Disc flattening technique

SS369

SS369

Well-Known Member
Hello all.
I have a 9 inch disc sander, variable speed and I am looking for some reasons and answers to flattening scale material, etc. on it.

I seems that I always have a non-flat outcome using it, in that the front and rear of the material always has more taken off it. Not flat.

I've tried many different positions and methods of hold it, flipping if this way and that and different parts of the disc surface, light pressure and heavy, to have the same outcome. I know some hand sanding, figure 8 style will eventually get it right, but, I have read and read that it is the correct tool to get things flat.

Anyone have some suggestions?
Thanks, Scott
 
It sounds like you are applying too much pressure. A disc seems to cut slowly, but if you press hard it will absolutely eat scales. You can make a trapezoid really quickly if you aren’t careful.

You’ll never get anything perfectly flat on a 1° disc, but we’re talking thousandths here. It’s flat enough. Epoxy will fill the void.
 
Hi John, thanks for the response.

My disc is flat, no bevel.
A trapezoid would be better than the front and rear of the scale material ending up smaller than the middle.
I have tried light pressure and different hand positions, even different orientation/locations on the disc to no avail.
The last scales were more than a couple of thousandths out of flat. They were flatter coming off the saw.
I know I must be doing something incorrectly, I just have not figured out what.

Randy, thank you for the suggestion. I'll look into it.
How do you keep the sandpaper and glue from pulling the cork off?
 
SS369 said:
Hi John, thanks for the response.

My disc is flat, no bevel.
A trapezoid would be better than the front and rear of the scale material ending up smaller than the middle.
I have tried light pressure and different hand positions, even different orientation/locations on the disc to no avail.
The last scales were more than a couple of thousandths out of flat. They were flatter coming off the saw.
I know I must be doing something incorrectly, I just have not figured out what.

Randy, thank you for the suggestion. I'll look into it.
How do you keep the sandpaper and glue from pulling the cork off?
Click to expand...
Glue your cork material on with 777 spray glue. Use feathering adhesive for your sandpaper. It might pull up a little changing papers but it pushes back down.
 
Check out Nick Wheeler on YouTube. I haven't finished mine yet but am working on a 9" grinder with a 1deg bevel for flattening blades.


This guy makes a beautiful Fighting Bowie. And is unbelievable stickler for details! He has a couple of videos on this subject.
 
SS369 said:
Hi John, thanks for the response.

My disc is flat, no bevel.
A trapezoid would be better than the front and rear of the scale material ending up smaller than the middle.
I have tried light pressure and different hand positions, even different orientation/locations on the disc to no avail.
The last scales were more than a couple of thousandths out of flat. They were flatter coming off the saw.
I know I must be doing something incorrectly, I just have not figured out what.

Randy, thank you for the suggestion. I'll look into it.
How do you keep the sandpaper and glue from pulling the cork off?
Click to expand...

Maybe there is something i’m not thinking of, but I really think this is a practice issue.
 
I'll probably pick up one of the rubber backers, but I can't understand how it will help.
Seems like the soft backing will possibly let the sand paper "bunch up" or warp slightly just ahead of the leading edge of the scale.
 
SS369 said:
I'll probably pick up one of the rubber backers, but I can't understand how it will help.
Seems like the soft backing will possibly let the sand paper "bunch up" or warp slightly just ahead of the leading edge of the scale.
Click to expand...
It works. Gives it a little cushion and keeps the chattering down. It's also very thin and more rigid than you think it would be. And being solidly mounted to the disc it doesn't bunch up like a belt on a 2X72. I wouldn't run mine without it.

This is what I use on mine. Gasket material from the local auto parts store. Boss' rubber gasket will work great too.

Gasket Material.JPG
 
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If you use cork, you will have have soft edges. It's a bit soft for this application. It will work and sand but the front and back edges will be slightly rolled over similar to what a flat platen does.
I use a thin piece of hard rubber glued to my disk with 3m 77 adhesive. It's more or less permanent.
On that I use 3M feathering adhesive that allows me to use 8x11 sand paper and switch them out.
The thin layer of rubber is actually an old printers mat used in off set printers as a backer to print rollers.
After using that, I sourced and sell some thin rubber sheets that will do the same thing.
 
