Specific Bushing Questions

N

Nomad

Joined
Oct 23, 1999
Messages
9
I am looking for a few tips about bushings for folding knives. I will appreciate learning what you have found in your experience.

The type of knife I have in mind could range from a simple pocket knife to a "tactical", but would not need to be as bullet-proof as a "truck knife". I would very much appreciate it if you could give me a few tips (or point me to a discussion published on the net) about:

1) Are bushings always synthetic/plastic or are there any good reasons to use metals like sintered bronze?

2) What particular materials are commonly being used for bushings currently?

3) What types can you recommend?

4) I have noticed that there is some variety in thickness. I can't imagine a reason to make them any thicker than the minimum thickness required to ensure that they never wear out (plus a little bit for tolerance). What is the reason for bushings to be any thicker than 0.10 inch?

 
Nomad,
Depends on how many makers respond as to how many different answers you will get. Here's my answers:
1-In my terms, a bushing is what fits in the blade and a washer (thrust washer) is what fits between the blade and the liner. I prefer Teflon or Nylatron but have used soft copper and brass. I just prefer Teflon or Nylatron.
2-On some blades, primarily damascus, i use a phosphorous bronze bushing, .250 od x .186 id. The pivot bolt goes thru this.
3-I normally use .015. The washer should be the same thickness as the detent ball sticks inside the knife (liner). On 4" plus blades I use .020 to allow a little more room for the blade to shift right/left on opening, without hitting the liner.
4-I'd say the best answer to your question is simply maker's preference and what he feels works best.

Now, let's see what other guys say. Might be an interesting thread.
 
Re: Sintered Bronze - I see some folks using a sintered bronze called "Oilite" which is microscopically porous and contains a lubricant. I think this is of no particular value in knives, because the manufacturer states that the lubricant is only drawn to the surface by centrifugal force while the part is in high-speed motion. I doubt a knife's pivot meets these criteria. Still, it is bronze and that's a fine bushing material in any form.

Re: Washer thickness - I really hate to disagree with Kit (and Pontiac) on this point, but thinner is better. If you're using metallic washers it doesn't matter much, but most non-metallic ones are compressible and this will create slight blade play. The best cure, IMO, is to keep them as thin as possible.

Some folks use thick washers to allow clearance for the detent ball. Benchmade does this (0.025" on both sides, even though only one needs to be thick) and it's the reason all of their knives have slight lateral play even when "tight." A more elegant but complex solution is to mill a space in the scale to let the liner "overtravel" slightly, allowing you to use very thin washers. Spyderco does this with the Military/Starmate (0.0025" washers, sometimes stacked in pairs), and can thus use very thin washers that allow no lateral play. Another solution I saw recently was used by Owen Wood, who thins the locking bar towards the locking face. I don't like the idea of thinning this bar as it supports the stress from the lock, but it was certainly an interesting approach.

Mind you, Kit makes a heck of a lot more folders than I do and I don't recall anyone ever complaining about blade play. I'm speaking mainly from what I've seen working with and disassembling hundreds of factory knives, which are several steps below Kit's in terms of quality and attention to detail. But from a purely mechanical standpoint I can't recommend buffering the blade between two fairly thick pieces of compressible material.

-Drew
 
Drew,
i wouldn't argue your points. The key is to allow for compression. The backspacer is the thickness of the blade and both "conmpressed" washers. If you don't allow for this compression, there will be play, no matter the thickness. With Teflon, you put the knife together and it is already smooth, unlike other materials.

Waiting on more ways of doing it.
 
personally...i think washer thickness is more a question of blade shape...if you have a flat on the blade....like a double ground tanto with flat all the way to the tip...and use a thin washer...just the lateral pressure opening the knife will scrape the ti and put scratches on the blade....thin washers work fine in hunter style blades that are hollow ground all the way up...with the refinements in materials these days i see no need for bushings...but thats my opinion...i use a straight pivot pin that fits just right and thats all....i agree with the method of relieving the back of the bolster to give the liner lock plenty of opening room....theres a million ways to solve your problems..try something inovative.
 
