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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: Model Turbines
12/03/2019 11:28:58

Mea culpa, I've got it wrong! If you are stuck with a low blade speed, there are advantages in the ejector idea, but you'd still be better off with steam only and very high speeds

Blade force is mass flow rate * change in whirl velocity, so increasing the mass flow rate for a given blade speed increases the blade force. If for steam only we have a mass flow of M kg/sec and a blade speed of B, the blade force is M * B * 2, and the power output is force * blade speed = M * B^2 * 2

Mixing in an equal mass of air, the mass flow is 2M, for the same blade speed, the blade force is 2M * B * 2, and the power is 2M * B^2 * 2, so there is an advantage.

You're still better off with higher blade speed tho as the change in whirl velocity increases as well.

11/03/2019 21:04:20

I'm not sure this ejector idea works. The force exerted on the blades is the momentum flow rate * the change in whirl speed (see below). For single stage impulse the change in whirl speed is roughly twice the blade speed (up to the limit of blade speed = half steam speed). The mix of air and steam from the ejector has the same momentum flux as the steam alone, so for a given blade speed the blade force is a constant. However, the power output of the turbine is blade force * blade speed, so the for a given blade speed the power is the same. However, if you can physically achieve it, you can have a much higher blade speed for steam alone, hence higher power.

Newton's second law, usually expressed as F=m*a can equally well be expressed as F = rate of change of momentum

Thread: Centre finder?
10/03/2019 11:11:15

I just have a bit of 3/16" rod about 12" long with a point on one end. Grip it in the tailstock chuck and poke the point into the centre punch mark and run your DTI on the outside. The point doesn't need to be concentric as the rod isn't rotating. What I do to keep the point pushed against the pop mark is arrange for the handle on the tailstock handwheel to be off balance to push the rod along the bed. Saves taking the chuck out to put the centre in. If I were to improve it I'd loctite a 1/2" diameter collar up at the pointy end, then you could use the dti with more eccentricity. The fact that I haven't bothered in all these years means that either I'm bone idle, or it wouldn't make that much difference

Thread: 15mm and 7/16" diameter HSS tool steel bar
10/03/2019 00:42:51
Posted by Adam Harris on 09/03/2019 22:04:07:

Duncan £100 is well outside my target!

Ow! I didn't look at the price. How about £16 for a set of 4


Edited By duncan webster on 10/03/2019 00:44:03

09/03/2019 21:31:40

How about one of these, loctite a sleeve on for the 15mm, turn it down a tad for the 7/16


Thread: Watchmaker's vertical slide
08/03/2019 23:51:32
Posted by Howard Lewis on 08/03/2019 21:38:39:

Maybe Herr Lorch did not feel a need for increments of less than 4 degrees?

Could you add another fiducial point to provide a vernier effect, giving a 1 degree increment? At first and not deep thought, this would seem to be 5 degrees from the first.

Anyone want to comment on that instant thought?


you'd need more than one extra mark, but I'd need to put my thinking cap on to work out how many and their spacing

Thread: Hardened Silver Steel Shattered - How to Avoid?
08/03/2019 23:48:53

Get hold of a copy of Tubal Cain's book on hardening, tempering and heat treatment, it explains everything. You can borrow mine if you want, but I want it back as it's very good.

Thread: LED lighting
08/03/2019 18:08:40
Posted by AdrianR on 08/03/2019 17:16:48:


The 10W is at the voltage of the LEDs, If say you need 60V, 10W is 167mA. Using a resistor you still need to have 167mA. The resistor will be dropping 180V @ 167mA, so would be need to be 30W.

A transformer converts the power, ignoring losses in the transformer, 240V @ 42mA in, 60V @ 167mA out .

Ah yes, forgotten that. The (burnt out) switch mode power supply increases the current from the mains, so it's definitely not a good idea. I wasn't going to drop all the voltage in the resistor, the dimmer would have reduced the effective voltage.

Thread: Sports springs and firewood.
08/03/2019 14:19:32

Vectras were renowned for breaking rear springs as well, makes the handling a bit interesting when you've got 4 people plus luggage in and only 3 effective wheels

Thread: Denford CNC Lathe ?Viceroy based
08/03/2019 14:16:44

How big is it? If it's based on the 5" manual lathe I would have thought you'd easily sell it as a fixer upper

Thread: Line boring question
08/03/2019 14:07:09

This might be a daft idea, but unless you want to bore all the bearings at exactly the same diameter (ie without moving the cutter bit) could you have several cross holes in a shorter bar and move the cutter along.