What kind of disc do you have? It is possible that the disc isn't flat? Are you using fresh paper? A used abrasive sheet might cut unevenly as you describe. Start with fresh, good quality 220 grit sheet. Also verify with a good straight edge that your disc is flat. As for the backer, that shouldn't really affect flatness if you have a flat and balanced disc, it will just help you get a better finish, and slightly extend your paper life.

I get rubber sheet from McMaster-Carr. They have just about any kind of rubber, durometer, and thickness you could need.
 
Sounds like I should go with rubber.
My disc is anodized aluminum, flat and runs true and well balanced (from a knife making supply company).
I've been using PSA discs that are fairly thick paper, 80 and 120 grits. Since it is just being used for flattening scales I don't have a need for finer grits, I'll be following up , by hand, with 60 grit on the flat surface.
I find that the finer grits load up and then heat up too quickly ultimately burning the surface, even using an eraser frequently.
I'm thinking my challenge is more about technique rather than equipment.
 
BossDog said:
If you use cork, you will have have soft edges. It's a bit soft for this application. It will work and sand but the front and back edges will be slightly rolled over similar to what a flat platen does.
I use a thin piece of hard rubber glued to my disk with 3m 77 adhesive. It's more or less permanent.
On that I use 3M feathering adhesive that allows me to use 8x11 sand paper and switch them out.
The thin layer of rubber is actually an old printers mat used in off set printers as a backer to print rollers.
After using that, I sourced and sell some thin rubber sheets that will do the same thing.
Click to expand...
I been using the cork but I'm going to try rubber backing. Just put in an order at your website. Thanks Boss.
 
PSA discs are ungood.
They are expensive and an absolute bugger to change out.
Feathering adhesive is a medium tack (like post it notes) that allows you to easily switch abrasives. You can reuse a sheet if you don't use it up all the way but need to switch grits. Most PSA's won't let you do that.
Mostly this setup allows you to use much less expensive 9x11 sandpaper. I use Rhynowet abrasive but another brand will work. If I knew of a better quality than Rhynowet I would use it but I haven't found it yet.

SS369 said:
Sounds like I should go with rubber.
My disc is anodized aluminum, flat and runs true and well balanced (from a knife making supply company).
I've been using PSA discs that are fairly thick paper, 80 and 120 grits. Since it is just being used for flattening scales I don't have a need for finer grits, I'll be following up , by hand, with 60 grit on the flat surface.
I find that the finer grits load up and then heat up too quickly ultimately burning the surface, even using an eraser frequently.
I'm thinking my challenge is more about technique rather than equipment.
Click to expand...
 
I agree, PSA disc paper is a real chore to change out. Plus the glue lets go in a cold shop when sitting for a bit. I keep small spring clamps around the outer edge during its down time.
I've feathering glue and Rhinowet, but nothing course enough at present and what I do have, 240, 400, etc. is too fine for flattening scales.
When I use up the few more PSA discs I have, no more.
 
BossDog said:
If I knew of a better quality than Rhynowet I would use it but I haven't found it yet.
Click to expand...

Rhynowet is really good stuff. I also like "Starcke Matador" Silicon Carbide paper quite a bit. It's a German brand, but seems to be of reasonably high quality, and pretty decently priced if you buy 50 sheets at a time.
 
BossDog said:
If you use cork, you will have have soft edges. It's a bit soft for this application. It will work and sand but the front and back edges will be slightly rolled over similar to what a flat platen does.
I use a thin piece of hard rubber glued to my disk with 3m 77 adhesive. It's more or less permanent.
On that I use 3M feathering adhesive that allows me to use 8x11 sand paper and switch them out.
The thin layer of rubber is actually an old printers mat used in off set printers as a backer to print rollers.
After using that, I sourced and sell some thin rubber sheets that will do the same thing.
Click to expand...
Got my sheet of rubber from you today Boss. Put it on and tried it out. I do like it much better than the cork. Thanks!!
 
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