I've made hex socket items before in stainless steel and just used a
section of allen key for a broach and pushed it in with a fly press,
that allowed me to have the tool locked in place so a stripper plate
allowed removal. I selected an allen key at the high end of size and a
driver on the low end. Worked well but for 50 off I think I would make a
tool out of silver steel and harden and temper it. Maybe look into
rotary broaching, I don't know how difficult it would be to make to
tooling yourself but worth a look.
Oh, sorry for the delay - thank you very much for the offer, and I may
take you up on it (with some real Loch Fyne kippers, I know a woman who
has known the herring and salmon smokers there for over 80 years :)
(actually she's my mother, so I know her quite well... but don't tell
However it would be of more use to me to be able to make these screws
myself, as similar future requirements will probably be many and very
Most of the screws in the engines I'm building can't be made from
ordinary stuff, as they either get too hot or too cold or both, and some
of them have to be inert to liquid oxygen and be inert and still very
strong in hot high pressure oxygen (currently 3,500 psi and 650 C, but
these figures may double) - and be able to take the thermal shock of
going almost instantly from 650C to -180C as well.
These won't be large screws, say from M1.2 to M5 at the largest - the
(design) engines are tiny, eg the 400N 40kgf version is 38 mm dia by
about 100mm long - it would fit in a Red Bull can.
It has a 6kW = 9HP turbine driving five different pumps all in about 1/3
of that volume - about the size of a shot glass. That's just for the
fuel pumps - total power is more than a Red Bull F1 racing car.
The numbers in rocket engines do get well beyond surprising.
I was initially thinking about something like David's fly press (to both
form the larger-diameter head from thinner stock, and also form the hex
Andrew's reply and king offer made me think about making or buying an
EDM machine, and some googling led me to think about ECM
(electrochemical machining, sort of reverse electroplating), which may
be the best solution.
The (an) other problem is getting hold of the right kind of stock. I
said I needed Inconel 600 for the first lot of screws, but getting hold
of that isn't easy - and then the iron content may be within the Inconel
600 specs, but still be too high for my purposes.
Thanks again Andrew for the offer I may ask you again about that, but
not quite now.
-- Peter F
One thing to bear in mind is that EDM will form microscopic surface cracks
on the work which in highly stressed situations could act as stress risers.
The commercial way round this is to etch the part after edm'ing if the job
demands it, but for normal usage it isn't an issue.
Do you work for Reaction Engines? This all sounds very whizz bang and bleeding edge type stuff, certainly the temperature range and rate of change are very like the figures mentioned in the last press release about the SABRE intercooler tests.
On 22/02/14 22:52, email@example.com wrote:
Well I am making rocket engines, but not SABRE type engines. 
My engines will be small regeneratively cooled liquid oxygen/kerosine
engines, fairly standard except they work in a closed cycle with an
oxygen-rich preburner, like some Russian engines
I am going to go on a bit now, please don't bother reading it all if it
What this means is that all the oxygen is burned with a small part of
the kerosine in a preburner. The output gasses from this drive a turbine
and then pass into the main combustion chamber, where the rest of the
kerosine in burned with them.
The reason for doing things this way is twofold - first, using a closed
cycle rather than an open cycle. In most American engines an open cycle
is used, and about 10% of the propellent is burned to drive the turbine
and then exhausted. This propellent does very little to add to the
thrust, and for a similar main chamber pressure a closed cycle will be
about 10% more efficient.
The reason for using an oxygen-rich preburner is that in order to obtain
high chamber pressures the turbine has to do a lot of work pumping the
propellents to high pressures - about 4,500 psi at the preburner inlet,
about 2,000 psi in the chamber for my engines, about 7,800 psi at the
preburner inlet/3.500 psi chamber for a Russian RD-170 engine - and the
amount of work needed to pump the propellents to those pressures can't
be obtained with a fuel-rich preburner.
The engine uses a lot more oxygen than fuel, and with an oxygen-rich
preburner the amount of propellents passing through the turbine is
therefore larger (all the oxygen plus a little the kero, rather than all
the kero with a little oxygen) than it would be in a fuel-rich preburner.
This means more turbine power, and thus higher chamber pressure - and
the higher the chamber pressure the faster it squirts out the nozzle,
and the the better the engine performance is.
Now comes the hard part: the output gases from the preburner contain
about 85% oxygen, and they are moving fast at about 600 C and at very
high pressure, so they are very oxidising - stainless steel would burn
up immediately, almost explosively fast, under those conditions.
This means at least four very exacting materials are needed - first the
preburner casing, second the guide vanes which pass the gas to and from
the turbine, third the turbine itself, and fourth the turbine bearings
and shaft seals.
Starting with the preburner casing, if we just made a simple tube at one
end it would be at -186C from the liquid oxygen, and the other end would
be at 600C. This is hard on materials which have to be light and contain
several thousand psi of pressure, so we cheat a little - there is a
copper alloy cylinder with a lot of holes in it inside the preburner
casing. There is a very hot flame in the middle, burning 15% of the kero
and 15% of the oxygen, and the remaining oxygen flows down outside the
copper cylinder and through the holes, mixing with the flame and keeping
the cylinder and more importantly the outer casing cool.
If you know a bit about flame design, you'll realise that that is a very
complex bit of kit. Deciding where to put the holes, and how big they
should be, is very difficult indeed.
The end-to-end temperature of the preburner casing is now about -150c to
250C. This rules out most ceramics, as they tend to crack when subjected
to such temperature ranges and especially the rapid temperature changes
experienced on startup and shutdown. However several alloys can be used
which are strong through those temperature ranges and non-burny enough
to cope - also the chamber can be made from a strong material and lined
with a weaker but less-burny material.
