see any difference in the wood being cut. So woodworkers can easily see a solid carbide cutter as being "perfectly" rigid. BUT WE should know better.

richard newman said:

Got into a discussion on a woodworking cnc forum about cutter deflection. Machine in question was clearly lacking in rigidity, but some were convinced that the cutters themselves flexed also. Hard to dissuade them...

Just to quantify things, how much can a rigidly held 3/8" x 2" spiral 2 flute cutter deflect before it snaps?

Click to expand...

Deflection is proportional to length and inversely proportional to the material modulus. Steels have a modulus of about 30,000,000 and carbide about 90,000,000 so all other things being equal, carbide deflection will be about 1/3 that of tool steel.

Rickyb said:

Deflection is proportional to length and inversely proportional to the material modulus. Steels have a modulus of about 30,000,000 and carbide about 90,000,000 so all other things being equal, carbide deflection will be about 1/3 that of tool steel.

Click to expand...

It is probably worth adding that deflection is proportional to the cube of the length. So if you have a 2" stick out, reducing it by 20% to 1.6" will cut the deflection in half.