Liner lock tolerance question

M

MJB206

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Mar 24, 2023
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Hello all

I'm still new to knife making - I've made about 12 fixed blades, and one friction folder. I'm thinking about making a liner lock prototype, but I have a question about spacers and tolerances - my reference is from watching quite a few YouTube videos.

Most makers I've watched try to match the width of the back spacer to the blade + spacers shim on the blades (or, I guess, thin bearings if those are used.) In some cases, the shims are .005 per. A lot of my stock is .125", so I'd be looking at .125" + .010" total thickness at the blade. I've seen standoffs to use in place of a back spacer, and those can be found at .125, but that would mean a .010 difference between the blade and the back.

My question is....how critical is a .010 difference in fit? I know I could remove stock on the blade to match the back, and frankly, after heat treat, removing decarb on 1084 I'd probably be removing that much or more anyway. I know folders need a lot more precision than fixed blades, but .010 seems negligible to my non-machinist brain. Wondering how the collective handles this.

thanks!
Michael
 
You have two options:
1) Flaten the blade tang to be .010" less than the backspring.
2) Flaten the backspring to be .010" more than the blade tang.
 
I agree; close tolerances are important for a liner lock
 
Thanks - yes, I've looked at a number of options to address - I guess I should have phrased my question slightly differently - I"m trying to understand what the functional impact is if it's off by .010?
 
It isn't as hard as one might thing to lap down a backspring/spacer. Or to lap down the tang.

Sure, a very good surface grinder is nice, but a granite surface plate and a sheet of 400 grit paper will do it in a few minutes.

After finishing the blade, the tang is almost always thinner than the backspring, so lap the backspring down to as close as possible to the thickness you calculate for the blade plus spacers. A good micrometer is needed for this level of fitting. A HF plastic unite won't work well.
 
Stacy E. Apelt - Bladesmith said:
It isn't as hard as one might thing to lap down a backspring/spacer. Or to lap down the tang.

Sure, a very good surface grinder is nice, but a granite surface plate and a sheet of 400 grit paper will do it in a few minutes.

After finishing the blade, the tang is almost always thinner than the backspring, so lap the backspring down to as close as possible to the thickness you calculate for the blade plus spacers. A good micrometer is needed for this level of fitting. A HF plastic unite won't work well.
Click to expand...
Thanks Stacey - that makes sense. I don't have a surface grinder, but I do have a granite plate, and I figured that might be the easiest way to go about it. I have a decent micrometer as well, so I think I'll be able to get it close.

One thing I've not had to worry about before is ensuring universal precision thickness on the tang for fixed blades - and - despite clamping my friction folder blade during tempering (it came out of HT dead flat) I had a tiny warp - but I suspect that may have been due to the flip part being smaller than the blade and cooling differentially. Hopefully I can mitigate that with the liner lock blade.
 
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