Concave radius cutter

Hey guys, this may be common knowledge, but I don't have it, so I'm asking.

I need a cutter to mill a specific radius on the edge of flat bar. In this case, 1/8", or 3/16" to fit into a milled slot with a radius end. i.e. flat to half of diameter of the cutter the slot was milled with.

I found a concave radius cutter, which is labeled 1/8" radius, but the diameter stock that fits into this is 1/4" (matching radius, seated exactly half the cross-section of the stock).


Should I be looking for a radius cutter half the diameter then? If I want to mill an end of a bar to fit into the end of a slot cut with a 1/8" end-mill, do I need a 1/16 radius concave cutter?


Is this standard across the different style of concave cutter?


I just want to verify before I order yet another incorrect cutter. The one I got was surplus, and it's possible it was reground or something. Also, if there's a better tool for this job, I'd be happy to know about it.

I'm sure I could make something similar, or duplicate a one-off concave radius cutter also, and I may, but I'd be curious to know how other people are accomplishing this.

I know I could do this on a rotary table, but I'm trying to find a quicker way, I need to do these one-offs in a few sizes, from different materials, not in batches, and setup time is the biggest factor here. With a concave radius cutter I feel I should be able to make these happen ad-hoc, pretty quick.

Thanks guys!

Of any given circle, the radius will always be half the diameter. That's not machining knowledge, that's just geometry. Keeping this in mind, it's not surprising that a 1/8" radius cutter corresponds to a 1/4" diameter piece of round stock (or 1/4" thick rectangular bar with completely rounded ends).

Corner rounding end mills are sold by radius. For example, look at the maritool site here. That's common practice since in normal use they are being used to cut a radius on a part edge and the drawing would call it out as such (meaning, not as a diameter).

So to answer your question:

Should I be looking for a radius cutter half the diameter then? If I want to mill an end of a bar to fit into the end of a slot cut with a 1/8" end-mill, do I need a 1/16 radius concave cutter?

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Yes, assuming you are talking about a 1/8" thick bar.

carbonbl said:

Of any given circle, the radius will always be half the diameter. That's not machining knowledge, that's just geometry.
Corner rounding end mills are sold by radius. For example, look at the maritool site here. That's common practice since in normal use they are being used to cut a radius on a part edge and the drawing would call it out as such (meaning, not as a diameter).

So to answer your question:



Yes.

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Yes, I understand that it's basic geometry, thanks for being condescending. I wasn't asking about geometry, but nomenclature, which is often, completely disconnected, in this industry, and there is VERY little standardization between manufacturers when it comes to tooling. Many places I looked for these cutters list diameter instead of radius for cut, but it seemed pretty obvious what the case was when I got the larger than expected one in, which is why I just wanted to verify before making another assumption.

Either way, thanks for clearing it up.

javan.dempsey said:

Yes, I understand that it's basic geometry, thanks for being condescending. I wasn't asking about geometry, but nomenclature, which is often, completely disconnected, in this industry, and there is VERY little standardization between manufacturers when it comes to tooling.


Either way, thanks for clearing it up.

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Sorry dude, I don't know who you are or what your knowledge level is so I'm assuming zero knowledge when trying to give an explanation. There's no geometry test to make a PM account and really basic questions do get asked here. Not trying to be condescending, but I don't blame you for reading it that way since there are a lot of condescending people on this site. Good luck with your project.

carbonbl said:

Sorry dude, I don't know who you are or what your knowledge level is so I'm assuming zero knowledge when trying to give an explanation. There's no geometry test to make a PM account and really basic questions do get asked here. Not trying to be condescending, but I don't blame you for reading it that way since there are a lot of condescending people on this site. Good luck with your project.

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Fair enough man, apologies for being snarky. It's hard to infer intent here, as we all know. I appreciate the response regardless.

javan.dempsey said:

Yes, I understand that it's basic geometry, thanks for being condescending.

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I don't think he was being condescending. I think he was being clear. I would have been inclined to say out loud something more like "What the Hell do you think RADIUS means, you lack-wit plebe", but then my social potty-training filter would cut in and I'd probably actually post "Radius is always half the diameter, so yes, that's what you get when you buy a 1/8" radius cutter". Oops, I guess I posted the potty-mouth anyway.

I can kinda see how it might be a little confusing, I still get messed up on the "included angle" vs. "side angle" thing. I get it when they give both but when someone says "a 45 degree angle cutter" is that 45 per side or 22.5 per side?

"lack-wit plebe" is potty mouth? Man, you shouldn't *ever* listen to me mouth off at the drivers around me when I drive...

I'm surprised no one has mentioned this yet...

Diameter is probably referring to the diameter of the whole tool at the widest point of the cutting edge. Corner round radius is referring to the radius of the arc that is left by the cutting tool.

So consider - if you’re trying to leave a 1/8” radius on the edge of the bar, you want the 1/8” corner radius. But the tool diameter needs to be larger than 2x corner radius. Remember that there is a “pilot” at the tip of the cutter - and that has width too...

See this example:
Harvey Tool - Carbide Corner Rounding End Mills - 2 & 4 Flute

If you look at the tools with .125 corner radius, you can get them with 3 different shank diameters from 5/16” to 5/8”. Depends what works better for your particular application.

Also note that the pilot diameter ranges from .060” to .250”.

So if you’re making a 1/8” slot and wanting to corner round the inside edges of the slot, you need the smaller tool. The 5/8” cutter - even though the .125” corner radius is correct - won’t fit in your slot. But the 5/16” version with the .060” pilot would fit in your slot...

Also keep in mind that the pilot extends below the corner radius tangent point. So you need your CAM (or manual g-code) to compensate for the pilot length properly.

Hope I understand your questions correctly...

Mooner said:

Diameter is probably referring to the diameter of the whole tool at the widest point of the cutting edge. ... Hope I understand your questions correctly.

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Not exactly, but corner rounding tools like the one you linked are a useful alternative to the concave half-circle cutters we've been discussing so far. OP wants to make one cut, not two, to round over the edge, so the corner rounder are less desirable.

For concave half-circle cutters, radius means what it does on a corner rounder, and circle diameter means twice the radius. The cutter diameter (cutter OD) is a completely different property than the circle diameter/radius which is the form that the cutter cuts.