Dime Thickness?

R

Runthemall

Joined
Jan 25, 2012
Messages
66
Is dime thickness good advice pre heat-treat? I measured one and got .05 inches. Then I see people recommending going to .02 or even less. My last knife was .05 and it was a lot of work to finish after it was hardened. What advice is best?
 
There are issues that lead to the choice of the edge thickness before ht.
1) distortion: it happens the worst the more difference between spine thickness vs edge thickness. Normalization and stress relieving help a lot avoiding distortion.
2) decarburation: without the proper athmosphere protection you need to know that the edge area exibits a lot of loss of carbon from the steel during the austenitization. PCB works well protecting carbon steels, inox foil envelopes for SS.
According to those points you have to find the best edge thickness that suits your knife and ht regimen.
 
A dime is .053" thick. That is way too thick for HT.

You used to read "file to about the thickness of a dime before HT". What I recommend is "about half the thickness of a dime". For carbon steels, you want about ,030". Stainless steels can be about .020" thick. With experience and a good HT oven, you can trim another .010" of both those.

What is as important as the thickness ( maybe more important) is that the blade be fully sanded to 400 grit and all coarse grit scratches and file marks removed.
 
Stacy can you please elaborate why people often think that SS edges could be quenched thinner than carbon?
 
When using SS you ALWAYS seal in SS foil which protects the edge from scale, decarb, etc. SS foil keeps the edge in good condition. I'd use SS foil on carbon steel if there was a way to get blade out of foil fast enough for a good quench. Foil just works good.

Ken H>
 
In a carbon blade the edge must cool 500 degrees in less than one second. A rapid and rather violent quench is the only way to do that. This creates a lot of stress that can cause serious distortion/warp. It is also exposed to the heat in an unprotected state, which creates as much as .005" of decarb to be removed post-HT.

Stainless steel is HTed while wrapped in foil or in a vacuum oven. This keeps decarb to around .001". It is air quenched most of the time. Sudden stresses and differences in time reaching Mf between the edge and spine are avoided.
In stainless you can take ten minutes to get below the nose and half an hour to reach Mf with no ill effects. Adding to that the use of quench plates to retard/diminish warp, and you can take much more metal off pre-HT. Finally, stainless can be quite tough to finish after HT ( CPM-S35VN being really hard to sand post-HT). You do as much finishing as possible and take to 400-800 grit before HT. Hopefully, all you have to do after HT is take a few thousandths off each side and sharpen.
 
It provides far less surface area for decarb, and makes post-HT clean up much simpler. Deep scratches are as much as five times harder to remove on hardened steel than annealed steel. On steels like CPM-S35VN, sanding post-HT is a real bear. My friend takes it to a mirror polish before HT. I can't recall the exact numbers, but consider that the Rockwell scale is logarithmic, not linear. The difference between 45 and 60 is not a 33% increase. It is hundreds of times as hard.

As to the surface area, I would have to do some math to be exact, but a quick mental image says that a 120 grit surface has at least twice the surface area of a 400 grit surface. This pushes decarb deeper into the steel, necessitating more surface grinding after HT.
 
Im new at knife making and I finished up with 220 grit. Im not planning on keeping it shiny I just want to leave it dark colored on the flats. Is sanding to only 220 okay fot that purpose?. Or should I still take it to 400 grit? I also grinded the scandi grind to 220 should that also go up 400 grit? Or 220 fine? Thanks
e2a4b33ea995a4c6a49d4ba4ab71e3f6.jpg


I
277f2b55a4bef270ea841a0f1efddb51.jpg
 
Last edited:
On O-1 it should be OK. The reason to go higher is to assure all deeper grinding marks are truly gone. That blade looks well sanded and prepared. After HT you should go back one grit size to start the final grinds. If you stop at 400, you drop back to 220. If you stop at 220, you drop back to 120.
 
You must log in or register to reply here.
Back
Top