Belt progression for flat grind to convex edge (rotary platen or slack belt)?

C_Claycomb

C_Claycomb

Joined
Dec 11, 2000
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I have read though a lot of threads on both convex grinding and rotary platens and none have spelled out what I want to know. Maybe I am being extra boneheaded.

What grit do you grind the main bevel to before you start convex grinding? What is your belt progression and process for grinding then finishing the flat and convex portions of the bevel?

When I had my Coote two wheel grinder I would flat grind on my platen right through to 600 or 800 grit cork (+green compound), leaving my edge between 0.015 and 0.02 inch thick. Then to apply the edge I would go back to a 120grit ceramic belt and use the slack areas above and below the platen to thin the edge until I got a burr. Then I would use 220 ceramic followed by 400, 600 and 800 cork with green chrome to finish. There was often some alternating between the slack and flat platen to remove errant scratches and blend. Final cutting edge would be finished with a stone and strop by hand.

This worked well enough for the robust but low angle (22-26deg total) edges I wanted for wood cutting but I was never happy taking the 120 grit to a finished blade.

Now I have a Reeder grinder with rotary platen and want to make blades with thinner convex edges and haven’t found a process and belt progression that I like. I either put deep scratches up finished flats, wash out grind lines, or swap back and forth between platens which is much more trouble now.

thanks

Chris
 
Basically, just fine enough to get the deep scratches out. Probably somewhere between 120 and 220 grit.
 
I've got a couple knives in the works that are flat ground with convex bevel geometry and the one thing I learned for next time is that I will finish the flats to whatever final finish I want and then follow the same grit progression to the same end point when convexifying the edge bevel. The outcome I had this time is that the flats are a slightly coarser finish than the convex planes and the discrepancy isn't great looking.
 
I go to 220 for the flat grind, then use a firm F3 felt piece on a metal platen to give some convex and blend an make the entire face a convex. I start back at 80 and work my way up to the final grit to do a full convex grind. This works pretty well on kitchen knives, but reduces the spine thickness a bit since the convex works both on the edge and a little on the spine portion of the blade.

If I an doing a scandivex grind (blade bevel 1/3 to halfway up the blade face), I do the bevel grind to 220 on a glass flat platen, then clean up the entire flat portion of the blade to the final finish. Then I use a S2-32 piece of very hard felt from Mcmaster Carr and put a slight convex to the blade bevel up to 220 and work down to a near zero edge with the very hard felt. After that, I switch to a leather faced platen and clean up the plunge area and the grind line where the convex bevel and flat meet and work my way back down to the edge, rocking slightly if needed, working my way up the grits to clean everything up. I finish with a cork belt loaded with Bark River Black compound so the bevel is a bit shinier than the remainder of the blade, which I usually bring to a blue or grey non woven abrasive belt finish. If I need more convexing, I go to the F3 felt platen and then to the leather. The felt edges round over a lot, so getting into the plunges is hard with the felt and keeping the transition from bevel to blade flat is harder with the felt, so I use the leather to clean up those areas since it is very firm, but has a slight give to it.

With a rotary platen, I would do my grinding up to 220 or 320 grit on a regular flat platen first (to remove all deep scratches first) and then use the rotary platen starting at 80 grit to provide the convex surface. The rotary platen use the rubber belt, so that will help keep your plunge cuts and bevel grind lines more crisp than the felt will, so I don't need to use the leather. I also used to use the rotary platen where it has a lot of tension on the rubber belt. I had the KMG rotary platen and would often work a blade in the smallest wheel gap I could fit the blade in for the smallest convex.

I want to get another rotary platen one of these days, but I have figured out how to make the felt and leather combo work pretty well for me. A Variable speed grinding is key here; the leather and felt can burn up if the belt is running anywhere near full speed.
 
Stacy E. Apelt - Bladesmith said:
Basically, just fine enough to get the deep scratches out. Probably somewhere between 120 and 220 grit.
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Thanks Stacy E. Apelt - Bladesmith Stacy E. Apelt - Bladesmith ,
So, 120-220 on the main bevels, same again on the convex, then from 220 to finish by hand sanding?

Between EDM stones and sand paper I know I can erase (cover up) a multitude of grinding sins, but I like the look of belt finished bevels with contrasting lengthways finish on ricasso and any remaining flats at the spine.

T Taz
Thank you for that reply! I have some hard leather, hadn’t thought of making a platen backing with it. Swapping platens on the Coote wasn’t really convenient and I guess I am still operating with that mindset a bit. I am probably doing something wrong on the rotary platen since getting into the plunge at the edge is something it hasn’t done well for me so far. I have been using the medium wheel spacing most. I tried the smallest spacing on a paring knife recently and it came out a lot thinner than I am used to, good for that knife, but might have been too aggressive for my woods knives.

