View Full Version : How to cut arbitrary corner radius?

05-27-2010, 09:55 PM
I have a piece of stock 1/2 x 1.5 x2.5, for example. I want to make a corner radius approx 1/4" along the 2.5" edge. But maybe I want it to be a little less. One way is to buy a radius cutting mill, but that would only work for one radius. One other way is to stand the piece vertically in the vise and use the CNC to cut half of it, and then flip over and repeat, but if I don't match up the two halves perfectly, there will be a little line in the middle. I could also clamp the piece horizontally in a vise and have the CNC make the radius in multiple long passes parallel to its axis.

Any other ideas that don't involve a lot of money?

05-27-2010, 10:03 PM
Angle grinder and a file will be fast and low cost.

05-27-2010, 10:06 PM
Re-grind an old endmill into a single lip cutter with the radius you want offhand ground in.

05-27-2010, 10:14 PM
Sometimes the old tools are the thing... a simple form tool in the shaper. no problem!
Joe B

05-27-2010, 10:19 PM
Get out your radius dresser for your surface grinder, and grind the correct profile in a 1/2" wheel, then grind it.

Or dress the negative of your profile on your wheel and then use that to grind an end mill.

Or pay a tool & cutter guy to make you the right tool.

I personally wouldn't worry about the "little line" -- a pass on a 3M deburring wheel would blend that in no problem. Unless this part is for a guided missile or something.


05-27-2010, 10:26 PM
If the answer is one and it's for appearance or feel then I don't think any above suggestons beat a grinder and a file.......

doctor demo
05-27-2010, 11:03 PM
Belt sander, Disc sander, File and a lenght of emery cloth.


05-27-2010, 11:23 PM
Take a 45 degree cut off each edge to minimize the amount of material to be removed by grinding. Draw file the radius to show the high spots, belt sand. Repeat the draw filing then belt sanding til it's right.

Or- use a radius mill of slightly larger size than you want, just don't cut as deep. Grind, file and sand the rest of the way.

05-27-2010, 11:51 PM
what machines do you have? how exact does it have to be?

your cnc idea is good. I dont have one working yet so I'd make an end mill, simple style, sort of like D bit. Turn the radius, mill half of it away, and file relief behind the cutting edge. harden and stone/ if you need it more exact, you're buying a grinder and radius dresser :)

05-28-2010, 12:02 AM
My first thought was to mount it on a rotary table that is mounted vertically. Use a tailstock to support the far end. Use the mill to remove the corner as the table rotates. The further from the center, the gentler the radius.


J Tiers
05-28-2010, 12:23 AM
I believe this is over-thinking........ I like the file idea and its relatives.....

As someone once said, sometimes it's just close enough....... you don't wipe your A$$ with 3 D co-ordinates and a color density meter.......

05-28-2010, 12:38 AM
Since he did not say what he wanted for the level of accuracy, any thoughts might help.

Besides, this is an amateur website where people share information, ideas and thoughts. The OP now has at least 4 ways to do it. That's not including router bits if he's doing aluminum.


05-28-2010, 12:47 AM
I think I have a 1/4" radius cutter so I know what I would do if I wanted it to be real nice. On the other hand many times I just belt sand a radius on stuff if it is just a nice looking radius of no particular size and close enough is good.

05-28-2010, 01:10 AM
Easy. Buy a Kearney Trecker 2D. I got mine for $0.09 per lb. Pretty cheap when you think about what meat goes for these days... :D

05-28-2010, 02:14 AM
If you don't like manual methods (file etc), you can make a lathe bit or single lip form tool to any arbitrary radius with good finish and accuracy. Just takes a little time. First, grind a close approximation for the bit using a radius gage or other check. Then turn or otherwise acquire a rod of the appropriate diameter in some appropriate material, and load it with Clover or similar compound (see "appropriate material"), choose grit or multiple stages according to need. Use that rod as a lap to finish bringing in the edge. Provide adequate clearance and go. Easier on a lathe form tool, but possible on a rotating cutter.

Then again, I would love that 2D...

05-28-2010, 03:43 AM
The level of accuracy I needed was "presentable", as in a gift or product for sale.

I think I will just stick with making many longitudinal passes on the CNC mill (piece mounted horizontally, ball end mill), because that gives the most consistent finish. And then sand it down to remove the tiny "tesselation" ridges.

I'm only going to do this two or three times (with a different radius each time), so I didn't want to grind a tool for it.
Thanks for all the suggestions.

05-28-2010, 04:19 PM
Can you mount the piece @ a 45 deg angle and cut it using CNC? Seems like a solution to me.

Your Old Dog
05-28-2010, 05:36 PM
The level of accuracy I needed was "presentable", as in a gift or product for sale....................................


Doing it by hand with a file will give you "evidence of hand" which I think is a desirable trait in a gift. In other words, it wasn't spit out by a machine.

If your familiar with drawfiling it would be a piece of cake to put a nice radius on the edge.

05-29-2010, 12:35 AM
I think I will just stick with making many longitudinal passes on the CNC mill (piece mounted horizontally, ball end mill), because that gives the most consistent finish. And then sand it down to remove the tiny "tesselation" ridges.

When you mentioned CNC that was my first thought. Using a ball endmill, make many passes just a few thousandths apart. Should not be too hard to plot the passes for the radius manually, piece of cake if you program the CNC from the computer.

We used to call it kellering.

tyrone shewlaces
05-29-2010, 12:58 AM
Carbide corner-rounding router bit
They're cheap.

05-29-2010, 01:09 AM
Grind HSS blank (With dremel round grinding stone predressed to the correct diamiter to make the radius? Would make getting the radius perfict pertty easy..), Insert into flycutter. Simple, Cheap, effective.

05-30-2010, 11:29 PM
Bunch of good suggestions mentioned above. I have used the wood router rounding bits too. They work decent in softer materials. Not many sizes availble though.
You mentioned cnc,
I made a quick little model with the radius on just one side and then created a few tool paths to get some numbers.
A 3d contour or parallel operation work nicely.
Using a 1/4" ball endmill with a max stepover of .01" will cut a .250" radius with a worst case cusp height of .0002".
That of course would be a very nice looking end result.
Kick up the step over to .030" and it can reduce machining time by 40% but also increases the worst case cusp height to near .002".
The time saved machining might be offset with the time needed to hand finish the radius.
These numbers will change with various cam programs of course.
And then it all boils down to how critical the radius needs to be..size, accuracy , appearance etc.
Hope this helps a little.