Collated from newsgroup postings.
I am having trouble sanding pieces while they are turning on the lathe both with hand held sandpaper and power sanding.
I sometimes end up with hills and valleys in the piece, (the piece becomes out of round). It doesn't always happen but when it does I can't figure out why. Someone suggested it could be the speed the lathe is turning at, too high or too slow. What speed should it be turning at for hand sanding and/or power sanding??? Most pieces I'm working on are 6 to 10 inches in diameter. For the power sanding I'm using 5" hook and loop disk in a right angle electric and/or air drill.
Does the speed of the drill matter also? But that doesn't explain it when I'm hand sanding.
This is a problem Richard Raffan talks about.
Basically, what is happening is the softer part of the grain is sanding out more than the harder parts, so "valleys" or depressions appear where the softer grain is.
Raffan's solution is to cut to an extremely fine finish, using difficult to master shear cuts. In this way, he does not have any sanding that requires coarse grits. He only has to touch up the object with finer grits.
If your piece is becoming out of round when sanding, perhaps it's because the wood is changing shape because it's not yet fully dry.
Sanding (power or hand) causes heat, which speeds up the drying process and can cause the piece to go a bit oval--especially sidegrain pieces.
A professional woodworker friend of mine once told me something I'll never forget about sanding. He said, never sand at too fast a lathe speed, because you'll invariably skip over low spots, and at too fast a speed, you're no longer sanding, you're polishing.
I usually power sand running the lathe at about 500 rpm or less, and the drill at a higher speed. If the power sander is faster than the lathe you should be cutting at 45 degrees to the grain, which is very efficient. I usually start at 100 grit, then move to 150, 180, 220, 320, and 400. Don't spend a lot of time on any one grit. If scratches or tool marks won't come out, back up to a coarse enough grit to take care of it, and then go through the grits again.
Oversanding with too fine a grit just builds up heat which not only can warp your piece, but put heat checks in it, as well.
Wood has varying densities in a single piece. Within each growth ring the spring layer is harder than the summer layer. The wider the growth rings the greater the possibility of more pronounced "hills and valleys" with poor sanding technique.
Also end grain is usually harder than face grain and in a bowl or vessel that is mounted cross grain on the lathe and you will be sanding harder end grain and softer face grain alternately. This can cause "hills and valleys" if you are not careful. Some woods are really bad if you have to do much if any sanding - any of the conifers, and all spalted woods.
The best solution is to perfect the technique of shear cutting the surface so you will have very little sanding to do. Sanding is no fun anyway so anything that reduces sanding time is worth the effort. That being said there are some things that you can do to while sanding to prevent "hills and valleys".
- Use wax or water to soften the wood fibres while sanding. Added benefits are less dust, less heat checks in the wood, and cooler sandpaper.
- Use a slow speed on the lathe and a slow speed on the drill. For the size pieces you asked about, I'd recommend a lathe speed of around 200-300 rpms or maybe less. Keep the sandpaper moving.
- Be sure that the sandpaper has a thick soft backing. Most Velcro disks in the 5" size are too stiff.
- Use a smaller and softer disk that will conform to the shape of the vessel easier. I use a 2" disk that has a 1/2" thick soft foam backing. I make my own disks and have written an article on how I do this. It is on my website at http://www.kestrelcreek.com/Sanding_Tip.htm.
- Use sharp sandpaper. This was a hard lesson for me to learn for some reason. Buy only top quality sand paper and don't expect it to last forever. Sand paper comes in various sharpnesses. Manufactures don't tell you this, you have to experience it for yourself. Use a light touch - let the sandpaper do its work.
- Don't be afraid to hand sand with the piece stopped on the lathe or off of the lathe.
- If you get "hills and valley's" don't be afraid to re-turn the piece if it hasn't warped too much.
- Practice your shear cuts.
The softer early wood sands faster than late wood, the face sands faster than the end grain. If this is your problem, I'd suggest a bit more attention to gouge work for starters. I like to make my final trip with a freshly sharpened gouge, bevel riding, and over a curved toolrest so I can get minimum distance from the work. The gouge is a spindle type, the angle is within 20 degrees of vertical, and the shaving is as thin as I can catch.
Practice is the only way to acquire proficiency with this technique, and, I would suspect, with shear scraping as well. Practice making a single pass, regardless of the fact that the final half is often obscured. With a bevel riding, you can do it by touch.
Until the technique is perfected, might I suggest using a flexible shaft for sanding, and using the hard discs in grits coarser than 150. Use the toolrest to support the handle and take light cuts. Since the disc is not "pressing" into the piece, it tends not to follow variations in the grain. Also, once you begin using flexible backing for your discs, use the largest disc possible, so it will bridge and average better than a small one which might drift into valleys.
Don't use huge discs with coarse grits though, they'll hurt your wrist
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