Spring calipers are small gauges used to let you know when you have reached the correct diameter of a section of your spindle turning when using your parting tool. Having a few calipers is convenient when turning spindles so you don’t have to continually change the diameter for different sections of your spindle, especially if you are turning four table legs and want to keep the same sizes for each of the legs. You can certainly get by with just one caliper to start with, and buy another one or two when you notice an inconvenience. Somewhere between 6-inches and 8-inches is the most common size, which can handle most spindle turning needs.
If you look at different woodturning caliper sets, you’ll see they usually come with three types of spring calipers: outside calipers, inside calipers, and spring dividers. For spindle turning I only use outside calipers. So I’d recommend that you don’t buy one of those sets.
I have found a lot of good vintage calipers, usually under $10 a piece, and often for a couple dollars at tool swaps or flea markets. Sometimes they’ll even be thrown in when buying a used woodturning lathe. You can find a lot of vintage outside spring calipers on Ebay (see link below). Because I have so many vintage calipers, I haven’t had to buy any new calipers. But most inexpensive outside spring calipers should do the job just fine. Here are a few affordable options that you can compare:
Some cheaper woodturning calipers come with sharpened tips. To prevent catches, you should blunt and round the sharp tips with your grinder or with a metal file.
If you don’t want to buy calipers just yet, you can also make a “Go No-go” gauge out of scrap wood. This is helpful if you are turning tenons or other repeated diameters. However, I find it a bit time consuming to make a gauge for every diameter of a spindle. Another option is Peter Galbert’s “Galbert Caliper” (found here). I haven’t used it, but it does look like it would improve speed and safety. The one drawback I see is that you have to rely on your eyes to reach the right diameter of the scale, as opposed to using spring calipers which just drop over the spindle when the correct diameter is reached. Also, spring calipers can take measurements directly from another spindle, whereas with the Galbert Caliper you need to know the measurement.