Horology: Silver brazing tiny part, old mild steel

[Eric The Viking]

Stupidly, I broke the lock on our 200+-year-old long case clock last week.

The key has been getting ever harder to turn. It's not the original key, but one supplied by the case restorer when it was done about eight or nine years ago. It doesn't fit, and it now looks like the lock has seen quite a few ill-fitting keys in its lifetime. Trying a different key, I managed to get the lock thoroughly jammed/broken (or so I thought) with the bolt half-way out.

With some trepidation I removed it from the door (a few scratches to the steel surface of the lock, otherwise no harm done), predictably to find it was not "openable" easily, to get at the works.

It's made of mild steel (pictures above), and hand worked. The cover plate (inside the rebate, facing the outside of the door), has what I can best describe as mortices in it, and there are two 'tenons' filed diametrically across the U-shaped frame of the lock mechanism. I think they were peined-over and filed down. A bit of gentle "persuasion" with a needle file has released the cover plate, to let me get at the mechanism. I can almost certainly lock it back in place with a centre punch to spread the tenons, as I've only removed a tiny amount of metal...

... it wasn't damaged or jammed (by me, anyway). 200 years of neglect has allowed grime and rust to build up inside, between the bolt and the main plate of the lock. It was just too much friction really. A bit of emery cloth and needle files used cautiously has smoothed it, and the bolt now responds to finger pressure (I couldn't shift it at all by hand before).

But now I get to the problem: the pin that locates the centre of the key's shaft was also held into the main plate by a "mortice & tenon" arrangement. It had been badly bent by abuse over the years - one reason why the lock wasn't at all smooth. Stupidly (probably) in trying to straighten it I loosened it, and it has separated from the plate. You can see the "mortice" slot in the second image, and how bent the pin has become (someone must have really forced it in the past!).

I've been mulling over how to repair it, and thought it best to ask advice.

As far as I can tell, both bits are mild steel (too little material to spark test!). I don't think I dare try any sort of weld, as it's all just too small, so I wondered about silver brazing it (i.e. high temp proper brazing, sometimes misnamed "silver soldering").

I know YouTube can be misleading to the ignorant -- that's me in this case! -- but I have watched a few brazing videos. The fact that brazing wets the surface so well, and that it spreads thinly and is strong, makes me think I can make a good repair. But it's still micro-surgery!

I have several good propane and MAPP gas blowlamps, so I can get enough heat in easily, but I'm not sure what type of rod to use, nor whether I should use separate flux, or one of those with an outer flux coating. I'm guessing the approach is probably to clean carefully, heat with flux and drop on a small piece of rod with tweezers. I can grind back any excess with my Proxxon later.

The idea is to get a good, functional repair as neatly as possible. I really don't want to replace the lock, as repairs are part of the clock's life and the open mortice in the case door was made to fit this lock!

Any advice will be much appreciated, especially if I'm making a poor choice of repair method!

E.

 

[DTR]

I've only silver-soldered once, but.... I had to join two parts of a crank handle together (i.e. the crank and the shaft). I used a solder kit from eBay with separate flux; the flux came as a powder which you mixed with water to make a paste. I successfully soldered the two bits together with just a common plumber's blowtorch, and the crank is still going strong several years later. The two parts aren't mechanically joined in any way, just soldered face-to-face.

My crank handle is many times larger than your lock fitting, and subject to a lot of torque. It hasn't failed yet, so I think silver solder would be ideal for your lock.

 

[Eric The Viking]

Thanks Dave, that's encouraging.

The plan is to heat the pin and straighten it (that bit is easy-ish), and take a tiny bit of metal off the sides of the top of the "T" so it's a slightly loose fit, to allow for the braze metal to wet the faces properly.

I haven't yet worked out how to hold it in place for brazing though. A magnet won't work because of the Curie point (bound to be way over that!). I'd use a piece of drilled brass or similar but that would get brazed to the steel. I think aluminium might burn, but if not, it's a possibility, and I might be able to contrive something with shards of ceramic tile.

On t'other hand, it might just sit nicely on a piece of flat ceramic, face down as per second photo above. That would probably do it.

Still wondering about the specific brazing rod to use -- your flux sounds like Borax.

 

[Phil Pascoe]

If you PM me your address I'll send you some rod and flux - it'll save your buying whole rods or tubs of flux. Make yourself a little hearth from a couple of engineering bricks and lay the lock flat on that.

 

[xy mosian]

From schooldays memory bank. Thank you Mr. Foster. Brazing flux used was a paste of Borax. These days I buy flux coated rods, shortish as I don't need them often, from Big and Quick. They may not stock them these days, and anyway Phil's offer sounds brilliant.
Will the loose part stay in the mortice with a push fit at all? If so you may get away with it. Failing that clamped in place, toolmaker's clamp springs to mind, you could perhaps manage with a fine flame.
Please tell me you will straighten the key guide before re-fixing.
Have fun
xy

 

[TFrench]

How about JB weld?

