Machine a Ball Turning Tool / Small Radius Cutter for your Lathe (2-in-1 tool)

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Offline goodfellow

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Here's a small project that doesn't require any big machining skills, but is very useful. Most "ball turning" fixtures are quite small and only useful for turning very limited size ball ends and curves. I not only wanted a tool to make larger ball ends, but also use it as an inside/outside radius cutter for stock up to 3+ inch diameters.

This is what I came up with -- all parts are from my scrap bin. It can be scaled down (or up) depending in the need. I used an 18" piece of channel iron and an old lathe bit toolpost ($5.00 on e-bay several years ago) -- along with a cheap chinese machinist scale.

Ball turning tools allow you to make nice round knobs and other projects were a concave or convex profile is required

Above pics courtesy of Steve Bedair

However, as you can clearly see from the above pics, most of these tools are made for cutting only small radii -- usually less than 1". I wanted an adjustable radius cutter that could produce radii out to 12", and thus could turn not only ball ends, but large convex and concave radii on round objects with a diameter of up to 3, or more inches.

Here is my approach -- by using the cranking capability of my rotary table, I'd be able to swing that cutter precisely (and without deflection) into the workpiece.

I used a 4"x18" piece of heavy channel iron and milled the top square (not necessary in most cases, but mine was warped). Then I center mounted the piece on my 6" rotary table

Next I used an old lathe toolpost and accurately marked and indicated the position of the cutting bit to the centerline of the channel iron bed. This means that the post needed to be mounted in an offset position on the channel bed. To mount the post in an adjustable position, I milled a slot along the length of the bed to allow the tool post to slide to the desired radius of the cut -- anywhere from 0"-12".

Centering the cutting bit and marking the offset slot position

Milling the slot -- and mounting the post in the slot with a screw from underneath the bed.

Shimming the cutter to the centerline of the lathe -- I used the tailstock "center" as my gauge.

I painted the radius cutter bed a bright color to reflect light and added a cheap $2.00 machinist scale along side of the bed. This will allow me to accurately choose the radius of the cut by sliding the toolpost up/down in its slot.

This simple device can't cut large diameters (you need a super expensive radius cutter for that), but it's much more versatile than a simple ball cutter and will cut diameters up to 4" with ease; simply by shimming the bed and/or the toolpost up or down.

What can you do with it??? Well, I made the following e-wheel anvils by hand --using a template and XY coordinate cuts; tedious at best and it took all day.  :angryfire:

I have since made other sets, and by using the radius cutter, five anvils can be easily machined to any radius from 0-12" in less than two hours.