I've been working on this idea for a while. The other files are screen shots from my CAD drawings. A professional product called "TRAVADIAL" is still available but mine is a little different.

To achieve direct diameter reading I use a gear that revolves once in 5 inches of travel. My design uses a 20 thread per inch acme threaded rod for the rack portion. (29 degrees acme/2=14.5 degrees for the pressure angle of the spur gear)

5"/PI=1.5915" pitch diameter is required.

A diametral pitch of 31.41593 (PI X 10) is used with 50 teeth to mate perfectly with the 20 TPI acme threaded rod. The pitch from tooth to tooth is .050"

The dial is graduated 0-10" in inches and tenths even though the compound only actually moves 5 inches. The doubled scale supplies the direct conversion from radius to diameter for you.

The threaded rod is used to adjust the readout to whatever the micrometer says the diameter is. Once set, it is accurate until you move or change tools for sharpening etc.

I have been using a similar device that uses a spool, small pulley, plumb-bob, and cord to move the pointer for several years now. It is surprisingly accurate. I routinely get down to about .005" over-size by eye just reading the dial.

This design is just an improved version that will display "thousandths" as well as "Inches and tenths".

Of course you still need to measure and adjust the readout whenever you disturb or change tools, but I find the readout a great aid in preventing the "under-size" turning jobs we all end up with from time to time.

I've also included a photo of the existing cord and spool setup I currently use. It's not fancy, but anyone can make one in an hour or two for free.

For now I'll just post the pictures. More to come.

Rich