Poor mans CNC lathe cutting radius on manual lathe

[thehomeengineer]

Hi All
This is a how I cut large radiuses on my manual lathe. I have used it for both convex and concave radiuses. For making such things as press tools to smoke box doors for steam engines. I made a jig a long time ago to fit my lathe but gave it to an apprentice school as they wanted to cut large radiuses and never got it back but this simply fix works and one day I will make another jig.

Thank you for watching
The Home Engineer

[olderdan]

Super simple and effective, a couple of questions,
What is the minimum radius that is practical for this method?.
I am having trouble visualizing how this works for concave rads?.
Great idea though and I am sure I will use it.

[mklotz]

I am having trouble visualizing how this works for concave rads?.

For concave radii the rod is pivoted against a fixed point on the tailstock side of the carriage. A locked-down tailstock might serve as the fixed point.

[thehomeengineer]

Hi Olderdan

The smallest I have cut is 1” rad which was for the press tool for the brass end cap for the Foden fly wheel. There is nothing stopping you going smaller. The jig I made had a pin system one attached to the bed and the other to the cross slide, so you drilled the holes to the radius to be cut in a flat bit of stock and slide it over the two pins which then produces the required profile.

To cut a concave radius you simply move the tooling to the opposite side (tailstock end) and use the tail stock locked to the bed as a positive stop.
Hope this answers your questions if not please ask again for better clarification

The Home Engineer

[olderdan]

Thanks The Home Engineer, pretty obvious once it is pointed out.

[Toolmaker51]

BTWWW [Before The World Wide Web] BCNC [Before Computerized Numerical Control] & BPWC [Before Phones With Cameras]....
I watched something just like this being done on a universal shaper.
They built a convex forming die set with about 12" radius, maybe 80% of a full arc. Probably a door or cover of some type, likely faster than rolling short pieces.
The bar; shopmade turnbuckle with Heim joints at each end, connecting bracket secured to floor and X axis of table. The turnbuckle was set by a large caliper, generating male [punch] side first. The curvature was adjusted for die next, accommodating material thickness. That was checked by narrow strips of that gauge, about .140 IIRC.
IWSY [I Was So Young]
As moldmakers, we did this at least once lathe-wise, a restaurant sized salad bowl mold. There were 2 radii; a flatter bottom with more vertical sides, kind of hyperbolic. IRTC [I Recall That Clearly]
Not sure why another lathe with hydraulic tracer wasn't used.

[Frank S]

Quite by accident I made a convex cut on a part I was making way back when I was a teen and trying to teach myself how to run a lathe I had just bought. There was some object wedged between the head stock and the edge of the cross slide that I didn't notice as the cutter moved across the part the cut became less and less I had started out making a pretty deep cut in aluminum but before I had gone half way to the center my tool wasn't even touching. That was when I noticed the object. Haven't thought about purposefully duplicating the result until your thread Thanks

[DIYer]

Thanks thehomeengineer! We've added your Large Radius Cutting Method to our Machining category,
as well as to your builder page: thehomeengineer's Homemade Tools. Your receipt:

Large Radius Cutting Method by thehomeengineer

tags: cutting, radius

[dobajara]

Where are the images?

[Isambard]

As the saddle has to travel backwards & forwards in the cut, I found best to rig up a cable & pulley with a weight to the saddle, to keep pressure on the cut...