why disk grinder???

K

Ken H>

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Dec 31, 2011
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Hello all, I've read one of the reasons for a VFD disk grinder is to flatten scales. What advantage does a VFD disk grinder provide over a 2X72" VFD grinder with platen?
 
The disc will get stuff flatter The platen will have flaws and not be completely flat and the belt can bow more on it.
 
I should have added "with glass platen" which I would think should be pretty flat. I grind scales on platen and when I hold them together to the light, there is no light showing thru. Am I missing something there?

Please do understand, I'm not trying to be difficult or argumentive, just trying to understand.
 
Glass platens (or even steel platens) can be perfectly flat, but the belts will not be. Whether it's crown from the wheels "bowing" the belt, or simply just slack generated from pushing against the running belt (it's very small, but the belt does "bunch up" ever so slightly, were the top of your piece pushes into the belt), you will get a little round over on the edges, at the very least.

Discs don't have this problem, because the paper is adhered across the whole plate, and can't "bunch up". Now, if you have a cork or rubber backer to your disc, you can certainly get some round over from the compression of the backer, but on a hard plate, you'll typically get much flatter than any other method, short of, say, a a lapping machine.

What's also nice is that you can get into much higher grit paper without getting into relatively more expensive belts.
 
The disc grinder was a game changer for me as I do mostly FFG. It reveals inconsistencies in your grind you will forever chase or never see on your belt grinder. I also don't have a surface grinder. If I have to hog off material to change the thickness on stock the disc grinder helps to clean that up.
 
Flatness: Properly set up belt > Disc with hard face > Hand lapped on surface plate > Surface ground (on a real SG)

Where they actually measure I don't know but would guess something like ~.010" > ~.005" > ~.002" > ~.001"

Actually, this would be a good experiment. I'll flaten some test pieces, set them up on 3 points and indicate flatness on a surface plate. If someone wants to send me a plate they've flatened with either a belt converted surface grinder or a 2x72 surface attachment, I'll include those results. Something like a 2x4" coupon.
 
kuraki said:
Flatness: Properly set up belt > Disc with hard face > Hand lapped on surface plate > Surface ground (on a real SG)

Where they actually measure I don't know but would guess something like ~.010" > ~.005" > ~.002" > ~.001"

Actually, this would be a good experiment. I'll flaten some test pieces, set them up on 3 points and indicate flatness on a surface plate. If someone wants to send me a plate they've flatened with either a belt converted surface grinder or a 2x72 surface attachment, I'll include those results. Something like a 2x4" coupon.
Click to expand...

Good points there!! I love my real SG for getting stuff flat... but I always have some bowing on folder blades that I can't remedy unless I grind the other side, and even then it's not perfect. I've tried everythign from flood coolant to only taking 1-2 thou passes. So I always finish up by hand on a granite plate w/ sandpaper.
 
razor-edge-knives said:
Good points there!! I love my real SG for getting stuff flat... but I always have some bowing on folder blades that I can't remedy unless I grind the other side, and even then it's not perfect. I've tried everythign from flood coolant to only taking 1-2 thou passes. So I always finish up by hand on a granite plate w/ sandpaper.
Click to expand...

This is likely due to deformation on the magnet. If you set the blade on with the magnet off, and use shim stock to find and short up any bows in the material, that problem will go away.

Or, lap one side flat before surface grinding the opposite, then flip for a consistent finish. That's more work though.

It can also be an issue with the grinder, since most of use are using old worn machines. If the grinder spent 50 years reciprocating in the same 6" of the ways there may be a dip/depression.

Machine tools are just about transferring flatness, parallelism and perpindicularity of the machine to the part. To find out if your surface grinder is grinding things flat, you can grind 5 blocks and compare them on a surface plate. 1 in each corner and one in the center. The deviation between them will tell you how capable your grinder is for flatness over the area they encompass. If they're not satisfactorily flat you could then try to remedy first by regrinding your magnet flat (they move over time) and then by just trying to find perhaps an area on your magnet (really an area of travel on your ways) that gives satisfactory flatness.
 
I find that the shape of the surface is easier for some things than the 2" platen. I use the disk for hogging off handle material and rough shaping handles, and also wa-handles to finished dimensions. Never tried a FFG on one tho, think I'd want the 1-degree crowned disc for that. And bigger. And more horses. And more true. :D
 
no CMM? :D


kuraki said:
Flatness: Properly set up belt > Disc with hard face > Hand lapped on surface plate > Surface ground (on a real SG)

Where they actually measure I don't know but would guess something like ~.010" > ~.005" > ~.002" > ~.001"

Actually, this would be a good experiment. I'll flaten some test pieces, set them up on 3 points and indicate flatness on a surface plate. If someone wants to send me a plate they've flatened with either a belt converted surface grinder or a 2x72 surface attachment, I'll include those results. Something like a 2x4" coupon.
Click to expand...
 
I don't like flatness off the cmm because of the averaging ours does. If we had a scanning cmm I'd do it.
 
Ken H> said:
I should have added "with glass platen" which I would think should be pretty flat. I grind scales on platen and when I hold them together to the light, there is no light showing thru. Am I missing something there?

Please do understand, I'm not trying to be difficult or argumentive, just trying to understand.
Click to expand...

I can understand your perspective Ken. I've just not felt a need to get one to date either. I'm not afraid of the purchase, I just can't figure what I would do with it that I can't do already.
Maybe it's because I almost never do FFG's, or folders. ?
 
Brock Cutlery said:
I can understand your perspective Ken. I've just not felt a need to get one to date either. I'm not afraid of the purchase, I just can't figure what I would do with it that I can't do already.
Maybe it's because I almost never do FFG's, or folders. ?
Click to expand...

I thought the same way until I got one. Dovetails or any multi piece handle is easier with one, really getting anything you want flat is easier on one. I use it in every knife I make and have only recently started using it on ffg bevels.
 
another issue with a belt is the seam. i have done pieces on the belt with ceramic platen, marked with sharpie, then sanded lightly on disc. was amazed at the small imperfections i found.
 
That's a really good test.

Get a block of wood "flat" on a belt. Mark it up with pen. Lap it on a plate with figure 8s and its easy to see
 
As said above dead flat fast. Not only that but extremely quick to set up a shelf for either square
or any angle. I'm down to running 3 of them now 1 @ 1/2horse, and 2 @ 1 horse. I'll bet its been
over 10 years since I haven't used one of mine on a knife for something.
Ken.
 
Good points all. I agree with the idea of the bevel grinding on a disk grinder. I've done that a couple of times and it really gets the bevels flat for a FFG. Lately I've been doing the lengthwise grinding as shown by Nathan in this video and have been impressed who well it works (this is not the best video, but can't find the other one):


I'll test grinding the wood thing tomorrow, then mark it marker and see how much difference I get. I've not tried that - just held the two scales together up to light, and if I can't see any light between them, call it good.

Thanks to all for comments.

Ken H>
 
once you get one you will want two, they are so cost effective and don't take up alot of room.
Brock Cutlery said:
I can understand your perspective Ken. I've just not felt a need to get one to date either. I'm not afraid of the purchase, I just can't figure what I would do with it that I can't do already.
Maybe it's because I almost never do FFG's, or folders. ?
Click to expand...
 
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