Is it possible to add a radius to a square end mill?

I have a project that calls for a 7/16 end mill. The challenge is that it should also have a corner radius between 0.020 and 0.015. I can buy this mill but it is more than I want to spend to run a couple of pieces. (It's hobby not a business.) Can I grind a radius or will that destroy its cutting capability? If it can be done, any tricks or methods would be appreciated. Thanks in advance!

Yes, and there's lots of ways of doing it. But what material are you cutting? If aluminum, then quality of the grind doesn't have to be high, the edge will hold up even if not optimally shaped. But if cutting a tougher material, then a badly ground radius may break down too quickly to be useful.

If you buy a high speed steel end mill you may be able to hand-stone a radius on the end. That would be hard to measure accurately without an optical comparator, or at least either a purchased or shop-made radius gage and a loupe.

The key with doing this would be to make sure you maintain the proper clearance angle, basically matching the relief on the back side of the flute with the stone.

What material are you working in? Maritool has carbide 3-flute end mills, 7/16 with .015 radius for $35. In a job shop you'd spend that much in time trying to cobble something together, but I understand trying to make something work from a hobby standpoint.

Grinding is fine..In a pinch we would grind a bevel and the hand hone the corners. one can look at a part corner in a shadow graph(optical comparator) and see what corner bevel fits a .015-.020 radius for starters.

michiganbuck said:

Grinding is fine..In a pinch we would grind a bevel and the hand hone the corners. one can look at a part corner in a shadow graph(optical comparator) and see what corner bevel fits a .015-.020 radius for starters.

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Yup, done that plenty of times. For carbide use a diamond hand hone to finish it off.

Honestly with a decent diamond hone thats only a couple of minutes to lose the point. Carbide goes away fast with a small DMT hone. IMHO its really not hard to do if you simply are just after some radius, if you want dead nuts exact, pony up the money, but if you just want not crack inducing corners, just have at it.

Most of the corner radius cutters that I've come across are somewhat imperfect.I often
use CR cutters for cutting 3D mold parting lines,so I need to figure out the actual CR on the cutter.
I normally do a test cut and check the resulting radius with gauges,pins or under the comparator.

Usually the manufacturers use a different grinding wheel to cut the corner radius,
and they tend to leave some stock on the radius( maybe 5 tenths),
so the corner radius ends up being non tangent to the main flutes,looking more like a chamfer
in extreme cases.
I've had some 3/16 Dia. Fullerton brand CR cutters that were marked as being changed from square cutters,
so it must be economical for them to add the radius from an existing cutter.

I am cutting aluminum and the radius does not need to be perfect. I just need to get rid of the sharp corner to eliminate the stress riser. Since I'm a hobbyist time is my friend. I'll follow the advise above and hand hone the corner. Thank you all for the input!

I usually hand grind a big chamfer on 3 flutes, that way I only have to work on making one corner nice

I like the 1 for 4 approach!

I have done this, by hand even and it can work out well. But when you are rounding the corner or corners, don't forget that any cutting edge must have relief or clearance behind it so that it can cut. So, you should continue that rounding all the way to the back side of the flute.

I do not do that one out of three or four thing as I would prefer that all or at least most of them share the cutting. But if you do chamfer some, that chamfer also must have enough clearance so that it does not rub.

If you have something with magnification you can overlay circles on, need not even be a real comparator, but similar you can actually hand hone all 3 or 4 corners pretty dang near. Yes you need some clearance, yes the cutters not going to be as great as one with a radius swung on it by a 5 axis cnc cutter grinder, but you can get pretty sweet results so long as you can see what your doing and have a rough feel for a clearance angle.

Clarence angle is pretty easy to judge based on the width of the flat compared to the other clearance lands, basically you want to keep the angle such it stays parallel. Its easier to do this if you knock the corners off to a 45 chamfer getting that clearance right and then round it from that.

''Is it possible to add a radius to a square end mill? ''



If you can't I've been doing it REALLY REALLY wrong for over 40 years

looking at the end mill side near the end corner one can see the clearance where the OD and end intersect (perhaps 8 to 10* clearance), it is good to pick up that same clearance angle. looking through a loop is not a bad idea to hold the cutter to a parked wheel a few times so you get a feel for clearance angle. Make a hand 45* bevel that looks like it near fits a radius gauge. Then hone off the edges of that bevel to make radius fit gauge. for a larger radius do the same and then round off the corners with additional small grinds on the wheel to finish radius. A secondary clearance can also be bumped on by hand.
I worked with fellows who did that same task in a high class cutter manufacturing shop on new cutters.
One can practice this talent on end mills that are going out for sharpening to get better at this skill.

Being fancy on larger radius one can 45* bevel. then 30 and 60* bevel to near fit a radius gauge, and then blend bevels together by hand to fit the radius gauge.

Another method it to nick a surface grinder wheel with the desired radius then come in to grind each corner..in a production need one can buy a radius diamond for wheel nicking .

OT: I worked with fellows who could hand spiral clearance back off the OD of step drill at the step diameter > got to be K goo at that but never good enough to look like factory work..still good enough to work fine.

^ I can just about reliably do a three flute countersink by hand so all 3 flutes cut well, but clearance on a drill flute, yikes, that's getting tough.

I do have a rats tail dresser though and its kinda usefull for little groves in grinding wheels, i have been known to dress a SG wheel, then mag a large block to the table of said SG and use it as a some what over sized bench grinder. Sure guarding leaves a little to be desired, but needs must and i can still count to 10.

It kinda always surprises me how in this trade no one ever considers human hand movements to do some of this kinda stuff, half the stuff i turn has features added by some weird form tool i have ground then diamond lapped to perfection. Maybe im odd, but i grew up wood turning, to turn, you need a decent single bevel on the chisel, what ever its cross section, has to be once smooth single bevel, if you can't get that on a tool from the bench grinder then you don't have the skill - hand eye coordination to do at all well turning generally either.

I rarely leave carbide EMs dead pointed as they are a lot more resistant to tip failure if you knock the point off. I use a five diopter Optivisor and an inexpensive diamond lap

EZE Diamond Hand Lappers | Gesswein

on both HSS and Carbide. I use the medium fine stone and may finish up with fine. I count strokes on each point trying to use the same pressure on each stroke and roll the hone to make a smooth radius while maintaining some clearance. Sounds tricky but is really very easy and result are excellent. A little Sharpie ink on the cutting end helps to show where the lap is cutting if in doubt. Also you don’t have to make a radius as just knocking the corner off at a 30 deg angle or so angle protects the corner too. It’s a simple skill worth learning.

BTW, the Sharpie ink can also be used to show you have adequate clearance if it remains after you take a test cut after radiusing.

Denis