Could use some design help with a belt grinder

MikeWi

MikeWi

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I'm designing a 42" belt grinder to attach to the right side of my new bench grinder. it gives me something to learn Fusion 360 with :) I have no trouble designing things, but I have no engineering knowledge at all, so I'm never sure what is enough or too much. Almost everything I see on the net is made of 1/2 plate or similar heavy materials, but I want to use what I have on hand, and I just don't believe it needs to be that heavy. The whole thing is about 20" tall. I having a lot of fun, and the design is parameter driven, so I can make adjustments to one item and everything else adjusts to suit. Nice! This is mainly intended to be an additional tool for sharpening and shaping lathe tools and drill bits. My old 6" grinder will keep the gray wheels for doing anything else.

Anyway, the questions:
1) I'm planning on using a 4" drive wheel, which will get me around 3600 SFPM. From what I've found that seems to be reasonable. Any thoughts?
2) I'm struggling on the platen support. I'm currently looking at a 1/4" plate that will hold an 1/8" plate with pyroceram plate glued to it's face. To hold it out from the tower, I'm looking at 1/4 again, anchoring where the two tabs are projecting, with an option of further contact at the top of the grinder base. There will be adjustment for belt contact and angle. Do you think that this would be rigid enough?

here's what I have so far. Clearly still a work in progress. :)
snapshot.png
 
Well even that company's grinders only do 6000 in spite of that article. I've seen anywhere from 3600 on up, but I was wondering if anyone is using one here. I don't even know when I'd build this anyway, but it'd be a fun project for the lathe as well as my CAD practice.
 
I have a KMG 2 x 72 that is variable speed. My top belt speed is a little shy of 5400 as I remember. Occasionally I feel that it would be nice if it was a touch higher.

I did some studying on belt speed recommendations by 3M and others and their answers kept coming out at 7000+ for their ceramic abrasives belts.
Not sure if you are planning for a variable speed drive or not. If it's fixed speed, I'd likely stick with the 3600.
 
Mike, if I was going to build a 2x42 belt sander, I would order the wheels from Sears that fit their belt sander. The lower drive wheel is 5". The upper wheel mount incorporates the tracking mechanism and it is simple and effective, and it bolts on to a flat chassis. For a chassis, I would use a single piece of 1/4" thick steel plate that mounts to the motor housing at the bottom. I would weld a stiffening piece (at the back of the vertical piece) and carry it under the lower wheel to stiffen the vertical piece.

The steel vertical chassis would allow you to easily mount an angle iron platen support (adjustable fore and aft to align with the belt) to which a ground steel platen can be mounted. A Pyroceram liner can be epoxied onto this platen and it would be very solid.

The chassis would also allow you to mount a tool rest solidly.

Okay, out of ideas.
 
My 4 x 72 belt sander has a piece of graphite impregnated material on the platen... it does a wonderful job. It has been there for 40+ years and probably has another 200+ years of home shop use left on it. My father originally used double sided tape to mount it... the tape strips eventually worked there way through the material and left lumps. I pulled up the material and replaced the double sided tape with contact cement... been working great since and the lumps flattened themselves out very quickly. I remember when my father first put the graphite impregnated material on the platen... it was like doubling the hp of the drive motor.

1/4" plate sounds plenty strong and ridged to me... I think the HarborFreight belt sanders use aluminum foil as their structural material.
 
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I've been considering building one too Mike. The speed thing seams to be all over the place. As Alan posted 3M recommends high speeds. Im inclined to think the belt might perform best at the recommended speeds. They make slow speed wheel grinders for sharpening carbon steel wood working tools but I can't imagine trying to remove much metal with them. An 8 inch grinder cuts a lot quicker than a 6 inch with the wheel rotating at the same speed. And that first section of the wheel wears a lot slower compared to the next inch once you've wore it down to say 7 inch dia and are running at a lower surface speed.
From their charts 6500 fpm seams like a good speed if your going to be operating with a single speed motor as I was planning. Thats the recommended speed for carbon steel and the low end for stainless and aluminum with fine grit belts.
Like you I'd appreciate some input from real world experience.
Im planning on 2x72 belt size, they seam to be a lot easier to find up here and expect they wear beter with the extra surface.

Greg
 
I haven't done a lot with mine yet. However, I read that ceramic belts work better at high speeds. My grinder runs off a VFD, so I can set it to many different speeds. I tried about 3000, it works but seems to make the part hotter. 6000ish removed material much faster and kept the part cooler. With a 36 grit belt, keystock is gone in seconds. For finishing work with aluminum oxide belts I expect slower speeds would be better.

Speeds are approximate as I don't have a tach, so I'm going by frequency from the VFD.
 
From whose charts exactly? I haven't found anything except a pdf from 3M that recommends 3500 to 4900 (roughly) for carbon steel. There are other places that state the higher speed ensures that the grit exposes new cutting edges and cuts cooler by taking the heat away in the chip. Many threads out there with the same confusion! LOL One source says that the "rule of thumb" for grinding belts is "a mile a minute" while the consensus among knife grinders is around 6000 sfpm.

I think I'll go higher with a larger drive wheel. I do like the idea of being able to use less pressure. That will keep wear and heat down.

There's been alot of comments I've read saying that 72 inch belts are the easiest to find, but I have no trouble finding any size. This place in particular seems to have anything you could want. http://www.trugrit.com/index.php?main_page=index&cPath=1_145
 
The numbers I was quotng were from a 3M pdf as well Mike, didn't book mark it just wrote the figures down. Could be these were for ceramic and not aluminum oxide. They also gave belt tension 15-20 pound per inch of belt width. Wonder if aluminum oxide needs slower speeds, more research in order.

Greg
 
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