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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: 08 Shunter in 2 1/2" Gauge
06/10/2015 12:36:59

HPC do 4mm chain and sprockets at not ridiculous prices. I managed to find a local gear cutter man who made me some sprockets cheaper than I could buy them, but so far back in time that I can't remember his name. I too have made sprockets my drilling holes on apitch circle then filing till the chain ran smoothly. You need something good on the radio as it takes a long time, but is very satisfying when it works.

Edited By duncan webster on 06/10/2015 12:38:25

Thread: axle pump
05/10/2015 17:56:39

Most axle pumps are too big. I've read somewhere that 1/250 of swept volume of cylinders is a good start.

Thread: help with drawings 280 nigel gresley
05/10/2015 17:49:21

It's not a Nigel Gresley, that valve gear looks decidedly odd. You best bet on the boiler is to join a club whose boiler inspector is qualified for steel boilers. Probably as rare as hen's teeth. Te Southern Fed booklet http://modelengineeringwebsite.com/New_boiler_test_code.html refers.

I don't want to be too pessimistic, but without material certs and welder certs I wouldn't be too hopeful.

Thread: sizes of superheater elements
23/09/2015 22:40:41

Definitely fit superheaters, and make them the radiant type. The theory and practical results are proven, see http://modeleng.org/articles/hall01.pdf, and an article by a chap called Wallbank in ME a couple of years ago.

As regards how big, I think the critical factor is the area exposed to the radiant heat of the fire, so as big as you can get in (probably 5/16" in your case). Put them side by side, not on top of each other, otherwise the top run is shielded from the fire by the bottom one.These will be well big enough to carry the steam.

Thread: Acme Rod
22/09/2015 19:10:39

see previous thread at http://www.model-engineer.co.uk/forums/postings.asp?th=65338

Thread: regenerative braking
21/09/2015 00:29:55

I've looked at the 4QD site, very interesting. It appears to be as I suggested in my 16:42 post, the motor is repeatedly shorted, allowing the dynamo back EMF to drive a current, and when this current is switched off the inductive spike pushes charge back into the battery. It must be a bit clever to know how long to short the motor to allow full current to build up without burning anything out.

As energy was taken out of the battery to accelerate the train, it seems that you can't overcharge the battery by regen braking as there isn't enough energy available. That is unless you start at the top of a long hill!

With a permanent magnet motor you can't increase the field to get more dynamo back EMF. Perhaps if you had a loco with 2 motors in parallel you could connect them in series for braking, or as Neil says switch batteries from series to parallel, but it's all getting a bit complicated.

I tried field weakening on a parallel wound motor once (Lucas dynamo, still got a couple under the bench). It worked a treat until I weakened the field too much when it stopped dead and drew a large current through the armature. Never understood that either. It didn't damage the motor as I switched it off quick time. Dead easy to reverse, just swap the filed, which is low current so toggle switch. All done with op-amps and a big FET. I'd use an Arduino now, but the dynamo is pretty big compared with modern permanent magnet motors

I think I've finally got it, (shades of Prof Higgins in My Fair Lady). Thanks for all your contributions

20/09/2015 16:42:20

Sorry Peter, I'm still confused.

I recognise that if the motor is being powered from a PWM you will get an inductive spike when the current is switched off (if you don't have a freewheel diode), but when the motor is not being powered, ie mark/space = 0, which is when you would want braking, there are no inductive spikes, all you have is the voltage generated by the motor as a dynamo.

Does it somehow short circuit the motor for a very short period to allow the magnetic field to build up, then break the circuit to create inductive spikes?

20/09/2015 12:15:05

Can anyone explain how regenerative braking works on a battery loco with a PWM controller? My simple mind says that the back EMF from the motor acting as a dynamo must be less than battery volts or you wouldn't be able to drive any current through the motor. If back EMF is less than battery volts how can it charge the battery? I'm obviously missing something somewhere

Thread: valve gear design sofware
01/09/2015 19:05:38

I've had a look at BBC basic, and it's tempting, but I'd have to rewrite everything again. Dosbox is solving the current problem, I'll keep slogging away at the graphics issue when time permits, but we retired chaps don't have a lot of time!

