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Slip joint
- Thread starter jrhart50
- Start date
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- Mar 28, 2007
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- 1,171
lots of practice and a bucket full of mistakes. Seriously most of it in my case was trial and error.
- Joined
- Aug 17, 2009
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Yep, what he said. One thing you can do is go to the flea market and pick up a cheap knife with a pattern you like. Take it apart and use it for a pattern by laying it on your steel and scribing.
That is also a good way to study slipjoints. Everytime I find one for cheap I get it and disassemble it. Helps me to understand how the relationship of the tang and spring, which is the heart of the knife.
Agreed with Sean -- get some old slippies (the older the better) that are still snappy in patterns you like and take them apart for patterns -- you only need the intact spring and tangs really -- so it doesn't matter if they have worn out or broken blades - just don't get ones with broken springs or no snap. But before you take them apart study how the mechanism works, how far do things move while the blade cycles, exactly where in the cycle does the snap to closed, to half-stop, to open happen,.... The totally deceptive thing about slippies is that they're so simple, until you try to actually make one. Everything depends on and is effected by everything else.
A few questions and a few rules of thumb: How hard are you tempering your springs? You need to shoot for about 48HRC, give or take a point or so -- but definitely no harder than 50HRC (don't do any fancy messing about with only hardening a spring so far back - spring temper the whole thing - especially temper it well past the center pin hole). How much does your spring rise and fall as the tang is cycled opened and closed? Typically you shoot for maximum rise of about half the spring depth under the tang -- the depth of the spring under the tang should generally be about 1x - 1.5x the spring stock thickness. How much pre-tensioning are you giving the spring? Pre-tensioning of about 1/32" or a little bit more is typically good. Where is your center pin? It should be no closer than the center point of the knife -- preferably further back than that on jacks - between 1/2 and 2/3 of the distance to the butt typically - the spring needs space to act and feel like a proper spring (and it much reduces the possibility of fatigue and breakage over time).
A few questions and a few rules of thumb: How hard are you tempering your springs? You need to shoot for about 48HRC, give or take a point or so -- but definitely no harder than 50HRC (don't do any fancy messing about with only hardening a spring so far back - spring temper the whole thing - especially temper it well past the center pin hole). How much does your spring rise and fall as the tang is cycled opened and closed? Typically you shoot for maximum rise of about half the spring depth under the tang -- the depth of the spring under the tang should generally be about 1x - 1.5x the spring stock thickness. How much pre-tensioning are you giving the spring? Pre-tensioning of about 1/32" or a little bit more is typically good. Where is your center pin? It should be no closer than the center point of the knife -- preferably further back than that on jacks - between 1/2 and 2/3 of the distance to the butt typically - the spring needs space to act and feel like a proper spring (and it much reduces the possibility of fatigue and breakage over time).
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It's tricky for sure. I made probably five or more before I figured it all out. My problem at first was my tang was too short and fat. I was also rounding it out too much which made for smooth opening and closing but made it not snap as well. Another mistake was I tried to compensate for the rounder tang by putting more pressure on the spring. This works ok but after about 100 openings it starts to get rough. Once I found the correct tang design I made a pattern on some formica and just redesigned the blade for each knife. I've had a lot of headaches over those little guys. Good luck.
Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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Several thing are involved in getting a good fit on a folder, regardless of the style or lock type. A slippie is the simplest lock, any devices to lock the blade open or closed will just require more fitting, but the same methods apply.
First, don't make the parts to final size in one fell swoop...at least not until you have made that style many times and have a good pattern.
Make a set-up jig on a piece of scrap steel sheet ( 1/4-1/2" thick). Drill the holes for the pins in this plate. When cutting out, filing, and doing the fitting.....place the parts on the jig, pin them in place, and test the fit and action. When everything is almost right - quit. The final fitting is done after HT. Mark the jig with style name/number and keep it for future knives. You can even drill it for several folder styles as long as you keep track of which holes are which.
As said in the earlier posts,
Leave enough tang to allow a firm and snappy open/close. Remove the excess a little at a time.
Don't try and use too much spring movement to make this happen....do it by having all the tolerances and radius of the tang dead right. If the rotation lifts the spring just the right amount, and the tang has the same amount of metal on both sides of the pivot...it will snap open and closed.
The back spring should not be sitting completely flat on the spine tang. It should be only contacting the blade spine notch and the heel of the tang with a slight angled opening between the two. As you fit the knife, the blade should open with a small angle left un-opened ( not quite fully open). Leave it this way until you are done with everything . You will slowly remove metal from the end of the backspring and/or the spine notch until the blade opens straight out. This will be the last fitting thing done before final assembly. On your first few slippies, leave this fit at about 175 degrees of full open and work it in after assembly. You will be amazed at how it wears in after 50-100 open/close cycles.
On the blade edge side, the tang surface should have a slightly hollow curved shape, allowing the blade to rest on the spring in such a position at the two points touching the spring place the blade edge just slightly above the spring ( the edge should not be sitting on the spring). Again, leaving extra metal in this area, and doing the fine adjustments and fitting last will make a clean action.
