Pivot Hole Slop?

Peter Atwood

Peter Atwood

Joined
Oct 26, 2000
Messages
2,468
I've been noticing that when I ream my pivot holes I am getting some blade slop. I'm using 7/32 pivots and I have the appropriate reamer, in fact two. I have checked everything with calipers and they all read the same but I'm still getting slop. It seems I get a more accurate hole if I ream to 5.5mm which is the press fit size for the 7/32 pivot and then use a regular drill bit to enlarge the hole to 7/32. Why would this be? I suspect there is a slight bit of wobble in my drill press which must be accounting for some of it but it's a decent drill press. How much slop is allowable?
 
to get a truly round hole (there is really no such thing) you need to BORE it on a mill or lathe. That is using a cutting tool with only one cutter and have the workpiece in a lathe fixture or clamped to your mill table. that way you are cutting around. Many drill bits are not accurate. Your drill press table needs to be checked to make sure it is ninety degrees to the spindle. any wobble in the bearings on your drill will affect the accuracy, and any flex in the drill press main shaft will affect things too.you have to make sure the down side of your work is perfectly flat in relationship to the drill bit..........a lot of stuff to consider. the most common problem is that no drill presses are very accurate, and so you get runout, and that affects your hole. Start with a center drill, then go to a 1/8" and then to an undersized (around .003 or .004 undersized) and then ream. BUY GOOD REAMERS.........carbide is what I use....they are much more rigid than the steel ones and are usually more accurate. ALWAYS do a test hole before doing a batch of blades. You could also test the pivots for concentricity. I could go on........good luck. :)

I will add.............for proper fit, if the pivot is well made and your hole is round with parallel sides, the hole works best at .0005 over the pivot size. .1875 hole and .1870 pivot
 
Thanks Tom. There's more to consider than I even thought! I'm sure there's a little wobble in the bearings on my drill press and that probably accounts for some of it. The table is 90 degrees to the spindle, or darn close to it. But I'm sure all the darn-close-to-its add up to make for slop in the end.... :)

I've been considering chopping my reamers down as well. I've never understood why a tool that's supposed to be precision is so darn long. I start all my holes with a #2 combo bit and use the short drill bits whenever I can to increase accuracy. So then there's the reamers- about 6 inches long!!! Why? It seems to me like they would be much more accurate if they were half that length.
 
yeap, I cut all my reamers down to min length, mostly so I dont have to move my drill press table as I keep it up close to the chuck.

they probably make them long for lathe work.
 
you could think that way or you could think that
a long shank would flex and center it self to a
 all but round hole with less wobble pressure
side to side from a spindle out of round or???
 a shorter shank just may wobble out the hole..
  if you sharpen your drills you can make them
cut larger holes or tighter holes by the angles
 or if one flute is cut different than the other.
as tom said you can go on and on...
 
Peter, I think Tom pretty much covered it. The one thing I would add is if you drill and ream your blade with the same side of the blade facing up you eliminate one variable. You might want to try this with a piece of scrap doing the same side up for one pivot hole and then alternate sides for the second hole. If the second hole is not as precise then your table is not square.

I also cut my reamers down to reduce flex, and use plenty of cutting fluid with a smooth steady stroke.
 
Hey Peter,

For what it's worth...I bought a bunch of reamers starting at .1855" up to .1885". I measure the pivot to .0005" and the last drill I use will be a hair smaller than the reamer that's a hair smaller than the pivot. For example, (if I remember correctly-my tables are not in front of me) a #15 drill, a .1870" reamer and a .1875" pivot. Then I lap the hole and pivot with, gosh what is it I have...1200 grit lapping compund. I do this before heat treat, and try to stop lapping just before I get some "wobble." If I look closly with a strong magnifier, I can tell that rather than lap a perfectly round hole and pivot, I've really just knocked off some high spots. That's when the fit is good. After heat treat, cleaning the hole with some abrasive paper gives me a fit I like. Not loose, but not too tight. Works well for me, but then I don't know any better.

BUT, I don't think you can get a near-frictionless fit and have zero wobble too.

Don
 
Sometimes I think there are advantages to NOT being trained as a machinist...You guys are scaring me with your ultra precision ruminations. My general approach is to drill the damned hole and be done with it! :D Naturally I want it to be right I just don't want to have to make a lifetime chore out of it. :p Interestingly, despite my seat of the pants approach, my folders are getting smoother and better all the time. So I guess I must be doing more things right than I thought!
 
one thing to always remember.............the hole that you are putting in the blade is what makes the knife work.......


that and having every other hole perfectly lined up and ninety degrees to the handle material and making the lock fit perfect and.........................................................
:p :) :p
 
Peter,
Try using a micrometer to measure your drills and reamers, calipers are not as accutate. A mic can be far more accurate just use the same pressure each time. If you want to double check measure the outside diameter with the calipers and then measure over the other end of the calipers (the side for measuring the inside diameter) with a mic to see if you get the same reading.
Chuck
 
Peter how fast are you running your drill press when either drilling or reaming ? Use a slower speed . drill to within .010-.020 of desired diameter and use a carbide very sharp reamer preferably with an odd number of flutes. What happens if you look close enough is your getting a boxing effect where your hole has four corners .This is caused by chatter and can be a nuisance. You can reduce this with an odd # fluted reamer and eliminate the chatter by slowing down the speeds and feeds as well as the ammount of material to be removed
I drill all my holes a couple thou under size and then heat treat. Then I ream them with a carbide reamer.But it can be done befor heat treat I just feel like I get a smoother and higher polished hole . go slow and don't force it let the reamer cut if you appily too much pressure you will cause unwanted chatter.Good luck!!
Oh ,check the slack in your drill press and adjust it out if possable if not then pitch your drill press and get one you can adjust it is important .
 
Thank you for all the excellent advice! I will check out all the various suggestions and see what I can improve... :)
 
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