Belt sander questions

[hornluv]

I'm building a low speed belt sander for sanding on the inside radius of bends that I make in tubing. The radius of the bends will range from a very gentle 6" radius down to a .250" radius. How much of a crown should I put on the wheels? Will a wheel with more of a crown result in more flexibility of the belt? If so, is the belt also more likely to come off in use?

Thanks,
Stuart

[Your Old Dog]

I'm told the tighter the radios on a belt the harder it is on a belt. If the belts are pricey this may be an issue for you. As for the crown on the wheel, it only has to be there to do it's job as an adjustment. If you over do it on the crown it will become much harder to center the belt on the wheel as any slight pressure/deformation of the belt on the work wheel will likely cause wild movement on the adjusting idler.

[darryl]

.250 inch radius- are you saying that you want the belt to go around a 1/2 inch diameter roller? More info needed.

If I was making a crowned roller for a belt sander of some type, I probably wouldn't put more than about a 10 thou crown over a 4 inch width roller. Considering the small deviation it takes to run the belt off either side of the rollers, that might even be a bit much. You can always leave yourself the option of re-turning the roller for more crown, or less.

I'm confused over one thing- how do you plan to accommodate the various sizes of bend radius with fixed diameter rollers? Maybe I'm not seeing this right.

[oldtiffie]

Why not try a "flap wheel"?

Deleted/edited-out

[Swarf&Sparks]

I must be missing something here, but I can't see the need to sand a bend unless there's something horribly wrong with your bender.

I've bent, welded and polished a lot of 316 tube.
Usually use a narrow, stitched sisal mop on the angle grinder. Start with grey compound, finish with green, maybe go to jeweller's rouge if a real mirror finish is required.

[hornluv]

Darryl: The radius on the bent part would be as small as .250". The wheels are going to be 2" in diameter. Also, there will be about 30 inches between rollers so the belt will flex enough that it doesn't just cut into the tube. There's a picture of a VERY large version of what I want to make here (the first photo on the page):

http://www.ferreestools.com/

Tiffie: I hadn't thought of a flap disk. I'm not sure it will give me the access or control that I want, but I'll give it a shot.

Swarf: These parts are various bent tubes for the custom French Horns that I make. They are made from thin walled (usually .015 wall, but up to .020 wall) Brass tubing and need to be completely round on the inside. The tubing wrinkles a lot while bending the more severe bends, so those wrinkles are tapped out while the tube is still filled with Cerrobend and the part is balled out afterwards to get it round again. The sanding is to remove the hammer marks and such. I also need to sand the tapered parts, which are made by drawing cylindrical tube through a steel washer onto a tapered steel mandrel. This leaves a lot of drawing marks (longitudinal scratches) on the tube that don't just buff out. I want a low speed sander to avoid the rapid material removal and grooving that happens with my current 3450 rpm sander. Removing that much stock is fine for some things, but not this. I hope this answers any questions.

Thanks,
Stuart

[Swarf&Sparks]

OK, got the picture

[micrometer50]

Have you tried filling the tubes with sand or a manderal?

[DR]

Have you tried filling the tubes with sand or a manderal?

He said he uses Cerrobend.....

[hornluv]

Micrometer50: Bending isn't my problem. I either use pitch for the gentle bends that I do by hand (rarely results in a wrinkle) or Cerrobend for the tight ones I do with my bending jig. I just want to speed up the sanding portion of the work, which is currently done by hand and takes me a solid hour for a 22" long part and results in a lot of blisters and more than a few sandpaper cuts. Those are the worst.

Stuart

[Your Old Dog]

Given what you're working on I'd stay with the 2 X humongously long belt sander in slack configuration rather than the flop wheel. I've owned a 2x72 at one time and it's a fantastic tool but won't do well what you want. The one pictured looks almost twice as long as 72". It will offer you much more gentle shaping.

Were I you, I'd be looking for 2x72 belt grinders and just make it larger. The one pictures looks to me like it might have some vibrations in it that may give you a harmonic/rhythmic blemish in the polishing. Maybe it's more sturdy then it looks.

[DR]

horn',

I have a 1"x91" slack belt grinder from G&P. A quick measurement of the rollers shows no crowning (to my surprise). The drive wheel is 3" diameter, rubber covered, also no crown, the others are 7/8" diameter plain steel.

I've done work similar to yours, polishing the inside radius of bent parts.

For your application I would order specially made belts in various widths down to 1/4" for the smaller bends. Custom belts are generally less than stocked sizes, but the usual minimum is around 20 belts.

When I need belts I call the makers and explain what I'm doing. They suggest an abrasive and the proper weight backing. I would think you want the lightest backing.

Google on "custom sanding belts" and variations of that to find sources.

[micrometer50]

Sorry I didn't read more carefully. How about scotchbright wheels?

[gellfex]

Adding to the "from the hip" ideas, what about a 3/8 x 13 air belt sander? you can just dial down the pressure for slow speed.

Or how about Leah abrasive on a leather belt on a 1x42, or a die grinder with a 2" sewn wheel. It's basically abrasive up to 400 grit in a glue base that you use like rouge or tripoli, but it can cut much faster, how fast depends on the grit.

Finally, since you got me thinking about this and I've been there sanding tubing. What you really want is for the strip of abrasive to wrap around the tube so you don't get a flat spot. Like you were pulling on both ends of a strip.

So....how about this: Make a 3" dia x 1" wide flat belt sheave with high walls, and mount it on a slow dc gearmotor with a footpedal "sewing machine" type control. then put a belt on with the grit inside, not out, and use the tube as the idler. you control the speed, and the pressure with how hard you pull against the drive wheel. It sure would be smooth. The sheave might not last all that long though.

[darryl]

Looking at that picture, it's hard to imagine the belt staying on the rollers at all. After all, you'd be putting pressure on the belt from different angles, not just straight on and centered. Something must be keeping it tracking, possibly a crown on at least one of the wheels, and maybe all of them. Still it shouldn't be much, though I'm just guessing since I haven't played with belt tracking much.

One machine we have at work has a 1 inch wide flat belt running on all straight rollers, with one exception- an idler with a very definite high center section. Actually, it's tapered towards center from both sides, and it's a straight taper. This roller is about 2 inches wide and long, and the center must be at least 1/4 inch larger in diameter than the outer edges. The belt often runs towards one side, but sometimes it's tracking dead center.

It almost seems to me that the wider the belt, the less crown is needed, though again this is just a guess.

Stuart, I might make a suggestion if you're going to go ahead and build one of those- make the rollers straight and leave enough material so you can turn a crown on one or more of them if you have to. Temporarily wrap a band of masking tape around the center of one roller to simulate a crown and see what happens. Go from there.

I did have an idea to make up a 'fork' of sorts which would have a ball bearing on each leg- the belt fitting between them with some minimal clearance. The fork would be connected to one roller and it would be able to tilt it. I think (not sure) that the direction it would tilt it would result in a self-centering action. Good luck with the project.