Your Choice Of The Most Difficult Ordinary Model-engineering Operation?

[Nigel Graham 2Participant @nigelgraham2]

By “ordinary” I do not mean exotica most of us would rarely if ever dive into, like making large boilers, accurate worms and worm-wheels, bevel-wheel differentials, nested-cube ornaments, long-case clocks and the like.

Hacksawing to a line? Perhaps laborious but not especially difficult.

Filing two edges square (ish) to each other? Perhaps.

Milling a bulky component using a vertical-slide on a small lathe? Very likely.

Marking-out and drilling several holes in two parts, by bench-drill, so all align fully? Certainly not easy.

 

For my money I reckon it must be silver-soldering small fittings onto pipes.

 

Having just spent an hour trying to do just that: one pipe, two nipples. And yes, I did assemble it with both nuts on and facing the right way.

I am not convinced it worked. I will need make an adaptor to test the pipe on the club’s boiler-test kit.

[Andrew TinsleyParticipant @andrewtinsley63637]

Usual explanation is that you have used insufficient heat. Suggest a larger torch.

Andrew.

[Chris GunnParticipant @chrisgunn36534]

I am guessing that this would be quite small piping, and would also guess cleanliness could be the issue. not easy to clean the nipple bores enough.

Heat could be the issue depending on the setup.  I noticed the other day when silver soldering a few small levers about 1/8″ diameter on top of a firebrick, i could not get them hot enough. as soon as I lifted them off the firebrick into the flame, they went red virtually instantly and the solder flashed. I guess the firebrick was absorbing all the heat.

Chris Gunn

[Bill PhinnParticipant @billphinn90025]

I can’t see anything intrinsically difficult about silver-soldering small fittings on to pipes unless the pipes are enormous.

Can you show us what you’re attempting?

[Chris Gunn]

On Chris Gunn Said:

I am guessing that this would be quite small piping, and would also guess cleanliness could be the issue. not easy to clean the nipple bores enough.

Heat could be the issue depending on the setup.  I noticed the other day when silver soldering a few small levers about 1/8″ diameter on top of a firebrick, i could not get them hot enough. as soon as I lifted them off the firebrick into the flame, they went red virtually instantly and the solder flashed. I guess the firebrick was absorbing all the heat.

Chris Gunn

Try to get hold of one of or a piece of, those very light firebricks which are – I think – compressed vermiculite.

I use broken ones from our woodburning stoves, they don’t steal as much heat from the torch.

[Phil PParticipant @philp]

Quote
“I guess the firebrick was absorbing all the heat.”

Chris

Thats what traditional firebricks are designed to do, they are often mistakenly advised for use in a soldering hearth etc, but are not what you should be using. What you need is a material that reflects the heat rather than absorbs it, something like Vermiculite board is ideal.

I found that out the hard way myself many years ago.

Phil P

[Simon CollierParticipant @simoncollier74340]

For me it is spotting through drilled holes in one part to drill matching holes in another part. No matter how careful I often end up having to drill clearance holes over size or even draw holes over with a file.
For small fittings I use 1/32 silver solder not the more common 1/16. By the time the fittings are hot enough to melt 1/16, you’ll likely melt the brass pipe tails.

[DiogenesParticipant @diogenes]

Most difficult thing? – ‘proper’ engineering chisel-work; ‘square’, clean, crisp and accurate rebates, slots, pockets – even flatting..

[JasonBModerator @jasonb]

The problem of drilling two plates can be made a lot easier if the plates are clamped together and then drop a pin or bolt through as you drill the holes. Same fort s[potting which is something I never do now that all my drilling is done on the mill.

 

[Nigel Graham 2Participant @nigelgraham2]

Of course, lack of practice could be another factor! I don’t have to hard-solder anything very often at all.

 

This is only a length of 3/16″ copper tube with a brass union nipple on each end, but I’d had to make the nipples. They are the flat faced type that fit pressure-gauges – as this will. The other end will fit a banjo connection that screws into the bush in the boiler, and I made that end the same 1/8″ BSP as the gauge.

So had to make the nipples.

I cleaned the tube with wire-wool but could not do much about the brass fittings. I tried using silver-solder paste, and 1/32″ wire. The work surface is one of the vermiculite tiles forming part of one of those proper silver-soldering hearths from CuP. I held the assembly in a small clamp-stand. The heat is from a plumber’s blow-lamp giving more than enough therms, although it having to be held all the time so it does not switch off, can be awkward when Nature’s given one only two arms.

 

I think the problems here are of:

– drilling the fitting so it fits the pipe accurately (loose sliding fit- one was slightly over-size);

– cleaning the inside of the brass nipples (long wait between making and using, allowing oxidation);

– actually making the flux and solder penetrate the joint without merely building up on the outside.

Cleaning revealed a visible gap between the pipe and the fitting wall at one end. Testing by washing-up liquid and blowing showed this is an open channel.

I’ve pickled the whole thing and could try re-heating and sealing that.

