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Is There a Maximum End Mill Length?

mlcarey1959

Plastic
Joined
Dec 30, 2021
I need a long end mill to cut EPS foam. I would like to know if there’s a maximum length for an end mill. There’s a company that sells and manufactures custom drill bits up to 72” in length. Can an end mill be this long?


Thank you,


Marcus
 
Last edited:

mhajicek

Titanium
Joined
May 11, 2017
Location
Minneapolis, MN, USA
What matters is the length to diameter ratio. The deflection of an endmill of a given diameter goes up with the cube of the length, so a 72" long 1" diameter endmill would be a wet noodle. It couldn't even handle its own inertia, and would whip around, destroy your machine, and likely injure someone.

So in short, no. Not unless it's approaching 10" diameter.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
Yes, they make infinitely long endmills for foam. They are special because they are very small diameter and they don't spin. You apply current to them instead and they cut foam pretty sweet.
 

DavidScott

Titanium
Joined
Jul 11, 2012
Location
Washington
I remember talking with somebody who used 3/4" x 7"-10" long endmills for machining polystyrene foam, I don't know if it was expanded or extruded. No idea of where to get them though. At least they do exist and are used for what you want.

They cut signs and the like on a router so the tools did spin.
 

Larry Dickman

Titanium
Joined
Jan 30, 2014
Location
Temecula, Ca
I've had to use 12" long end mills before, not a lot of fun (2" 6fl)

A customer of mine cuts foam for layup molds. They use tapered ems, so a 12" long tool is still pretty stout.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Yes, they make infinitely long endmills for foam. They are special because they are very small diameter and they don't spin. You apply current to them instead and they cut foam pretty sweet.

Almost right, they're infinite-1 long. But that only hold for the diamond monocrystal versions that are direct Sol powered.

For the rest of us, a heated Chromel or similar resistance wire, an insulated support structure, and a variable current low voltage power supply works pretty well, but you need to play with the current and speed settings to get things to work nicely, especially over long lengths.

I didn't do anything like 6 foot depth, but I did make foam stowage trays for Shuttle use up to 6" deep. Where blind holes were needed I did a through profile, then fit an insert to get the final pocket depth. A little silicone fixative to hold things in place, and presto!
 

mlcarey1959

Plastic
Joined
Dec 30, 2021
Yes, they make infinitely long endmills for foam. They are special because they are very small diameter and they don't spin. You apply current to them instead and they cut foam pretty sweet.



A hotwire might work with a small diameter rod or tube that doesn’t bend over longer lengths.

Thanks for the reply.
 

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Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
With longer lengths and larger surface areas it can be hard to control temperature, which means you can wind up with sections of the "cutter" being coated with melted foam (bad), or increased fire risks (bad bad).

You should also pay attention to smoke/fume evacuation, I did as much of my work as I could under a fume hood.
 

mjr6550

Aluminum
Joined
Oct 7, 2012
Location
Lansdale, PA
If this is an ongoing process and hot wire does not work, perhaps you could modify a bandsaw. Cutting foam with a blade that has a decent depth or width (not thickness) the deflection would probably not be a problem.
 

boosted

Stainless
Joined
Jan 4, 2014
Location
Portland, OR
A 6 to 1 ratio is about max...ie 1/4 endmill-1 1/2long 0r 1 endmill-6inch long and the machining feed rate is almost zero...Phil

For the OP that is not a bad rule, but FWIW 6 to 1 should have pretty minimal drama in nonferrous. We have standard tools that run nearly 6 X D stickout.

Over 12 X D is when things start to get really tricky. :D
 

Strostkovy

Stainless
Joined
Oct 29, 2017
If this is an ongoing process and hot wire does not work, perhaps you could modify a bandsaw. Cutting foam with a blade that has a decent depth or width (not thickness) the deflection would probably not be a problem.

An oversized scroll saw would work great, as those can support omnidirectional blades.


As for a 72" endmill, you'll exceed the critical speed of the shaft, where the centrifugal force of any slight offset exceeds the strength of the material (very oversimplified explanation) and it bends or shatters.
 

CarbideBob

Diamond
Joined
Jan 14, 2007
Location
Flushing/Flint, Michigan
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As for a 72" endmill, you'll exceed the critical speed of the shaft, where the centrifugal force of any slight offset exceeds the strength of the material (very oversimplified explanation) and it bends or shatters.
72 inch endmill 12 inches in diameter? Shaft critical speed or whip still a problem?
Drills of this length very different than endmills due to the guiding.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
72 inch endmill 12 inches in diameter? Shaft critical speed or whip still a problem?
Drills of this length very different than endmills due to the guiding.

Actually, yeah. Some math I didn't verify the accuracy of seems to indicate a critical speed of 2755 RPM, ignoring any flutes. That's a mere 8600 SFM. That's only 8 times higher than the highest sfm recommendation I could find for any type of foam.
 








 
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