I'm looking at pure nickel liner surrounded with basalt fiber in a
nickel matrix for these.
Next, the nozzle guide vanes. These are at the full 600C - in fact they
are at at a higher temperature in places, as the output from the
preburner is not completely mixed and is hotter in some parts than in
others - part of their job is to even out the temperature differences.
They also have a very high flow rate. Fortunately they don't need a lot
of strength, and again there are several nickel alloys which can do the job.
Next the turbine itself. Again a horrible chemical environment, though
the temperature is now a bit more stabilised at 550-650C. Flow rate
again is fast. The turbine is spinning at a tip speed of 250 m/s
(120,000rpm) or more - so it has to be very strong, like high tensile
steel strong - at 650C.
I know how to make an alloy like that, but I don't know of anyone who
actually makes one. There is an available alloy used in high performance
engine exhaust valves, Inconel x-751, but it's not specified tightly
enough - it's strong enough, and at the lower ends of the chromium,
aluminium and iron content ranges it might be OK against burning, but
the ranges are wide enough that while one batch might do another one
I'm not entirely sure what I'll do about material for the turbine.
There is another guide vane after the turbine, its purpose is to screen
the turbine against sound and infrared from the chamber and to support
the final fuel injectors.
The bearings? Fallback is a rotating sleeve bearing as used in early
turbochargers but lubricated with liquid oxygen, but it may be possible
to use some ceramics instead - the potential difficulty here is the
startup and shutdown transients.
-- Peter Fairbrother
 I don't think SABRE-type engines (or SKYLON) are the way to go - in
fact I think they are a waste of time and space and attention and money.
Perhaps more important, I don't think a SSTO (single stage to orbit) is
the correct way to go either. It's sexy, but a TSTO (two stages to
orbit) makes far more sense economically.
SSTOs are far more expensive to develop and build than TSTOs. Their
cargo capacity is less. Where TSTO boosters can return to base quite
quickly - basically the booster just goes up and gives the second stage
a bit of a boost, then returns to where it took off from - it can refuel
and fly again quickly (with a different second stage), whereas a SSTO
takes far longer.
For a start, consider a typical mission. The spacecraft is in orbit for
a few weeks - during that time it can't be used to launch anything into
orbit. If it's an SSTO the lift-off part of the technology is just
sitting there doing nothing.
There are many more reasons, I won't go on and bore you even more.
That's a continuing problem. I used to use McMaster-Carr a bit, but they
don't post outside the US anymore. There were a couple of websites like
smallparts.com which are now gone :(
There are http://www.williamgregor.com 's http://www.alloyshop.com/
which has a range of nickel alloys, and www.shopmetals.co.uk who have
nickel alloys and many uncommon metals, but alloyshop are very expensive
and often don't have the sizes I need, and shopmetals prices are just
(williamgregor have other shops for titanium and PGMs, but I don't know
There is occasionally something on ebay if you keep your eyes open.
Amazon have some surprising stuff.
And some things, like welding rods for inconel (which are made of
inconel) or cast iron (which are made of nickel), and monel rivets can
be bought in reasonable quantities from normal outlets at sane prices.
But it is a problem. I am even considering building a melting furnace
(and foundry? wire drawing? rolling mill?) and mixing my own alloys.
I think they are still in business, but I'm not sure how - while
alloyshop are expensive, goodfellows prices are totally ludicrous.
But their catalogs were great, full of interesting facts. And they
did/do sell unusual stuff which is hard to find elsewhere - they sold
uranium round and flat bar at one time, don't know whether they still do
(it's still legal afaik).
-- Peter Fairbrother
Maybe a high mark-up on small quantities to the general public. I know
someone that recently got a quote for some silver nitrate solution,
glucose, and ammonia plus some beakers and other bits to do some
was a teacher and she got the stuff from the supplier her school uses
there is a minimum order to be met imposed by the suppliers supplier. I
found that out when the 2nd order for about twice the quantity as the
first order cost very little more, I asked why and they mentioned the
minimum order value. I they had mentioned that on the first order I
would have bought more.
A quick comparison:
6mm dia nickel rod, 1000mm:
now judging by the alloyshop differential price the "real" small-scale
orders isn't unreasonable).
I leave you to draw your own conclusions, but you can see why I don't
buy small orders from alloyshop. And why I don't buy anything at all
from Goodfellows or shopmetals.
-- Peter Fairbrother
Ouch on the shopmetals and goodfellows prices. I did note that
shopmetals is part of William Gregor according to their logo on the
site. BTW I did try to weld that thin tube you gave me to try a couple
of times and although my TIG set may be able as it goes down to 5A I
struggle to see the weld. Have now got a magnifier fitted inside the
helmet so will have another go, need to play with the pulsing also to
see what benefits that has. They eyes aren't what they used to be but
that is typical as approaching 50.
Yes - so are alloyshop. Why there is such a difference between two
sources which are both part of the same larger organisation (and that
sort of price difference is typical for all their overlapping products)
I do not know.
They get worse once you pass 60.
One thing which helps me, I get lots of specs from the optician with
different strengths - I can still see detail with the right pair of
lenses, it's just that I can't focus as easily as I used to.
each, no need for fancy frames as you are only using them in the workshop.
And keeping them clean helps a lot too. Put spectacle cleaning cloths in
the wash (in a mesh bag) every time you use the washing machine
Laser surgery is for the young, unfortunately it doesn't do us older
folk much good.
Also make sure you have no cataracts - removing them is simple and quick
(if a little unpleasant) but it can make all the difference.
Of course for some people the rods and cones go, and specs and cataract
ops don't help much then :(
-- Peter Fairbrother
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