Technique for using a barrel lap?

JC in SC

JC in SC

Joined
Feb 28, 2009
Messages
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I've been reading and rereading every WIP or tutorial I can find on slip joints. I see where some makes ream the blade pivot hole to size and others ream it ever-so-slightly undersize and then barrel lap it following heat treat. I'm working on my first slip joint now, so I ordered a reamer and a barrel lap. The problem is, I'm not exactly sure how to use the barrel lap.

I've searched here and only found reference to it by those that seemed to know exactly what to do. A Google search results in 1,000,000 places to buy a barrel lap, but nothing on how to use them. Obviously, I need to refine my search skills as well.

I thought from the tutorials/WIPs it would be intuitive, but it isn't or at least isn't to me. Maybe I'm missing something obvious, but it looks like a piece is absent. The lap itself has four (I think) slits in it. Don't know why, but I was expecting something to protrude slightly from these slits that would hold the lapping compound. Is the recess in the lap simply filed with lapping compound and it exits through the slits? I'm loosely familiar with honing, so perhaps that is tainting my perception of how I think lapping should work.
 
I use two different size laps. One for 1/8 holes and one for 3/16 holes. The 1/8 is a split lap that uses a small tool to widen the split and the 3/16 lap has a small stud that screws in to widen the brass lap. The slits are too allow you to widen the lap as it wears. I drill and ream my holes .0005 to .001 undersized (depending upon what reams I have on hand) and heat treat. After heat treat I chuck the appropriate lap into my drill press. I adjust the lap so it is snug in the hole and then apply a bit of 400 grit lapping compound to the part of the lap that will be spinning in the hole. Turn on the drill press and run the lap in and out of the hole a few times. I clean the pivot hole and then test fit the pin/pivot. I repeat this unit I get a nice fit between the pivot pin and the hole. Your drill press or mill must be trammed square. If it is not square the hole will be at a slight angle and the blade will not sit square.
 
They are actually very simple to use. You need to spread the bars apart slightly in the middle so that it fits snug in the hole. If your are planning to "Lap it In" it is far easier before doing the HT. get it Real Close and then walk it in by polishing. Picked up a set of Diamond Lapping Compound dispensers on Fleabay fairly cheap (But I think I got what I paid for) in various grits. Squirt some into the center of the lap where the bars are spread.
I use it chucked in my Foredom but that was a mistake. Did not keep the hole at a true 90 deg angle.
Next time it will go into the drill press at the highest speed. Note: It is easy to overdue your hole and ruin your blade at this point. Just remember this is a finishing step and needs to be done with care.

BTW the adjustable ones that Arthur mentioned are far easier to use :(
Hope that helps
 
for a slip joint with no pivot bushing in it just ream the hole oversize 3-5 thou and peen it all together. If you are using a pivot bushing then follow the instructions above.
 
Thanks for all the responses. I'm planning on using a 3/16" pivot bushing, ream the blade to size prior to heat treat and then lap for final fit following heat treat.
 
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