You could also use a bar nearly up to the drill size but file flats where the tool pops out to leave some space for chip clearance

Holding one end of the boring bar in a 4 jaw rather than between centres makes the whole setup stiffer as well, just don't take the boring bar out till you've finished.

Thread: LED lighting
08/03/2019 14:00:10

Having read the above I think that is going in the 'not quite such a good idea' box. However, to clear up some issues;

the 40V was rectified but not smoothed, so the peak would be 56V or so. This must have been enough to light them up dimly. If it were 2 lots of 12 I think I'd have let the magic smoke out

I wasn't thinking of using a capacitor, but a trailing edge dimmer with a say 100 ohm resistor. If 10W at 240 V is 40 mA, the power in the resistor is way less than 1W. When I come across a decent size 100 ohm resistor I might resurrect the idea

Why bother? I just can't bear seeing things go to waste if they can be used for something else, and it keeps the grey matter churning

Thanks for the replies it clarifies one's thinking.

Thread: Hoover bags
08/03/2019 00:05:44

You will find the bags clog up and the vacuum loses its suck. Didn't stop me doing the same trick when we had a bag vacuum, but need to replace now and again. I now have an ancient Hoover Aquamaster which has both a crude cyclone and a cloth bag. Made the mistake of washing the bag once, had a bit of a job getting another, but now I just batter it on a tree with me upwind to clear it out.

Thread: LED lighting
07/03/2019 23:42:09

I've been swapping all domestic lighting from CFD to LED for a while now. The LED's don't last as long as they claim, but seem to manage a couple of years. I've now got 2 where the built in switch mode power supply has died, but the LEDs are still OK. Tonight I coupled one up to a transformer rectifier and fed it from a variac. With the DVM reading 40V the LEDs just light (can't go any higher). As there are 24 I guess they are in series. The master plan is to connect 2 (or more when more bulbs die) and a small value resistor in series, and then drive them off a LED dimmer switch. Is this daft? If not, can anyone suggest a suitable resistor. I imagine it's going to be a big wire wound job, so I don't want to 'try it and see'.

Once it's running I would measure volts and amps across the LED and adjust the dimmer to get 10W as the original bulbs.

Thread: Mystery make Slip Rolls
06/03/2019 14:08:18

So if it's outside your area of expertise why are you writing H&S docs? Find someone who is familiar with this type of machinery.

Thread: EN1a vs EN3 steel
06/03/2019 10:20:22

Not all EN1A contains lead, see **LINK** for more info. You problem is more likely to be minimum order charges, my local stockist wants £35 before he even speaks to you. As others have said, EN1A machines better but EN3 isn't that bad

Thread: 8255 peripheral port adaptor
06/03/2019 10:08:35

Whilst having a tidy up (rare event!) I've come across a little board labelled Centronics port 8255 adaptor. If anyone wants it it is available for the cost of postage, send me a pm if interested

Thread: AutoCAD substitute
04/03/2019 17:06:06

Draftsight is free and is very very similar to AutoCad

Thread: Larger ball check valve lift
04/03/2019 17:04:59

For full area flow you need more than 1/4 D lift, but I've read that for a high speed pump the ball lift should be well less than 1/4 or the ball doesn't have time to get back on its seat. They certainly get very noisy if the lift is excessive. I think Gordon Smith did an article in EIM some while ago about clack valves, unfortunately I don't get EIM, so can't supply copy.

Thread: Low rate automatic house plant watering system
03/03/2019 00:31:01

If you have the tipping trough thing why do you need a syphon? Just have it tip into a chamber which then has feeds to each plant pot.

It would be a lot more fun to have a little gauge 1 track along the wall above the plant pots, a small tipping trough tips into a jubilee skip type waggon, then the loco pulls it to a plant and another actuator tips the water into the plant pot. Small microprocessor controls the whole thing to make sure each plant gets its share. Over-engineered? definitely, fun? Oh Yes

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