If you've ever tried to free hand a circle on the bandsaw - particularly on thicker stock to prepare a bowl blank for the lathe, you'd know it can be a trying experience - its easy to snap a blade, it can be difficult to get a nice smooth cut, and if you're lucky it'll even turn out looking roundish.

This handy jig makes it a no brainer. Set your radius, drill a shallow hole in your blank, and twirl away.

Materials

18mm ply scrap (I’m sure thinner would have been fine, but I had that on hand),
an offcut of hardwood,
aluminium t-track,
a bolt,
a washer,
a wingnut
a shelf pin.

Too lazy do to all this reading? Watch the build on youtube

Step 1: Size Your Plywood

The base for your jig should be a little bit longer than the length of your bandsaws table, and at least 10cm wider than the distance of the blade to the edge of the table. This will mean you'll have a nice zero clearance plate for the blade and overhang to attach the bolt to lock the track down.

Using your bandsaws fence, cut into your plywood about halfway lengthwise, making sure there is some overhang on the front of the table.

Step 2: Cut and Attach Runner

Cut your runner. Most bandsaws will use a standard 19mm (3/4") track. I used hardwood (blackwood), but plastic or metal may work too. About 10mm (3/8") thick is good, you want some gap underneath the runner for sawdust so it doesn't effect your sled.

Don't try and get the runner absolutely perfect on the first pass, slowly work your way towards it by using a tablesaw, plane, or thicknesser. That way you can sneak up on the perfect fit rather than overshooting.

Attach your runner using screws, glue, or both. Raise it up with washers or coins so its not touching the bottom of the track.

Step 3: Stop

While the sled is on the bandsaw with the blade in the furthest position, mark underneath the sled where it will hit the table.

Take the sled off the table, and attach a block of wood (plywood, hardwood, whatever) using glue, screws, or both. This will act as a 'depth stop' for the sled so you know when to start turning the blank.

Step 4: Getting Groovey

While the sled is in the mitre track and touching the blade, extend some lines perpendicular to the mitre track that line up with the front of the blade. This needs to be the dead center of where your pivot point will be.

As I was using 19mm wide t-track, I drew out lines 9.5mm either side of the first line to indicate where the t-track will go.

Using a crosscut sled or router table, cut a dado between the lines deep enough for your t-track (9.5mm for my track).

Step 5: Lock It Down

Drill a through hole in the middle of the groove, about 50mm in from the edge that hangs off the table. This should be as tight a fit to the bolt as possible.

Step 6: Pivot Pin

For something durable, I went with a metal shelf pin by just pulling it out of the plastic housing. Any smooth metal rod/dowel would work just fine.

Drill a (slightly) undersized hole in the t-track, about 25mm in from one edge.
Pound the shelf pin into the hole you just drilled in the t-track. This will create a very snug fit that won't come loose on you.

Step 7: Wrap It Up

Fit the bolt through the hole with the head sitting in the dado, slide the track - upside down with the pin facing upwards - onto the bolt. You're done!

Find the center of your bowl blank, drill a 5mm hole (if you used a 5mm shelf pin) slightly deeper than the amount of the pin sticking up out of the track.

Pull the sled back, put your blank on, turn the bandsaw on and push forward until it hits the stop on the underside of the sled. Then start turning. You should have a near perfect circle in no time.