Making a knife with a disc sander

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Dr Heelhook

Joined
Jul 24, 2007
Messages
3,285
I can't justify spending money on a proper belt grinder at the moment, but I've noticed a regular 12 inch disc sander is edit: not great, but passable, depending on the kind of blade, for grinding knife bevels. The only problem is, the disc generally only spins in one direction, limiting me to making chisel ground blades only. I know the machine is likely not made to be used that way, but is there any POSSIBLE way of reconnecting the circuits to make the disc spin the opposite direction on a machine for an example like this one (it's a Holzmann)?:
TS305-thumbnail-450-450.jpg

That way I could grind the plunge line and bevel on both sides of the blade. I know it's a shot in the dark, but I have to ask.
 
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Would have to see schematic of the motor. Many singlephase motors are easily reversed.
Also be sure its disk anchoring will permit to spin either direction.
 
Those units won't run backwards. They also spin too fast for good knifemaking.

If you are going to make knives with it, get rid of the table and mount the grinder horizontal by screwing the feet to a piece of 2X12. Screw the 2X12 to the end of a workbench or clamp in a vise.

You don't have to reverse a disc sander to make a normal double bevel. It is nice to have, but not a requirement. Think about it ... a belt grinder only goes one direction.
 
Stacy E. Apelt - Bladesmith said:
Those units won't run backwards. They also spin too fast for good knifemaking.

If you are going to make knives with it, get rid of the table and mount the grinder horizontal by screwing the feet to a piece of 2X12. Screw the 2X12 to the end of a workbench or clamp in a vise.

You don't have to reverse a disc sander to make a normal double bevel. It is nice to have, but not a requirement. Think about it ... a belt grinder only goes one direction.
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I'm not gonna lie, I've been using a jig, so I've been dependant on the table so far. If the disc would just spin in the other direction, it'd be so simple to just do the same thing, but on the other side. I should probably just stop being a baby though and start grinding my bevels free hand. Mounting the grinder horizontal sounds like a good idea in that case, if I'm gonna keep using this machine. Thanks!
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Dr Heelhook said:
I can't justify spending money on a proper belt grinder at the moment, but I've noticed a regular 12 inch disc sander is great for grinding knife bevels. The only problem is, the disc generally only spins in one direction, limiting me to making chisel ground blades only. I know the machine is likely not made to be used that way, but is there any POSSIBLE way of reconnecting the circuits to make the disc spin the opposite direction on a machine for an example like this one (it's a Holzmann)?:
TS305-thumbnail-450-450.jpg

That way I could grind the plunge line and bevel on both sides of the blade. I know it's a shot in the dark, but I have to ask.
Click to expand...
It's probably possible to rewire it; but not with external circuitry (or does it have a plug with reverse polarity protection?). I'd also look out for stuff like left/right-hand threads that might come undone if the wheel spun the other way round.
 
If you use that jig and the table, just flip the blade on the jig and go to the other side of the table. One side will be an up-grind and the other will be a down-grind. Otherwise, it should work the same.
 
Stacy E. Apelt - Bladesmith said:
If you use that jig and the table, just flip the blade on the jig and go to the other side of the table. One side will be an up-grind and the other will be a down-grind. Otherwise, it should work the same.
Click to expand...
Yes, that seems like what I'm gonna have to do. I'm working on a similar jig setup for another blade right now and I'd prefer this one to be a regular double grind and not chisel.
It will be a bit scary with all the sparks going upwards, when doing that "up-grind", that's all.

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I have one similar that I use for certain processes. It sounds like you are grinding at the 9:00 position, as the machine is intended, with sparks shooting down. You don’t want to grind at the 3:00 position because sparks will shoot straight up. Can you grind at the 12:00 position? That is what I do. Does that make sense? Sparks would shoot straight to your left as you are facing the disc.
 
JHAller said:
I have one similar that I use for certain processes. It sounds like you are grinding at the 9:00 position, as the machine is intended, with sparks shooting down. You don’t want to grind at the 3:00 position because sparks will shoot straight up. Can you grind at the 12:00 position? That is what I do. Does that make sense? Sparks would shoot straight to your left as you are facing the disc.
Click to expand...
I need the table to rest my jig on, so 9 and 3 are the only possible options for what I'm going to do. No, grinding on the right side with the sparks going upwards is certainly not optimal, but it might be the only way here.
 
If your motor is less than 1/4HP this should work. (This is the motor schematic for that particular sander. It doesn't say what size the motor is, but it doesn't show a start cap.) If it is larger, and capacitor start, you would do the same by reversing the polarity on either the start winding or the main (run) winding, but not both.



The permanent mod would look like this:


The only thing stopping you would be the mechanical part... is the disc plate screwed on, keyed or pressed? If threaded, it won't work without some way to ensure the plate doesn't back off of the shaft, like a set screw. If pressed or keyed, then you're good to go.


**Note, you'd most likely have to open the motor up or use a multimeter to determine which wires are which.
I'm an industrial electrician so this kind of stuff is commonplace for me, but might not be so straight forward to others.
 
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Or the simpler solution (maybe) would be to rig a guard over the disc to route the sparks back towards the floor...
 
Or just pull a motor assembly out of an old top loading clothes washer.

as always
peace and love
billyO
 
There's a reason these things are designed to use a "down-grind". Imagine grinding up and anything catches; the blade will shoot up into your face/across the room. Had that happen at work once, not fun.
 
SV-97 said:
There's a reason these things are designed to use a "down-grind". Imagine grinding up and anything catches; the blade will shoot up into your face/across the room. Had that happen at work once, not fun.
Click to expand...
That's the other thing to keep in mind.
 
Like Stacy said, these things really aren't very good for making knives :D. What I mostly realized was that it's impossible to cut a nice and crisp plunge line with it. I went ahead anyway and it is what it is. I got the bevels down decently on both sides and if heat treatment goes well (I'm gonna let a grownup take care of that) and I manage not to mess up the edge, it should cut pretty all right at least.

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