Slipjoint tuning - am I too picky?

[coffeecup]

For all that feel like leaving the knife open in the most "backspring stretching" position for a few days helps; it is good to hear. But if you had some scientific way to measure it, I am afraid we would be disappointed at the true result. I'm not going to go as far as some and say there is no way; but I know every factory I have ever asked the same question laughed at me.

I do have a device to measure the amount of force required at the nail nick to open a knife; it is one of the things I made after getting bashed for being "too picky" a couple years ago. I played around with measuring the effect of various methods touted to reduce perceived spring tension on slipjoints and lockbacks.

Cutting to the chase, in most cases there seems to be little actual reduction in the amount of force needed to open a knife when left in the "backspring stretching" position for any reasonable amount of time. Repeated cycling of the knife did seem to have some slight effect on pull, but the sample size was exceedingly small. Note the weasel-words there: "in most cases," "any reasonable amount of time," "slight effect," and "sample size was exceedingly small."

In a few cases "backspring stretching" appeared to cause some reduction; these knives all began with low amounts of force required to open them (10# or less), and the amount of force needed was inconsistent over the range of motion of the tang as the blade was opened. I'm inclined to think the springs weren't very good to begin with. Other than those instances, and one spring that broke (the spring was probably too hard) there was no significant reduction noted.

I really don't know what would happen if you left a knife half-open for months or years; I used periods of up to two weeks. I just didn't have the time or interest to extend the time period longer--I would have forgotten about it before I got back to check.

I tested 5 knives using the "open and close it a thousand times" method. (Why only 5? Do you want to sit there and go through more open-and-close cycles than that?) Average reduction in pull was slightly less than 5%. Perceived speed/"smoothness" of opening was increased, but I have no way of measuring this.

 

[ea42]

You really don't have to go taking a knife apart in order to weaken a spring. As I've stated before the area of the backspring right over and slightly forward of the center pin can be taken down to ease tension. You'll need a belt grinder or something similar for the task, but it takes very little material removal to make a difference. Mind you this is only good on a single blade knife unless all blades on that particular end are heavily sprung. There's also another way, kind of like the above mentioned three point method, but done with the knife together. You can take a backspring (oe something similar of about equal width) and wedge it between the tang and the offending backspring with the knife about half open. Now place the partially opened knife face down against something solid and press down, pushing the blade back against the wedge. This will force the backspring to flex. Don't go crazy cause you don't want to snap the spring, just one or two flexes pushing the spring about a sixteenth to and eight inch beyond where it normally rests against the tang can make a difference. I've added a couple of pictures below to show the process:

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Eric