Blade pivot

K

Kunklec

Joined
Aug 21, 2006
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I recently bought some precision barrel pivots for folders. These are the ones that have a "precision" fit barrel that fits in the blade. The pivot pin fits through this. They are advertised as being fit within 0.0002 to minimize play and maintain fit. Problem is, after fitting one up, the blade play was excessive. When I miked the pin OD and the barrel ID, there was over 0.002 difference. I checked 3 others, with about the same results. Needless to say, that's pretty sloppy. From now on I'll go back to turning my own and using a barrel lap to get good fit. It will take longer and be more costly initially, but at least I won't have to do it twice.
Has anybody else run into this?
Chip Kunkle
 
.002 is a fair bit of slop, but still better than a lot of folks who simply drill their pivot holes. (drilled holes aren't even round)

.0002 is a very precise fit. A lot of home shop lathes won't even turn something round to .0002 . In fact I'd wager that there are very few on this board can really honestly measure .0001 with true honest accuracy. I can't.

Anything under a thou is frequently a challenge to me, and I'm not exactly a novice. To reliably produce pins and bushings with .0002 clearance is out of my league and I have a pretty nice CNC turning center.

Some bushings are oversized until pressed into a specific sized hole, where the ID tightens up. Though not .002 on a pivot sized hole...

exactly .0002 clearance is an unreasonable expectation, but .002 slop ain't too hot.

my 2 cent...



Edit to add, I think of blade play as more a function of squareness and parallelism in your overall mechanism. Just relying on a tight fit between your pivot and bushing isn't particularly robust.
 
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I buy precision dowel pins from MSC and drill them for 2-56 or 4-40 screws depending upon if they are 1/8 or 3/16. Normally they are .0002 or better. I drill my blade pivot hole .001 under size and lap the hole for a nice fit.
 
I've had this same problem in the past, and after purchasing so called "precision" bushings from just about everybody who has offered them, I still have never been happy with any of them.

My solution has been to purchase the oil impregnated bronze bushings from MSC. (I use a .125" pivot, with a .250"OD X .125"ID bushing in my folders) My most commonly used size is .250 OD X .125 ID. They actually come in at .253-.254 OD, and .123-.124 ID. Since the material is bronze, I devised a little holder that fits into a drill chuck, and I "spin fit" them to each pivot hole so they are a perfect fit for the specific pivot hole. I use a .125 reamer for the ID. Since going this route, I have eliminated most of the slop that always seems to be present with I used someone's "precision" parts.
 
Thank you Brian.

Any level of precision can be achieved if you are willing to pay for it.

Our pivots and bushings are expensive if you only look at the price. When you consider the time required for you to make the same parts with our tolerances, we offer an excellent value.

We pay lots of money to have the parts made. They are precise. I know they are precise because I check them. The tools to check the tolerances cost more than $500. The CNC Swiss screw machine that makes our pivots and bushings cost more than $500,000. The machinist who makes our pivots threw away more than 500 pivots because they were .0005" oversize. The parts were outside our tolerances.

How much are you willing to pay for precision?
 
Ed, would you happen to have a pic of your bushing device?? i use similar bushings and always have a heck of a time reaming out the I.D on them. i generally use a .186, .187 and .1875 reamers, holding the bushing in my fingers. a real P.I.T.A.
 
Balibalistic: I saw one of these devices once, made one and it works quite well. Cut a piece of spring steel.....1095, 0-1, etc will work well. Cut a piece of approx 1/8" thick stock or thinner if necessary for whatever thickness bushing you are using. Cut this piece of stock about 1/2" wide and 4" long. Bore an oversize hole about 1/4" from one end and centered in the 1/2" width that is a bit larger than your OD of bushing. Now slit the 1/2" piece from opposite end up the middle of the piece to the center of 3/16"+ hole you bored. You may have to make this a wider slit depending upon various busings used. Now put bushing in hole and squeeze the two legs together. It is a small vice and works quite well. You may really make this any size required, longer, shorter, wider, etc. This method works for all sorts of oddball size thingys. You should HT and temper this pc to a spring temper. If you taper the two legs you may use a ring to secure bushing.
 
Chuck Bybee said:
Thank you Brian.
Click to expand...

Anytime Chuck.

The reality is that your normal pivots are so good that I don't even use bushings anymore...
Ream the pivot hole undersized, lap it to size, and they open silky smooth.

I used bushings on every knife until recently.

Now I made them myself, on high end equipment so there was no slop.

I would actually lap the hole in the blade to fit the OD of the bushing myself.
But I went on the theory of the blade pivoting around the bushing.

Many folks, like Chip here I think, press the bushing into the blade and have the blade and bush pivot around a pivot pin.
 
I need to say that I did not get these pivots from Chuck Bybee, much to my sorrow. Can't remember where I got the first ones, which were OK, but the last ones (again not from Chuck) were really sloppy. In a liner or frame lock, this was enough to let the blade scrape the frame when closed. After I made my own, and lapped them correctly, the same blade sits center.
Thanks to all who responded. I'm glad that the problem was not due to my pin alignment.
Chip Kunkle
 
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