 

Otherwise…

I’ll need make new fittings, might need a new length of tube if cutting the ends off the existing makes that too short, will clean the interiors of the fittings by degreasing and citric-acid pickling directly before assembling.

 

 

[Nigel Graham 2]

On 17 December 2025 at 17:22 Nigel Graham 2 Said:

By “ordinary” I do not mean exotica most of us would rarely if ever dive into, like making large boilers, accurate worms and worm-wheels, bevel-wheel differentials, nested-cube ornaments……

That eliminates all the interesting stuff; been there done that, except for building a large boiler. The second list is basic metalwork or, in the case of milling on the lathe I’ve never needed to do it.

I will admit that it took me a few attempts to get going with silver soldering once I started using propane instead of oxy-acetylene. The two keys were a bigger burner, equals more heat, and preheating the silver solder rod in the flame before applying to the work.

Julie

[Bill DawesParticipant @billdawes]

For me by far the most difficult was silver soldering the boiler on my 5″ gauge loco, in fact I have had to admit defeat after two test failures.

Early days of the boiler making is the old issue (I now know) of too small a burner.

However I think I have fallen foul of one too many reheats, I’m just chasing leaks around, I also suspect I have unsoldered the crown stays on the firebox as the roof is curving in to the fire box rather than out following last pressure test, nothing burst but time to call it a day I think, will probably get some Roubles out of my Swiss bank account and get one made, I live not far from Western Steam.

If I were to start again and make a new one (which I’m not!) I hope I would benefit from my experiences to make a better job generally but admit I have a bit of a phobia now about the silver soldering so can’t afford to waste time (at 84 something I’m running out of) and money to finish up with the same problem.

Other than that I think the other struggles I have had are dealing with fiddly little bits and pieces and even smaller fixings that hold these together and maneuvering into place, I have relatively slim fingers so don’t know how you guys with ham size fists get on. Oh and not forgetting dropping aforesaid bits on the floor to disappear forever, or until you walk in one day and there it is smiling at you.

Bill D.

[Bill PhinnParticipant @billphinn90025]

On 18 December 2025 at 08:33 Nigel Graham 2 Said:
“I cleaned the tube with wire-wool but could not do much about the brass fittings. I tried using silver-solder paste, and 1/32″ wire… -cleaning the inside of the brass nipples (long wait between making and using, allowing oxidation);

– actually making the flux and solder penetrate the joint without merely building up on the outside.”

(quoted text not in usual format because submit post button didn’t work, and I’m not editing the full quoted text again.)

To clean the fittings after pickling and prior to soldering, cut a small strip off a scotchbrite pad and run it back and forth through the fittings until bright and shiny.

I’m not sure what the 1/32” wire is doing. Is it binding wire? If you’re using solder paste, you shouldn’t need solder wire as well.

Once you’ve made the new fittings, if you can post a photo of them in position for soldering it may be possible to see any obvious problems before you attempt the job again.

Whatever you do, both the mating surfaces have to be clean prior to soldering, i.e. free of contaminants and oxidation. Both surfaces can be lightly fluxed prior to assembly, so you don’t need to worry about flux not reaching all the way into the joint. If you’re using silver solder paste, then obviously you’ll be painting this on rather than flux.

[Dave HalfordParticipant @davehalford22513]

Having bought a terrible Minnie boiler very cheaply for the gears, drawings and castings that came with it I decided to rescue the fittings bosses for the fire door etc. Heated it way passed the green flame stage and still the best I could do get the silver solder to the consistency of thick treacle. Based on that silver solder does not appear to remelt and run like lead/tin solder does and so leaks are more likely caused by ss not flowing sufficiently on the first heating and not your reheat.

It helps if you can apply SS one end and watch it appear on the other side. Joints can’t be too tight, each grade of SS needs a different clearance to flow though.

[Nigel Graham 2Participant @nigelgraham2]

Julie –

I did not say making accurate worm-gears, bevel gears and the like is not interesting. Just the opposite, but their formidable mathematics and machine-setting needs considerable experience and skill. I have quite a lot of the former but less of the latter!

I excluded such things as being operations I think most model-engineers would use only rarely – or never.

I am still considering between the gamble of cutting my steam-wagon’s transmission gears or certainty of commercial ones; all spur wheels. I have no drawings, and the gears are largely hidden so not too dictated by scale and appearance. I would have to buy the worm and its wheel for the steering-gear, if I use that rather than screw-and-nut.

I made the differential many years ago, by modifying an ex-BMC front wheel drive unit to traction-engine form, on through and cannon shafts. It’s not very true to Hindley practice but not far from the Foden wagon axles of the same period.

.

Jason –

Oh, I often use that method for drilling, especially pairs of parts including things like handed angles. (Clamp their other webs to an angle-plate so they act like a T-piece.)

I tend to choose between mill or bench-drill depending on the nature of the work, but with due precautions to obtain the necessary matching, by either process. Building the workshop’s overhead travelling-crane, of almost all bolted construction, I drilled all paired components clamped together so even if the holes were slightly out of nominal position they still matched and the whole thing went together with mimimal “adjustment” (to my surprise!) .

