I have a need to sand some concave surfaces. A bobbin sander is too small a radius, I want a big, slow radius, so I've built a rig to turn my belt sander into a concave Linisher and it works rather well!

Supplies

Belt sander (mine is a Bosch PBS 75A)

Abrasive roll (mine is Abranet P120)

Some MDF and plywood from the scrap box and a few nuts and bolts.

Step 1: Make the Curved Platen

Let's start with the curved platen. It's made up of three layers, with the centre layer having an M8 hole and a 9mm gap in it. Set it aside for the glue to cure.

Step 2: Find a Way of Mounting the Sander

Your sander may well be different to mine, but we have to find a way of holding it securely. Mine has a couple of M6 holes in the side, so I used them. I cut a piece of plywood to fit so that it cleared the dust port and the air vent. This bracket also has a cylinder nut for mounting.

Step 3: Make the Baseboard

The baseboard is nice and big, about 500x400mm and is held down on the bench with a couple of holdfasts. A notch is chopped into the back edge to take and upright mounting plate. This has a slot routed into it, using the "Dropping-On" technique. I have a YouTube video of that technique if you need it.

The mounting plate is glued in, nice and square.

Step 4: Mount the Sander

A Bristol lever through the back slot secures the mounting bracket, and thus the sander, to the mounting plate. A small spacer (the green MDF in the photo) lifts the sander off the baseboard to prevent it scouring a groove. I found that out the hard way...

Step 5: Shape and Mount the Platen

When the glue is fully cured, bandsaw the platen into a curve. Mine is flatter towards one end and rounder towards the other, giving me a range of radii, rather like French Curves. Two M8 nuts are embedded into the underside of the baseboard and the platen is mounted onto a pair of matching studs..

Step 6: Make the Belt

If you can find a ready-made belt of the required size, then great, but I could not, so I made my own. The abrasive has to go around both rollers of the machine and the platen, with an overlap of, say, 20mm for the glue. The ends were cut at 30 degrees or so. I discovered that the abrasive is in two layers, the mesh itself and the velcro-type backing. The latter can be removed with a combination of thinners and scraping. This makes the joint less bulky.

I used contact adhesive, for its flexibility, and clamped the joint up overnight to let it cure fully.

Step 7: Let's Go!

The belt must be installed so that the joint trails, rather than leads on the platen, and is tensioned using the adjustment afforded by the slot in the platen. I found that I had to have it quite tight to avoid it stalling. There is not as much traction provided by the drive roller as there would be normally, becuase the belt is in contact for only about 1/3 of the circumference rather than 1/2. So I have to take it easy.

By removing the platen entirely, I can use the setup for flat and convex surfaces too.

Please watch the video and if you have any questions I'll do my best to answer them.

Enjoy!