4 Jaw Work

I'm not sure if this is the correct Topic Section for this. If not, can a Mod move it appropriately please?

Started from a small aluminium billet. 4 spigots, each in line on their respective centre-lines.
1/4" ~ 3/8" ~ 7/16" ~ 1/2"
Accurate to 0.0005"

Probably not the best turning in the world, and certainly not the best photography, but I'm pleased with the result.
Approx 3 hours. So I'm unlikely to be offered a job in industry

Edited By Jack Foreman 1 on 02/10/2014 15:47:24

Edited By Jack Foreman 1 on 02/10/2014 15:47:53

Edited By Jack Foreman 1 on 02/10/2014 15:48:32

Nicely done, looks like brass. It looks like an injector body, perhaps you should try making one as your next exercise in precision work?

Neil

The brass effect is a combination of the lighting in the workshop and converting Raw into jpg on Photoshop Neil.
Definitely ali.
I enjoyed doing it - and working to that degree of accuracy. I'm beginning to get to grips with the calibration markings on the Bantam cross and compound slide wheels.

What is an injector body?

This afternoon I set up a 1-1/2"bar in the 4 jaw chuck using a dial gauge to true it up. That's a first too.

Jack

I am sure that is not an easy part to make and now I'm wondering how you did it!

If the block was accurately rectangular I could imagine marking it out on a surface plate, then you would have to centrepunch each centre and pick those up in the 4 jaw. That would rely on your centrepunching being better than 0.0005" so good eyesight and steady hands are needed.

Without any pre marking out I could imagine setting up the block in the 4 jaw using a DTI on the corners, that would work for the concentric bosses but only if the block corners were identical in terms of their precise corner geometry (no, or identical radii). That technique would be much more difficult with the offset bosses.

All in all a neat job, but how?

Ian P

Ian, the blank was simply an off-cut from a standard aluminium rectangular 35mm 20mm section x 58mm long.
I marked everything out on the blank and used a finely pointed awl to mark the intersection points.
Then used a brand new half centre with a still very sharp point in the tailstock to position the blank in the 4 jaw chuck each time. I used a DI to check the face of the block in the chuck each time it was repositioned. Then it was a matter of turning the spigot to each of the designated sizes, facing off the two side surfaces & two end surfaces of the block on the last pass. The two sides without spigots were dressed by hand, upon completion.

I do appreciate that using the pointed awl on bright steel may not be so clearly effective - but on the ali, it worked very well.

Edited By Jack Foreman 1 on 02/10/2014 21:17:32

A cunning device for pumping water into a boiler using its own steam pressure with remarkable efficiency.

The black bit is the body. See what I mean?

Neil

My word Neil - that looks a sophisticated piece of kit. And presumably it has interconnected passages between each of the connection points too.

I do see what you mean about the similarity in profile.

And presumably the brass fittings are also made and threaded on the lathe too?
If I can source a drawing, I would like to attempt making something like that.

Next though is a double ended die holder with an arbor and a 3MT shank, for the tailstock.
Then I want to make the external and internal depth stops, which are in Harold Hall's book.
And then, a set of hole gauges and a set of dimension gauges, also described in Harold's book.

Jack

In my experience standard ali extrusion rarely has sides that are very flat and I think there is probably more than half a thou allowance regarding the nominal size. For your part the exact dimensions are not important but centering the bosses to your tolerance is no easy feat, so I'm impressed.

Ian P

Thanks for the tip Roy. Perhaps I'll put that one aside 'for-the-future'
Although I am now intrigued to know what the inside of an injector body looks like.

This is a fairly typical injector.

Jack, don't worry about the stutter, a good many of us do it.

It seems impossible that the steam from the boiler can push water into the same boiler, but it does. The basic mechanism is that the jet of steam converts the cold water into tiny droplets and accelerates them so that their inertia is sufficient to overcome the pressure inside the boiler. In practise, the inlet water must be cold, they don't work very well, if at all, if the water is warm.

The machining requires very high precision, especially for ones intended for small 3.5" or 5" locos. Many people just give up before they get one that works. The larger ones, whilst still critical, are a little easier to make.

I'm afraid I don't quite follow that? If a fixed mass of input steam has a certain amount of potential energy associated with it's pressure how can it accelerate a greater mass of water to a higher pressure? Where does the extra energy come from?

Andrew

It's counterintuitive, but this is how I understand the mechanism in simplified terms:

The steam accelerates as it goes through the first cone, the extra energy has to come from somewhere so it gets colder but it's kinetic energy is proportionately increased. As the stream passes through the next cone it combines with the water and accelerates it through the next cone, energy for this is partly from the latent heat released when the steam condenses. Eventually the water and steam re-enter the boiler having only wasted a fairly small amount of energy as heat radiated from pipework and the injector.

A work of thermodynamic genius!

Neil

...a work of thermodynamic genius. ...

And dangerously close to witchcraft

They work by black magic.. But work they do !

I have an O level " A" pass in Physics , and i'm buggered if i can get my head around how they work still

My Dads really, really brainy...can even do a Rubik cube !!  But, he can't see how a jet engine can work ! And lets face it, they're really simple !!

Edited By Hacksaw on 03/10/2014 17:27:13

Edited By Hacksaw on 03/10/2014 17:29:02

I thought that the steam cone converted the potential energy due to the pressure to kinetic energy, no extra energy was added? At the outlet of the steam cone the steam pressure is below atmospheric so water is sucked into the space. This is then accelerated by the steam jet, which also slows down; conservation of momentum? As Neil correctly says the feed water also condenses the steam, releasing the enthalpy of evaporation, which heats the water jet. This is where the extra energy comes from. The water jet is then slowed, converting the kinetic energy to pressure energy.

I think that is why warm feed water isn't good, as it doesn't condense the stream properly, so the extra energy needed is not available.

Being an awkward so and so, when the time comes, I am planning to have a go at designing and building my own injectors for my traction engines.

Regards,

Andrew