How Flat It Is....

C

Cushing H.

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Jun 3, 2019
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though I probably should have realized this a number of months ago, I just had a rather unsettling revelation made obvious to me.....

As my grinding skills have progressed (hopefully...) I have continued to periodically struggle with getting a nice flat grind on my bevels (or at least flat enough to satisfy my) .... and have been somewhat surprised at the lack of flatness off of the grinder revealed when I have gone to hand sanding. Well ..... I have been gearing up to put a glass platen onto one of my metal platens, with the hope that it would reduce temperature and also help a little with the flatness issue.

WELL ..... thinking that ceramic is rather brittle, I thought I should use my newly obtained mini mill to "touch up" the platen so that the **brittle** glass platen was not resting on any low or high areas. So .... while doing that, I was REALLY surprised to find that there was quite a concavity from top to bottom on that platen (i.e. it was bowed so that the top and bottom protruded further out towards the surface that the belt rides on). After advancing, step by step, the endmill enough to produce a finally flat surface, I was REALL surprised to find that the difference between center and ends was at **least** 12 mils. The photo below is taken from the back-side of the platen (which had the same curvature, but I did not mill it) - the light shining through under the scale showing the gap:
upload_2021-1-17_13-10-17.png

the photo below is taken of the side that I did mill flat (showing no obvious gap):
upload_2021-1-17_13-11-32.png

Arrrrgggghhhhhhh. no WONDER I was struggling to get nice flat grinds - especially on FFG grinds on wider blades - which is the majority of what I do as I focus on culinary knives.

So .... I guess I mean this as a heads-up to all (myself included) to not assume that a platen is really all that flat - especially if you are grinding wide blades. I guess I will not be doing any significant bevel grinding until I get that ceramic platen installed :-(
 
It's nice to have precise tools and I hope your grinds continue to improve. I doubt however that this has been a big factor. When I think of my own set up and no doubt worse platen, I have a thick belt moving rapidly, not adhering perfectly to the platen despite firm tension, perhaps overhanging one side or the other... me holding the piece freehand and applying various pressures... overall a few thou in the platen likely won't show up.
Thanks for the reminder to replace mine though, I have one sitting by and my current one is both worn and chipped.
 
Keep in mind a belt grinder doesn’t give you a true flat surface even with the glass plate. The belt bunches ever so slightly while making contact. I know Several people use a disc sander to flatten their bevels off the grinder and before hand sanding and it significantly reduces hand sanding. That said on kitchen knives an ever so slight convex toward the edge is good it helps food to be less sticky.
 
I knew someone would bring up the disk sander :-). I do know the issues with belt bunching and chatter.... I was just surprised to see just how non-flat the platen was....
 
i am glad you noticed ! everyone should put a straight edge on their flat platen with light behind it to show any gaps. my metal platen gets a worn spot from doing profiles right at the work rest height. i even wore into a pyroceram glass platen face. now i just touch up the metal platen on the surface grinder attachment every year or so. dykem it blue to make sure its all flat. i think the bunching up of the sandpaper as joshua mentioned happens, but to different degrees depending on how hard you are pushing, how sharp the belt is, and i think it would happen much more on a spring loaded tension than a ratchet tension system. i do not own a disc sander, but i see people mention using them to flatten, which seems odd because the outside of the disc is sanding away more material than closer to the center of the disc because there is more surface travel, so i do not understand how the sanded surface could become one plane. i hope to try one someday and find out.
 
john april said:
i am glad you noticed ! everyone should put a straight edge on their flat platen with light behind it to show any gaps. my metal platen gets a worn spot from doing profiles right at the work rest height. i even wore into a pyroceram glass platen face. now i just touch up the metal platen on the surface grinder attachment every year or so. dykem it blue to make sure its all flat. i think the bunching up of the sandpaper as joshua mentioned happens, but to different degrees depending on how hard you are pushing, how sharp the belt is, and i think it would happen much more on a spring loaded tension than a ratchet tension system. i do not own a disc sander, but i see people mention using them to flatten, which seems odd because the outside of the disc is sanding away more material than closer to the center of the disc because there is more surface travel, so i do not understand how the sanded surface could become one plane. i hope to try one someday and find out.
Click to expand...
The people I’ve seen using a disc sander to flatten blades prior to hand sanding usually have a disc with a tapered face or atleast that’s what I looked into when I was thinking about getting a disc sander, that would allow just the slightest clearance while still maintaining a flat surface.
 
Joshua Fisher said:
The people I’ve seen using a disc sander to flatten blades prior to hand sanding usually have a disc with a tapered face or atleast that’s what I looked into when I was thinking about getting a disc sander, that would allow just the slightest clearance while still maintaining a flat surface.
Click to expand...
The tapered disks will also give you the slightest concavity when grinding. It’s very slight, but it’s there.
 
john april said:
which seems odd because the outside of the disc is sanding away more material than closer to the center of the disc because there is more surface travel
Click to expand...

True the outside moves faster than the inside, but the whole surface removes metal. And that whole surface is flat.

If you took a piece of material and pushed it into the disk so that the whole face, from center to edge engaged the material at the same time, with the same pressure across the whole surface, you'd end up with an angled plane. But that plane would be FLAT.

You can vary the pressure you put on the blade as well as draw it across the face of the grinder, to get the grind you need.

Will it be perfectly flat? No. Will it be better that a belt? Probably.
 
Sooooooo Cushing, seems like they are saying that you might have fixed a minor problem...but the problem is mainly operator error:D:D:D.

When my wife is complaining about some problem she is having with her computer I tell her the problem is between the back of her chair and the screen.
Honestly, when you think of all the variables it is pretty cool that we do get things so accurate.
 
Randydb said:
Sooooooo Cushing, seems like they are saying that you might have fixed a minor problem...but the problem is mainly operator error:D:D:D.

When my wife is complaining about some problem she is having with her computer I tell her the problem is between the back of her chair and the screen.
Honestly, when you think of all the variables it is pretty cool that we do get things so accurate.
Click to expand...
Hmmm. Maybe I will just resort to files
 
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