View Full Version : Turning A Radius

05-17-2005, 06:55 AM

I finaly got my lathe up and running. I've never used a metal lathe before last week. I Have succesfully turned a front and rear axle and extended shock mounts for a sportster that I am putting a 250mm tire on. Now I would like to turn some dies form my plannishing hammer and make a set of edge turning dies for gas tanks but I have no idea how to make a radius needed for these jobs. I was wondering if anyone could give me a lesson in turning a radius. Thanks for all the helpfull info

Battle Creek, Mi

05-17-2005, 08:13 PM
Kerry Pinkerton wrote a great radius turning tutorial for ewheel anvils. It translate nicely though for any type of radius turning.


05-17-2005, 08:50 PM
Chad, there is a big difference between cutting anvils the way Kerry did and say cutting a planishing hammer die. Kerry had the anvils on a mandrel, and you will be chucking the dies in the chuck.

Making a radius cut when the part is in the chuck means you will be cutting from the outside to the center. You can step it as Kerry did, or most using hand tools will use a grinder to get it rough shaped then use sandpaper.

There is a radius cutter for manual lathes, Holdridge radius/radii cutter is one of them....they are expensive if you don't use them a lot. I built one for my lathe and it works OK, but it is usually faster to do it with a grinder or sander.....just remember to clean the machine well after you grind on it..........john

05-18-2005, 12:14 PM
Chad, Check this link out. Mech works shows a radius cutter that he made for his lathe. These can also be purchased from different suppliers.

Brian McCollim
05-18-2005, 07:20 PM
There is an easy way to do a radius on the face of a part, but you are somewhat limited as to size of radius.
If you put a punch mark on the headstock somewhere below the chuck and a corresponding punchmark on the cross slide (compound). Then use a pin or rod with the ends ground down to a point. Put the pin in between the center punch marks and keep slight pressure (with apron) against the pin while feeding across you will turn a radius on the face. With some experimenting you can get quite a nice radius. You will have to hold the pin in place (with the chuck STOPPED) to get a starting point and just feed across to get an idea of what to do.

The idea is to get the pin to act as a stop and as you feed the cross slide in the pin moves closer to center (becomes straighter) and moves the carriage away from the chuck. There are formulae out there somewhere to get exact but I don't remember where to look right now.
Brian McCollim

05-25-2005, 11:36 AM
Thanks for posting that discription....makes perfect sense...one of those "why didn't I think of that?" ideas.

A couple of points:

The length of the rod will be the radius of the cut, IF you use a tool with a sharp corner. If you have a radiused tool, the part radius will be the rod length minus the tool radius.

If you have a carraige stop, you could put the punch mark on that instead of the headstock.

The rod needs to be parallell to the headstock axis (bed of lathe) exactly when the cutter is crossing center. If before center, you will cut a torroid (donut) and if after center you will cut an ogive (HP rifle bullet, or rocket nose cone). Fine if that is what you want.

08-09-2005, 09:28 PM
An easy way to turn a radius is to use your compound rest as the radius tool. Position your cutting tool in front of the center of rotation for concave radii and behind the center of rotation for convex radii. You need to loosen the locking bolt or bolts just enough so you can swing the compound slide to generate your radius. It helps to attach some sort of bar that you can hold to help control the rotation, then just swing the tool around as you feed it into the rotating part.


Russell Arney
08-14-2005, 06:32 PM
Along with the radius for dies, how do you make a convex for bending dies, does anyone have a clue?

08-14-2005, 08:58 PM
Russel, I would turn the die out on the lathe, get the od, the id right as well as the thickness, then turn the convex radius in the part from the side of the part as described in the posts above. You should have a 360 degree die made........john

08-15-2005, 12:17 PM
There is one more method that might work- See http://lautard.com/ballbook.htm
I'm going to try this method someday.