Disc Ginder?

[Pelallito]

Hello,
I am thinking about making a 9" disc grinder/sander for my shop. I am thinking of using 1/2" steel plate and attaching a boss(if I am spelling it right),with a key way cut into it, to it, so that I can attach it to the motor. After attaching it mechanically with bolts and perhaps welding it into place. I plan to turn everything as one unit on my lathe to balance it, get rid of any run out and make it vibration proof.
I have been told that the way to do this right is to turn down a 1.5" by 9" steel plate to make something vibration free. I don't know why my idea won't work, so long as my boss is carefully centered.
Can you tell me any reason why this won't work? Any suggestions on making it better would be happily accepted.
The reason for 9" is that I can use normal sandpaper on it. All I have to do is glue it on and trim it in the shape of the disc.
The motor will be reversible, and the disc will have a 1 degree taper on the face.
Thanks for the help.
Fred

[cecilstrange]

Your plan should work if you balance everything that rotates with precision. A heavy duty motor or one with oversized ball bearings should be used for this kind of direct mounted tool. This is how a bench grinder differs from an ordinary motor. End play in the shaft is particularly undesireable in a disc grinder.

A sturdy guard and a way to keep dust out of your lungs would make it a lot safer. A sturdy fence that is easy to adjust should be a must have. Lots of finger and hand injuries are the result of work jamming between the face of a disc and the table.

You may be confused if you think you can balance the rotating mass in your lathe. Dynamic balance requires bearings, true to the rotating axis, that allow the part to oscillate as it reaches critical speeds where the harmonics of the imbalance forces create movement in rhythm. If you haven't built a fancy fixture to turn your lathe into a balancer, you should take your assembly to an electric motor balance shop for assistance.

The balance is critical to both safety and a decent service life. Although dynamic balancing is the only way to achieve your goal of "vibration proof," a static balance job might result in a useable tool, requires less specialized equipment, but still requires significant skilled labor.

Finally, if you use bolts, rather than welding to join your hub to your disc, add a locating dowell and make the fasteners vibration proof. Loostening or missassembly here would destroy the balance.

[Racer Al]

I have a 12" disk sander, and it's pretty annoying how rapidly the sandpaper disks wear out. IMHO, if you're going to the trouble, I would build a larger disk.

A brief search on disk sanding machines reveals that the common sizes are 12" and 20".

Also, a belt drive will allow you to adjust the speed of the disk by changing the diameter of the pulleys and will protect the bearings in the motor --- unless you're adapting a buffer or grinder, they're not designed to hang something that big and heavy off the shaft.

[Pelallito]

Cecilstrange,
You gave me a lot to think about. I did not think that putting a 9" disc would be that hard on the motor. I had planned to use the bolts to hold it togther while I welded it, to minimize distortion. The fence is on the to do list. I am not sure about the guard, I need to get close to the face on both sides. I will talk to a motor shop that we do business with about this.
Racer Al,
I will put this on a 3 phase motor operated through a VFD that will allow me to control speed and direction of rotation. The 9" plat is used so that you can use regular sand paper. Just glue it on and trim to size.
It won't be used for heavy grinding, mostly finish work with fine grit paper.
I will consider follow your advice and not hang something that heavy on the end of the motor.
Thanks for the help.
Fred

[moonlight machine]

You may want to make it 12". self stick discs are easy to buy in 12". Racer Al is right the paper wears out right away, Thats why I put my home built 12" aside and built a 24". It uses a 5 Hp motor with the disc mounted directly to the shaft. It's 3 ph with a vfd. good power from about 25 rpm all the way to 1725 rpm where it positivly devours anything you care to grind. I get the discs from Klingspor, they last forever, Klingspor makes the best abrasives I have ever used.

[machine1medic]

Originally Posted by Racer Al

it's pretty annoying how rapidly the sandpaper disks wear out. IMHO, if you're going to the trouble, I would build a larger disk.

That much effort would be better put toward a belt sander, of any width.
Disc sanders are easilly clogged with even real discs.
Trust me, make-yer-own will be even worse.

