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radkins
02-06-2010, 11:40 AM
Ok, my inexperience is going to be very apparent here, both in not knowing how to do this and not really knowing for sure how to ask! I have a rectangular 1"x1-1/2" hole milled all the way through a 2" block (4140) and I need to clean up the corners. I have the hole milled to size but the corners have a 1/4" radius (?) because of finishing with a 1/4" end mill. How do I finish these corners?



The fit is not critical here (a part for a plow) and I could finish it with a file but I am trying to get it as perfect as possible because I need to learn how for future projects. How would I approach this if it was indeed to be a precision part with both fit and appearance being critical?

Carld
02-06-2010, 12:19 PM
Actually with a 1/4" endmill the radius is 1/8" but to clean the corners the easiest way is with a square file. I do that a lot and the way I approach it is I finish out the corner with the smallest endmill that will cut the corner top to bottom and then file it square. Of course that will not work on a blind pocket, the hole has to go all the way through to file it.

You can make a broach with a piece of HSS tool steel and work the corner out. It's slow but it works and you have to take small cuts at a time. I broach square holes in boring bars the same way but that is one pass and takes a lot of pressure.

Flatten the end of the HSS cutter and grind a V from side to side leaving four sharp corners sticking out. that is your broach.

Toolguy
02-06-2010, 12:46 PM
Carld has a good suggestion about making a broach out of a HSS lathe bit. Once you have the broach, you need to make a holder for it - a piece of steel with a hole in one end that the broach will slide into and stop at the end with enough length sticking out to do the job you're working on. A round hole for a square bit is fine. Drill and tap for a setscrew in the side to hold the broach in place. Put the broach and holder in a collet in the mill spindle and run it up and down with the quill. Like drilling a hole, but with the spindle off. Orient the flats of the broach parallel to the sides of the milled hole and work the corners out.

Lew Hartswick
02-06-2010, 12:51 PM
Or EDM it. :-)
...lew...

Dan Dubeau
02-06-2010, 01:42 PM
If it's not critical you can drill out the corner like shown below

http://img31.imageshack.us/img31/406/squarecorner.jpg (http://img31.imageshack.us/i/squarecorner.jpg/)

But a file can be surprisingly quick. i've got a couple safe edge files (face smooth, edge tooth, and vise versa) that make quick accurate work of stuff like this. A square file with an edge ground flat and smooth, lets you just slap it on, and file till your corner is square. Another handy tip it to have a sharpy marker handy, and "blue up" the area you dont want to remove material from. marking up the area lets you see your progress as you file.

radkins
02-06-2010, 02:03 PM
Somehow I kind of thought the idea of using a file would be frowned on but I guess it is OK after all, of course that's why I asked. Certainly using the file on this particular project would be OK, as would the idea of drilling the corners that was suggested, but since I am using this as an opportunity to practice I will use the broach idea and see just how precise I can get it.


That square file "with an edge ground flat and smooth" is something I had not thought of but I will certainly add one to my tool box.:)

ptjw7uk
02-06-2010, 03:22 PM
Another way is to grind the face of a flat file and use the cutting edge easier to control as a larger surface to keep flat the square files are liable to tilt.

Peter

SpyGuy
02-06-2010, 04:07 PM
Hint: A keyway broach can be used for more than cutting keyways. :cool:

beanbag
02-06-2010, 04:52 PM
I would have drilled out the corners in advance with something like a 1/16" bit or so, not overshooting the corner (i.e. still inside the rectangle), but big enough that the end mill still cuts into this hole. It will save you time in terms of metal removal later on.

http://i139.photobucket.com/albums/q286/beanbag137/cutcorner.png

ehhh, maybe 3/32 drill would have been better.

You could also try a long 4 flute carbide end mill in the corner only.

Sometimes your customers will care about surface finish, and having a lot of filing marks around is bad.

radkins
02-06-2010, 06:38 PM
Hint: A keyway broach can be used for more than cutting keyways. :cool:




I was thinking along those lines and was considering waiting until my Broach set arrives, the 3/8" is flat with a 3/8" body so I was kind of wondering about doing that.:confused:

All good suggestions and the idea when I asked was to learn something, I got a lot of good info here and I'm definitely taking notes!

Thanks to everyone

JoeCB
02-06-2010, 07:04 PM
I'm surprised that no one mentioned using a cold chisel, specifically a cape chisel. In days past that would have been the tool of choice absent a shaper.
Maybe I'm just showing my age!
Joe B

darryl
02-06-2010, 08:56 PM
The chisel idea works- I've done that. I've also considered getting an arbor press and turning it into a broaching machine- also considered making a power filing device to take square or round files. I guess the idea pretty much is to shape the corners, whichever way you do it. Hmm- vertical shaper- I see another future project in the works-

Paul Alciatore
02-06-2010, 09:44 PM
If it's not critical you can drill out the corner like shown below

http://img31.imageshack.us/img31/406/squarecorner.jpg (http://img31.imageshack.us/i/squarecorner.jpg/)

But a file can be surprisingly quick. i've got a couple safe edge files (face smooth, edge tooth, and vise versa) that make quick accurate work of stuff like this. A square file with an edge ground flat and smooth, lets you just slap it on, and file till your corner is square. Another handy tip it to have a sharpy marker handy, and "blue up" the area you dont want to remove material from. marking up the area lets you see your progress as you file.

I usually do something like this unless the part just won't allow it. But after the main hole has been cut, it is best to use an end cutting, end mill to take the corners out. And it does not need to be as extreme as Dan's drawing shows, you only need to take it out up to the actual square corner. This could be done with the 1/4" mill you used to cut the main hole. With the cutter above the work, just go out at 45 degrees at each corner until you reach the point where cut will reach the point two sides would meet, then take a plunging cut down and through. But a 3/16" end mill would be wide enough to do it and would take out less unnecessary metal. To reach the point where you are cutting to the corner, touch the sides of the hole and then move outwards 15% of the cutter's diameter (3/16" x .15 = 0.028"). Do this for each side of the corner to go out at 45 degrees.

If you are interested, the formula for finding the minimum diameter of the cuitter needed to do this is 0.707 x Diameter of Cutter that left the corner radius. So 0.25" x .707 = 0.177" and 3/16" is just slightly larger than this at 0.188". It is best to be a little bigger than the minimum here and the 3/16" does this quite well.

Carld
02-06-2010, 11:28 PM
I don't think a broach will do the job. For one thing the back part of the broach is wider than the part that cuts the keyway and that would keep it out of the corner. Another thing is even if you ground the sides of the broach flat with the cutting part how would you hold it against the sides of the hole. Of course you would have to make a special plug to fit the hole. Then you just wasted a broach and the time to make the plug you may never use again.

Just file it or a chisel or the square broach I described.