Re-building Disk Sander, direct drive or belt?

[matt tennessen]

I know it is worth my while to invest some time in learning about electric motors, but with that said...
I found an old 12" disk sander a while back for ridiculously cheap and the motor finally gave out. The repair shop I took it to said it was too old and too shot to bother fixing. So I've got this sander minus a motor and I'm going to just replace the motor but then I see this old issue of shop notes sitting around and now I've got questions. I was going to find a reasonably powered/priced motor and just attach the disk. In shop notes, they used a lower powered 1/2hp, a couple of step pulleys and a link belt.

My two questions are: is there any advantage to having variable speed for a 12" disk? And, if I can use a smaller/cheaper motor by just adding a pulley and a belt (and keep the motor housed away from dust), is there a reason why you wouldn't? I always figured that the belt drive was easier on a motor and the stand that came with sander has room for me to mount it low, so... (Is the real question: how can I be so stupid to not understand this?)

[glenn bradley]

I'd do step pulleys in a heartbeat. The main dissatisfaction I find with most sanders is that they won't go slow enough. Higher speeds are great but I often wish I could reduce based on grit and requirement.

[Dennis Ford]

I would go with the belt drive but think that 1/2 hp would be a little wimpy.

[Jesse Tutterrow]

The first thought that comes to mind is that usually direct drive tools are less powerful than belt driven (think of table saws). So go with the belt driven method. That said, I would use step pulleys.

I agree with Dennis, 1/2 hp may be too small. It depends on what you are doing and how fast you want to do it.

[Ryan Hellmer]

I would have to disagree that direct drive tools are less powerful. Direct drive tablesaws like the portable jobsite ones, yes, because they rely on overrated universal motors. For many years (and to this day) many industrial tools use direct drive induction motors with outstanding results (such as my crescent 16" 5HP jointer or Tannewitz GH bandsaw). Using an induction motor (which is undoubtedly what OP has) would be great for a disk sander, although 1/2 HP is a bit low for a 12" disc. Direct drive also solves the added expense and hassle of an additional driven shaft and bearings. If the motor is TEFC, I wouldn't hesitate to make it direct drive. Open would be a good candidate for belt, and if you're going belt, you may as well add the variable speed feature.

Ryan

[Scott T Smith]

My two questions are: is there any advantage to having variable speed for a 12" disk?

Matt, a variable speed is not necessary for a disc sander, because all you have to do in order to slow down the feet per minute of sanding is to move your workpiece closer to the center of the sanding disk.

For every revolution of the disk, the sandpaper near the outside is traveling 37.68", the sandpaper 1" from the center is moving 3".