End mill as router bit: Cautionary note

[Jim C Martin]

Hello All:
I do a lot of slot mortising. Years ago I read a tip about buying spiral end mill bits to use as mortising bits. They can be had cheap and seem like a perfect alternative. I have a whole box of Hanita HSS end mills that I got on ebay and they worked great... until today. Looks like it had a small crack that led eventually to catastrophic failure. Not sure how much use this one had on it. Even though I still have quite a few brand new ones I may toss them all and go with real router bits. Price of good bit is a lot less than a copay at the ER.
Boy I'm glad it didn't impale me.
JimEndmill.jpg

[Keith Outten]

Jim,

The same thing happens with router bits. I expect that your end mill that failed had a defect in the material.
.

[Keith Weber]

Wow! That surprises me that it would snap off like that. The end mills (US-made) that I use on my Bridgeport cut through steel like butter, you would think that wood would be nothing compared to steel. Mind you, my depth of cut is usually less than .050" at a time. The speed on a HSS end mill on milling machine is a LOT less than the speed of a router (like tenfold), so given the much higher speed, cutting depth, and varying feed speed that you were using, maybe it's not so surprising. I'm sure that the cutting geometries (rake, etc.) would be a lot different on the end mill than it would be on a router bit, too. I've never looked at them side by side. I'd probably stick with real router bits for your woodworking, and buy yourself a milling machine so you can use up your end mills.

[Jim C Martin]

Yep, I agree on all points. I just always thought it was good enough. In fact, it cut really nice mortises. And yes, the fact that I already own a few dozen dollars worth of end mills certainly justifies the purchase of a milling machine!

Wow! That surprises me that it would snap off like that. The end mills (US-made) that I use on my Bridgeport cut through steel like butter, you would think that wood would be nothing compared to steel. Mind you, my depth of cut is usually less than .050" at a time. The speed on a HSS end mill on milling machine is a LOT less than the speed of a router (like tenfold), so given the much higher speed, cutting depth, and varying feed speed that you were using, maybe it's not so surprising. I'm sure that the cutting geometries (rake, etc.) would be a lot different on the end mill than it would be on a router bit, too. I've never looked at them side by side. I'd probably stick with real router bits for your woodworking, and buy yourself a milling machine so you can use up your end mills.

[Mike Heidrick]

You were using a router? Where was the end of the collet? That end mill has a flat designed for a tool holder set screw.

[Jim C Martin]

Yes, I was using a router and the end mill was in the collet up to just shy of the flutes. Do you think that to be some sort of issue?

You were using a router? Where was the end of the collet? That end mill has a flat designed for a tool holder set screw.

[Pat Barry]

That bit looks to be burned. I suspect it overheated due to a dull cutting edge and then the shear stress from the routing cause it to fracture. I think the danger of a end mill is not optimized for cutting wood consequently it tends to overheat

[Phil Thien]

What was the diameter of that bit?

[Jim C Martin]

7/16" cutting diameter, 1/2" shank.

[Bruce Page]

I have used endmills as router bits for years. Sometimes they just break just as router bits sometimes break. I wouldn't get too worried about it.

[Mike Heidrick]

I would not think mount would be an issue as long as the collet was fully bedded on the shank and not on the flat. I agree with Bruce. Toss it and get out a new one.

[John Coloccia]

You were using a router? Where was the end of the collet? That end mill has a flat designed for a tool holder set screw.

Yes, I was using a router and the end mill was in the collet up to just shy of the flutes. Do you think that to be some sort of issue?

That may or may not have been designed to just stick in a 1/2" collet. I'm not sure what to make of it as I've never seen an end mill with that feature, but it may have well just not have been secured properly, and that could certainly cause problems, though I'm sure you would have felt excess vibration before breaking if that was the case. You were probably getting excess vibration anyhow from the unbalanced shank. I have to think that whatever that's designed to fit into, the mating holder has a bit of metal that matches up with it and offsets any vibration or I couldn't imagine it being useful for milling.

Anyhow, everyone breaks router bits from time to time. Good idea to be wearing eye/face protection (I use a Trend Airshield pro) and a nice, heavy shop apron. They've both kept flying things from leaving anymore behind than a startled John.

[Bruce Page]

Flats on the endmill shank are fairly common in the machine shop world. They can be used in a collet or solid tool holder. Bridgeport type mills aside, most machining centers use solid tool holders with a setscrew in them to lock the endmill. I have a 1" diameter solid R8 tool holder with setscrew for my garage mill. I worked in a shop once that used a quick change tool holder system in all of their Bridgeports, they did not use collets at all.

[Lee Schierer]

I've had carbide router bits do the same thing when I hit a knot or hard reversed grain. Carbide does not like shock or being dropped. It is a brittle material and even a slight chip in a cutting edge can cause a failure like you saw. I would keep on using the end mills.

[Mike Heidrick]

My cnc knee mill is nmtb30 and all tooling gets dedicated tool holders and is measured and managed in my post processor and cam software. The quick change setup allows every tool mounted to always get mounted at the same height. At 4500lbs there is not much in the way of vibration. The tool holders have a set screw for the flat. Er32 and Er40 collet chuck tooling is used for smaller than 3/8 shank tooling.

Flats on the EM shank are fairly common in the machine shop world. They can be used in a collet or solid tool holder. Bridgeport type mills aside, most machining centers use solid tool holders with a setscrew in them to lock the endmill. I have a 1" diameter solid R8 tool holder with setscrew for my garage mill. I worked in a shop once that used a quick change tool holder system in all of their Bridgeports, they did not use collets at all.