Making a practice SFIC
Posted: Thu Aug 08, 2013 4:50 pmThis was done some time ago. I have been meaning to post this but some minor surgery followed up by accelerated pace classes has led me to pout and do little of use this summer. Now on to the goods.
Quickly tiring of rekeying SFIC locks the hard way and unwilling to spend any money on a block I wanted to thread them up like other practice locks. I read Jeff Moss used a 5/40 forming tap. Now I would just prefer to roll with a known-to-work method, however my pedantic ways got the best of me.
I measured the holes on several of my locks, SFIC and not. It varied slightly from brand to brand even when they used the same diameter pins. That is no good. For a perfect match some reckoning needs to be done. On BEST branded SFIC the holes were measured at .113 inches. Looking up here shows that is drill size #33. That site conveniently also lists the thread size as #6-40 UNF.
The holes are spaced so close together. Could the soon to be cut threads overlap? Measuring, I found a gap size of .72 inches. The major diameter of the threads that are to be cut is .1372, some not so fancy math later (major diameter-hole radius < spacing/2) means a tiny .023 between the threads. Keeping threads parallel will be important.
There are three types of taps, differing in the number of tapered cutting surfaces. Taper taps, as is implied, has the greatest taper, having several turns that help spread the cutting pressure out. These are the most common, are least likely to break, and are the easiest to center. So, of course, I purchased this type in H2 steel. The other two are the bottoming tap, which could yield an abrupt end to the threads but is more likely to break or be off center, and the plug tap which is somewhere between the two. Along with the tap I purchased a box of 100 cap screws and a #33 drill. The drill is to create an SFIC capping block from scratch when I get around to it. Unfortunately the minimum quantity at mcmaster for 6-40 cap screws was a box of 100. Worse, the smallest they came in was 1/8 inch. Why this is a problem will be obvious in a moment.
I disassembled a core, put one drop of oil in each hole, held the tap with a pair of vice grips and hand turned the core around the tap. It went in quick and easy. Getting all the burrs out takes longer than cutting the treads. For that task it is recommended to use a toothbrush and some toothpicks or hose the whole thing down somewhere you don't mind brass burrs. To fully seat the caps to make the core usable I needed to turn the tap about 6 1/2 times. Unfortunately, as can be found in the other thread on capping off SFIC, leaving screws at this depth wont allow all key profiles to be used. For proper operation the cap screws cannot go in more than 2 1/2 turns or so. Here are pictures of the results:
One problem is the limited depth of those cap screws. They aren't very secure. If I were to do it over again I would also get a bottoming tap at minimum. Start threads with the taper tap then finish with the bottoming tap. It would give a nice firm base so all that would be needed is to tighten the caps down instead of remembering the issue of depth. Though the threads appear deep they do not interfere with any of the pins but may change the feel of the springs. Not really a burden to the learner or a challenge to the experienced at this depth.
Total cost of the project was around 20 dollars including shipping complete with enough cap screws for over a dozen cores and a drill fit to make a capping block. Oh, and an important note on finding screws, though we are using them as capping screws the proper term for them would be 'set screw'. What we want is best found using that term.
So to sum up the whole process so that it can be done on any lock:
1. Measure the hole size
2. Cross reference with drill size and determine what tap sizes are most appropriate
3. Check that the cut threads wont overlap
4. Decide on what tap or taps you want to use
5. Tap it by hand! These guys are tiny and break easily.
That is it. A simple bit of reasoning and a very modest investment can save quite some time re-pinning certain practice locks over and over.
Quickly tiring of rekeying SFIC locks the hard way and unwilling to spend any money on a block I wanted to thread them up like other practice locks. I read Jeff Moss used a 5/40 forming tap. Now I would just prefer to roll with a known-to-work method, however my pedantic ways got the best of me.
I measured the holes on several of my locks, SFIC and not. It varied slightly from brand to brand even when they used the same diameter pins. That is no good. For a perfect match some reckoning needs to be done. On BEST branded SFIC the holes were measured at .113 inches. Looking up here shows that is drill size #33. That site conveniently also lists the thread size as #6-40 UNF.
The holes are spaced so close together. Could the soon to be cut threads overlap? Measuring, I found a gap size of .72 inches. The major diameter of the threads that are to be cut is .1372, some not so fancy math later (major diameter-hole radius < spacing/2) means a tiny .023 between the threads. Keeping threads parallel will be important.
There are three types of taps, differing in the number of tapered cutting surfaces. Taper taps, as is implied, has the greatest taper, having several turns that help spread the cutting pressure out. These are the most common, are least likely to break, and are the easiest to center. So, of course, I purchased this type in H2 steel. The other two are the bottoming tap, which could yield an abrupt end to the threads but is more likely to break or be off center, and the plug tap which is somewhere between the two. Along with the tap I purchased a box of 100 cap screws and a #33 drill. The drill is to create an SFIC capping block from scratch when I get around to it. Unfortunately the minimum quantity at mcmaster for 6-40 cap screws was a box of 100. Worse, the smallest they came in was 1/8 inch. Why this is a problem will be obvious in a moment.
I disassembled a core, put one drop of oil in each hole, held the tap with a pair of vice grips and hand turned the core around the tap. It went in quick and easy. Getting all the burrs out takes longer than cutting the treads. For that task it is recommended to use a toothbrush and some toothpicks or hose the whole thing down somewhere you don't mind brass burrs. To fully seat the caps to make the core usable I needed to turn the tap about 6 1/2 times. Unfortunately, as can be found in the other thread on capping off SFIC, leaving screws at this depth wont allow all key profiles to be used. For proper operation the cap screws cannot go in more than 2 1/2 turns or so. Here are pictures of the results:
One problem is the limited depth of those cap screws. They aren't very secure. If I were to do it over again I would also get a bottoming tap at minimum. Start threads with the taper tap then finish with the bottoming tap. It would give a nice firm base so all that would be needed is to tighten the caps down instead of remembering the issue of depth. Though the threads appear deep they do not interfere with any of the pins but may change the feel of the springs. Not really a burden to the learner or a challenge to the experienced at this depth.
Total cost of the project was around 20 dollars including shipping complete with enough cap screws for over a dozen cores and a drill fit to make a capping block. Oh, and an important note on finding screws, though we are using them as capping screws the proper term for them would be 'set screw'. What we want is best found using that term.
So to sum up the whole process so that it can be done on any lock:
1. Measure the hole size
2. Cross reference with drill size and determine what tap sizes are most appropriate
3. Check that the cut threads wont overlap
4. Decide on what tap or taps you want to use
5. Tap it by hand! These guys are tiny and break easily.
That is it. A simple bit of reasoning and a very modest investment can save quite some time re-pinning certain practice locks over and over.