Help with Slipjoint designing

autogateman

autogateman

Rod Braud
Joined
Jan 6, 2017
Messages
180
Are there any books or web articles that talk about actually designing a slipjoint? I have two books on slippies from Culver and Robinson, but neither go in to detail about the ins and outs of tang size, offset pivots etc. I don’t want to just copy someone else’s designs on knives, I would like the ability to take a blade shape and draw up a slipjoint myself. I am having problems with getting the correct angle on the blade in the closed position and not hitting the center pin by the time I get it tucked in to the handle. A larger spring can move it up, but at the expense of the exposed run up on the blade. Just looking to see if there are any ‘rules of thumb’ out there concerning this.

Thanks in advance

Rod
 
Steve Culver has a article on his website or he might sell them as a hand out now that talks about tang to spring ratio and about designing that I followed.
 
Yes that’s the book I have. It only addresses the size of the tang. Two to one tang to spring ratio.
 
I think autogateman and i started out around the same time. I too struggle a bit with the geometry. It used to drive me crazy. I became completely obsessed to be honest.

Knife designs like the lambsfoot, teardrops, serpentines, swell backs, canoes etc arent the way they are just because they look good, it is my opinion they are designed to get around that dang center pin.

I think a very important part of the spring/tang geometry is the geometry at the other end of the spring. Patterns like lambsfoot have the end of spring opposite the pivot bend backwards, which allows a center pin with little or no hump to get over. Same with teardrops, serpentine, etc.

Swell backs are just cheating in my opinion haha.

A lot of blade profiles have the tip of the blade angle down a tad, especially on clip points. This is to keep the tip low so it sits in the handle, but also the mid portion of the blade has more clearance from the center hump.

A lot of straight back knife handles often have the pin off center, and that helps a great deal avoiding the center pin.

A lot of spring ends have what i believe is called a shelf, the end is fatter than the main leg of the spring. For this to work without excessive run up, you need a pattern like a canoe where the very end of the end of the spring can be made small (matching or even smaller than the main leg of the spring) but there is enough shelf so that a tang with a minimal kick can hold the blade at a good angle.

I have no earthly idea how guys make congress pattern, it is the one pattern i have been putting off.

I make a wharncliffe knife that has a saddlehorn type handle. I cut two springs for it at once by diagonally cutting a rectangluar piece of steel. The spring end has a shelf, and the kick comes off the shelf and hits the leg of the spring. The blade edge runs parallel to the diagonal line essentially, and so is basically guaranteed to avoid the center pin. The blade profile makes the whole knife look evenish from one end to the other.

I find the geometry to be what keeps my interest the most. It is so difficult to get right. I have stared at blades and springs for hours thinking about how to profile them.
 
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A lot can be avoided by putting the center pin closer to the edge, but it looks bad if that section of handle scale is beveled/curved. I probably have twenty springs and a hand full of blades I’ve scrapped from grinding away too much. I’ll start off with a .40” tang square and by the time I’ve got it dialed in that sucker is .24” lol.
 
With a serpentine handle like this
iipsrv.fcgi

I would grind the center top of the spring at an angle, sloping up and to the right. Notice also how the right end of the spring is not exactly horizontal, that allows for almost no hump in the center. Also, you can see how the main clip point tip angles down a bit. That lets it sit down in the handle tighter, and avoids the very ugly clip point blade profile with parallel spine and edge.
 
This is one of the complexities of slipjoint design. There are no golden rules here, lots of opinions, and mostly, it comes from experience. Start with a known functional pattern, and then make minor adjustments until you start understanding what works for you and your style.

Many designs do necessitate a non-center pivot, that's one of the advantages of bolster and shadow bushing construction, it'll allow you to hide this off-center aspect, however, many designs can incorporate the pin centered in the bolster area, if you compensate with spring and tang geometry, and proper pre-load. You can have a much thinner spring than many use if you understand how to properly load a spring to the design as opposed to having a "one size fits all" mentality.


