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Radius/Ball Turning Tool - No Mill Required
Radius/Ball Turning Tool - No Mill Required
This post discusses a tool for turning spheres (balls) and other curved surfaces on a lathe. A key feature of this tool is that it can be made without the use of a mill.
A short while ago I purchased a used mini-lathe (I'll be doing a post on that in the near future). As I am sure happens with most every other would-be home machinist, one of the first things I began doing with my new (used) lathe was to look for ways to improve it, and to add new capabilities. Among other things, I wanted to be able to turn spheres and other curved shapes.
There are of course tools that can be purchased for this purpose, as well as a number of very good plans available online for making your own tool. My inclination is to build it myself before buying whenever practical, but all of the online plans I could find required the use of a mill to build the tool. Unfortunately, I don't have a mill at present (though I plan to get one eventually). Consequently, I set about to design a radius turning tool that could be built using only a lathe and other tools I already had available, and that specifically did not require a mill for its construction. The result is the tool you see below:
I'll get into the details a bit later, but here are the basics of its construction:
- There are two steel plates:
- The bottom plate sits on the cross slide of the lathe (with the compound rest removed); it is locked in place with two cam-locks (see on the right side)
- The top plate rotates and holds the cutting tool
- The cutting tool is held with a brass screw, which is used to adjust the radius of the turning
- A handle mounted at 45 degrees on the top plate is used to rotate the tool while cutting (and yes - that ball on the end was made with this tool)
Here is a view of the tool from the bottom:
To the left you can see two fixed posts made of brass (to prevent marring the cross slide); on the right you can see the two cam-locks. The cam-locks are essentially two brass posts drilled off center; when rotated they wedge against the cross slide and lock the tool in place. Also visible is the circular cut-out in the bottom plate which allows the top plate to rotate.
Here are two views of the tool mounted on the lathe:
Tool from the bottom.
Tool mounted on cross slide.
The next photo shows a first attempt at turning a sphere in aluminum. This failed for several reasons.
First, the mandrel was made using a too small diameter - the mandrel was not stiff enough and this led to vibration during the cutting. Second, there were design deficiencies in the tool, which need to be corrected, and third, the tool was not properly positioned so the radius was not in proper alignment.
After several modifications, as well as better understanding how to best position and use the tool, I obtained the result below:
First attempt - aluminum.
At the left you can see a completed ball, turned in brass. The turning was completed with very light cuts (a few thousands or less), and the finish is quite good. The ball could probably be used in this state for many purposes with no further work.
Note that a new mandrel was used - in this case made from a 1/2 inch steel rod.
I wanted to see how good a finish I could get with a bit more work.
The result is shown in the photo to the right. The ball was polished with emery cloth and steel wool. In just a few minutes I was able to achieve a mirror finish.
To further test the capabilities of the tool, I tested it on a brass rod.
Here, the end of the brass rod has been turned to a hemisphere.
I continued with the same brass rod to turn a "ball end."
To the right we see the finished products - a ball end on a brass rod, and finished brass ball.
The final picture below shows the results of the first use of the tool.
From left to right:
- The very first attempt (in aluminum). The tool was off center, and there was a great deal of vibration and chatter during the cutting due (in part) to a too small diameter mandrel.
- A second attempt - the cutting radius of the tool was not properly adjusted.
- A bit better - still off center.
- A bit better - cut too deep.
- Badly off center. Also, at this point I found I need to adjust the cutting tool height slightly with a shim.
- Success!
At this point, I feel that I have a good working radius turning tool, though I can see a number of things that might be done to improve it.
In a future post, I will publish detailed instructions for building the tool, as well as a link to measured drawings and other related information.
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