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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: Steel for machining
05/10/2018 11:00:09

Cast iron is not stiffer than steel, in fact it's Youngs Modulus in tension is considerable less. Considering that the load is put into the overarm via the much smaller arbor it is quite obvious that high tensile steel is not required, as others have said the original would have been cast iron. Using a 'better' grade of steel will not reduce elastic deflection. Making this out of one piece is just making life difficult, Neil's suggestion is far more sensible. I would just make it out of 2 pieces of EN3 (070M20) or S275, but the latter would not machine as well. If the OP really must have it from solid then a plasma cut blank would be a better starting point

Thread: More powerful batteries to make steam?
30/09/2018 23:27:43
Posted by Robert Atkinson 2 on 30/09/2018 22:22:05:
Posted by duncan webster on 30/09/2018 17:33:26:

AC still electrolyses the water, it just swaps the anode and catode around so you don't erode the anode.
It's not unsafe s long as the case is well grounded. Early designs of electric water heares and showers had a bare resistance element immesed in the water.


I'm sure you're correct, but I'm confused (no change there then) If the water is still electrolysed by AC, then it is split into hydrogen and oxygen. This sounds like a recipe for a bomb unless there is a divider beween the anode and cathode extending below water level, which you cannot acheve with AC. According to


it is all down to current density, keep it low and the gasses recombine as they are created. I'm still not going to make such a boiler

30/09/2018 17:33:26

I think I might have been the gunner when SOD got shot down, but back in the 1920s/30s there was an article in ME about a mains heated boiler. Heat was generated by passing electricity through the water. They just connected neutral to the shell and had a big insulated electrode inside connected to live. You could actually make a safe version of this if you had a big enough transformer to get the volts down to something non lethal. You could then ground the neutral. This would generate enough steam for a little stationary engine. It has a built in safety feature, if you let the water run down, there is no path for the electricity, so it doesn't overheat. You have to use AC so you don't electrolyse the water

Thread: scam?
29/09/2018 18:20:32

I've got an ad in classifieds, and have received an e-mail from a Nicole Curtis. It reads

Good Day to you.Am interested in your advert posted and i will like to know the present condition of it and the last price with pics..Mind you,i will be responsible for the pick-up of it at your locations okay.Hope to read from you soon.


I think this is a scam, as the grammar is a bit odd, and Ms Curtis would not usually sign with just a surname. Anyone else had dealings with this person

Thread: non slotted screws
29/09/2018 18:10:19

Using countersunk screws in shear is not good practice. For starters for shear you should have a non threaded length through the hole so that the bit in shear is full diameter and the sideways force is passed from the hole to a close fitting full diameter shank. Second if relying on the countersunk head to transmit shear you will find it comes loose as Journeyman says. As others have said, hammering 1/8 rivets into a countersunk hole is not all that taxing. It makes it easier if the snap supporting the round head is mounted on something heavy so that all the hammer blow goes into deforming the rivet, not deflecting the support

Thread: gents slave (unipolar)
28/09/2018 10:33:11

Thanks for confirming my thoughts on resistor. There must be some limit on this, you couldn't have it on for an hour then off for 150 hours. Must be to do with how hot it gets during the on time.

John, I've done the dodge of using a quartz clock movement driven from my electronics to get me going on another clock project, but I want to make my own, preferably one that moves every 30 seconds.This is shown in Michael's link, but the Gents moves 1/4 turn every step, and looks subtly different to the Lavet. I've got one of those as well, but I don't want to destroy it by taking it apart to measure everything up. Looks like I'll have to experiment, I was hoping someone would have done the spadework, I know, idle so-and-so. I've also done it the other way round and used the electronics from a quartz clock to give one second pulses to drive a normal Synchronome slave, again via some electronics

here's a couple of pictures of one of my stepper driven slaves, the brass gear is just an idler which connects the stepper to the extra gear on the minute shaft 1/48 of a rev of the stepper (little steppers are typically 48 steps per rev) moves the minute shaft 1/240 revs, ie quarter of a minute. The electronics then advances 4 steps per minute. All done with stock gears, I'm not brave enough to make my own yet, but it's on the list. On my rescued Blick, I geared it 1/48 to 1/120 and advanced every half minute. I'm planning to take the Blick apart again soon to fit microswitches to the one rev per day wheel, I'll take some pictures then

stepper clock 2.jpg

stepper clock 1.jpg

27/09/2018 22:11:13

I think I'm answering my own question here, but if someone could sanity check it I'd be grateful
I'm building some electronics to drive a Gents slave from another clock. I've found out via Google that I need 220 mA pulses lasting 200ms every 30 seconds, and the coil resistance is 4 ohms. I have a dropper resistance, actually 3 off 82 ohm in parallel giving 27ohm effective. I'm driving it off 8 V, so I will get 255mA, actually a bit less as I haven't allowed for the FET. So far so good. Now for the question! If it was energised full time each resistor would dissipate 592 mW, but it is only on for 200ms every 30 seconds. Can I get away with 0.25W resistors?

