Double disc grinding and roughness averages (Ra)

REK Knives

REK Knives

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Knifemaker / Craftsman / Service Provider
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Gonna have 20 folder blades dd ground after ht. Most dd grinding services seem to be around 30 ra (which I hear is pretty coarse) and it seems to get finer than this you need to switch to lapping. Can anyone tell me what grit (roughly) 30 ra, 15 ra, and 5ra would be from experience? Ideally I want to be able to get these from grinding and lap them w/ a single grit - 600 grit on a granite plate w/ out having to do multiple grits/steps.

From charts that I look up online it looks like 30 = 150 grit, 15 = 240 grit, and 5 = 500-600 grit. But I guess I'm just looking for any personal experience.
 
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I know that in a machine shop getting a 32 ra finish it's a pretty decent finish, but the double disc ground parts I've seen were pretty crummy looking and would need additional finishing before use on a knife.
 
Thanks Nathan, yeah I think I'm going to have to have them lapped to a finer finish.
 
What kind of grinder? A couple of years back, I talked to Greg Larkin and IIRC, he said that with "normal" steels, he was getting closer to that 5 ra finish with the Berger flat grinders. Is that different than what you are talking about?
 
jdm61 said:
What kind of grinder? A couple of years back, I talked to Greg Larkin and IIRC, he said that with "normal" steels, he was getting closer to that 5 ra finish with the Berger flat grinders. Is that different than what you are talking about?
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I don't know what type of grinder they use, my understanding is that it grinds both sides simultaneously so you don't have any stress build up from grinding one side at a time, and it's extremely precise, parallel, and flat. I did contact larkin but they only take it down to 30 ra or so and then tumble.
 
My understanding of double disc grinders is they grind a hollow grind. That will not lap smooth because it is not a flat surface.
 
I had a work project that put me in contact with these folks: https://arrowgrinding.com/capabilities/grinding-services/double-disc-grinding/

We ended up not using them (not doing a double disc grind), but they came highly recommended by a partner manufacturer I work with. Unfortunately, I don't have first hand experience with the surface finish, so can't comment on the roughness.

We were flat grinding plates of D2, on a blanchard with a magnetic chuck. The magnet would pull the warp right out of the plates, so after it was ground & the magnet released, any warp would spring right back. Massive PIA.

For those not familiar, double disc grinders work a lot like a centerless (cylindrical) grinder, and there is no added stress from a magnetic chuck of a typical surface grinder. 2 large grinding wheels remove material from both sides at the same time, producing surfaces completely flat and parallel. The downside is that you need to leave adequate grind stock - at least 0.020/0.030''.
 
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