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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: Chain Hoist Tripod in MEW 264
07/08/2018 17:31:50

A lot depends on definition of safety factor. I tend to use proof stress as my criterion and apply a safety factor as defined in the BS for cranes. If you exceed the design load by a lot, it could permanently bend but you'd need even more to break it, but for buckling if it goes it goes with a wallop, hence higher safety factor. Was involved in the enquiry after a scissor lift cylinder gave way, not a pretty sight, fortunately no-one was underneath.

Thread: O rings for pistons
07/08/2018 17:14:59

Thanks for the link, I'll read it with interest, but I'm going to use O rings to start with to get it going. I'll fit Peek later on, but at least I now know how. I'll pm you if (when) I get stuck! Just from a quick scan if I use the methodology in the link but halve the expansion coeff I should be OK. Looks like the section is 0.1" square, but not at all clear.

Edited By duncan webster on 07/08/2018 17:19:19

Thread: Minnie 1" boiler miricale needed
07/08/2018 14:46:44

I'm not sure I've got this correctly. Usual sequence is to silver solder the tubes into the firebox tubeplate before they are fixed to the smokebox tubeplate, so if OP has got the tube out how is it 'in the middle of all the others'?

A photo would help

Thread: O rings for pistons
07/08/2018 13:08:09

Before I make an expensive leap in the dark I hope I can get some advice. I'm about to bore my gunmetal cylinders. Diameter is 28mm. I can get Viton rings in 1.5, 2, 2.5 and 3mm sections. Which ones do I want? Viton seems to be OK at steam temperatures.


Eventually I want to try Peek rings. Anyone done this? If so what grade and section (width/depth) did you use, and did you put an O ring underneath to energise the ring?

Thread: Engine plans
07/08/2018 12:32:56

Why don't fraction-o-philes have micrometers which read in 1/1024" (the logical progression from 1/2, 1/4 etc)? Simple answer because fractions are extremely unwieldy when you get beyond simple stuff. This from someone who started out bi-lingual in cgs and Imperial, but for the last many years of employment used only SI for calculations because it's less prone to errors introduced by the wierd 32.2, 384, 770, 550 factors which you have to remember. As for someone brought up in metric being encouraged to learn fractions, let's get real. If young people are to take up model engineering they are not going to learn a new illogical language.

Thread: Having trouble turning grooves
07/08/2018 10:57:28
Posted by Andrew Johnston on 06/08/2018 19:21:30:

I hate 4-40UNC screws! They're the standard for screwlocks on RS-232 connectors. Albeit brass, but I've lost count of the number of times I thought just a final nip up, and ping the **** screw breaks. I've had the same happen with 3/16" BSW bolts, so I'd expect 1/8" BSW to be similar. All basically too coarse a pitch for the diameter.

In my experience "professional" far eastern tooling uses metric screws, although made of cheesium and therefore need replacing. Hobby grade far eastern seems to use BSW for some reason. No idea why?


I totally agree about 3/16 BSW, for some reason it's the tap most likely to snap. Must be to do with the thread depth/core diameter relationship. However 1/8 BSW is 40 tpi, and seems benign by comparison. All academic, I use metric for preference nowadays

Thread: Crankshaft repairs
07/08/2018 10:26:53

Welding of alloy steels is best avoided. The rapid heating and cooling gives rise to a hard and brittle zone. Yes it is possible to avoid it but not easy

06/08/2018 14:24:00

I wouldn't even think about welding, the heat would do all sorts of unpleasant things to the base material. You would still have a lot of machining to do, make some new ones. As others have said, EN24 is probably a good start, not quite as strong as EN27 (3% Ni) but I doubt that would be a problem, at least you can get hold of it fairly readily

Thread: Chain Hoist Tripod in MEW 264
06/08/2018 14:13:25
Posted by Ian Austin on 06/08/2018 11:28:25:
Posted by duncan webster on 06/08/2018 10:47:16:

That's not a structure, it's a mechanism. Even with the bottoms of the legs tied together it can all move.

Wouldn't that only be if the lugs holding the tube sockets bent across the plane of action? As in, two of the lug-pairs would have to bend as the third leg moved over the centre? Or am I not visualising that right?

Re scouts - yes, we made one like that at scouts too -- which has just reminded me of a description of that type (rope and timber poles) in the Admiralty Manual of Seamanship 1952 edition vol2, pp 218-19, as an "extempore gyn" -- I'd forgotten about that, just looking at that now.

Edited By Ian Austin on 06/08/2018 11:34:41

the gap between the vertical lugs looks a lot wider than the blade on the end of the upright, and there will be clearance on the pin, so it will wobble until some clearance somewhere is taken up.

I too used to be a scoutleader, and have supervised the making of dozens of tripods out of rope and pioneering poles. Always either tied the bottoms of the legs together or dug a hole and buried them

06/08/2018 10:47:16
Posted by Ian Austin on 06/08/2018 08:35:23:

Here is a sketch of the scaffold-pole tripod head that appeared in a second-hand advert, mentioned above.

scaffold pole tripod head sketch.jpg

That's not a structure, it's a mechanism. Even with the bottoms of the legs tied together it can all move.


