Clearance size drill choices in assembling locks

[rich pierce]

Looking at tables for drill sizes for drilling, tapping and drilling clearance holes for screws such as 6-40, 8-32, etc the tables offer a "close" and a "Free" drill size.  What do you use say for a sear screw hole which acts as a bearing surface?

Example:
6-40 major diameter is listed at 0.138"
A #25 "Free" clearance drill is 0.1495"
A #27 "Close" clearance drill is 0.144

[D. Taylor Sapergia]

Rich, I never stick to charts.  I measure the shank of the screw, then cut a hole that is appropriate to that measurement.  There is too much variation in commercial screws as received to rely upon a chart.

[P.W.Berkuta]

There is not an easy answer for your question. First all screws will be slightly different in diameters. Measure the un-threaded portion with a good measuring tool. Take a scrap piece of steel and drill a hole two number drill sizes under the size you measured on your screw then change to a drill bit the exact size you measured on your screw and drill your scrap steel. Try your screw it should be a very snug fit. When you get this snug fit drill your sear and either polish the screw to a slip fit with a very slight drag or polish the hole. Or buy a reamer for the exact size of the screw's body then polish screw to fit. This way you can get the fit you want. It boils down to how much "play" you are looking for. The best fit is by drilling a undersized pilot hole first - then the exact size hole is drilled into the work then polished to your fit of choice. Remember that when you harden the sear the hole dimension might change every so slightly.

[jerrywh]

Like Taylor I do not use clearance drills. I measure the existing screw and use number drill .001" to .003" over.

[David Rase]

Rich,
In your particular situation, I'd go with a # 28 if the screw diameter is truly .138".  A #28 is .1405".  A 9/64" is .1406". 

A lot of times I will go to a zero clearance fit and then diamond lap the hole, especially on tumblers and frizzens where a tight tolerance is critical.
David

[Bob Roller]

Rich,
In your particular situation, I'd go with a # 28 if the screw diameter is truly .138".  A #28 is .1405".  A 9/64" is .1406". 

A lot of times I will go to a zero clearance fit and then diamond lap the hole, especially on tumblers and frizzens where a tight tolerance is critical.
David

I make all my own screws for locks.My 6-40 is .130-.132. The 5-40 which I seldom use is .120-.122.
Bridle screws are .108-.109 and a #35 drill is .110. Sear is REAMED .1094 and screws are .109.
I finished a Rigby "4 pin" recently and the sear turns on a polished drill rod pin of .1094 thru a .1094
hole. With the rigid bridle and a properly cut full cock it is a very stable assembly.
Helpful hint. Any reaming must be done with a sharp reamer and flooded with cutting oil or one of
the commercial tapping fluids.

Bob Roller

[flehto]

As a tool and diemaker sometimes a hole had to be made w/ a number drill {no reamer was available} but also had to be very smooth.....like that produced by a slowly fed  reamer or smoother. Took the drill and ground a radius or curve thereby eliminating the corner at the cutting lip. After first drilling a .005-.010   undersized hole depending on the final hole size and then using the reground drill w/ plenty of cutting oil, the hole was w/in .00001s.  The first 4 flintlocks I used for my builds starting in 1977 were made from Siler kits and the above procedure produced very accurate and close fitting holes where needed....although sometimes a hole even had  be slightly lapped for close slip fit ......Fred