Whilst I'm on does anyone know where I could get drawings for a silent bipolar slave? Or even a silent unipolar. I've built 2 slaves using stepper motors, but there is a certain fascination in making the whole thing using a ready made stepper is cheating!

Thread: O ring Pistons cylinder Clearance?
25/09/2018 23:51:15

if you go down the small clearance no ring route machine some vee shaped grooves in the piston. I'm not sure whether it acts as a labyrinth seal or just traps a film of oil but it seems to work by all accounts

Thread: A question for Thomas the Tank Engine fans
24/09/2018 15:18:15
Posted by Neil Wyatt on 24/09/2018 12:42:06:
Posted by Alan Jackson on 24/09/2018 12:19:40:

Politically corrected, the Fat Controller has been renamed Mr Topman

I think he was always known as 'Sir Topham Hat'...

Only in the USA version

Thread: Pins for a Blick Clock
22/09/2018 15:01:12

Bike spokes seem to be too big diameter, I'm not fiddling about trying to turn them down.

Edited By duncan webster on 22/09/2018 15:01:24

22/09/2018 11:20:10

I'm bringing and old Blick master clock back to life. The synchronous motor was completely beyond repair, so I've replaced it with a stepper and that is all working fine. What I want to do now is fit pins into the 24 hour wheel so it will ring bells (and drive senior management to distraction). The holes in the wheel are 12 BA, so I need to make 96 off about 10mm long with a short bit of thread. I can get 1.2mm TIG filler wire, which would be absolutel ideal, but could someone out there measure a bit and see how accurate it is? For it's intended purpose as filler it wouldn't matter if it was 0.1 up or down, but would make all the difference to a 12 BA thread

Thread: Convex buffer face
20/09/2018 12:04:24

Industrial locos ofetn had collosal buffers to cope with sharper curves

Thread: Cycle chain drawing
19/09/2018 19:57:08
Posted by Alan Vos on 19/09/2018 17:57:20:

I believe this works. The key is making GH equal to the radius of the two circles.

tangent construction.jpg

AB is the line between the centres of the two circles
CD is the defined line, parallel to AB
E is the centre of the desired circle

Construct the perpendicular bisector of AB, this intersects CD at G)
Construct H such that GH=AF=BI
Construct the perpendicular bisector of AH
Those two perpendicular bisectors intersect at E
The desired circle has radius EF=EG=EI

As a check, construct the perpendicular bisector of BH, which should also pass through E.

Neat, making 2 equilateral triangles. Wish I'd thought of that

Thread: Today's mystery object
19/09/2018 14:33:48


Why didn't I spot that! I've had a bit of a dismantle and found there is a laser pointer down the middle, obviously replacing whatever was there before, and the level on top is diffenent as well, but I'd still like to know what it was before it was modified, the rack and pinion thing would seem to be of little use in any long range instrument, but then the bottom bit with the 360 degree protractordoen't look like it was originally part of the top bit, the paint is completely different. Perhaps it's the ultimate bitza, a microscope body mounted on an old theodolite base?

19/09/2018 13:55:25

img_3366 (small).jpgBeck of London were into making microscopes, and this looks like some kjid of surveying device, but you can't see through it, there is a plastic? disc in one end as seen in photo. There is a rack and pinion to move the top member back and to, and levelling screws underneath. Anyone got any ideas?img_3365 (small).jpg

Thread: M5 x 0.5 threaded bar supplier
19/09/2018 00:43:09

make a die holder to go in the toolpost and then you can power feed the die at the right pitch. Relying on the die self feeding will guarantee pitch error which might or might not matter. If you have a travelling steady you could screwcut it down to say 3/4 depth then finish with die

Edited By duncan webster on 19/09/2018 00:43:29

Thread: Cycle chain drawing
19/09/2018 00:40:21
Posted by HOWARDT on 18/09/2018 20:42:53:

The circle command is TTR, tangent-tangent-radius.

But you don't know what the radius is, you need ttt

18/09/2018 19:57:20

The easy way is to use the ttt command in CAD. This gives a circle 13.66 diameter centred 9.08 from the centreline.

Otherwise I think you are into Algebra rather than Geometry.

call the radius ofyour big circle R. The distance from centre of big circle to chain pin is (R+4.25) From centre of big circle to centre line between pins is (R+2.25), so by Pythagorus (R+2.25)^2 + (12.7/2)^2 = (R+4.25)^2 which can be easily solved as te R^2 terms cancel out.

Thread: Small Carronade model - 68 pdr.
16/09/2018 10:15:47

No confusion, the recipe calls for both

Thread: Arc
15/09/2018 23:20:51

If we're getting into politics (I thought that was banned?) perhaps after March Ketan will have to be Arc Not Trading With Europe

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