Here's an example of a centered pin, on a non-bushed, bolsterless design, it's got a pretty short (height) spring by most standards, but it has a snappy action with good walk and talk, due to plenty of spring length between the front pin and the tang, and heavy pre-load, with proper HT.

IMAG0224.jpg
 
I do want to add though, IMO one of the best resources on slipjoint building is How to Make Multi-Blade Folding Knives, by Eugene Shadley and Terry Davis. However, it's out of print, and doesn't really go much into design aspects.

The thing is, if you look at all the vastly different configurations, even just among traditional slipjoint styles, you'll see that, there simply are NO RULES, so any pontification about "how it should be done" would be simply stroking the writers ego, in an attempt to entrench their opinions into dogma, and ultimately, a disservice to the art.
 
javand said:
This is one of the complexities of slipjoint design. There are no golden rules here, lots of opinions, and mostly, it comes from experience. Start with a known functional pattern, and then make minor adjustments until you start understanding what works for you and your style.

Many designs do necessitate a non-center pivot, that's one of the advantages of bolster and shadow bushing construction, it'll allow you to hide this off-center aspect, however, many designs can incorporate the pin centered in the bolster area, if you compensate with spring and tang geometry, and proper pre-load. You can have a much thinner spring than many use if you understand how to properly load a spring to the design as opposed to having a "one size fits all" mentality.


Here's an example of a centered pin, on a non-bushed, bolsterless design, it's got a pretty short (height) spring by most standards, but it has a snappy action with good walk and talk, due to plenty of spring length between the front pin and the tang, and heavy pre-load, with proper HT.

IMAG0224.jpg
Click to expand...
Thanks so much for the explanation, even if it’s not was I was hoping to hear haha. I get confused to when I should be setting the pin off center, or swelling the center up, or bringing the end of the handle sloping down. That is a beautiful knife! I would love to see pics of that before it was assembled to see the tang/spring relationship.
 
autogateman said:
I don’t want to just copy someone else’s designs on knives, I would like the ability to take a blade shape and draw up a slipjoint myself.
Click to expand...

no advice other than what has already been mentioned,
I just want to add that IMO what you are asking is very difficult to do, and to do well. - "...take a blade shape and draw up a slipjoint myself"

I learned slip joints in a class where I did bring in buy own design and fortunately the instructor had immense experience and skill, and in a day and half we made it happen. I'm no expert and frankly I think I would need to make 10 in a row before I had some confidence in doing it well.

I see you don't want to copy someone else's designs etc but I think it's an efficient way for you to learn and gain alot of ground and you. might reconsider that.

if you would like my pattern to play around with, I'd be happy to share with you, but I don't guarantee it works :-)
 
HSC /// said:
no advice other than what has already been mentioned,
I just want to add that IMO what you are asking is very difficult to do, and to do well. - "...take a blade shape and draw up a slipjoint myself"
...
Click to expand...

I have a propensity for doing things backwards, but i always decide on and profile the blade after i have already fitted the tang to the spring. I will take small trim cuts off the blade blank as needed while fitting, but i basically make profiling the blade come late to the design. I like to profile the blade and handle at the same time.
 
HSC /// said:
no advice other than what has already been mentioned,
I just want to add that IMO what you are asking is very difficult to do, and to do well. - "...take a blade shape and draw up a slipjoint myself"

I learned slip joints in a class where I did bring in buy own design and fortunately the instructor had immense experience and skill, and in a day and half we made it happen. I'm no expert and frankly I think I would need to make 10 in a row before I had some confidence in doing it well.

I see you don't want to copy someone else's designs etc but I think it's an efficient way for you to learn and gain alot of ground and you. might reconsider that.

if you would like my pattern to play around with, I'd be happy to share with you, but I don't guarantee it works :)
Click to expand...
Yes I’d love to see your patterns!
 
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