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Home built surface grinder concept


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Just curious as to some input on this concept I've been playing around with:

 

 

Surface Grinder Assembly.JPG

 

It uses a 1/2 hp motor that I have laying around the shop, with a 3" x 3" drum sander attachment and three 2" x 1" x5" lifting magnets that have also been laying around looking for a purpose.  All the bracketry is 1/4" mild steel that I'll fab up at work.  The whole thing will cost me about $100, if it works I'll be able to surface grind up to 2.5"ish x 1.5"ish x 17"ish blocks.  She won't be the most powerful thing in the world (and probably not highly accurate either), but I'm thinking that it'll produce better results than trying to flatten out my forgings on a platen.  Thoughts?

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Cool idea. You will need some way to move either the motor, or the work table up and down in very small (think thousandths of an inch) increments. Is that what that screw on the left is for?

 

For an extra $85 or so, you could replace that drum sander with a contact wheel, Take that drum sander wheel and use it as an idler wheel (or buy an actual idler wheel). Then put the idler wheel on a tensioning rod above the contact wheel and use your 2x72 belts.

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Yeah, the bolt on the left is the adjustment to raise and lower the motor.  Currently it's a 3/8-16 which as it sits will give me about .030" or so per revolution.  It's a bit coarser than I would like, but I can always change out the screw down the road if I need to.  I had thought about building it to take belts, but the house needs new windows so this one is on a shoestring for now.  I also toyed with the idea of adding angle adjustment to the slide for tapering tangs or adjusting distal taper, but that's going to be backburnered as well.  If it wasn't for the bearings for the 80/20 rail, the whole thing would only cost me about 20 bucks.

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A heck of a first start, but the design has a problem. You have the grinder assembly hinged on one side. Instead, it needs to move up and down in relation to the table, but with a solid mechanical mechanism that doesn't allow for movement once grinding starts. As you have it now, there is a good chance that the bolt and spring assembly will allow the motor to buck up and down slightly, not grind smoothly from one side to the other. Maybe instead of a hinge, think of the motor moving up and down on guide rails, with a heavy piece of all-thread to the side or back to move the motor up and down, which can be locked down tight to stop vibration.

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Yeah, I don't disagree.  I would love to build something with a bit more rigidity, but the budget isn't going to allow that right now.  My hope is that the trampoline spring will produce enough downward force to to allow it to grind somewhat efficiently.  If push comes to shove I can always add a couple more springs to increase the tension.

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I hate to be a negative Nellie, but I bet you are going to be very disappointed with the results for a few reasons.

 

As already mentioned, the spring-loaded pivot design isn't a good basis for grinding.  It will lead to a "Washboard" pattern on your stock because the motor/drum assembly will bounce up and down as the stock passes underneath.  Adding more force from the spring is not the answer as you will ultimately stall your 1/2HP motor long before you stop the oscillations.

 

What you have designed is more of a power sander.  The drum will be under a mostly constant force so it will more or less follow the contour of the original surface removing the same amount of material all the way across.  A surface grinder needs to be rigid so that it removes more material at the high spots.  This would require the force to be higher on the drum at the high spots.

 

You will end up in a climb milling situation in one direction or the other.  With something in the force range of a trampoline spring, I suspect the motor assembly will start bouncing up and down rather violently, and very well may eat itself or the person using it.

 

If the budget is tight, I would put your $100 somewhere else. 

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Power sander.  Hmmmm.  Fair point. 

I have some ideas that I'll have to model up and see how they actually look. 

Thanks for the input guys!

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I am rarely, if ever, mechanically inclined....so this may be a dumb idea
what if instead of a hinge and bolt mechanism, you used a bolt on either end, threaded into the base, and tighten/loosen simultaneously as needed to lower or raise the abrasive...?  Use two nuts on either end of the plates to prevent loosening/tightening as the machine vibrates...

as I write this I wonder about how strong that would be and if there would be rotational or shear forces(?) that might bugger the bolts....

Edited by Jaron Martindale
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@Alex Middleton, I am horrible at describing things but I'll give it a go:

What if you got rid of the spring. Make the hole on the arm slightly oversized so the bolt spins freely and hold the arm at the top of the bolt with washers and a double nut. Then thread the base plate so the arm moves up and down by screwing it into the base plate. That way the motor and spindle are at a locked fixed height based on how far up or down you have screwed the bolt and don't bounce. You would have to add some blocks underneath to account for the travel of the bolt. You would also have to angle the hole through the base plate to the same arc as the swing of the arm (which could be a right bugger). Hope that made sense...

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@Jaron MartindaleI think I follow what you are saying.  With what I have pictured in my head, I think it would have a hard time holding the motor on a consistent plane without making thins overly complex.  I might be picturing it wrong though.......

 

@Bill SchmalhoferI had played with that idea.  With the rear pivot point design, you would have to leave a bit of slop in the motor bracket where the head of the bolt passes through.  I'm afraid it would end up being in the same boat as the original design and not quite sturdy enough.

 

After wasting waaay too much of my bosses time :ph34r:, I'm gravitating toward something like this:

Surface Grinder Assembly v2.JPG

 

The adjustment mechanism is the same, but the two knobs should serve well enough to tighten things down and hold the motor relatively solid.  It might be overkill considering the weight of the motor, but I want to keep the spring as the tension that it will provide will help to keep things steady while adjusting the height.

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Is your 80/20 rail supported along the bottom?  It's hard to tell from that angle, but if not it'll flex a lot on you.

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How about losing those two knobs and doing the height bolt slightly differently?

Thread it into the base plate. Have it pass though the motor frame arm. Put a nut under the arm to secure the arm to the bolt head. Ned to adjust height? just loosen the nut, turn the bolt head, tighten the nut.

Eliminates the possibility of the motor arm bouncing.

 

Edited by Joshua States
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I think tha could be made to work as well.  The more time I spend thinking about it, the more I'm leaning towards having 4 good contact points instead of three.  @Brian Myers and @Brian Dougherty comments have gotten me to thinking a bit deeper, and a little bit of overkill isn't going to hurt.

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