Bushes

[Dave420g]

All,

What is the common accepted method of fitting bushes to a copper boiler?
I am building a 2" scale Fowler Showmans traction engine (Plastow Drawings) from a part-built set of parts I recently obtained.
The copper boiler has been started; the barrel and firebox wrapper are made, as is the firebox itself (with tubes but I will be replacing these as there are issues with them). Front Tube plate and rear bulkhead are made
There are no bushes fitted for the guages, steam manifold, clacks etc. I will be using Phosphor Bronze for these but I am unsure of how to attach these, particularly on the curved surface of the barrel.
Do I simply drill a hole to fit a plain bush - if so how to hold in place whilst (silver) soldering?
Or do I use stepped bushes - fitted from inside or outside? How do I take into account the curved surface at the junction with the step? The Plastow drawings arn't very clear (to me) in these aspects.
I am not seeing my club boiler inspector again until after Xmas and I want to progress during the holidays.
Any advise here welcome (as you will guess, this is my 1st boiler build).

Cheers
Dave (UK)

 

[GWRdriver]

Hello Dave,
You've asked some astute and pertinent questions. First of all you are most definitely headed in the right direction in using phos bronze. That's the optimum material. Two quick answers to dispense with first, by all means install bushes from outside, and then don't worry about the curvature as in smaller bushes there is so little gap to account for the solder will fill (or fillet) it up.

When fitting bushings I always want them to be, so far as possible, "self-jigging", that is, I want them to hold themselves securely in place during the soldering operation. To this end I Always drill the bushing holes first. For this and all holes in copper I now use a "Unibit" style single-flute drill for sheet metal which has virtually eliminated 100% of the snagging problems associated with drilling in copper. I then turn the bushing shank to a very snug or even a thumb press fit in the hole I've made. Done the other way round, the turned bush is unlikely to ever fit the drilled hole the way it needs to. If the bushing turned to fit the hole ends up being a bit shy of a thumb press fit, that is, not fully holding itself in place, I simply center pop the copper on the backside of the hole in three places which serves to tighten things up. If I lack the clearance to get behind the bush (inside) I'll pop the bushing shank to raise the metal a bit. You don't want them so tight that they prevent solder from flowing through the joint, but you absolutely don't want the to fall out while in the midst of soldering.

I size the OD of bushing faces and "collars" to suit their purpose, some benefit from larger faces than others, and then I make the body (or shank) step just slightly smaller than the collar. It need only be enough of a step to prevent it falling through its hole. The attached sketch will give you the idea. You are going to consume a certain amount of material sizing the face OD so this allows me to retain "meat" in the thread area. Using a rat-tail file I typically notch the bush hole IDs slightly in three places. This will give the solder some channels to encourage solder flow to the back side of the shank without affecting the fit in the hole. BTW, the depth of the collar in the sketch is drawn a bit on the thick side. Collars need not be proportionately this thick. How thick they should be typically depends upon whether they need to show through the jacket or cleading or are hidden behind the jacket.

I hope this gets you going.

View attachment BUSH.pdf

 

[Dave420g]

Harry,

Many, many thanks for the comprehensive answer and sketch. That is very clear and I should be able to continue over the holidays. Thm:
Once I get going, I will post some pictures (they may not be pretty!)

One question for you (or anyone else) is with regard to the bushes for the water guages. I read somewhere that it is best to do these last, once everything else is built, and do them with the guage hardware fitted (but with a metal rod in place of the glass). That way you do not need washers etc to get everything to line up. I sounds sensible to me, is this standard practice?

Cheers
Dave

 

[GWRdriver]

Dave,
Glad I could be of help. I don't know that "mocking up" a water glass assembly was ever a standard practice although in some instances there might be some benefit to it. In the UK where most of you lads use straight threaded fittings you would never be more than one thread pitch away from alignment between the two and that isn't enough to fret over.

Let me digress for one moment, . . turn your bushings and drill for the thread but don't fully tap the bushes at this time. It's helpful to me to put a tap in for 2-3 threads to start the threads square and act as a guide when I come back after soldering to complete tapping. The reason for this is the thread crests, which are very thin and have relatively little mass, will heat up far more quickly than anything else around them and can easily go white hot and burn. In many cases this is almost impossible to avoid if they have been fully tapped and are left unprotected.

I suppose if your gauge glass bushes must exactly lined up you could make the bushes per above, run the tap in for 3-4 threads and assemble the upper and lower fittings to their bushes and, after noting their position, synchronize the rotation and mark the bushes so that they are soldered in those positions. Even so movement in the heads due to thermal expansion and contraction during soldering could be on the order of one thread pitch thus negating your careful work.

Harry, . . One question for you with regard to the bushes for the water guages. I read somewhere that [snip] do them with the guage hardware fitted (but with a metal rod in place of the glass). . . . is this standard practice?