Questions, lock building, tap drills, thread height,clearance drills and fit.

[Rolf]

I'd like to hear opinions on what is acceptable tolerances for a good quality Lock. I'm working on a scratch buildt lefthanded 4 screw percussion Lock.
Bridel screws are 4-40
Recommended tap drill options in the chart are:
 #44 = 0.0860 in = 2.1844mm which gives 80% thread heigth
#43=0.0890 in= 2,2606mm which gives  70.8 thread heigth
#42= 0.0935in =2.3479mm which gives  57.0% thread height

Wire drills sizes are not available in Norway. An Norwegian industrial supplier can get me drill With 0.1 increment. 2.1mm, 2.2mm, 2.3mm, 2.4mm, ect.
His Charts recommend 2.3mm for a 4-40 screw. Would this be acceptable?

What would an acceptable size clearence hole in a bridel be for a 4-40 screw?

Best regards
Rolf

[P.W.Berkuta]

From my experience when drilling a hole with any drill bit in metal the hole size will be a few thousands larger than what the drill stated diameter is. Also the acceptable standard thread engagement is 75%. If it were me I would choose a drill bit a few thousands under the diameter you want.

[rich pierce]

Why not just change the lock screws to something metric? Just an idea.

[kudu]

Humm? got a few friends that could just mail you a few drills? couldn't be too much cost involved.

[kudu]

just read your post again- 2.2 mm should be perfect you'll get a high percentage thread which is good!
 should be ok in Mild steel. Just use oil and a sharp tap and break the 'chip' when Tapping. (turn in a little and back a 1/2 turn) you got to have "feel" for the tool. i been a Machinist for 20 years so ive done it a couple times. 

Now if your power tapping, or running a CNC all bets off , no "feel" they just break!

 

[Bob Roller]

For YEARS I made my lock screws 4x40 or sometimes 4x48 and they were usually .108 to.110.
A #34 drill will give adequate clearance.A #33 certainly will.I drilled a#43 hole and then counter
sunk it the depth of ONE full thread on both sides,put in a drop of Rapid Tap or some similar thing
and power tapped them. No broken taps in the 1018 steel I used or any of the cast lock plates
made from who knows what.A 2.75MM diameter will easily support a #4 American thread
and a 2.80MM will give ideal clearance.
   What kind of lock are you making? Flint or percussion?
Bob Roller

[Rolf]

Thank you for all Your replys. I'll try to answer the questions. This past year, my Focus has been trying to learn to build Locks. I have several cast Lock sets from Blackley and Kibler collecting dust. I have not dared start on them, out of fear for messing up parts that are almost impossibel to get replacements for. To me, it made sense to learn by building from scratch. If I mess up, a chunk of steel is simple to replace.

I've started on a left handed four screw stanton percussion Lock using patterns and measurments, Bob Roller kindly sendt me.
His Lock uses 4-40 screws. The closest metric size would be a 3M screw which is 0.118 inches. I really hate turning screws that skinny, so I thought buying #4 screw blanks form Brownell would be a smart alternative.
Brownell Norway had the taps, dies and blanks in Stock, but for some reasone not the tap and clearence drills.

Did some tests With the drills I got from the industrial supplier. Think the problem is solved.
Tap drill 2.3mm threads easy, but maybe shallow. Tap drill 2.2mm is a tight fit. Snapped the tap on the first attempt. Lucky I had a spare. 2.2mm tap drill Works if I back out the tap and Clear the hole for every quarter turn.
The #4 screw blank from Brownell is 2.8mm thick. The 2.9mm drill gave a Close fit.

Here is a couple Pictures of the lockplates and bridel blanks. The plates and bolsters are milled out of 7mm steel, no soldering. Bridel blanks are 8mm thick With screw clearence holes drilled. Plan to use an endmill and a turntable to mill out the posts. 





Still working on the 1772 pistol Locks. Finished two, four to go. Hope to complet the four Cocks this week end. These Locks are my main Project for the last year, but I needed a break from them. Thats why I started the percussion Locks.


Best regards
Rolf

[Clint]

Rolf,
I have used Brownells screw blanks extensively and they are great. The shank diameters run a little oversize and I use metal polish to "hone" them into the holes. It is a good way to get a perfect fit for sears and frizzens. You can set up your tumblers so they are a little tight under the bridal and use valve grinding compound (for automobile valves) then metal polish to get them to ride smooth. The metal polish I use in called Simi chrome and I use it for honing wood chisels also. Good luck and have fun. Clint