Thanks for the suggestions.
I doubt that the ice would work well. Once I've done my bit I need to
hand it over to an external machine shop to cylindrically grind the
OD. If the ice didn't melt on the journey I'd be mighty suprised.
However I've done a test this afternoon. Inhibited sulphuric acid (aka
decent drain cleaner) does appear to dissolve the foam. Overnight I'll
discover quite how inhibited it really is on steel.
OK, it was just an off-the-wall thought - and you didn't tell us about
the second stage!
Sounds highly likely the steel will corrode.
Have you actually tried solvents? From Googling, it seems polyurethane
foams vary in their solubility, some are very resistant but some can be
dissolved by acetone, butanone, dichloromethane or the like.
Charles,you probably know this,but it is usually inhibited
hydrochloric acid that is used to pickle steel.It will not attack the
Why are you bothered about finish if it`s getting ground after
turning,or,are you heat treating it in between and worried about
How thin is thin is thin wall?
The only reason that I used the inhibited sulphuric was that it was on
the shelf.I would have used inhibited hydrochloric if I had some.
The piece in question is like this:
The cylinder is ~ 5" long, 4" diameter. The end flanges are part of the
cylinder and are a little over 1/4 deep. The main wall thickness is ~
75thou. The inside needs to be turned parallel whilst it's held in a
steady and finished to a decent turned surface as a minimum. The ouside
is the part that will be ground after preturned end plates have been
fitted to enable it to be held between centres.
The idea of filling the cylinder when the ID has been finished and
whilst the OD is still oversize is really to reduce any resonance
causing issues with surface finish, at either the turning r frinding
stage. That's why, in my simple mind, something stiff like a PU foam
might do the trick. But it's also important that the whatever cleans out
the foam doesn't wreck the surface quality. I don't give a monkeys about
staining. Nobody is ever going to look in there.
Other suggestions welcomed.
In that case the old non-shrinking "Baker Mixture" might work for you:
Sulphur 2 oz
powdered lampblack 3 grains
camphor in alcohol 2 drops
or 10 parts sulphur by volume plus one part powdered graphite.
Heat slowly so none of the sulphur overheats and thickens.
Get some thick-ish walled alloy tube, turn it to just fit in the bore,
split the side and arrange to expand it once inside. Expand by, say, a
tapered screw down the split at either end or screw into tapered hole
depending on your preference.
I doubt a saw blade thickness slot would have any noticable effect on
the circularity of of the job, if you really want a thin slit then a
010 slitting saw would do the job.
You could make the bore of the thick walled tube tapered and stuff a
cone up it in some way.
Make the alloy tube a sloppy fit and use some low temperature
breakable glue to stick it in. Some Loctites possibly
turn up a wooden plug that will just barely fit in the bore from some
seriously rubbish softwood that has been in the airing cupboard for a
week or two. Once placed in the bore, stand the wood in a saucer of
water over night, actually just atmospheric moisture will probably be
enough, just slower. Whittle it out at your leisure.
now I`ve seen the pic I would do it differently if I was limited in
I would finish the OD before machining the ID.That way you can wrap
the OD with rubber bands to kill the chatter.Cut up inner tubes work
well for this.
I would not use a steady on something as short as that.
Alternatively put a rubber bung inside it and expand it with a bolt
Have you tried machining one to see what finish you get? I think
you`re thinking too much into this.
Quite possibly thinking too much but that's probably because I have
much more thinking time that doing time.
I haven't tried to cut the material yet (very thick walled tube) but
I'll give it a go at the weekend.
I've heard of the rubber band technique but never tried it so I'll
test that route but I'm still not certain about having such a length
held in such a short length of jaw but defer to your greater
Will post on my progress
Charles,thick walled tube might work against you.It`s traditionally
crap material manufactured to a mechanical spec and can be difficult
to get a finish on.
Better with a nice bit of EN8,more turning but mirror finishes.
I`m assuming you are going to hold it in a nicely machined set of soft
jaws when you come to bore it.
Well the supplier in Nottingham reckoned that it was good stuff but
I'll tell you next week! It's 125mm OD with a 75mm bore. That's what I
call a thick wall.
I have a billet of EN8 as a substitute if it'sunworkable.
And thankfully Rotagrip can supply soft jaws for an Emco branded 3 jaw
(evidently made by Elliott)
I used to find Rotagrip expensive.JS flagged up a company who made
them a while back but I could never get them to answer the
phone.Midland Engineering or MES or similar.
I use UEW in Halifax for our soft jaws.
Watch these suppliers in Nottingham,they,ll tell you
anything.,especially once you get out to the Long eaton side. :-)
Now I'm curious. Should have asked this first, what is "inhibited
Another off-the-wall thought: How about filling the cylinder with
plaster of paris? it should provide perfectly good support, but it is so
soft that you could, I imagine, dig it out quite easily afterwards.
Would be as well to protect the inside with some inhibiting oil to
prevent rusting and facilitate cleaning up; a poly bag might even be
Sulphuric acid which has been inhibited, ie it has had something added
so it won't attack steel.
The added something varies from hydrofluoric acid (which forms a thin
layer of steel fluoride which the sulphuric acid can't dissolve, but
this is mostly used in very high tech products only) to tin/antimony
(which see below) to inorganic phosphates or phosphoric acid (which
forms iron phosphates which ditto) to organic compounds in great
profusion, including amines, amides, esters, sulphamides,
Exactly what you get will depend on the manufacturer, the intended use
of the product, the price range, and so on, but the drain cleaner types
are looking for extreme cheapness and have highly proprietary and
supposedly secret inhibiting additives, which are usually just organic
amines like hexamine with some phosphoric acid - or so I hear :)
There is an actual 2official" formula for inhibited hydrochloric acid,
hydrochloric acid with tin and antimony chlorides added, though
unspecified organic additives are far more frequently used instead these
days, as tin and antimony are expensive.
The tin and antimony chlorides form a very thin layer of tin/antimony
metal on the steel surface, which the HCL isn't strong enough to attack.
You can easily form this sort of tin layer just by dipping clean steel
or copper in tin II (stannous) chloride solution - this was sometimes
used to protect steel or as a base for soldering, though I don't think
it is used much nowadays, except maybe for home tinning of copper on PCB's.
-- Peter Fairbrother
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