My indexable end mill: why doesn't the specified diameter agree with actual cut width?

[homebrewed]

Some time back I bought an R8 indexable end mill. It was sold as a "1-1/4" end mill. Today I used it to cut a slot in some steel and to my surprise the slot was way off -- it measures 1.311 inches wide! That's pretty far off from 1.25".

I thought perhaps I misunderstood the 1.25" specification and it actually referred to the body of the EM, so if the inserts stick out past that (and they do) it might explain things -- but the body isn't 1.25" in diameter either!

The body width and cut width also don't come out to be an even number of millimeters.

What gives here?? Am I out of line to expect that an indexable end mill with a specified diameter will actually _cut_ that diameter?

 

[markba633csi]

Sounds like revenge of the imports to me...
How bout a picture?

 

[ChazzC]

Some time back I bought an R8 indexable end mill. It was sold as a "1-1/4" end mill. Today I used it to cut a slot in some steel and to my surprise the slot was way off -- it measures 1.311 inches wide! That's pretty far off from 1.25".

I thought perhaps I misunderstood the 1.25" specification and it actually referred to the body of the EM, so if the inserts stick out past that (and they do) it might explain things -- but the body isn't 1.25" in diameter either!

The body width and cut width also don't come out to be an even number of millimeters.

What gives here?? Am I out of line to expect that an indexable end mill with a specified diameter will actually _cut_ that diameter?

My indexable end mills consist of a set of Denitools (the straight, 16mm end mill measures 16.0mm across the inserts and 15mm on the body) and a Shars 3/4” 60 degree dovetail cutter that measures 0.750” across the inserts. The rest of my insert tooling is for my lathe, where actual size only matters for minimum hole size for a boring bar.

Note: Due to a number of factors, the only time I would rely on a 1-1/4” end mill being on-size would be if I needed a 5/8” radius (like for a fluted knob); if you want a 1-1/4” slot, I use a 1-1/8” diameter end mill (or smaller) and clean up the slot with final passes.

All that not withstanding, I would go back to the vendor for a replacement, credit or refund.

 

[benmychree]

Indexable end mills are not intended to cut to nominal widths, same for face mills, they are made to remove material, not to cut to finished dimensions.

 

[homebrewed]

Sounds like revenge of the imports to me...
How bout a picture?

I'm not sure what you are looking for, but here are a couple of photos.

 

[homebrewed]

Indexable end mills are not intended to cut to nominal widths, same for face mills, they are made to remove material, not to cut to finished dimensions.

Good to know, I had thought they were functionally the same as HSS. Except, of course, they can't be used for side milling.

Some seem to cut closer to the specified diameter than others. I have a 3/4" indexable EM that's pretty close. YMMV I guess.

Thanks for the reality check, I learned something today.

 

[atunguyd]

Note: Due to a number of factors, the only time I would rely on a 1-1/4” end mill being on-size would be if I needed a 5/8” radius (like for a fluted knob); if you want a 1-1/4” slot, I use a 1-1/8” diameter end mill (or smaller) and clean up the slot with final passes.
.

There's another scenario where you want the end mill to be on size. That's for tool offset, especially if when you are using a dro. I find that the edges of my part! Set zero my dro and then apply tool offset for each end mill that I am using.

Even without a dro and using the machine dials most people would zero on one edge and then use the mill radius as part of their calculations for the opposite edge.



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[atunguyd]

Good to know, I had thought they were functionally the same as HSS. Except, of course, they can't be used for side milling.

.

I did not know this (only had two indexable milling cutters test are hss).

Can someone verify this? To me looking at the endmill I would think side cutting is all of can do as well that is the only exposed cutting edge. The bottom cutting edge is actually relieved so it is the side ward action that does the cutting (ie you cannot plunge cut like you can with a center cutting hss endmill)



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[homebrewed]

There's another scenario where you want the end mill to be on size. That's for tool offset, especially if when you are using a dro. I find that the edges of my part! Set zero my dro and then apply tool offset for each end mill that I am using.

Even without a dro and using the machine dials most people would zero on one edge and then use the mill radius as part of their calculations for the opposite edge.



Sent from my SM-S908E using Tapatalk

Yep, that also is something I was counting on for another step in this particular project. I think it still can be used for that but the radius offset won't be a "nice" number.

 

[higgite]

You say the slot measures 1.311" wide, but you don't say what the insert edge to edge measurement is. Is it not 1.25"? If it is, what size and how rigid is your machine? Have you measured runout of the spindle or the end mill?

Tom