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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: Potty:- Helping Dad Grasshopper Engine
26/10/2015 15:43:10

I'm going to make one! The straight line linkage is fascinating to watch, but not at the speed shown on UTube!

Thread: Phase converter / inverter
25/10/2015 20:23:43

provided you can reconnect the suds pump motors as delta you can run them at fixed speed from 240v single phase using capacitors. See

Edited By duncan webster on 25/10/2015 20:24:04

Thread: How to determine the spec of a pinion gear wheel
25/10/2015 16:50:41

HPC list 15t 0.75 module, for about £10. I daresay if you send them the bits they will advise

Thread: Elf n Safety police
25/10/2015 16:44:01

For some reason many people seem to think that the old way of doing things which involved hundreds of people being killed or maimed at work every year was a golden age. I don't. Unless doing work paid for by a council, they have no authority to impose Health and Safety procedures on private industry, although they do seem to employ some jobswoths who think they do. All that is required under HSE legislation is that you consider what might go wrong, how likely is it, how badly would you get hurt, and then take action to reduce unacceptable risks. It's not rocket science, and having done that you can tell any interfering busybody to take a running jump. Even the much promoted PAT testing is not a legal requirement, all that is required is that you have procedures in place to ensure the safety of portable electrical equipment. If you have another way and someone competent to do it get on with it.

If you think Health and Safety is expensive, try having an accident. and someone getting hurt. There is an unfortunate tendency to use H&S as an excuse for inaction.

Edited By duncan webster on 25/10/2015 16:44:56

Thread: What did you do today (2015)
23/10/2015 22:20:31

Just goes to show what dangerous people these gardeners are. Introduced Rhodedendrons, Dutch elm disease, asian hornets into France and so on. Leave the garden alone, something will grow, no doubt the butterflies will love you and you will have more time for the workshop.

Edited By duncan webster on 23/10/2015 22:20:52

Edited By duncan webster on 23/10/2015 22:21:11

Thread: Engineer's Blue
21/10/2015 21:39:33

Rouge is iron oxide, not lead. Still widely availabl in powder form. Don't know how finely it is ground. Also used as a very fine abrasive for polishing.

Thread: Drilling big holes in 304 stainless steel
19/10/2015 21:04:08

If you only take a little bit with a drill it is only cutting right on the corners and so will blunt very quickly. If the lathe will take it go for 15mm then 19m and bore the last bit to get a finish, HSS for preference. Keep it cutting or the stainless work hardens.

Thread: Climb Milling
17/10/2015 17:17:20

Hi Graeme,

is your Chinese model one of these?

If so can you let us know how well it works and how much air it uses? Don't want my compressor running all the time, but I have a little air pump I can use.

Thread: Motor Input and Output Power
13/10/2015 19:50:07

Kewtan asks 'So how does one prove it ?, can anyone suggest a simple way?.'

I'd use a rope brake, pulley on motor, rope over the pulley, weigth on one end , spring balance on the other. This measures torque output, you can get RPM with a non contact tacho, multiply them together in the right units and you have output power.

Note all the power is converted into heat in the pulley, so be quick about it and use a hemp rope so it burns rather than melts. if you want prolonged runs you arrange a chanel in the pulley and spray water into it for cooling, but it gets a bit messy at this point.

Thread: JB weld
12/10/2015 20:23:31

Depends on how much it has wandered off centre but you could consider making a drilling jig to centralise over where hole should be, drill it out say 4 ba tapping, make a stud threaded 4 ba one end, 7 ba the other and face off the 4 ba end so that it dissappears down the hole leaving the step just underflush

Thread: Climb Milling
12/10/2015 15:23:33

Thanks Emgee, another one on the 'to do' list.

12/10/2015 00:38:02


any more info on air mist system? If it extends cutter life it would be of great interest to me at least

Thread: Centre finding small holes
11/10/2015 01:14:10

The button method suggested by Nobby removes the potential error from concentricity in the chuck, but a 7mm drill will follow the 5mm hole anyway, so unless you were going to bore the hole it probably wouldn't achieve better results

10/10/2015 20:31:32

put a short length of 5mm bar in the chuck, enter it into the hole, clamp down, swap to 7mm drill and away you go.

Or if you're crude like me, just put the 7mm drill in the chuck and hold the workpiece by hand!

Thread: Propane Or LPG powered instead of coal?
09/10/2015 17:01:10

see also

09/10/2015 11:56:57

There was an article many years ago in ME about making a burner for a 5"g Metro, it seemd to work quite successfully. No doubr someone will find the exact issue number

Thread: Injectors
08/10/2015 19:17:38


you can't simply apply ideal gas laws to steam as it is a vapour, ie it can be liquified by increasing the pressure alone. However at pressures above the critical point (210 bar, 374C) the latent heat becomes zero, which has all sorts of good effects on cycle efficiency. Some power stations work up in this range, I wouldn't recommend trying it on your next model loco.

The bizarre effect is that liquid water cannot exist at above 374C, which sounds daft until you remember that liquid oxygen cannot exist above -119C. This implies that as long as the temperature is above 374C you can have steam at whatever pressure you want.

08/10/2015 18:06:42

Got a bit careless with mu brackets in earlier post try

density*speed^2/(2*g) = 62.4*136^2/(2*32.2) = 17922 lb/sq.ft = 124 psi,

08/10/2015 14:44:18

But we're not trying to put steam back into the boiler, we're putting hot water back in. The following simplified sum ignores nozzle inefficiencies:

If you expand steam from 80 psig through a properly designed nozzle to atmospheric it will have a speed of 1493 ft/sec.

If you then mix this steam with say 10 times it's own weight of water it will have a speed of 1493/(10+1)=136 ft/sec, but the flow is now water. Using Bernoulli equation if we then slow this flow down to rest it will have a pressure of density*speed^2/2*g) = (62.4*136^2/2*32.2) = 17922 lb/sq.ft = 124 psi, which leaves enough to cope with nozzle inefficiency etc. Nothing to do with latent heat, as Howard says it is to do with compressibility.

If you really want to make your brain hurt try an exhaust steam injector where steam at a few psi was used to feed water into the boiler against full pressure. I'm sure I'm not skilled enough to make a live steam injector, but can I challenge these demigods who can to make an exhaust injector?

Edited By duncan webster on 08/10/2015 14:47:21

Edited By duncan webster on 08/10/2015 14:48:29

08/10/2015 01:12:40

Julian'sexplanation is on the right track. The steam is expanded through the steam cone and converts its pressure into momentum. The water then mixes with the steam, and condenses in the combining cone. Mix of water and steam then has lower velocity than the steam jet, but higher density and so has the same momentum. The delivery cone then exchanges the momentum back into pressure to push the water back into the boiler. If the water is too hot it won't condense, so it won't work. The latent heat of the steam doesn't contribute, in fact if it were lower we should be able to use hotter water. This is simplistic as it doesn't allow for the inefficiencies of the nozzle.

Edited By duncan webster on 08/10/2015 01:13:41

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