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Help! I need to generate a .003R max on the edge of a hole in Inconel 718.


Aug 31, 2015
The Moridor
I have a part that is made of Inconel 718 that has 4× Ø.0625 ±.001 thru, and 1× Ø.0315 thru. Material is .175 thick. Putting in the holes is not the problem. Generating a .003R max edge break with no burrs is what's troubling me. Before you ask, NO this can not be a chamfer, it must be sharp up to .003R. Does anyone know of any tool makers that make corner rounding endmills this small? Harvey Tool makes one that has a .003R, but the pilot is too big to fit in the Ø.0315 hole.

Normally, I would just get one made as a custom/special tool, but this is kind of a rush job. I have about 500 of these to make, so the time and skill required to do these by hand just isn't feasible.


Feb 24, 2018
Greenfield, Mass
Worst case, this might end up being a hand operation to deburr your holes, which is a pain in the rear, but sometimes necessary when you are dealing with tight tolerances on these nasty alloys.

If you can get any "Cratex" type abrasive, you can use either a stick (sharpened to a point with a belt sander) or the bullet shaped on a mandrel. Put em in your cordless or corded drill/ a drill press, and go to town.

With the proper amount of pressure, it will remove any light burr and sometimes leave just the slightest radius, which should satisfy your customer's requirements. Make sure to change your drills if they are leaving burr that cannot be removed by the abrasive.


Jul 6, 2006
Hillsboro, New Hampshire
IMO Cratex is risky here, they might be soft enough to deform and lead to marks beyond the .003" callout.

If this is a flat plate, I'd lap the surface on some WoD to get the burr off (if it's heavy, adjust your process so it's not), then take a fine mounted stone that's been trued by a diamond into a cone and spin it by hand within the holes. Don't power it, you might take too much material too quickly.

A small orbit of the hand while spinning the stone should produce something close to a fine radius, some playing around may be needed to get the technique down.

If you must, must do it on the machine then you'll be chasing thermal drift in the machine and spindle, so check every few parts.

Speaking of checking, learn how the customer will check the parts - this could be a killer or open the process up.


Dec 14, 2017
Since sharp is allowed, lapping might be the way to go. Sharp is likely easier than R.003. If the part is small it might not take too long to scrub it on some PSA abrasive paper stuck to a flat. It's hand work but sometimes you gotta do what you gotta do.


Jun 23, 2002
Vancouver BC Canada
Hi Zeuserdoo:
Since these are through holes you're not spoiled for choice...I assume both sides need to be deburred.
If you just lap the plates you will push the burrs into the bores.
Even if you can get the holes just about perfect on the entry side they will have burrs on the exit side.

About the most efficient way I can think of is to wire EDM all the bores to bring them to size and make them burr free.
Is that an option?
That assumes everyone is good with dead sharp corners.
If they are just flat plates with holes, you can stack up a bunch of plates and wire them all at once to save cost on each hole.

If you actually NEED a radius and it may not be a chamfer you have no choice but to poke down a form tool of some kind.
You have to do it to both sides of the plate.
You may not go too deep and put a ring around the edge of the hole.
You have no room to interpolate a smaller cutter, so ideally the corner rounding tool is piloted right to size and spun in by hand with a positive depth stop so you can't go too deep.
You need one for each size of hole.
It will be a lot faster to spin the tool in a slow speed right angle handpiece like a Foredom or a Dental Surgimotor.
It's all way to little for a pistol drill IMO.

Spinning it by hand will not work well, but an old fashioned bow drill may do just fine if you make your tools as diamond coated hardened steel mandrels.
There are places that will plate your mandrels with whatever grit you want.
Kinda like those diamond coated precision hones for one pass honing.
I don't know how small they will go.


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Jan 14, 2007
Flushing/Flint, Michigan
Most certainly I can measure this from .00004 to .010 and control it's size and shape down way, way inside and control it.
Also have been doing it for 40+ years and seen the oh-shit.
Can these go through a vibe? Yes one needs very small stones in her and you knock the rad everywhere.
Then comes how good of a rad or the shape to it.
You would like a custom corner rounding cutting tool to do a .0025 rad. Have you lost your mind?