platen troubles...again

Kumdo

Kumdo

Joined
Mar 7, 2000
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Hello all. I recently bought a new platen plate for my KMG, and also one of those pyroceramic (?) platen liners from Darren Ellis. I was having a hard time with flat grinds because the old one was was bowed out on the sides. So I epoxied my new liner to my new plate and fired up the grinder, and lo-and behold, it's like nothing changed. It's still not flat... convex in the center, bowed on the edges!

What is going on? Is there ANYTHING I can do to get a flat platen? I mean perfectly new flat platen plate (I assume) + perfectly flat liner (I assume) = bowed platen. Am I going nuts or what?

Thanks in advance....
 
I am not sure what the problem must be, but have you tried to press a piece of steel flat aginst the platten with the belt running. That was long. that I am trying to say is to surface a piece of steel aginst the platten. Mark it with Dychem or paint to show exactly where it is cutting. Also take the plate off and check with a straight edge. If it is indeed bowed, someone with a mill can take care of that.
$.02
 
Is the epoxy an extremely smooth even layer on the entire ceram surface so that it can't flex? I've seen some fotos of others' liners where it's just stuck on with 3 dabs of whatever adhesive; not a good idea for flatness.

Also, the "bow" and "convex" you are speaking of, is this how it grinds or is this how it looks with a machinists square checking the straightness?
 
John, heat the platen to release the bond, then flatten the old platen on a 9" disk. then round off the leading, and trailing edges(top, and bottom) of the pyroceram on a 220 grit, followed by a 400 grit belt. That will ease the belt on, and off the platen, then epoxy it like Mike Fitzgerald mentions above. An even layer of epoxy.

Keep in mind though, that when you have a moving belt going over a stationary platen, your grind is never going to be exactly flat. Even slight twitches of the hand while grinding can cause a small convex.
But it will be far flatter than with the metal platen itself, and if you want perfection, get the rubbing block out and hand sand to finish, after using the platen to get almost there.

Edited to add; Make sure that the platen with pyroceram liner stand out above the normal line of the belt. In other words that the belt where it goes over the platen sticks out a little bit. That will keep it flatter. You don't want any slack at all in that area.
 
sure the belts aren't bowing? it happens.....that's why Mike's advice to knock the corners/edges off is a good one.
 
Hi John,

I'm not sure what to add to all of the good suggestions the others have made...I would check those things out. You had me worried, so I just went and took a random sample of 25 liners off the shelf and checked them all with a machinists straight edge and they were all dead flat...well, as flat as the naked eye can tell... If you determine that there is a problem with the liner I sent you, just let me know and I will be happy to replace it free of charge!

:)

-Darren
 
Something else to watch out for is that if you are pushing the blade into the platen with force on the point and on the handle area, the blade will bow. It is especially easy to do with thin stock. Make sure the stock is flat. It will cause the grind to kind of miss the center of the blade and take a lot off the tip and ricasso area. I hope that make sense.
 
Hey Fitzo, I'm the wacko that uses pressure tape. But the tape is behind the glass - not on top! How could that make the platen un-flat?

Kumdo - are you saying the belt is bowed in the center or the platen is actually bowed? You must be talking about the belt, not the glass. If you push the belt in the middle (with the machine off!) is there some give?

Some medium weight belts do that. They are not stiff enough to keep flat and not flexible enough to recover from the wheels on each end of the platen. Like suggested you can move the platen out a little further from the wheels and you can increase the tension on the belt. But some belts just do that.

Steve
 
Knife makers are COOL. :) Thanks for the help...

Yesterday evening I was having the fore-mentioned problem, and got frustrated and "closed up shop" for the day. Today I will try to determine if the platen is in fact warped, or if it's just the belt doing a funny little dance. I'm pretty sure that it's the platen and/or liner because I think that I could see it with the motor stopped (looking down on the platen).

Fitzo, the entire platen surface is covered in epoxy, so there are no gaps that can give.

Lots of great suggestions here. I'm going to try and determine a few things and implement these suggestions as need. I will, of course, let you all know what I find out...

Thanks again!
 
Steve, ok, so you're a wacko, but that's ok, so am I.... :D

My point was that, despite the ceram having very low elasticity, having it unevenly supported against the platen by having "airgaps" so to speak will certainly give it a greater chance to flex than if it was evenly supported by a smooth, continuous layer of adhesive. Pressing HARD against the glass could cause some SMALL dishing, I'd think, if it was only glued top and bottom. Consider an old windowpane. Press at the edge and it doesn't move in the frame, press in the middle and it'll flex. This very effect is essentially the appeal of the KMG rotary platen.

Well, Kumdo just added in on this, so we will have to await his further investigations. Kumdo, check it with a square, and let us know. Thanks.
 
Ahhhh yes Mike I see what your saying. I'd have to agree - YOU ARE A WACKO. ;) ;)

hahahah - I see what you mean. Now I'm wondering if we are talking about bowing from top to bottom or left to right?

You gotta love this on-line crap with a bunch of inarticulate metal dust sniffers.

Even while I don't understand the question I reserve the right to defend my answers to the death.

Steve
 
Generally I've found that when Pyro-Ceram flexes it also cracks :( Three times Iv'e found this out :rolleyes:

Jerry
 
Nut Jobs!! Every darn one of you :D

let us know what the straight edge told you about the platen......
 
J. Shorter said:
Generally I've found that when Pyro-Ceram flexes it also cracks :( Three times Iv'e found this out :rolleyes:

Jerry
Click to expand...

Been there, done that, experiencing it currently.
 
Okay, party people, I checked out my platen. It's flat as a...a very flat thing. Aparently it's the edges of the belt that are flaring outward, mainly when it's spinning, but even a bit when still. Is that possible? I wonder if thin belts distort over time- I am waiting for a new order belts, but my last 120 grint (which I'm using now) is old and too worn to be using.

Anyway, heading Mike's sage advice, I pushed the platen out a bit more and that did help quite a bit. It is still doing it a little, but much less now. So here's my dilema; do I round off the edges of the platen liner, like Mike suggested? I hate to do that because I enjoy having a fairly square eges for my plunge cuts. I don't know how I can get crisp shoulders if I have radiused edges on my platen.
 
I wouldnt "Round" it over but just make sure that the edge doent flair up a little. That would make the belt dig in. I would think that you would see if the sides had some kind of "rise" to them.....??? Dont round it over alot but just a tad with a hand held sanding block.
 
Kumdo,Do the radius on the leading and trailing edges {the edges in line with the belt} It will run smoother and the belts will last longer..
 
Sando said:
Ahhhh yes Mike I see what your saying. I'd have to agree - YOU ARE A WACKO. ;) ;)
hahahah - I see what you mean.
Steve
Click to expand...

You actually understood me, eh? Now you got me worried about ya.... LOL! :D


And, BTW, John, when you can see the backing fabric from the grit side of the belts, ya just GOTTA throw that one away!! :eek: :)
 
Kumdo said:
Anyway, heading Mike's sage advice, I pushed the platen out a bit more and that did help quite a bit. It is still doing it a little, but much less now. So here's my dilema; do I round off the edges of the platen liner, like Mike suggested? I hate to do that because I enjoy having a fairly square eges for my plunge cuts. I don't know how I can get crisp shoulders if I have radiused edges on my platen.
Click to expand...
John, if you're referring to my reply, DON"T round the edges. I was referring to the top, and bottom, where the belt come on to, and leaves the pyroceram, NOT the sides. All you have to do is round the corners there so it causes less drag, and wear on the inside of the belt. It can also help to keep it flowing flat along the surface better.
 
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