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View Full Version : Inside radius, lathe work. Help?



JRouche
05-19-2009, 10:26 PM
Im turning some bushings and I want a very small inside radius at the corner. Its at the inside base of a flange where it meets the shaft. The flange is 1" OD and the shaft is .750", all one solid piece.

I want the shaft as smooth as I can get it, using mico abrasive film to get a mirror finish on the shaft.

So my question is how do you guys go about doing the radius. Do you turn the shaft section down to close to final size then cut the radius? Then finish turning the shaft and polish? So like if its a .0625" radius do you leave the shaft large by .0625 then do the radius then finish cut the shaft (same with the flange face?).

What Id like to have is a good transition from the shaft to the inside radius without too much under cutting of the radius. Prolly looking for pie in the sky. Is it rainin pies now!!! Yeah!!! I love pie. Oooh, I drifted of course :)

Its 4140 PH and Im using carbide. I ground a tool for the radius.

My goal is to have a flange tube and a nice lil radius on the inside corner to help reduce the stresses involved from a sharp edge.

Any help guys????? I know yer all saying, darn dude?? Cant you even cut a simple radius. Hahaha.. Evidently NO.. :) Thanks, JR

shadoof
05-20-2009, 04:48 AM
I copied this from my training manual,

http://i156.photobucket.com/albums/t20/shedfull_2007/eitb.jpg

Might give you a start

Lee

kvom
05-20-2009, 08:26 AM
It sounds as if your tool will be similar to a corner rounding endmill. To get these to avoid undercutting you would normally "index" it on some test material: touch off and move it in .001 at a time until the first undercut appears. Then you know how much to cut.

TGTool
05-20-2009, 10:29 AM
I think my approach would be to determine which feature (diameter or flange) is most important and favor the more important one. So, assuming your polished diameter is more significant, I'd machine the shoulder .062 short and take the diameter down ready to polish. Then work the shoulder back to position leaving the .062 step to be cleared by the radius. Dig out the radius with a form tool to perhaps a thousandth or so below the required finish diameter, then finish the diamter to size. My rationale is that if there's a bearing or something that's a close fit on the .750 diameter, there's a tendency for the diameter to be large where it meets the blend radius. That is, the polishing to size will slight the radiused end so it needs particular attention to bring to size and the very slight undercut at the radius won't hurt anything.

Paul Alciatore
05-20-2009, 12:13 PM
I would attempt to make the shaft OD and shoulder in a single cut. First I would rough out both with a sharp tool leaving about half the radius of the corner on BOTH the OD and on the shoulder. That will leave enough material in the corner for the full radius.

Then I would change over to the radiused tip tool and take out the remaining material starting from the tailstock end and cutting toward the shoulder. When I am approaching the shoulder I would then feed outward with the cross slide to finish it. For a 1/16" radius you would have about 0.032 to remove on both surfaces and could take several cuts at 0.010 and a final cut to finish both. This leaves you opportunity to measure after each cut. Keep track of the dial numbers as you go. Leave enough to use your abrasive paper to finish to final size, probably a few tenths.

If you are having trouble with this technique due to resetting the cross slide to the same distance on subsequent passes, you could finish the shoulder on the second to last pass, leaving a few thousanths on the OD and then finish the OD on a final pass stopping a thousanth or two from the shoulder to not disturbe the finished surface there. The difference on the corner radius will be all but impossible to measure or see.

For the radiused tool, you will need a tip angle that is less than 90 degrees. There are many inserts available and I don't know what angles are available, but I would just use a 60 degree insert and set one side close to the angle of the OD of the shaft to get a better finish there. Of course, if you are grinding a tool, you could make it 88 or 89 degrees and get a better finish on both.

JRouche
05-20-2009, 08:30 PM
I think my approach would be to determine which feature (diameter or flange) is most important and favor the more important one. So, assuming your polished diameter is more significant, I'd machine the shoulder .062 short and take the diameter down ready to polish. Then work the shoulder back to position leaving the .062 step to be cleared by the radius. Dig out the radius with a form tool to perhaps a thousandth or so below the required finish diameter, then finish the diamter to size. My rationale is that if there's a bearing or something that's a close fit on the .750 diameter, there's a tendency for the diameter to be large where it meets the blend radius. That is, the polishing to size will slight the radiused end so it needs particular attention to bring to size and the very slight undercut at the radius won't hurt anything.


Thanks.. The outside bushing will be made by myself so thats gonna be easy. Thanks for the help.. JR

JRouche
05-20-2009, 08:50 PM
I would attempt to make the shaft OD and shoulder in a single cut. First I would rough out both with a sharp tool leaving about half the radius of the corner on BOTH the OD and on the shoulder. That will leave enough material in the corner for the full radius.

Then I would change over to the radiused tip tool and take out the remaining material starting from the tailstock end and cutting toward the shoulder. When I am approaching the shoulder I would then feed outward with the cross slide to finish it. For a 1/16" radius you would have about 0.032 to remove on both surfaces and could take several cuts at 0.010 and a final cut to finish both. This leaves you opportunity to measure after each cut. Keep track of the dial numbers as you go. Leave enough to use your abrasive paper to finish to final size, probably a few tenths.

If you are having trouble with this technique due to resetting the cross slide to the same distance on subsequent passes, you could finish the shoulder on the second to last pass, leaving a few thousanths on the OD and then finish the OD on a final pass stopping a thousanth or two from the shoulder to not disturbe the finished surface there. The difference on the corner radius will be all but impossible to measure or see.

For the radiused tool, you will need a tip angle that is less than 90 degrees. There are many inserts available and I don't know what angles are available, but I would just use a 60 degree insert and set one side close to the angle of the OD of the shaft to get a better finish there. Of course, if you are grinding a tool, you could make it 88 or 89 degrees and get a better finish on both.


That is how I ended up doing it, mostly. Thanks for the info....

So... I am reducing 1.750" rod down to .750" and 1.5" for the flange (the 1" I wrote before was a mistake) and its pretty hard. Only supposed to be 28-32 hrc but its slowing me down some.

So what I did is set my carriage stop to where the carriage will stop at "close" to the length of the shaft I need, just a lil short for cleanup of the shoulder later. And I just ran my carriage back and forth, power feed for the cut, with a very light clutch on the release for the drive. It hit the stop, not fast at all and I released the drive and just cranked the carriage back. At 1.75" I was only able to take 30 thou cuts, some really pertty colored chips, and HARD. I am using a ccmt insert with it positioned so it wipes the shoulder at the end of the cut. When I was about .075" from my final shaft OD I swapped in the radius cutter, brazed bit. And cut the final .075" in one pass. MAN!! I was surprised, that brazed bit I ground up cut super nice. I think I coulda cut the whole bar with it and faster. But I wont, dont wanna break my radius. So I just ran it in, from the tail side, to the shoulder, hit the stop, disengaged the drive and cranked the tool out to form the shoulder. Worked out well... Thanks for all the advise. JR