Coarse vs Fine

[paul]

Am I right in thinking that fine threads are generally prefered for their strength compared to coarse? Also, are fine threads likely to prove any more steam tight than coarse?

(I'm talking new money here not that old-fangled stuff!) ;D

TIA

[Tel]

Depends on the application I guess. If we're talking, as I suspect we are, boiler fittings, then the fine would be better, but I'd still prefer the ME series of threads

[paul]

Yus, it's boiler bits. All my (very limited collection of) taps and dies are coarse. I splashed out (pun) on a water gauge today to see what they were all about and of course (pun) it's fine pitch.

Could make one I suppose as they look pretty straightforward. I'm not using it on the current boiler anyway but I thought I should take some advice before I do make the bushes.

[GWRdriver]

Paul,
My choice of threads usually has little to do with strength although in certain instances it certainly may. I choose a thread based primarily upon the application, and then to some extent the material. For example, I will use a courser pitch for a thread which I know will be disassembled on a regular basis because the courser thread will be easier/quicker to disassemble and less susceptible to damage or cross-threading. Since brasses are more prone to damage I tend to lean to courser threads in non-ferrous metals, and conversely I see finer threads as more of an option in ferrous materials. A connection which I know will rarely be disassembled has the option of being made a finer pitch, if there is any benefit to that. In fittings, especially where space, material clearances, or depth is often at a premium, a finer pitch is usually needed. I do quite a bit of work for others in the smaller scales, Ga1 in particular, and I prefer to use 32tpi for most of this work and stay clear of 40tpi if possible because of the tendency of beginners and non-ME steamers to be ham-fisted with small threads. Many threads on the models of a well-known Ga1 mfg are MX x 0.5, .5mm pitch metric threads, which are just plain delicate and are very easily damaged and cross-threaded. In any case, I suit my thread to the application first, and then material, and any strength worries I have typically aren't with the thread itself but how and where the thread is used.

[houstonceng]

Engineering practice I learnt "at my Fathers knee" was that :-

1. A Coarse thread - such as BSW - should be used in soft or friable materials such as Aliminium and Cast Iron (respectively), since it will have a deeper thread and more material between the crests - ie it will be stronger.
2. Fine threads - such as BSF - are easier to turn with a spanner, but should only be used on stronger materials such as brass, steel and monel metal
3. Studs into cast iron (eg engine Blocks and Exhaust manifolds) should be coarse threads into the casting and fine threads for the nuts on top.
4. Metric and SAE 60 degree threads are not as good as BSW - of the same diameter - for cast iron.
5. Of course (no pun), choose the thread to suit the wall thickness available. Sometimes, it's necessary to use a very fine thread because a coarse one would leave "nothing" of the tube behind.
6. "Stilsons", "Mole Wrenches" and adjustable spanners are for Pipe Fitters - not for Engineers. Always use the correct spanner.
7. If you can use a ring-spanner or a socket do so. Only use an open-ended spanner if you have no other choice.

The advice has served me well over the years.

In general, my addition to the list would be similar to GWRdriver. "Coarse" for often removed. "Fine" for things left in place.

[mutley]

Hi Paul
You say you spalshed out on a water gauge. I dont know of any one making boiler fitings with metric threads.
Andy

[paul]

mutley said:

Hi Paul
You say you spalshed out on a water gauge. I dont know of any one making boiler fitings with metric threads.
Andy

It is honestly Andy. A J Reeves is the place to get one @ £10.00 + VAT. The label on the back says 'metric water gauge'. Now I haven't actually tried it in a metric 'ole because I don't have any fine pitch threading equipment but I'm going to assume it is what is says.



The top and bottom bits appear to be 'dummies' - basically it's just 2 elbows and a glass with rubber seal at each end. I think I could make one pretty easily if needs be.

[GWRdriver]

Paul,
What you have there is a gauge glass made by Fyne Fort Fittings (I.O.W.), now no longer trading or in existence. I would be interested to know who, if anyone, has filled their shoes as suppliers of decent and inexpensive fittings in small sizes. I can't come close to making one for £10-worth of time and materials.

[Smifffy]

"The label on the back says 'metric water gauge'." Be careful, this means it can only be used on Metric water........

