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Making High Speed Steel Injector D Bits

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julian atkins17/06/2019 22:45:21
1232 forum posts
353 photos

My own experience is that silver steel injector 'D' bits, especially the 9 degree taper ones, last about 5 injector cones then become useless and blunt. The 13 or 12 degree ones are somewhat more durable.

I have proved that the 6 degree taper is not required on small injectors of 12 oz per minute delivery on the delivery cone, and 9 degrees will do the necessary, contrary to what has been written.

HSS reamers are the way to go if you want to make lots of these gadgets.

Boiler Bri (Brian) must dismiss any thought that the LBSC drawings and descriptions will produce a working miniature injector except by luck.

It is not a black art. Eric Rowbottham described via Basil Palmer the smaller injectors in ME around 1976, and Laurie Lawrence described his own 'standard' design was it 1977? The Laurie Lawrence articles were repeated around 1982. Derek Brown's book is indispensable and available via TEE publishing.



Nigel Graham 218/06/2019 07:27:47
578 forum posts

It should not be a "black art", no; but over the years I have heard or read of so many people finding injectors so problematical that there must be surely be a lot more to making and fitting them than meets the eye.

They do seem one of those fittings people prefer to buy than try making, along with pressure-gauges. Possibly, DAG brown's book will lift some of the mystique - I have a copy and he does go into detail on the plumbing as well as the injector itself.

As I say though, I suspect there are other, unquantifiable influences at play, making the device work in a much more critical range than it will anyway.

I have a copy of a heat-engine designer's text-book published in the 1930s, and although it does not go into much detail it does say the injector's team demand rises as pressure reduces, and as a pump it is inefficient although that is balanced by it heating the feed-water. The particular injector it describes was a Holden & Brookes lifting type with a single control. It put the steam on before the water, presumably to draw the water up first.

Stewart Hart18/06/2019 08:05:22
627 forum posts
334 photos

Thanks for your valuable input again Juilian

When I attempted to make a 12oz injector I wondered about the need for the 6 deg reamer for the delivery cone at some point in the future I'll have another go at making a 12 oz. For my first attempt I used the 24 oz basic cone sizes one of the difficulties with this was the hole depths its bad enough drilling a No 70 hole but when your trying to drill it nearly 1/2" deep is a bit tacking. Especially with dodgy quality drills I got some of flea bay supposedly German Manufacture of a make I'd not heard of 50% were next to useless with the point not ground correctly or not at all. When I worked for a living we use to swear by Titex small drills but these are eye watering expensive unless I can drop on a good supply of cheaper drill I'm going to have to bite the bullet and buy some.

For my next 12oz attempt I'm going to reduce the length of the cones to give myself a fighting chance of drilling the holes.

I would agree with Julian and Nigel its not a Black Art its just a matter of gaining an understanding of how they work and how to make them (I have a good coach) and this only comes with having a go, being patient and persistent learning by your mistakes and not giving up and taking the easy rout and resorting to cheque book engineering and buying one.

As an aside I've just tested the second vertical injector to my own design, and it worked though we did have to reseat the ball to get it to work. The test boiler is electrically heated so when we got it to 90psi we turned the electric off started the injector and watched the pressure drop as it put water into the boiler:- when it got to about 55 psi it started to splutter (stop and start) but it didn't stop completely until it got to 45 psi.

The design for this vertical injector is very much in the development stage I want to try refine it further.


Nigel Graham 218/06/2019 09:05:51
578 forum posts

I did stress I assume the injector is correctly and well made, but though my remark about "black art" was a bit tongue-in-cheek injectors in general do seem to have a certain reputation.

I expect you have seen the satirical poster about injectors having very wilful minds of their own, sold I think by Walker Midgely on their exhibition stands - I know many who would agree with it!

I think most model-engineers prefer to buy rather try making the things, but why must be a matter of personal choice. For me, though I might be tempted to try it to see if I can, I do not feel able to make one well enough to replace having to buy one.

My main point though was that I think many who blame the injector might be overlooking the rest of its system - an aspect DAG Brown covers well in his book, but which I have not seen so well considered elsewhere.


I mentioned for example the commercially-made injector on a Ken Swan " Wren ": it needs the water on full, then the steam on full, and rapidly, then easing the water back almost to off. I might unwittingly have solved it in my further remarks from a 1930s book, that the steam: water ratio increases with falling pressure. So, by revering the logic, could the Wren's injector actually have been designed for a higher pressure than the loco's 90psi? Once started, with sensitive water control it will operate down to about 40psi, just.

If so, then is an example of what I mean: don't just jump to conclusions and blaming the device itself!


Your test boiler: interesting idea, using electric heating. I can see this would lend itself to automatic pressure control from anything up to W.P.

You may recall Ron Jarvis' precious-metal-winning miniatures of various historically-significant engines. He was a fellow-member of my own society, whom he delighted in telling us of the secret of the boiler on his fully-operational Newcomen Engine. The boiler is about the size of an orange - complete with the "orange-peel" forged texture - and as per prototype, has a WP of only 2psi. Ron had to use electric heating to attain this successfully, and joked that with its electronic sensor and microprocessor discreetly hidden in the base, the machine was the world's only 18-Century CNC steam-engine!

Andrew Johnston18/06/2019 09:50:20
5381 forum posts
617 photos
Posted by Steamer1915 on 17/06/2019 19:03:40:

If you are grinding between centres, the centres should not rotate. This removes any inaccuracy created by the spindle bearings.

Well, I never knew that. Having looked at the sectional drawing of the workhead spindle on my grinder it does indeed look as if the centre does not need to rotate. However, on the real workhead, where the nose fitting is different, the centre does rotate. The spindle has a indent lock, but it locks the whole spindle, not just the centre. I can't work out how to get the nose fitting off, and if I use too much BF&BI something is going to get broken.

For the time being I'll stick with what I've got. I can grind to size and parallel to tenths, which is the minimum I can measure, and fine for my needs.


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