Newsgroups: rec.crafts.metalworking
Subject: Re: Lapping internal threads
From: Robert Bastow <teenut@home.com>
Date: Mon, 11 Oct 1999 14:29:40 GMT

I have a couple of sets of the cheaper version, no name brand I bought at $8.00
a set for cheap drill press packers.

They have exactly the same problem..Maybe they haven't heard of "plus five" taps
in ...wherever.

To add insult to injury, the clearance holes are even smaller than the tapped
holes..so there seemed little point in wasting time on the threads.  I do have a
couple of "Good" sets if required.

If you "Really"  need to open up the threads don't waste time and money
buggering up a couple of good taps..'cause it WILL bugger up the tap!!

Make a brass or even mild steel lap, screw cut to fit the existing "thread" 
drill full length and tap the end half the length for an 8-32 setscrew, split
the end with a fine saw about 2/3 the length of the lap.  No need for fancy
tapers inside..just insert the screw,loosely, coat the lap with 180 grit Clover
Compound and lap away. Twitch the screw up as it gets sloppy.  

If you want or need a better lap..for this or another job.  Make it out of Mild
Steel, forget tapping the end..see if you can split it half way from each
side..kinda like the eye of a needle, and leaving the end solid.  You can
"adjust" for wear and sizing, to very fine limits, by inserting the tip of a
screw drive in the slot and tweaking gently.

Commercial "Mini" or "Micro laps" work on this principle.  It works on threaded
or plain laps..the smallest I have is about 0.045" diameter and are used with
diamond paste. (Which is invaluable to have around and isn't as expensive as one
might expect.  Especially as a tiny tube of a couple differnt grades will last a
lifetime for most people!)

It will work quite quickly..as any Tool and Die guy will tell you.  If he tells
you he NEVER forgot to use an oversize tap..you can tell him to his face he is a
Lyin' SOB!!!
 
Makes you wonder about the current quality of B&S though...

"How are the mighty fallen...!"

teenut


"w.j.ward" wrote:
> 
> I bought a pair of those Brown & Sharpe "Ultra Precision" 1-2-3 blocks.
> Unfortunately, I couldn't get the supplied screws into the threaded
> holes.  They are 3/8x16 (I think).  I don't know if the holes shrunk
> during hardening, or if they are just fouled up with scale.  These
> blocks are supposed to have been made in the U.S.  At least, that's what
> the MSC catalog said.  Since there is no indication of the country of
> origin on the blocks or the package they came in, I can't say for sure.
> I have a suspicion that they are from China.  Anyway, they are really a
> disappointment.
> 
> Any suggestions of how I can clean up the threads?  The steel is really
> hard.  I don't have a carbide tap, but I was thinking about lapping with
> a bit of threaded rod or a bolt.