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Silver soldering a blind 'mortise and tenon' joint.

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Robin Graham04/03/2019 21:45:06
631 forum posts
143 photos

I want to make a joint between a brass disc and a stainless rod: something like this:


Questions are:

(a) If I flux the parts, assemble and feed solder (455 / aka silver flo 55) from the shoulder of the tenon, would capillary forces draw the solder into the blind joint, or should I make a 'bleed hole' as shown in the sketch?

(b) Would it be better to 'tin' the tenon first, flux everything, then assemble and heat? I've never tried tinning with silver solder, so don't know if it's an OK technique.

Or (c) is this an 'if I were going there I wouldn't start from here' question?

Any advice appreciated,





Edited By Robin Graham on 04/03/2019 21:50:11

Michael Gilligan04/03/2019 22:24:17
14570 forum posts
633 photos

Two options [slight variation on a theme] that I believe are much more likely to succeed than (a)

Place paillons of solder, or a ring of thin solder wire, [with flux] at the end of the blind hole; insert the rod; and heat.

... A combination of gravity and capilliary action should do the trick.


Paul Kemp04/03/2019 23:14:15
362 forum posts
18 photos

Just be aware if you mix your flux with water and the hole is blind when the water boils off it will push the rod out. You can get over that by putting in a hole as you show or filing a small flat on the side of the rod to give the pressure a path to relieve through. I do a similar job on the canopy support for my traction engine at least once a year, soldering a 3/16" pin into some 5/16" square bar and I know the solder works its way in because I have to drill out the pin when it shears off flush! (It's a carp design but there is not another way to do it that looks good and still allows easy removal of the canopy so I accept its a consumable item and renew when it breaks!).


Speedy Builder505/03/2019 06:45:12
1865 forum posts
130 photos

Also I suggest Tenacity 5 flux for the joint as you are using stainless steel which quickly oxidises and prevents the solder wetting the joint.

JasonB05/03/2019 06:59:33
16872 forum posts
1815 photos
1 articles

I'd go with the small flat filed onto the pin to let any steam out and that will help the solder in. Also apply your heat to the large disk which will also help as the solder will flow towards the hotter material.

CuP Alloys 105/03/2019 08:05:26
211 forum posts

Hi Robin.

Michael's approach is underestimated- it will work!

It simplifies the heating process by creating the ideal heat pattern to promote that all important capillary flow. When the 455 melts and flows, the capillary forces will centre the spigot in the hole.

The weight of the spigot and the capillary forces will ensure that the spigot sinks into the hole. Just make sure that your joint design allows for this.

No need to file flats on the spigot or worry about steam or flux entrapment.

Michael's approach underlines the simplicity of the silver soldering process when it is done correctly.



Ron Laden05/03/2019 08:08:35
1535 forum posts
262 photos

If the drawing is something like scale then the pin is around 2.0mm diameter, how thick is the brass disc, if thick enough I would thread the end of the rod 1/4 or M6, tap the disc and fit it with Loctite retainer.

Just a thought.

Martin Kyte05/03/2019 08:57:41
1536 forum posts
24 photos

I wouldn't bother with the thread just a cylindrical loctite joint.

regards Martin

IanH05/03/2019 10:13:01
62 forum posts
44 photos

In the original post Robin mentions “tinning” with silver solder. I would be interested to hear Keith’s (CUP Alloys) view on whether this is a viable technique.

In Morgan Three Wheeler land, recovery of used chassis lugs that have been previously silver soldered on to chassis tubes is achieved by machining the tube out of the lug. Removing the lugs with heat is not recommended for fear of “de-zincification” leaving the steel lugs brittle and prone to cracking.

If this is real, then tinning and reheating silver soldered components to sweat them together sounds like it might have the same effect? I would be grateful for any clarification.



Robin Graham05/03/2019 21:06:52
631 forum posts
143 photos

Thanks for replies. I shall try the method suggested by MichaelG and endorsed by Keith@CuP. By doing it that way I can be fairly sure that if I see an even ring of solder emerge at the shoulder of the joint the internal surfaces have been properly wetted. A bit like using Yorkshire rather than end-feed fittings in plumbing I suppose.

The disc is thick enough to allow a threaded joint, but that is difficult because (I didn't tell you this - sorry!) there will be another similar disc at the other end of the stainless rod, and the two discs need to be coplanar. I have done it this way in the past, using a locking nut at one end of the rod, but even with threadlock the joint has failed. The joints are subject to both and axial shear and torsional forces - the torsional forces were strong enough to loosen the thread . I have also tried retainer on an unthreaded spigot, but that has also eventually failed. Maybe I used the wrong adhesives, or perhaps my technique was poor.

In case anyone wonders why I want to do this, it's to do with constructing systems of coupled ('chaotic'  ) pendulums. Apart from my own interest in these things, I give them away as presents - people find them fascinating even if they don't understand what's going on.





Edited By Robin Graham on 05/03/2019 21:08:11

Michael Gilligan05/03/2019 21:25:47
14570 forum posts
633 photos
Posted by Robin Graham on 05/03/2019 21:06:52:

In case anyone wonders why I want to do this, it's to do with constructing systems of coupled ('chaotic' ) pendulums. Apart from my own interest in these things, I give them away as presents - people find them fascinating even if they don't understand what's going on.


Those with an enquiring mind, and an iOS device, might like this:


Double Pendulum Simulator by loo kang wee


CuP Alloys 106/03/2019 12:24:41
211 forum posts

Hi Robin.

See private message for my views on "tinning"



Robin Graham20/03/2019 23:16:10
631 forum posts
143 photos

Other things having intervened as they inevitably do, I've only just got round to trying the paillon method Michael suggested. This is a poor photo (I rue the day I 'upgraded' to an iPhone!) of a test using scrap pieces:


Despite the poor focus you may be able to see that the solder migrated from the bottom of the blind hole and has wetted the stainless - there's actually a reasonable fillet as well. So thumbs up for this technique!

This is nominal 5mm stainless in a hole drilled with a 5mm bit. . As best as I could measure the stainless was 4.974 mm and the hole 5.015mm. Plus or minus a tad obviously*, but rather less than the minimum clearance recommended in the CuP guidelines. Anyhow it works!

I heated very gently at first to dry the T5 flux paste, no problems with steam forcing the thing apart.


*In a previous life I had to mark lab reports from undergraduate students. One of the tasks we set was was to estimate the distance between the hydrogen atoms in water molecule by a nuclear magnetic resonance technique. One pair came out with something in the region of five metres, quoted to 12 significant figures. It's what the calculator said, numbers on an LCD display, so it must be right....




Edited By Robin Graham on 20/03/2019 23:26:04

Edited By Robin Graham on 20/03/2019 23:36:26

Michael Gilligan20/03/2019 23:27:57
14570 forum posts
633 photos

Thanks for the report, Robin yes



P.S. ... Not quite up to your hydrogen atoms, but I remember an incoming graduate in the vibration test lab who insisted that his answer was right ... he had calculated that the corners of a 4' square Aluminium alloy 'head expander' for a large shaker [1" plates separated by 4" of honeycomb] would resonate at 4 point something Hz.

JA20/03/2019 23:31:11
830 forum posts
48 photos

My comment is a bit late but here goes.

A couple of years ago I tried a "tinned" silver soldered joint to see if it worked. It was successful and good. But for the life of me I have not idea why one would ever use such a technique!


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