Filing screw head slots & blueing

[jdm]

Here's few tips on these subjects by a master watchmaker.

Slotting
For certain things a circular saw is the cleanest. Small screws might have slots of 0.10mm. In general they'll  be between 0.15-0.13. But you'll need a way to hold the screw and the saw precisely, so at a minimum a lathe with a milling attachment. 

It's a lot of trouble for a one off screw. I've grabbed up screwhead files any time I've seen them over the years. Very handy.

For larger screws you can often slot with a jeweler's saw or hacksaw.

Blueing 
The most traditional is simply heat blueing, you can get a very dark nice blue this way. If you want a blacker blue but still a hint of blue there are techniques used by gunsmiths but they're quite dangerous. Any "cold blue" chemical treatment tends toward pure black (well, very dark grey), and aren't as tenacious as methods involving heat and caustic chemicals. 

In WOSTEP they would use a molten salt bath sometimes. This produces a very even blue but it's more pale than a simple heat blue. Also very dangerous as the bath is over 200 degrees as I recall. A spill would be disastrous, as would a drop of sweat hitting the bath.

One way to get black, more fore machine parts, is old motor oil. Heat the part below whatever tempering temp you use, dip in oil. You can repeat this and even ignite the oil on the part. It isn't always perfectly even, but can be quite pleasing,  almost like the mottled effect seen on parts that were case hardened. 

Case hardening involves packing low carbon steel parts in a sealable steel box with bone charcoal and holding it at critical temperature for some hours.

[jdrichard]

29 minutes ago, jdm said:

Here's few tips on these subjects by a master watchmaker.

Slotting
For certain things a circular saw is the cleanest. Small screws might have slots of 0.10mm. In general they'll  be between 0.15-0.13. But you'll need a way to hold the screw and the saw precisely, so at a minimum a lathe with a milling attachment. 

It's a lot of trouble for a one off screw. I've grabbed up screwhead files any time I've seen them over the years. Very handy.

For larger screws you can often slot with a jeweler's saw or hacksaw.

Blueing 
The most traditional is simply heat blueing, you can get a very dark nice blue this way. If you want a blacker blue but still a hint of blue there are techniques used by gunsmiths but they're quite dangerous. Any "cold blue" chemical treatment tends toward pure black (well, very dark grey), and aren't as tenacious as methods involving heat and caustic chemicals. 

In WOSTEP they would use a molten salt bath sometimes. This produces a very even blue but it's more pale than a simple heat blue. Also very dangerous as the bath is over 200 degrees as I recall. A spill would be disastrous, as would a drop of sweat hitting the bath.

One way to get black, more fore machine parts, is old motor oil. Heat the part below whatever tempering temp you use, dip in oil. You can repeat this and even ignite the oil on the part. It isn't always perfectly even, but can be quite pleasing,  almost like the mottled effect seen on parts that were case hardened. 

Case hardening involves packing low carbon steel parts in a sealable steel box with bone charcoal and holding it at critical temperature for some hours.

Thanks for the info, appreciated.

[rodabod]

For slots, I usually cut with a piercing saw and then finish with a slotting file if I have one of a suitable size. If you are finishing it properly, then on larger screws with a visible slot, the slot itself should be polished. I sometimes use strips of thin wood (coffee stirrers) to do this using a paste abrasive.

[mikepilk]

1 hour ago, rodabod said:

For slots, I usually cut with a piercing saw and then finish with a slotting file if I have one of a suitable size. If you are finishing it properly, then on larger screws with a visible slot, the slot itself should be polished. I sometimes use strips of thin wood (coffee stirrers) to do this using a paste abrasive.

I've been looking for a slotting file for use on balance screws, but I'm confused about their size.

This is from the Vallorbe catalogue. The files are 75 mm x 9mm.
For the No 8, I was assuming the "Thickness" 0.35 mm must be at the cutting edge, as it seems too thin for the centre of the file - just measured a craft knife blade at 0.5 mm.  

Is the thickness the centre thickness?  How fine a slot can they cut ? 

[rodabod]

Hi Mike, I obtained most of my files second-hand. The slit width cut is by trial and error. I’d also be interested in how they spec the width. It’s very possible that the slot cut will be noticeably wider than the file width as we are filing by hand. 

[mikepilk]

1 hour ago, rodabod said:

Hi Mike, I obtained most of my files second-hand. The slit width cut is by trial and error. I’d also be interested in how they spec the width. It’s very possible that the slot cut will be noticeably wider than the file width as we are filing by hand. 

Hi Rod, what do you reckon is the finest slot you could cut? Fine enough for balance screws ?

[nickelsilver]

I don't think Vallorbe makes the thinnest ones anymore. I also have many acquired secondhand over the years. Last I actually measured I have a couple that are about 0.12mm at the edge, which is probably ok for balance screws except for the really small ones. 0.35 would be way too big. 0.20 might be ok for some pocket watches.

 

[rodabod]

8 hours ago, mikepilk said:

Hi Rod, what do you reckon is the finest slot you could cut? Fine enough for balance screws ?

Probably not, unless it was for a simple posing job. If it were me, I’d try to make one for that: easiest option might be to dress a carbide Stanley blade with a coarse diamond lap. 

[mikepilk]

1 hour ago, rodabod said:

Probably not, unless it was for a simple posing job. If it were me, I’d try to make one for that: easiest option might be to dress a carbide Stanley blade with a coarse diamond lap. 

I was wondering about using a knife blade. Good idea.

[rodabod]

If you were trying to make really cleanly finished balance screws then it might be worthwhile making a jig to hold the screw and centre the file. The BHI described how to do this somewhere, but I’ve forgotten what was suggested.