Why are most disc grinders 9 inches in diameter?

B

Bob McRee

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Oct 7, 2012
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I am making two disc grinders: a vertical double disc and horizontal disc grinder. Both have DC motors with speed/reversing controls and I can choose up to 18" discs. Sandpaper in most grades and grits can be purchased here in wide rolls. I am wondering why most knife specific disc grinders available use 9" discs. Is the reason because sandpaper size and cost are factors or are there other considerations?
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IF you had a choice of 9", 12", 14", 16" or 18" discs and IF the paper size and cost was not a big factor, what size disc would you choose?
 
Yep, numbers is spot on. Have you checked the prices on pre made sanding disc paper?? Crazy. With a grinding wheel just barely under 9" in diameter you can utilize all kinds of different abrasive sheets for a fraction of the cost.
 
I understand the economics of the 9" disc. However, are there any "technical" reasons for that size? It seems to me that a larger disc would have some advantages, i.e., a 2X72 belt has advantages over a 2X48 IFthe cost of sandpaper were not an issue what diameter disc would you like to have?
 
Doest answer your question but I am getting a 12" one for Xmas. Think it will be big enough.
 
20" disc are used in woodworking. Talk about crazy prices .
My guess with 9" being standard is most sandpaper is 9" wide
 
Bob McRee said:
I understand the economics of the 9" disc. However, are there any "technical" reasons for that size? It seems to me that a larger disc would have some advantages, i.e., a 2X72 belt has advantages over a 2X48 IFthe cost of sandpaper were not an issue what diameter disc would you like to have?
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I don't know, but, from what I understand, it is convenient just to get sheets and a 9 inch disc will do pretty much anything necessary. Even if the cost wasn't an issue, it is still convenient to use the same stuff for both hand sanding and on the disc.
 
IF cost of the paper was not a factor then for me it would be the largest possible to prevent counter rotation grinding. BUT cost is a factor. Even if you can get large format paper you are paying much more per disc caused by the size. I can get a 9x11" piece of ao paper for $.40 ea. If you pay for pre=cut discs they run $2-6+ each. Then getting them centered on the disc can be challenging also. I simply take a full sheet stick it to the disc with feathering disc adhesive, then trim to the disc. Perfect fit every time. Also I have ground 24" blades on my 9" flat disc. No real need for a larger format in my opinion. I have used a 9" disc with great success for over 20 years now.
 
To OP

If I lived in China And could get sand paper in any size I needed for about the same cost per square inch I would use a disk big enough so that the longest blade I made would only reach to the center of the disk. but you are going to run into speed issues. the outside of the disk runs proportionatley faster in SFPM. so if you use a twenty inch disk it is going to be running over 18000 FPM at the outside edge of the wheel on a 3600 RPM motor. A 9 inch disk is only going to be just under 8500 SFPM on the same 3600 rpm motor. Now you are going to be using a DC motor so you can slow it down to a reasonable speed. the problem is when you slow down a DC motor you also lose torque. So slowing down a 20 inch disk to the point that you can actually grind on it. will have the motor so low on torque that the motor will stop as soon as you touch the blade to it.
 
Plus, you would need a larger motor to run the larger dis so the initial cost and run cost would be greater.
 
If you already have a vfd for your grinder you could get a toggle switch and a 3 phase motor and then you could control the speed.
 
I do live in China. I can get most types of sandpaper in 60 inch wide rolls which makes trimming the paper to any reasonably sized disc easy and also gives perfect alignment. And, yes, I understand that the rim speed of the disc does increase with each step-up in diameter. This is why I am asking about diameter considerations of the disc. So, it seems to me, that the 9 inch discs are a good compromise based on the cost and availability sandpaper. From reading several forums, 9 inch disc grinders have found favor with some makers. My thinking is, if you have an suitable sized, speed controlled motor and affordable sandpaper, a 12-18 inch disc would even better. But that is my thinking and why I am asking for your thoughts and advice.
 
Last year I turned down a 3HP 3PH disc sander from a big metal/wood fabrication shop. It had a 24" discs. They wanted $100 for it, but I would have needed a fork lift to move it. Something like 800 pounds. It had 3 speeds.
 
It depends on how big the pieces you are working on are, and how you work. If I was going to make up a horizontal disk sander, with variable speed, and paper cost was not a factor, I'd be tempted to go with a size of 18". I'd probably use it just for final flat grinding and sharpening, running very slowly maybe even with a water drip. We had a 12" vertical disk sander in the metal shop some years ago. I found that the paper wore quickly and took a while to change if the glue got overheated, or cheap PSA paper disks got used, after a couple years it was replaced with a 6" X 48" belt sander. There's about 3 times the usable abrasive in a 6X48 belt that there is in a 12" disk. Yes, there's more abrasive in a bigger disk, but a lot of tasks require sanding at the "edge" of the belt or disk, and the abrasive in the center of a big disk won't help then. I'd take a belt sander in the metal shop any time over a vertical disk sander of any size, but we do have a 12" disk sander in the wood shop that is very handy. It all depends on how you do what you do
 
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