This idea is not new to me. I saw one in a live demo last week and simply copied and adapted the principle. My thanks to those who are more original than I.

Some items, when turning on a wood lathe, are better (more easily) made using a chuck to hold the piece. I happen to own a Nova G3 chuck but there are many others out there in the wild and each has a rabid following. These chucks have four jaws that scroll in and out together - as opposed to independent-jaw chucks which are generally poor for wood turning and require that each jaw be adjusted independently.

Unfortunately, these small chucks have a limited radial movement and so are useful only within certain ranges unless you have the wherewithal to purchase multiple jaws of different sizes.

Holding a piece in a chuck requires that you either cut a TENON (or SPIGOT - a stick-out portion) or cut a MORTISE (a cut-in portion). The chuck closes on a tenon and opens to engage a mortise. Therefore you need to know the maximum and minimum sizes that your chuck will grab onto.

Step 1: Materials and Tools

Luckily, TeknaTool (maker of Nova chucks) publishes a table of maxima and minima for all its producrts. I simply copied out these figures and made a rough sketch.

The only material I used was a scrap piece of 1/4" ,plywood, about 3" by 6".

The tools I used were:

Pencil
Sharpie
Ruler
Digital Caliper
Bandsaw
Sandpaper

Of all the above, the bandsaw is the most dangerous so respect it, wear proper safety equipment and be careful!

Step 2: Making the Gage

I have two sets of jaws - 50 mm and Small Spigot, so the gage is going to be two-sided. The Small Spigot jaws are shown here and the 50 mm jaws are in the first step.

First, I drew a line across the approximate center of the plywood.

Next I used the digital caliper points to mark the dimensions from TeknaTool's chart.

Then I cut out for the tenon maximum and minimum dimensions.

Finally, using my homemade centering ruler, I marked the maximum and minimum mortise dimensions.

Just hit the edges with a piece of sandpaper and we're done!

Step 3: Using the Gage

I'd like to show it in actual use, but I'm currently replacing my lathe's motor so here is a picture of a bowl I turned recently showing the tenon. As you can see, it falls about the middle between maximum and minimum.

I'm entering this in the Trash to Treasure and Pocket Size contests. Please vote for me.

Any and all comments are welcome. Thanks for looking.