 

[AndyT]

If brazing doesn't work, "someone" nearby with a metal lathe could turn a new pin with a big lump on the end, then file the lump so it would fit in the rectangular hole and be gently peened into place. You may be making it hard for yourself trying to preserve the original crumb of steel.

Can you see any reason why they didn't just drill a round hole, make a shouldered pin and hammer it in tight?

 

[Eric The Viking]

I've PM'ed Phil. "Someone" is also very kind, too!

Why repair it by brazing? I did think about an alternative, for example tapping a circular hole in the middle of the rectangle to 4BA (probably) and fitting a threaded pin (well, a 4BA* stud or bolt), but the surface in the first picture is visible, albeit inside the door and I'd still have to end up with it neatly flush somehow.

I don't know, but I'm hoping this will be easier, and leaving it looking neatly repaired, rather than untouched at one extreme or roughly bodged** at the other. The latter I've had plenty of practice doing!

E.

* keeps it traditional, and the BA thread form is better for this anyway, at that size.
**No offense to chairmakers!

 

[hawkeyefxr]

Silver solder is the way to go, i have done loads of silver soldering over the years. When it's applied and the heat is hot enough it runs like water. It will spread out very easily if there is flux liberally put on, try to keep the flux to the area where you want your join and only use the tiniest bit of silver solder, it goes a long way. You will have to get the metal cherry coloured to do this.

 

[CHJ]

Personally I would turn & file up a replacement part and if the casing is sound enough peen rivet it, alternate would be silver solder it in, in either case securing the original out of harms way within the lock casing for historical record.

 

[Eric The Viking]

Phil P. is very kindly sending me some silver brazing rod (I literally need one small drip, probably). I can heat as necessary, and can construct a tiny hearth in which to do it, I think. Nothing will be harmed by the heating process either, as the part(s) needing heating are all of the same metal, and the spring won't be i place at that point.

I would make a new pin, but it would take a while longer to fit it, and brazing is still probably the strongest way to fit it in place.

The following step will be to find a new key, as I'm pretty certain the current one won't then fit, although it might if re-shaped. The advantage from the strip-down is that I can see the key working and get measurements, so it ought to be possible to make it fit really nicely.

Those things done, it should then be good for another 200 years or so.

Thanks everyone.

 

[dickm]

Just one comment- the hearth needs to keep the heat in as much as possible. It's an important part of the brazing process, not just something to rest the object on!

 

[AES]

Missed all this post up to now too (see earlier post above re making a special bolt - my PC has been off sick).

+1+1 for "silver soldering". +1 also for the flux (Borax is the boyo for this sort of "dirty metal" repair - mix into a smooth creamy paste with cold water and an old small kids paint brush to coat evenly wherever you want solder, even if the rod you get sent is pre-fluxed).

As always with any "soldering", get as much muck off as possible before you start. And just as dickm has said, the hearth isn't just a flat lump of brick to rest the job on - it's a very important part of the job. Very worth while visiting your local DIY or garden centre and getting a couple or three suitably sized "yellow" refractory bricks (as used in BBQs) if you've nothing handy. Plus buy a tin of Pyruma (or whatever brand - heat-proof cement, ready to use) and doing a proper job by making a decent little hearth up.

For a job this small it's VITAL that the job doesn't move even a fraction of a mm while you're heating then flowing the molten rod all round the joint - allow PLENTY of time to cool afterwards too.

Think about that hearth configuration a bit too, and imagine a drop of water running all over the joint. That's the hearth you need, and an advantage of BBQ bricks is that they're pretty easy to butcher (old saw, angle grinder, etc,) to get exactly the set up you need.

Sounds a fiddly but fun job. Let's know how it goes - and pix please!

Good luck.

AES

 

[CHJ]

I picked up some (4 off) 1 " thick heat reflecting slabs of what look like polystyrene at first glance from a model engineering show about 10 yrs. ago. and a few rods of silver solder and borax flux. Not used often but the shear pleasure of just being able to take them off the shelf and get the job done more than compensates for the initial outlay.

 

[hawkeyefxr]

If you want a really small an
mount of silver solder, file the silver solder, collect the filings mix a small amount of the silver solder flux with a few drips of water to make a paste then drop the iling in the paste and mix again. Dab the paste where you want it and just heat up, it will bubble a litle but when the silver solder filings melt the will be less than a papers thickness of silver solder.
Silver solder is a little like mustard, the makers make their profit on what you waste, that was drummed into me on my apprenticeship lol.

 

[Eric The Viking]

Thanks - that's a good tip.

Phil's envelope arrived yesterday: he's kindly given me enough for a whole museum full of grandfather clocks!

I probably can't get to it this week, as work went a bit ballistic (it's Sunday but I ought to be doing stuff now really), but I promise to report back ASAP.

And yes, I will practice first on something that doesn't matter!

Later,

E.