Relationship of spring thickness to blade thickness

[coffeecup]

Plas62 was trying to source replacement blades for two knives he's working on, and asked a question:

What is the maximum allowable difference in thickness between the blade tang and spring thickness?

I'd like to re-frame that question and ask, "In a new knife, what should the difference be between the blade tang thickness and the spring thickness?

I've torn apart enough worn-out knives to know that sometimes blade tangs wear a bit thin, and that this wear seems to contribute to the side-to-side play seen in many well-worn knives. But I've also seen new knives with excessive side-to-side play, and have to wonder how much of that play is due to the thinness of the tang of the knife? In one case--a new knife that had more than 1/4" of side-to-side play at the tip--the tang was .010" thinner than the spring. (And before someone asks, that knife was taken apart to measure, and measured with a Starrett 0-1" micrometer.)

In my mind, I marked that one down to general poor QC at the factory, but was it? Was that difference something that would be considered "acceptable?"

I'm hoping to hear not only from folks who repair knives, but from some of our members who work in the cutlery industry.

Thanks,
Jim

[Iron Hoarder]

The blade should really only be about .002 thinner than the spring. That allows .001 on either side of the blade so it doesn't bind on the liners. Pin diameter and fit is important as well. There should only be about .001 or less difference between hole size and pin diameter. I would say .0005 would be ideal. Just enough gap to rotate.

[ea42]

I agree with Iron Hoarder as far as spring to blade thickness comparison, you could possibly go to .005 difference max but when you get above that you lose the liners' ability to hold the blade tightly. all that's really in contact with the tang is the top front edge. It wouldn't take much twisting or torqueing to loosen the blade.

I'm not sure about that hole size ratio though. If you're referring to the blade hole I think the hole should be .005 to .008 larger than the pin. The reason for this is that when you hammer the pin it tends to swell up a bit, and if the hole is almost the same size as the pin you'll wind up binding the blade tang on the pin. This will make it more difficult for the blade to pivot freely, plus you run the risk of actually rotating the pin in the bolster when you try to move the blade. This is especially important with lockbacks where you have far less spring tension. You can lessen this effect by making the bolster holes a near match to the pin, but in my experience you still get some swelling regardless.

Eric

[Elvis]

1/1000th thinner than the backspring is the tang thickness I try to shoot for. I used that method even before I found out that Tony Bose does the same thing. I think I picked it up after measuring a lot of Case knives I'd worked on.

[coffeecup]

Thanks for the responses, guys!

I'll mark that knife down to poor QC at the factory, and another example of why I avoid current-production knives.