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Member postings for duncan webster

Here is a list of all the postings duncan webster has made in our forums. Click on a thread name to jump to the thread.

Thread: Clarkson's Horizontal Compound
29/08/2018 10:10:50
Posted by FMES on 29/08/2018 07:51:55:

Hi John, am I guessing you are running on air?

There is no expansion available using air so the pressure supplied to the second cylinder wil be lower, also you are relying on the exhaust from the first engine to be exactly in time with the inlet to the second.

If the two cylinders are the same bore then I would suggest that you run it as a 'twin' rather than a compound.


There is some expansion running on air, but not as much as on steam. For instance if you start with 40psig steam (55psi absolute) and double it's volume you will have 27.5 psia, 12.5 psig. With air you get a pressure ratio of 2^1.4=2.64, so 21psia, 6 psig.

I agree if cylinders are same bore it will run a lot better as a twin, but I don't understand Jason's suggestio at all. If the OP has managed to get it running sweetly with 4 bearings in line, leave it alone

Thread: Yet another parting tool question...
28/08/2018 23:44:51

If I buy one of Ketan's SLTBN parting tool holders and bolt it down rigidly through the tongue onto a block at the back of the headstock with the blade in upside down does this sound like a recipe for disaster? Would I be better with the holder upside down as well, even if it does make getting at the securing screws a lot harder?


Thread: Colchester Headstock Sleeve
27/08/2018 16:54:02

When I had a Chipmaster with the taper thing missing I convinced myself it was Jarno, which is as


Doesn't help you in finding one of course, but I never needed one, for between centres just chuck a bit of bar i the 3 jaw and turn a point on it

Thread: Yet another parting tool question...
27/08/2018 13:29:06
Posted by Grizzly bear on 27/08/2018 13:25:02:

Riley tool post mentioned here:


Thanks, it's a GHT mk2

27/08/2018 11:16:03
Posted by Brian Oldford on 27/08/2018 09:43:01:

I must confess I've never had a great deal of success with parting off until I invested in a Riley style rear tool post. Since then life has been pretty much stress free.


So what's a Riley style rear tool post look like. I've tried Google but it just throws up lots of GHT type

Edited By duncan webster on 27/08/2018 11:16:20

Thread: What did you do Today 2018
25/08/2018 15:37:22
Posted by Andrew Johnston on 24/08/2018 10:03:50:

The smokebox is over scale thickness. But the bolts are long enough for nuts and washers to be fitted internally, as were the now defunct studs.


Seems better to have nuts on outside then not exposed to corrosive atmosphere insoide smokebox and easier to get off again.

Thread: Aluminium eccentric
25/08/2018 12:22:15

Old Triumph twin motorcycles had the big end pin running direct on the aluminium con rod. They seemed to work quite well, but for some reason aluminium eccentrics doesn't feel right. As it's a difficult job to replace the eccentric and easy to replace the strap I'd stick to the conventional way. The mass of the eccentric is not going to change your life.

Edited By duncan webster on 25/08/2018 12:25:29

Thread: Solder..?
24/08/2018 00:33:45
Posted by Roderick Jenkins on 23/08/2018 23:36:51:


You'll note that I said "suspended" in water. Being an empirical sort of guy I have just measured a cut off a rectangular steel bar and calculated the vol at 94.67 cc and weighed it at 737g. This gives a density of 7.79 which seems reasonably correct for mild steel. I then suspended the bar from a thread and lowered it into a bowl of water that was sat on a tared scale. The indicated weight was 95g which equates, to sufficient accuracy, to 95cc. I'll let you do the algebra, I can't remember how.



That's not what you suggested in your first post ' weigh it suspended in water'. This to me at least means having the lump of lead/steel/whatever hanging from a balance, not what you've just described. Yes this second way measures the volume directly since 1cc of water weighs 1 gm. You need some conversion factors to do it in Imperial units

23/08/2018 17:56:38


not quite

dry weight is density * volume

weight of water displaced is (density of water) * volume

so wet weight is (weight in air) * volume - (density of water) * volume

go through the algebra and you get density = density of water / (1 - wet/dry)

as a worked example, if a lum of stuff had a dry weight of 17 gms and a wet weight of 15.5 gms, its density would be 1 gm/cc /(1-15.5/17) = 11.33 gm/cc

Thread: Tractive effort
23/08/2018 17:36:20

The loco has to start all the trucks, well something has to. If it is loose coupled it doesn't have to start them all at once, but if they are on roller bearings it doesn't make a lot of difference as static and dynamic friction of the bearings are much the same. For full size rolling resistance work on about 10lbs per ton rolling resistance, pull needed to go up gradient is load * gradient.