After HT sand the parts to just shy of a perfect fit,then polish the mating metal surfaces. Reduction of friction is a must for a walk-and-talk action. Don't expect oil to do it for you - smoothness is the only way it will work. Mating surfaces should be dead flat, not rounded by sanding/polishing. If there is any roundness, they will rub on the tangential contact points, and will wear to a grindy action after a few hundred open/closings.
Small flat stones, such as EDM stones, are great for getting the surfaces flat and smooth. You don't want any round edges in the lock. Cutting your abrasive paper into strips about 1/2" wide, and backing it with a corresponding piece of precision ground steel, will work well for fitting the joint.
Stacy
First, don't make the parts to final size in one fell swoop...at least not until you have made that style many times and have a good pattern.
Make a set-up jig on a piece of scrap steel sheet ( 1/4-1/2" thick). Drill the holes for the pins in this plate. When cutting out, filing, and doing the fitting.....place the parts on the jig, pin them in place, and test the fit and action. When everything is almost right - quit. The final fitting is done after HT. Mark the jig with style name/number and keep it for future knives. You can even drill it for several folder styles as long as you keep track of which holes are which.
As said in the earlier posts,
Leave enough tang to allow a firm and snappy open/close. Remove the excess a little at a time.
Don't try and use too much spring movement to make this happen....do it by having all the tolerances and radius of the tang dead right. If the rotation lifts the spring just the right amount, and the tang has the same amount of metal on both sides of the pivot...it will snap open and closed.
The back spring should not be sitting completely flat on the spine tang. It should be only contacting the blade spine notch and the heel of the tang with a slight angled opening between the two. As you fit the knife, the blade should open with a small angle left un-opened ( not quite fully open). Leave it this way until you are done with everything . You will slowly remove metal from the end of the backspring and/or the spine notch until the blade opens straight out. This will be the last fitting thing done before final assembly. On your first few slippies, leave this fit at about 175 degrees of full open and work it in after assembly. You will be amazed at how it wears in after 50-100 open/close cycles.
On the blade edge side, the tang surface should have a slightly hollow curved shape, allowing the blade to rest on the spring in such a position at the two points touching the spring place the blade edge just slightly above the spring ( the edge should not be sitting on the spring). Again, leaving extra metal in this area, and doing the fine adjustments and fitting last will make a clean action.
After HT sand the parts to just shy of a perfect fit,then polish the mating metal surfaces. Reduction of friction is a must for a walk-and-talk action. Don't expect oil to do it for you - smoothness is the only way it will work. Mating surfaces should be dead flat, not rounded by sanding/polishing. If there is any roundness, they will rub on the tangential contact points, and will wear to a grindy action after a few hundred open/closings.
Small flat stones, such as EDM stones, are great for getting the surfaces flat and smooth. You don't want any round edges in the lock. Cutting your abrasive paper into strips about 1/2" wide, and backing it with a corresponding piece of precision ground steel, will work well for fitting the joint.
Stacy
I have now built several hundred slipjoints and my pile of junk occassionally grows in heighth....albeit...very slowly now. T. Bose, for example says that the contact point between spring and blade should be ground or sanded with no finer than 400 grit as any higher grit will not hold oil...which, in my opinion, is necessary. A horizontal grinder is almost a necessity or a setup on a vert. grinder so that you may grind perfectly 90 degrees to blade and spring faces. As the previous poster mentioned...the spring must only contact the blade in all three positions...closed, halfstop, and open..on two spots. I always had problems getting good talk in halfstop but finally ground a very small hollow so that spring only touches both corners. You cannot leave the tang corners sharp..they must have a very small round on them. If not round..they will dig into the spring a bit and create a terrible grinding sound and feel. Remember your blade is ...say...R60 and your spring is...say...R 48. The harder blade corners will dig into the spring. When first building the slipjoint try positioning the blade pivot a bit farther back in the handle...about 1/16" so that in closed position the spring is longer than end of blade by a good 1/16" As you progress you may try to even these up a bit. This helped me a lot. I do position the spring pivot pin farther back than 50% also...it allows the spring to work..well..as a spring easier than if it were closer to blade pivot. It takes many, many attempts on a slipjoint before you have made all the mistakes possible. They are actually endless...unfortunately. As they say...there are 50 ways to loose your lover...but a thousand ways to screw up a slipjoint. Good luck.
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- Mar 28, 2007
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This is a good point John. polished ends are kinda a no no. the steel will gall like no tomorrow. there has to be something to hold the oil and break the surface tension between the two pieces of steel. Ive got another bucket of ruined springs because of this, took me a while to figure out what it was.
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- Dec 8, 2005
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Chris Crawford has a 4.5 hour set of dvd's for 50.00 that will put you on the right track with slipjoints. Just google Chris and go to his site for all the info.
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- Jun 26, 2005
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SCOTHBRITE THE CORNERS. nEVER PUT IT TOGETHER WITHOUT OIL.
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- Dec 23, 2008
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I second Chris Crawfords Video! I have it and its way underpriced for the information provided. I review it every couple of months and get more info each time!