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

[homebrewed]

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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If you mean the tool can only traverse across the work in the horizontal plane (x or y, not z), then yes that's the case. Attempting a plunge cut would produce a groove whose depth is limited to the distance the insert protrudes downward from the holder. I think things would get pretty weird before you went that deep, especially if attempting it on a small mill. The cutting load would get pretty high toward the end.

As @ChazzC mentioned, it would be possible to use a tool like this to cut a radius on the edge of something. But it's not absolutely necessary for that -- a standard HSS end mill can do it, along with a boring bar setup. Probably a fly cutter, too.

 

[homebrewed]

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

I measured the edge to edge distance between the inserts and it matched the slot width pretty well (the outside edges of the inserts are at 90 degrees so that was easy to measure) . That's why I was surprised by the discrepancy between the result I got and the specified diameter of the end mill. It just couldn't be what it was claimed to be. It was a pretty short trip between result, hypothesis and a measurement that proved it. Surprised the heck out of me, but, there you go: yet another example of the dangers of making an unvetted assumption.

I have one more indexable end mill I need to check. It is a smaller one with just one insert so I will cut a slot in a piece of scrap and measure that to see how the cutter stacks up. I'll record all these actual diameters, you betcha.

 

[ChazzC]

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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Maybe because I have a manual Mini-Mill (with DRO), I don’t rely on tool offset for my work. I’ll touch off when cleaning up an edge, but use an edge finder for locating and measure features to determine how much needs to be removed. This probably takes more time, but I don’t need to worry about tool wear as long as the cutting edges are sharp and as a hobbyist I’m not worried about speed.

 

[homebrewed]

Maybe because I have a manual Mini-Mill (with DRO), I don’t rely on tool offset for my work. I’ll touch off when cleaning up an edge, but use an edge finder for locating and measure features to determine how much needs to be removed. This probably takes more time, but I don’t need to worry about tool wear as long as the cutting edges are sharp and as a hobbyist I’m not worried about speed.

That all depends on what sort of operation you're doing. For cutting a boss at a specific location with a specific width the most expedient approach (I think) is to use tool offset calculations. I can think of at least two or three other approaches but they either are much less convenient or are difficult to get accurate results with; or impose restrictions on workpiece size. The latter issue actually would come into play for me.

 

[RJSakowski]

With the Tormach CNC and SprutCAM, tool offsets are a given. But even when operating in manual mode or on the RF clone, I use tool offsets. When I make my working drawing, I lay out my actual tool paths with start and end points for the intended tools. I find this reduces errors on my part. I use the TTS system for tooling and an edge finder for locating my work edges and a dial indicator for setting my z axis zero. I set up workplace coordinates just once at the start of the job and rely on my tool offset table for the necessary corrections.

 

[homebrewed]

I checked the actual cutting diameter of all of my insert-style end mills. A grand total of three, specified by their respective vendors to be 1-1/4", 3/4" and 1/2" diameter. Two were bought from Shars and one from Little Machine Shop. I cut three nested slots in a piece of 1/8" aluminum stock, installed with the thin edge up. The slots measured 1.315", .751" and .500". So the largest-diameter one is seriously noncompliant and the others are OK. It's likely the .751" measurement is just measurement error on my part.

Unfortunately it's been a few years since I bought the <quote> 1.25" <endquote> EM from Shars so it's unlikely they will replace it or refund my money. I'm sure the receipt has been recycled.

Now that I've characterized it, I think I can use the 1.315" EM for what I wanna do. I'll grouse about it but, hey, I got more than what I paid for, right?

 

[rabler]

Just out of curiousity, are these Shars supplied inserts or generic inserts from elsewhere? I've heard others comments here that Shars inserts are not always to "standard" spec.

 

[homebrewed]

Just out of curiousity, are these Shars supplied inserts or generic inserts from elsewhere? I've heard others comments here that Shars inserts are not always to "standard" spec.

They were purchased from Shars. It's hard to believe that their inserts would be that off from the industry standard. They'd have to be about .033" off-spec. But, what the H, it's easy enough to check.

 

[homebrewed]

They were purchased from Shars. It's hard to believe that their inserts would be that off from the industry standard. They'd have to be about .033" off-spec. But, what the H, it's easy enough to check.

Oh, and the 3/4" EM I got from Shars has their inserts installed and it _does_ cut a .750" wide slot. Same insert size and type that my larger EM uses.