 

Bill Dawes –

I was going to construct the copper boiler for my steam-wagon, and have machined the major components; but saw some sad and sorry examples displayed at exhibitions of what can go wrong with such hefty assemblies… So crossed Mrs. Verrall’s palm with plastic. The machined parts forlornly keep the loft spiders company.

.

Bill Phinn –

Thankyou for that tip on cleaning the interior of fittings. The hole for the pipe is 3/16″ dia but then continues to the end at a smaller diameter to leave a socket for the male part of the connection. This socket of course must not accumulate solder.

Otherwise I was being thorough and thought I was doing All The Right Things.

The silver solder I called 1/32″ diameter is really 1mm dia, 455 grade.

The paste is the same 455, but of thick consistency, dispensed from a syringe. It cannot be brushed or wiped on. It’s probably better for assembling flat surfaces to each other. Pushing the fitting onto the pipe just scrapes all the paste off.

I tried sealing the gap but it is too large, probably around 0.01″ wide.

 

I cannot satisfactorily photograph anything so small. The mimimum range for my camera is about 1 metre.

The nipple is a flanged bush; flange 0.3″ dia X 0.06″ thick, stem 1/4″ o.d. X 5/16″ long. Pipe o.d. 3/16″.

Placed for soldering with the pipe vertical, flange face supported on the refractory slab. Held there by a small clamp-stand clipped to the pipe well away from the joint.

Proper refractory, not firebrick – one of CuP’s small silver-brazing hearths.

[Dave Halford]

On Dave Halford Said:

Having bought a terrible Minnie boiler very cheaply for the gears, drawings and castings that came with it I decided to rescue the fittings bosses for the fire door etc. Heated it way passed the green flame stage and still the best I could do get the silver solder to the consistency of thick treacle. Based on that silver solder does not appear to remelt and run like lead/tin solder does and so leaks are more likely caused by ss not flowing sufficiently on the first heating and not your reheat.

It helps if you can apply SS one end and watch it appear on the other side. Joints can’t be too tight, each grade of SS needs a different clearance to flow though.

I have sweated silver solder, as one would do with lead/tin solder, just to see if it could be done. It appeared to be successful but it was just a test piece.

I think silver soldering is all about practice, following the basic rules and being aware that the rod of silver solder can rob the heat from the potential joint.

JA

[bernard towersParticipant @bernardtowers37738]

I find the most difficult operation to be keeping the workshop tidy and having a small piece of clear bench.

[duncan webster 1Participant @duncanwebster1]

I went to a presentation from CuP Alloys where the chap pooh poohed the idea that metals had to be surgically clean, that’s part of what the flux is for. Didn’t really believe him so took a piece of copper pipe which had been buried in the garden for a couple of years,  flattened it out, fluxed it and silver soldered just fine. Where the flux had been was bright copper.

I still clean the job up, it can’t do any harm

[bernard towers]

On bernard towers Said:

I find the most difficult operation to be keeping the workshop tidy and having a small piece of clear bench.

👍yes indeed..

[John HaineParticipant @johnhaine32865]

…or finding that small bit you just put down somewhere safe 5 minutes ago…

[Charles LamontParticipant @charleslamont71117]

Trepanning.

[bernard towers]

On bernard towers Said:

…the most difficult operation to be keeping the workshop tidy and having a small piece of clear bench.

Got it in one, and is partly why my workshop activities have spread into the hall (assembly area), kitchen (paint shop) and sitting room (3D printing).

Julie

[bernard towersParticipant @bernardtowers37738]

Yes Duncan and i bet that bloke was Keith Hale, a clever man when it came to joining metals.

[JAParticipant @ja]

I agree with Bernard about tidying the workshop. As for the bench, the more the better. Another difficulty is heating it.

JA

[Nigel Graham 2]

On Nigel Graham 2 Said:

Julie –

I did not say making accurate worm-gears, bevel gears and the like is not interesting. Just the opposite, but their formidable mathematics and machine-setting needs considerable experience and skill. I have quite a lot of the former but less of the latter!

I excluded such things as being operations I think most model-engineers would use only rarely – or never.

Fair point; I would make the following comments:

• The mathematics is fairly simple, basic arithmetic and some trigonometry, so early secondary school level. Knowing what to calculate is more of an issue.
• When I started on my bevel gears and worm gear set I didn’t have any experience. That only comes by reading, designing and then machining prototypes. Surely that is true of any engineering, professional or otherwise.
• I would encourage you to machine the spur gears, not difficult at all. A dividing head helps, but can be done using a rotary table.
• I happen to be fascinated by gears so naturally wanted to make my own.
• There are regular queries on here about bevel gears so some people are keen to try making their own.
• I have a backburner project looking at skew bevel gears and spiral bevel gears. Watch this space, but don’t hold your breath!

Julie

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