Lorien said:
I've got a couple knives in the works that are flat ground with convex bevel geometry and the one thing I learned for next time is that I will finish the flats to whatever final finish I want and then follow the same grit progression to the same end point when convexifying the edge bevel. The outcome I had this time is that the flats are a slightly coarser finish than the convex planes and the discrepancy isn't great looking.
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Hi Lorien,
Did you get any scratches on the flats while doing the convex? Sounds like you avoided needing to go back to the flats, which sounds pretty efficient to me. I expect that my convex edges will be a finer finish than the flats since the edges will get sharpened to at least 800grit and stropped with Flitz or similar. Over time they get sharpened to 1000 or 1200 grit and stropped.

cheers!

Chris
 
not really. I take the platen off and the belt won't really even hit the flats, it also doesn't hit the top of the knife since I hold it edge up and have fingers along the spine to keep it angled right
 
I flat grind to 120 grit. I leave appr 0.5 mm at the edge. Then switch to hard felt and convex to almost zero thickness at edge with first 120 ceramic then 220 ceramic. I typically finish with bluegreen Scotchbrite. I put on a small convex secondary bevel afterwards.
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Scaniaman said:
I flat grind to 120 grit. I leave appr 0.5 mm at the edge. Then switch to hard felt and convex to almost zero thickness at edge with first 120 ceramic then 220 ceramic. I typically finish with bluegreen Scotchbrite. I put on a small convex secondary bevel afterwards.
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It looks in those first two knives as if the convex thins the spine compared to the tang thickness. Is that the case? Does it offer a cutting benefit? They look an interesting design, sort of compact outdoor chef’s knives.

I keep thinking I should get another Scotchbrite type belt, I have one of the grey extra fine ones and sometimes use it, but it seems like the finish has to be pretty fine already and by the time I have it far enough I invariably wind up finishing with cork or hand sanding.

Lorien Lorien
Thank you for the clarification. It sounds like you are forming a steeper convex, so you have more clearance with the main bevels. Also, a bigger blade gives more finger clearance.

What folks have said makes me think I may have been trying (or hoping that there was a technique) to have my cake and eat it too. Looking forward to trying the next blade. I have a number at different stages to experiment on.
 
C_Claycomb said:
It looks in those first two knives as if the convex thins the spine compared to the tang thickness. Is that the case? Does it offer a cutting benefit? They look an interesting design, sort of compact outdoor chef’s knives.
Click to expand...

They are a little tapered yes. They are an allround profile, food prep included :)

I also have the grey Scotchbrite but rarely use it, it's too fine like you say
 
I've been enjoying structured abrasive belts. I find them to be a bit more idiot proof, and that's compatible with where I'm at
 
Lorien said:
I've been enjoying structured abrasive belts. I find them to be a bit more idiot proof, and that's compatible with where I'm at
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Structured abrasives, which have you had best fortune with? 3M Gators, Trizact, Norton Norax AO or ceramics? I haven’t had much luck with Trizact either AO or ceramic, but love the Norax ceramic structured belts. The Gator belts are nice but I wasn’t sure whether they were meant to be good for convex grinding. The stiffer backs should be better than the flexible ones.
 
I like the 347 gators for convexing since they can handle wet grinding. I found the norax I had arent waterproof, so I am looking norax that are waterproof. With the gators, get the wheel dressing stones since they glaze over quickly. This will expose fresh abrasive and they last a long time.
 
only ever used Norax
 
The Norax were working great until I went up in grits and started wet grinding them them...lol! Ruined one of the brand new belts I had. I think there is one that you can use when wet grinding, maybe U936 or something? Trizact 337 aren't good with water either (abrasive comes off in chunks like the Norax). but the 347 Trizact are good in water.
 
Yes, Norton U936 ceramic work fine with water. I really like how they cut on hard stainless. Prior to finding the U936 I used U254 AO Norax which were nice when new but even with dressing the seemed to dull while there was lots of grit still. Maybe the fault of too much pressure against a platen. For convex grinding I have not found that the stay flat, tend to bow when not supported.
Just got some 3M Gator 347FCs in a range of grits, looking forward to trying those too. Will be nice not to have to wipe the water off after dipping.

Since trying the U936s and 337DC (dry) Gators my collection of AO U254 have not seen much use. I expect the purchase of the 347FCs will relegate them to a rare fine finishing job at most.
 
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