Thread: How to change M12 to 1/2" BSW drawbar thread?
01/09/2015 12:15:17

How to weld cast iron is described at http://www.lincolnelectric.com/en-gb/support/welding-how-to/Pages/welding-cast-iron-detail.aspx.

Nickel rods are stunningly expensive!

If you have access to oxy acetylene then sifbronze might be more attreactive

http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-oxyacetylene-and-bronze-welding-of-cast-iron/

Thread: valve gear design sofware
31/08/2015 19:18:36

When I moved from Basic to Pascal the book kept saying 'pascal is so much better because you can't do such and such in Basic', well you could do most of it in both BBC and GW basic, in fact the only thing I ever wanted to do that I couldn't was pass a function to a procedure, but I think I'd struggle to do that in C now anyway. The biggest difference I noticed was the speed, in the valve gear program on the old Beeb I had a message come up to say 'calculating, go and have a cup of tea,' as it took 20 minutes or so to do the sums, then it would give you the results and moving pictures. On my first PC with BorlandC++ Builder it drew the pictures as it went along, and I had to insert pauses so you could see what was happening, but Borland won't run on W7 unless anyone knows different?

The interweb reckons I can import the Borland Graphics Library, but so far I've failed. This is as easy as BBC graphics

31/08/2015 13:15:01

It must be 35 years since I learned BBC basic, then switched to GW basic, then to Pascal, then C. Is this an invitation to second childhood? Be there soon enough, but thanks for the suggestion

30/08/2015 20:11:10

I thought I'd replied to this earlier, but it hasn't appeared.

Thanks to everyone who replied, I took the hint from clivel's reply and installed Dosbox on W7, works a treat. Why didn't I think of that?

Thread: Thin heat shield for soldering
28/08/2015 23:56:24

You can get Ceramic Paper

http://www.hightemperaturetextiles.com/needlemat-blanket-paper/refractory-and-ceramic-blanket-and-paper/ceramic-paper.html?gclid=CLWJmrrvzMcCFYYfwwodr6YH8w

I scrounged a bit from a local furnace repair company for doing a very similar job

Thread: Check valves
28/08/2015 23:50:59

The seat needs to be perfectly round, drilled hole no good. Reamed or better still D bit. Having said that I use O ring seated valves, which do not leak. Anyone want the drawing send me a pm, no idea how to attach it to this site.

Thread: valve gear design sofware
28/08/2015 23:47:15

Bit off the metal bashing themes, but I never cease to be amazed by the expertise out there..

Many moons ago I wrote some software to design and analyse valve gears using Borland C++ compiler and the Borland Graphics Interface. It ran on Windows 95, which I think in reality means it ran on DOS.

I cannot for the life of me get it to work on Windows 7, and it didn't work very well on Windows XP, it ran until it tried to save data then crashed. I know there is Dockstader version, but mine worked out the maximum die displacement, max swing of expansion link etc, which others don't. Apart from finding a geriatric computer anyone got any ideas how to get it to run? Linux?

I've started rewriting it in CodeBlocks, but getting graphics has defeated me

Thread: Drawing Software?
28/08/2015 15:47:03

Draftsight is free and good. Transition form Autocad was seamless, similarly Progecad. No experience of any others. Version of Draftsight for Linux available I think

Thread: Stent Cutter grinder
28/08/2015 15:43:02

Let us know if you put it on e-bay

Thread: Draw bar thread size for a 2 MT Clarkson autolock milling chuck?
25/08/2015 15:14:56

Mine is 3/8" BSW

Thread: Why reverse a lathes direction?
23/08/2015 21:49:56

Michael, if the motor is single phase with a centrifugal switch on the start winding, switching direct from forward to reverse without waiting for it to slow down for the centrifugal switch to re set will surely result in the motor carrying on going forward.

This is because reversing a single phase motor alters the connection of the start winding, but if the start winding is disconnected by the centrifugal switch, it won't know.

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