Grit size pre ht

Alden Cole

Alden Cole

Well-Known Member
I've been grinding some knives pre ht, specifically grinding a shallow trough in the handle with a wheel to reduce weight and increase epoxy adhesion. My question is whether it is ok to leave this trough at 50 grit (which is what I ground it at), or whether I should take it to 120 to reduce chances of bad things happening. This is D2 and is being air/plate quenched, so it isn't like I'll be water quenching or anything. Thanks!
 
just reread your post. You said the trough. - I think any grit would be ok there.
Just to be clear are you saying the entire blade is roughed ground to 50 or just the fuller in the tang?
I typically grind my blades to 120 grit prior to heat treat. But that’s my process, others may do it differently.
 
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I've heard folks warn against quenching a blade with low grit grinding scratches, citing cracks, but I've never had this happen. The reason I go to higher grits pre-HT is because I reason that it's easier to sand/grind a softer steel than a harder one.
Alden Cole said:
shallow trough in the handle with a wheel to reduce weight and increase epoxy adhesion
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FYI, lightening the tang and making fit up easier are the only 'real' reasons to do this. Having a bigger space for more epoxy to collect doesn't accomplish this. The way to increase epoxy 'adhesion' is to increase the amount of undercuts in your scratch pattern. So coarse grits and circular scratch patterns are what we want here. Drilling holes to simply to create "bridges" of epoxy does not increase the strength of the tool or bond.

Well, I guess hollowing out the tang will increase the surface area slightly compared to a flat surface, which would increase adhesion...
 
billyO said:
he way to increase epoxy 'adhesion' is to increase the amount of undercuts in your scratch pattern. So coarse grits and circular scratch patterns are what we want here.
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absolutely correct. I believe one of the epoxy manufacturers recommend the low grit, circular pattern is best, too.
 
Dan Johnson, longtime rep for Bader, advocated for stopping at 60 grit. He maintained that the "peaks" of the individual abrasive lines didn't harden and would grind off more readily post HT starting with a 60u 3M mylar belt. I tried, didn't notice a huge benefit.
 
Thanks guys, that is what I was hoping to hear. Will leave tang at 50. Rest of blade is ground to 120 or 220.
 
I hear a lot of cement-adhesion reasons for grit levels, but not much in the way of metallurgical/material science in the choices. I would rather have a superior blade via limited stress risers than a superior handle bond via epoxy. One can always put a new handle on a good blade, but a cracked blade with a new handle is a waste of both. Go to the appropriate grit the for the alloys heat treatment and worry about the handle adhesion after the blade is good.
 
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Kevin, (or any one else who knows) please explain what are "stress risers" to me . I'm certain that I'm the only person here who doesn't know!
 
billyO said:
Drilling holes to simply to create "bridges" of epoxy does not increase the strength of the tool or bond
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I agree if there is no companion recess hole in the scale. But by drilling matching shallow holes in the scale (that line up with holes in the tang), it would seem that the pocket of bridged epoxy should increase the shear strength of scale.
 
MTBob said:
should increase the shear strength of scale.
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Perhaps.... but wouldn't this be true only when the strength of the cured epoxy is greater than the strength of the scale?
Everything is only as strong as it's weakest part.
I want to say that I read somewhere that one of the main reasons these 'bridges' aren't as strong as we'd like to think is because larger deposits of epoxy also most likely have a lot of tiny bubbles ("..in the wine....") that end up weakening the bridge, whereas epoxy applied in a thin layer on the surface has less of these bubbles, and thus a more solid/stronger epoxy joint.
 
Bruce McLeish said:
Kevin, (or any one else who knows) please explain what are "stress risers" to me . I'm certain that I'm the only person here who doesn't know!
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A stress riser is a spot/place/area that will undergo more stress than the areas surrounding the spot. When a maker breaks the spine with the plunge, for example, they have created a stress riser. If the knife breaks, it will probably break at that point. Basically, the maker has created a weak spot... he raised the level of stress that the point will have to endure. He created a stress riser.
 
Bruce McLeish said:
Kevin, (or any one else who knows) please explain what are "stress risers" to me . I'm certain that I'm the only person here who doesn't know!
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Most transformations in steel during heat treatments are driven by levels of stored energy within the steel, where there are greater levels of potential energy, there will be greater, and perhaps disproportionate, levels of transformation. This means certain surface features will result in points of higher strain during hardening. One very good example is cutting very sharp 90° corners for the guard shoulders, I have seen blades simply let go of their tangs from such shapes. But I have also seen quench cracks initiate from course grinder scratches, as not only are these very sharp lines that are very conducive to stress concentrations, they also will have a level of deformation at the bottom of the scratch that will multiply the rate of austenitic transformation and then again with the martensitic transformation.

When the blade expands during hardening, and approaches maximum HRC, that coarse scratch is like scoring glass and then giving it a tap. I personally would never go less than 120X before heat treatment to avoid this issue, and advise all my students to go to 220X.
 
Its all right. A good friend, and really good knifemaker, Kevin Casey, who used to live here in Michigan, is yet another "Kevin."
I am always apologizing to Kevin Casey, because of the number of times knife publications give me credit for his work.
 
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