You can easilly make even a 6" wide belt-sander.
It's incredibly simple lathe-work, and some fabrication for your frame.

Belts are self-shedding, and long lasting.
Cheers
M1M

[cecilstrange]

Hard on the motor? -- Ultimately that will depend on how much you use it. If the motor is not expensive or if swapping bearings is no big deal, you might try your original idea.

For precision grinding of metal or wood you need a rigid disc and a table. Otherwise you might try building my all time favorite grinder for fabrication work. It uses 9" aluminum oxide or Norzon disks with a 7/8" arbor hole. They are fiber backed but flexible, not thin like sandpaper. These disks are intended for handheld grinders.

I have two of these machines. The backing plates differ. One has a factory made stack of spring steel discs secured to a hub, and I have it mounted directly to the shaft of a 1725 rpm motor. The steel discs vary in diameter so flexibility is greatest at the OD. The other is home made, a 9" disk cut from 1/8" plexiglass, and it runs on a 5/8 arbor with plain bearings and is powered by a 1/4 HP 1725 rpm motor. The disk speed is about 2500 rpm.

Because the disc is free to flex, off-hand grinding is always smooth. With no table, the edge of the disk is free to fit into tight places. For heavy deburring, grinding welds, shaping, even roughing tool bits, it is fast and convenient. I use it mostly for steel but it does double duty on wood and aluminum when I need it. I have used disks from 24 grit to 80 grit.

I have fashioned guards, but it is not a particularly safe machine so you have to pay attention. Both of mine are mounted on heavy free standing pedestals so I can present large and small objects at all angles.

[strokersix]

Belt versus disc: In my experience belt sanders remove more material and don't clog up or wear the abrasive as quickly as disc sanders. But the platen (correct term?) behind the belt wears and is no longer flat on every belt sander I've ever used which degrades accuracy. I personally chose disc over belt for this reason. I use a 12" Grizzly disc in my home shop and it's been fabulous and I use it all the time. I once used a large disc sander (24" perhaps) that came out of a pattern shop. Now that was a sander!

Want to remove material quickly or are just doing deburring go with belt. Want more accuracy choose disc. My opinion of course.

[moonlight machine]

Yeah I really like my 24". In 1988 I was working at the March race car factory in Bicester, England. Down the road a bit was there wind tunnel and on the shop floor under the tunnel was there pattern shop. In that shop was the mother of all disc grinders. this thing had a huge motor, I don't know how much HP but it was about 24" in dia. It had 48" discs on each side of the motor! It had real nice machined tables around each disc. The Foreman warned me that it was turned on at 7:00 AM and not shut off till quiting time. You could be standing next to it or even leaning on it and never know it was on, amazingly smooth and dead silent.

[kpotter]

You need both a disc and a belt I built both of mine. I was agonizing over this same problem several years ago and decided that I didnt have to choose I could have both. I have a 7.5 2speed dual shaft motor that I used for my belt sander it came from a ski grinder and a 5hp 20inch disc sander. The disc grinds flats and the belt contours and with two belts I keep one for finishing one for roughing their is a pic in the shop pics or maybe a video.

[Pelallito]

I already have 2 2" X 72" belt grinders. I wanted the disc sander for flattening some surfaces. I also have a surface grinder for some jobs.
I want it for knife making, and the norm is the 9" diameter. I can not argue the the other larger discs have a lot of plusses. Klingspor makes great abrasives belts, I assume their discs are just as good.
The grinders that you have described are awesome. I hope to see some like them one day.
Thanks for all of the responses and advice.
If you have any more I would welcome them. I have not gotten any materialas yet fot this project. But I will start soon.
Regards,
Fred

[precisionworks]

A brief search on disk sanding machines reveals that the common sizes are 12" and 20".

Also, a belt drive will allow you to adjust the speed of the disk by changing the diameter of the pulleys and will protect the bearings in the motor -

+1

My Delta has a 12" disc, and there's about 5" that you can use. On a 9" disc you have only 3.5" of usable disc - the 1" radius from dead center is useless as there is practically zero surface speed. 12" would be the smallest I'd use.