[houstonceng]

Paul

It could just be that the glass is a metric size - like 5mm - but the threads are ME (either 32TPI or 40TPI).

Beg, buy or borrow an set of Metric (60 degree) and Imperial (55 degree) thread gauges (or use a plug taps) to check the threads.

5 x 0.5 might indicate that the threads are 5 mm with a pitch of 0.5mm.

[GWRdriver]

Andy,
Typically that is how Fyne Fort labeled their goods. This would be an M5x0.5 thread on both ends.

[circlip]

If the top and bottom "dummies" you are referring to are the hex ended plugs Paul, how do you insert the gauge glass when you've screwed the fittings in? Also, when you screw them in, you need to be able to slide a drill or plain rod through both to ensure that you've lined them up correctly. Screw them onto the boiler with a SOFT copper washer and when you tighten the hex nuts onto the glass sealing rubber gaskets use FINGER tightness only. If the glass is 5MM as Houstenceng says, make sure you get some with the 0.5MM pitch threads on the end.

[Tel]

Simpler still, get aholt of an M5x.5 nut and see if it runs on the threads.

I bought a small pressure gauge recently that has an M5 thread.

[paul]

smifffy said:

"The label on the back says 'metric water gauge'." Be careful, this means it can only be used on Metric water........

Yeah I realised that - I've bought a special container that only holds litres of water

Hmmmm... coarse for oft removed yet also for non-ferrous. My bronze bushes have plenty of spare metal and so shall have coarse (saving £££ ker-ching!).

The gauge is definitely metric thread.

Now another slightly off-topic but related question:

How tight a fit should bushes be in the hole in the boiler? I planned to make them very snug but one hole is slightly misaligned and, as the bushes are quite long (they need to project through the housing) this might be fairly noticeable - i.e. the safety valve and regulator thingy would be out of kilter when fitted. By making one bush a slightly looser fit I should be able to angle it slightly to regain alignment. I suppose as long as it's well soldered and passes a hydraulic test it's ok?

I thought of making a jig to hold the two bushes at the same angle while soldering.

[baggo]

Hi Paul,

the bushes should have a thou or two clearance in the holes to allow the silver solder to penetrate the joint.

Back in the 'old' days writers such as LBSC and Martin Evans used to advocate making bushes and boiler tubes a tight fit in the plates which is now frowned upon as it leaves no gap for the solder to flow into. Martin used to tell you to file a few nicks around the hole to let the solder penetrate but although this would allow the solder to flow through to the other side you could still end up with no solder in the middle!

John

[Tel]

Baggo's right Paul, either a slackish fit or, if you want 'em snug for location, file a few nicks around the edges to allow some penetration

[havoc]

Some ream their holes to h/H7 but I just drill them with the drill of the same size as the bush or pipe that has to fit in. I'm not a proponent of the filed nick after seeing a nice boiler fail the test at those nicks.

[chameleonrob]

havoc said:

Some ream their holes to h/H7 but I just drill them with the drill of the same size as the bush or pipe that has to fit in. I'm not a proponent of the filed nick after seeing a nice boiler fail the test at those nicks.

if drilling large holes on thin sheet then the frilling of non-round holes will provide those nicks, even if the bush is a tight fit, in theory.

as to the strength of fine thread vs coarse threads then the strength is the same, double the number of threads each of which is now half the size, it cancels out. if made longer the fine thread would be stronger as the core dia would be larger. in ductile materials a fine thread is also stronger as there is less deflection in the threads themselves meaning that the tips of the thread are taking more load rather than just bending. all that being said it is assumed that you have the thread engagement which is harder with a fine thread due to the chance of drilling oversize.

rob

[Shawki Shlemon]

I fully agree with Andy on all the points ,he must have worked in aircraft industry . On holes for boiler tubes etc. ,I drill the holes in the lathe ,the drill/s in head stock and the plate mounted and clamped on a small table in the tail stock ( same as I described in boring cylinders between centres) , sometimes I have to clean the holes to fit the tubes in ,Yes clearance is very important for the solder to flow through and form a ring around the tube .The job is done once and no more attempts .

[Noddy]

If you're worried about steam leaks on threads, just wrap a bit of ptfe plumbers tape onto the male thread.

for strength of bolts, has everyone seen Al's simulation?:

www.varmintal.com/

Keith