4 Jaw Work

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4 Jaw Work

This topic has 38 replies, 16 voices, and was last updated 25 October 2014 at 14:49 by Howard Lewis.

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2 October 2014 at 15:40 #15733
Jack Foreman 1
Participant
@jackforeman1
An accuracy exercise
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2 October 2014 at 15:40 #165341
Jack Foreman 1
Participant
@jackforeman1
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
2 October 2014 at 16:00 #165346
Neil Wyatt
Moderator
@neilwyatt
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
2 October 2014 at 19:30 #165366
Jack Foreman 1
Participant
@jackforeman1
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.
2 October 2014 at 20:51 #165381
Ian P
Participant
@ianp
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
2 October 2014 at 21:14 #165393
Jack Foreman 1
Participant
@jackforeman1
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
2 October 2014 at 21:16 #165394
Neil Wyatt
Moderator
@neilwyatt
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
2 October 2014 at 21:27 #165397
Jack Foreman 1
Participant
@jackforeman1
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.
2 October 2014 at 21:41 #165400
Ian P
Participant
@ianp
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
2 October 2014 at 22:47 #165409
wheeltapper
Participant
@wheeltapper
Posted by Jack Foreman 1 on 02/10/2014 21:27:59:

"snip".

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.

Snip

I think if you saw the inside of one you may change your mind.

Roy.
2 October 2014 at 22:51 #165411
Jack Foreman 1
Participant
@jackforeman1
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.
3 October 2014 at 08:34 #165436
Gary Wooding
Participant
@garywooding25363
This is a fairly typical injector.
3 October 2014 at 11:18 #165454
Ian S C
Participant
@iansc
Jack, don't worry about the stutter, a good many of us do it.
3 October 2014 at 12:57 #165460
Jack Foreman 1
Participant
@jackforeman1
Posted by Gary Wooding on 03/10/2014 08:34:11:

This is a fairly typical injector.

My word Gary, that is considerably more complex than I imagined.
At first I thought that the body must be a casting – then I saw that it can be machined on the lathe, following the correct procedure. The inserts look fiddly and time consuming.
Yes. I might have a go at that at some point in the future.
3 October 2014 at 12:57 #165461
Jack Foreman 1
Participant
@jackforeman1
Posted by Ian S C on 03/10/2014 11:18:59:

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

Thanks Ian – that's reassuring though i will try not to repeat the mistake.
3 October 2014 at 16:07 #165487
Gary Wooding
Participant
@garywooding25363
Posted by Jack Foreman 1 on 03/10/2014 12:57:08:

My word Gary, that is considerably more complex than I imagined.

At first I thought that the body must be a casting – then I saw that it can be machined on the lathe, following the correct procedure. The inserts look fiddly and time consuming.
Yes. I might have a go at that at some point in the future.

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.
3 October 2014 at 16:44 #165489
Anonymous
Posted by Gary Wooding on 03/10/2014 16:07:23:

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.

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
3 October 2014 at 17:03 #165491
Neil Wyatt
Moderator
@neilwyatt
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
3 October 2014 at 17:23 #165493
jason udall
Participant
@jasonudall57142
…a work of thermodynamic genius. …

And dangerously close to witchcraft
3 October 2014 at 17:26 #165495
Hacksaw
Participant
@hacksaw
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
3 October 2014 at 17:31 #165496
Anonymous
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
3 October 2014 at 17:35 #165498
Hacksaw
Participant
@hacksaw
Strewth , i never realised they were so easy to understand
3 October 2014 at 17:41 #165499
Ady1
Participant
@ady1
At sea It's called an eductor and works on the venturi effect

The motive fluid fires into the tapered nozzle at high speed and sooks fluid and anything else from the inlet, gas liquid or other bit

Great for sooking crap out of cargo holds full of bits which would gum up or damage a pump

Edited By Ady1 on 03/10/2014 17:42:28
3 October 2014 at 17:46 #165500
Ady1
Participant
@ady1
They make shower heads which use the same principle

You can feel the air being sooked into the hole at the bottom of the handle if you put your thumb on it

A sandblaster which uses high pressure water uses the same principle to sook the sand up the pipe and into the nozzle

Edited By Ady1 on 03/10/2014 17:52:28
3 October 2014 at 19:51 #165513
Gray62
Participant
@gray62
If you want the full lowdown on how these 'mysterious' devices work and how to make them, get a copy of Miniature injectors inside and out by D.A.G Brown. Fascinating read.

Edited By CoalBurner on 03/10/2014 19:54:32
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