I think this topic is important and interesting, and I want us all to be clear. To that end, I drew up a simple diagram of the three methods I discussed:

View


Blade is shown in black. Washers are red. Liners are green. Detent ball is blue. Scales are gray.

The top picture is the "thick washer" method. This is the simplest but may cause lateral play if washers are too compressible.

The middle picture is the "milled slot" method. Note that the milled area does not extend all the way to the spine, only for the width of the locking bar. I prefer this approach.

The bottom picture is the method Owen Wood (sometimes) uses. The locking bar is thinned at the front to allow clearance for the ball.


------------------

-Corduroy
"Why else would a bear want a pocket?"

Little Bear Knives
Drew Gleason:
adg@student.umass.edu
 
I Agree with Kit. Nylatron GS is the best material for washers I have seen also. I use the same formula he does (thickness) for knives. This material is availble at McMaster Carr sales in small amounts if you want to cut your own. Punches such as leather or gasket punches work or you can buy a punch set to make several different sizes of washers..

------------------
Web Site At www.infinet.com/~browzer/bldesmth.html
Take a look!!!




 
Thank you all for the informative answers. This provides me much food for thought. I believe that I will try out the thin washer approach and mill out the scale since I have the equipment for this. I like the idea of minimizing the amount of play in the blade and keeping overall width to a minimum.

About two weeks ago this question came to my mind while working on my latest project. As luck would have it, on the day *before* that I had met and had a twenty minute conversation with none other than Allen Elishewits. Talk about a missed opportunity…
 
Nomad,
As you can see, there are many ways to "skin a cat"-No offense to cat lovers intended.

Another equation that comes into play is the angle of the back of the blade, where the lock bar meets it.

I use 7 1/2 degree. The angle changes slightly depending on the thickness of washer you use. I grind the back of the blade on a 10" smooth, hard contact wheel, not a flat platen. If you take a fine honing stone and hone it from side to side, there is a little radius. With the locking bar cut at a 90 degree, it mates up to the blade with good contact. Much harder to disengage than one done on a flat platen.

Again, I'm sure there will be more to follow.

All you other makers that do this kind of knife, jump in at any time.
 
One of the reasons I would prefer to maximize side-to-side rigidity is to minimize geometric variations at lock up. This can be critical with a liner lock since any flex that will occur happens when there is force
being applied to the blade and this is the worst time for the lock to fail. Since a thinner washer allows for less angle where the lock bar meets the back of the blade (given the same length lock bar) this
provides a better lock up geometry.

I haven't completely decided to make this project as a liner lock. I have been thinking of a number of possible locking methods for some time, focusing on three principles : lock up stability, simplicity, and ease of
fabrication. I have not seen all of the mechanisms that today's knife makers are using for locking blades open but I have a degree in mechanical engineering and a few years experience as a design engineer
and have some of my own ideas on the subject.
Since my projects are neither mass-produced nor profit oriented I can afford to hand fit these parts.

My "holy grail" in this search is to develop a locking mechanism that is easy enough to operate but has the opening button out of the way while using the knife. The locking liner is a truly elegant design but I dislike the fact that the tab that is pressed to open it is located in a place where the index finger could inadvertently release the mechanism when a good deal of force is applied. I know that my Teutonic roots are showing when I state that I just don't feel safe knowing that my safety is riding on a thin sliver of sheet metal ? I prefer a billet of machined and tempered steel (go ahead and say it : overkill).

At the same time I am striving for simplicity. I don't want to spend several weeks of my precious free time fitting together a chain of intricately formed and machined ferrous materials. I do like the look of a Luger but don't intend to create one just to keep a blade open. I have an idea for a push button design that would have only one more part than a liner lock but a more positive lock up. I can't say that it has or
hasn't been done before since I don't have access to all the designs on the market today (I don't get back to the US all that often).

(Maybe this should be a new topic?)


 
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