For your 6000 ton train on a 1 in 100 gradient this comes to 6000*10 + 6000*2240*1/100 = 194400 lbf

With a 6000 ton train you probably need to add a bit for curvature unless the railway is very straight.

Thread: Make some Money From Your Workshop
23/08/2018 09:25:38

Bill, PM sent


Thread: How long to build?
21/08/2018 20:34:53

nip one side down to give a bit of friction and leave the other slack. Then it will rotate around the first. I normally get mine within a couple of minutes, but there agin 0.01 mm is 4 tenths, do you really need it that good?

20/08/2018 23:58:36

It took me 25 years to make my first loco, but then I did get married, build 2 extensions, have 3 children during the same time frame. The second loco only took about 2 years!

Edited By duncan webster on 20/08/2018 23:58:57

Thread: Calling all Kiwis
20/08/2018 22:09:45
Posted by DMB on 19/08/2018 12:27:21:

Still is rip off Britain.

I bought a Proxon saw including delivery direct from a retailer in Germany for less than the UK importer pays for it. No hassle at all, just pay by paypal or credit card. Had to change the plug, but that's no big deal. Always worth checking price in other countries.

Thread: turning on switch mode PSU
19/08/2018 22:08:09

Thanks, I get that. I'll attach my oscilioscope to it and see what wave form I get out with the fan and valves off. It won't be soon as it's not easy to get to at the moment, but I'll let you know what happens.

19/08/2018 17:24:30

In the latest issue of ME Mr Editor describes a mains to 48v switch mode power supply and how he has arranged for it to have a load before it is switched on. Is this general for all such devices? On my Topsy organ I have one giving 24v (5A max) which is permanently connected to a 24-12v converter, which in turn drives a permanently connected microprocessor, so the 48v one has a load on start up, albeit very small. It only takes serious load when the 24v fan starts up and the 12v valves start operating.

I could fairly easily arrange a bigger load which switches off after 5 seconds, or attach a somewhat bigger load permanently, but I'd rather leave it alone!

Edited By duncan webster on 19/08/2018 17:34:45

Thread: Thermic Syphon
16/08/2018 22:34:41

I didn't express myself all that clearly. Yes a Chartered Engineer with professional liability insurance would sign off the calculations (assuming they are correct of course), but you are going to pay for his time and the insurance will be built into his hourly rate. This is going to get expensive, and I doubt ME or MEW is going to cover the cost for designs they publish. The sums are not terribly difficult, and are well set out in the Australian code. I don't think there is any requirement in UK regulations for independent checking, so do them yourself. What we don't have is a standard for acceptable stresses which is traceable back to national standards. LBSC said this or Martin Evans said that will not wash I'm afraid, even if they were correct. 

The chances of a reasonably designed copper boiler which has passed a 2 times pressure test and a steam accumulation test exploding in service are very small as copper is such a ductile material, so let's not get too carried away. There are loads of known good designs available, just adapt one of those and check the plate thickness and stay spacing.

Edited By duncan webster on 16/08/2018 22:35:07

Edited By duncan webster on 16/08/2018 22:53:05

Edited By duncan webster on 16/08/2018 22:59:11

16/08/2018 11:48:44

The difference in thermal conductivity between steel and copper firebox makes no sensible difference to the heat transfer. The main heat drop is from the hot gasses to the metal, temperature drop through the metal is very small

Thread: Aluminium thread strength
15/08/2018 21:51:36

This website goes into thread depth in some depth (sorry)


Thread: Vertical Boiler Fittings
15/08/2018 21:30:44
Posted by Neil Wyatt on 15/08/2018 18:28:43:

There's always Boss White for the impecunious.


sounds like 'Harwich for the continent, Frinton for the incontinent'

Edited By duncan webster on 15/08/2018 21:30:58

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