Edited By duncan webster on 06/08/2018 11:07:50

Thread: Prototype Mechanical Crab
05/08/2018 18:57:32
Posted by Neil Wyatt on 16/07/2018 17:12:10:

Lovely job.

Now you need to make a big one

Edit reading this Klann's linkage has many advantages over Janssen's (in the video).

Edited By Neil Wyatt on 16/07/2018 17:19:30

Wonderful, you need to get someone to do a construction series for a smaller one

Edited By duncan webster on 05/08/2018 18:58:07

Thread: Having trouble turning grooves
05/08/2018 18:48:48

On my to-do list is to make up a small version of Tubal Cain's (the English one) Gibraltar toolpost for my Perris/Cowells. Why? The top slide is always in the way unless you angle it a bit when it doesn't support the tool all that well. If I want to turn tapers I can very quickly put the topslide back on

Thread: A Replacement circuit board
04/08/2018 22:21:53

I've tried goog;ing the chip, no success, has anyone else found what it is? I'm guessing PIC of some kind, but having a pinout would be a good start

Edited By duncan webster on 04/08/2018 22:22:09

Thread: Milling Table "fault"
03/08/2018 22:58:09

Can you masure the thickness of the table from top to bottom sliding surface front and back. If the 2 measurements are the same then I think the fault must lie elsewhere

Thread: Inverter Drives for Motors
03/08/2018 19:15:36
Posted by Samsaranda on 03/08/2018 18:24:12:

Duncan in respect of PAT testing if you read the government schedule that relates to it there are two levels, the so called full electrical test and the lower level of a visual only check, the requirements for each are defined. It is not compulsory to carry out full PAT testing as some would have us believe, the relevant gov document outlines what and when is required. You seem to be of the opinion that it is a con and I concur.

Dave W

I think the relevant legislation is PUWER, Provision and Use of Work Equipment Regulations (or at least it was when I was still working), which require there to be a system in place to check that equipment is safe to use. I have absolutely no problem with this, but the guy doing it should know that you can't check earth leakage on a plastic earth pin, and he should check the cable, in fact I'd say the latter is the more important, as you can easily have bare conductors or damaged insulaton and not find either with a test box. Having the same regime for desktop computers which live in a nice friendly environmet and power tools used in a workshop is nonsense, but that's how some companies do it.

03/08/2018 18:01:24

When I was still working for money they used to send a man round every now and again to PAT test all the 'portable' equipment. All he did was plug it into a box of tricks and if all green lights he'd attach a sticker and go on his way. When I asked him what he was testing for he maintained he was checking for earth leakage. He couldn't understand that for eqipment with a plastic earth pin (double insulated), the concept of earth leakage is a little difficult. Never even looked at the state of the cable, checking for damage/abrasion. Not a very competent peron in my book, and this was a big company.

Edited By duncan webster on 03/08/2018 18:02:54

Thread: Extra axis on lathe?
02/08/2018 22:11:03

There used to be a thing called 'Quickstep Mill' which clamped in the toolpost


If I came across one of those Arrand spindles I'd buy it like a shot (see Roderick above)

Thread: help with threading
02/08/2018 10:07:10

Leave the topslide set parallel, but every time you advance the cross slide by 10 thou, advance the topslide by 5 thou. This way you generate the thread angle and the tool only cuts on one side. Then you can have top rake on the tool, it only cuts on one face, and you don't get 2 ribbons of swarf colliding. If you set cross and topslide dials to zero when the tool is just touching the OD, the topslide keeps a record of where you are up to as well.

Works every time. You do actually get a very light scrape on the non cutting side, just cleans it up.

Thread: Steam Turbines Large and Miniature
02/08/2018 09:59:59

many years ago I worked for Greenwood and Batley on small impulse turbines, deLaval, Curtis and Rateau type. The major reference books are by Kearton and Stodola. The big problem with very small turbines exhausting to atmosphere is windage on the rotor, a lot of the power is absorbed just turning the wheel. The wheels should be as small and fast as possible to minimise windage, with a close fitting shroud against the wheel. Stumph type, whilst attractively simple to make, have the blades too widely spaced for efficiency. G&B's nozzle/blade design man reckoned that even with vacuum exhaust a reciprocating engine would give better efficiency at less than a few hundred horse power. It would however cost a lot more, and need a man to look after it full time, whereas a turbine would just run.. The smallest deLaval types ran at very high speed and had bulb root blades, but the Curtis type ran slower and had tee slots round the wheel into which blades were fitted, much cheaper.

When the DO stores ran out of drawing paper because they couldn't pay the bills I left and moved into gas turbines. Pity really, G&B could make anything bar a profit!

Thread: Thread cutting with carbide inserts
28/07/2018 22:33:12

I use HSS for screwcutting, and much else. Carbide seems like a waste of money for a lot of work, fine if you want big cuts at high speed, but then you need lots of power and rigidity, which most hobby lathes don't have.

Even using HSS I wouldn't take anything lie 20 passes to cut 1/2" BSF, it's only 60 thou deep

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