I will put this on a 3 phase motor operated through a VFD

That is exactly my setup, using a 3 hp motor. IMO, 2 hp is a minimum size & 3 hp is better because you have more torque to work with.

I'd also go with belt drive as you can use a pair of large ball bearing pillow blocks for supporting the shaft that drives the disc. A shaft 1" diameter or larger should work well.

-------------
Barry Milton

[wtrueman]

Hi all; I have some info that MAY be useful to you and others. I also have a 2 by 72 belt machine but I used a waterpump from (I think), a Chev car. Any way, I use a waterpump from a car because of the about 2 inch bearing and the support that comes from the housing. The 6 inch drive diameter from a 1/3 old electric motor drives a 5 inch driven waterpump. I cross glued and turned some maple boards for both drive and driven; put a small crown on the pulleys as well as several coats of finish on them and this has worked well for me. I am in the process of building a similar 6 by 48 inch wide driven system but need a stronger drive motor than the previous. This just means a longer look for that! In any case, for home use, my afor mentioned system is working well for me. Wayne.

[gradstdnt_99]

I have several disc and belt grinders. They both have their place as pointed out. Disk indeed produce flatter surfaces. Belts are great for contours and deburring.

One thing I have discovered is that the usefulness of the machine can be greatly enhanced by making them variable speed. If you start with a three phase motor, simply adding a VFD does the job. My Burr King runs far too fast for delicate work. After adding a VFD I find myself rarely using it at full speed. I actually swapped out the single phase motors for three phase and added VFD's. Something to consider.

[Les George]

I have a 9 inch disk and it works great. With a 9 inch you can use spray glue and stick on 9x11 wet dry paper and cut it to size with a quickness. Then you can use any grit you can get your hands on, I have from 60 to 1000 grit.

Make the disk 8 7/8 dia and you will save yourself A LOT of drama sticking the paper on there.

I bought mine here: http://www.beaumontmetalworks.com/disks.html

VFD is priceless!

[Peter Miles]

I've had a 20" and a 24" diameter sander in my hobby shop. They both did very nice work. PSA Discs would last a very long time. Large tables allowed easily working with large parts, cleaning up steel plates, etc.

However, if you want to change from a coarse to a finer abrasive, you have to throw the old disc away.

I've since bought three different large belt sander/grinders ranging in width from 8" to 18". Two of them can be run as wet or dry. I probably won't keep all of them long term but I like being able to quickly change and reuse belts.

[Robert Campbell Jr.]

Fred, if you do fab your disc, I'd bore it to fit a reduced section on the hub. The "true" shoulder on the hub will let you press in the hub to keep them concentric in both planes while you tack, then weld both sides.

Both hub and disc should be oversize to allow clean-up turning to true, as the welding will warp the disc.

The weld is then only asked to serve as torque transfer and "keeper", the disc being mechanically borne on the hub.

With the bore and all surfaces then turned mechanically true and therefore symmetrical about the bore, a 9" disc at 1725 shouldn't require any balancing.

Bob

[ColoradoBoy]

Originally Posted by Pelallito

I want it for knife making, and the norm is the 9" diameter. I can not argue the the other larger discs have a lot of plusses. Klingspor makes great abrasives belts, I assume their discs are just as good.
Regards,
Fred

A few thoughts from using a 9" disk for knifemaking. Flating handle material & shaping to fit finger guards mostly.

CZ cloth disk abrasive doesn't last long and is a waste of $$. There isn't enough grinding preasure or speed to break abrasive grains. Have been using surplus 9x11 sheets from Red Hill Abrasive they cut from floor sander belt material, 50gr. or 60gr(?). Cost vs life they are the best on the 9" disk for handle material. If they still sell it? A rubber belt cleaner is a must for the disk.

If you are going to taper tangs on the disk? A better option is making a flat belt grinder platen overlay from 1/4" O1. Still using the first one 10 years later but needs re-flat on the disk every 400-500 knives & solved wear probems with factory flat platens or breakage of ceramic overlays.