A length of black flat bar shows one end acid-pickled to reveal clean steel under the oxide.

A simple pickling bath for long items made from a length of 100 X 50 rectangular plastic downpipe. After removing one edge by band-sawing, the ends are softened with a gas torch and bent over a wooden former to close them. Draw all edges upwards to ensure that the closures are leak-proof.

Workshop Tips

Cleaning files.
When files become pinned (clogged) they not only tend to be less effective in performing their primary function but may also produce score marks on the metal being filed. The use of a wire brush to clear material from the teeth is seldom completely successful. There is, however, a very quick and completely effective way to deal with the problem.

All that’s required is a length of 3 mm or 1/8” thick copper, about 15 millimetres wide by about 50 millimetres long. Starting at one end of the file, force the narrow edge along the length of the diagonal “grooves.” The copper will soon match the form of the teeth. Work your way along the length of the file till all the teeth are cleared.

Several such tools, each dedicated to a particular pitch or “cut” will take care of a range of files. A small hole drilled through the centre allows one to hang it from a nail on a shadow-board.

Smoothing screw-cut threads.
When one has cut a thread in the lathe it is often (usually!) difficult to achieve a good surface finish on the thread. This is especially so if one has used bright mild steel or medium-tensile bright bar rather than a free-machining steel.

A simple way to improve thread finish is to smear a small amount of fine grade automotive valve grinding paste onto the and apply pressure with the end of a thin wooden strip whilst the work-piece is rotating. Repeat several times to bring about a rapid and substantial improvement in thread quality.
For very fine threads finish off with a metal polish such as Brasso. In both instances run the lathe at a slightly higher speed than that used for cutting the thread and don’t overdo the finishing process.

Corrosion prevention.
The default lubricant for most lathes, but especially geared-head lathes, should be hydraulic oil. Unlike motor oils, it’s detergent-free, formulated to deal with high tooth pressures, and is an extremely effective lubricant of bearings both plain and ball or roller. Because of its properties it is also suitable for lubricating slideways, making for a one-stop solution to workshop lubrication issues. Prolube’s Hyplex 68, sold by Enzed and other hydraulic equipment suppliers, fits the bill admirably.

NOTE: motor oils are totally unsuited to and should never be used for machine tool lubrication in any way whatsoever. This default lubricant, however, also has another use. Dilute in the ratio of about 1:4 with mineral turpentine in a spray bottle (1) and spray the mix over all bare metal surfaces, whether on tools or machine tools. The turps will evaporate, leaving a thin, even film of oil on metal surfaces which is extremely effective in keeping corrosion at bay.  Repeat as required.  Since the mix has no deleterious effect on painted surfaces it can also be used on models which usually have a mix of bare metal and painted finishes.    

(1) Not all spray bottle mechanisms will stand up to prolonged contact with hydrocarbons. One that will contains a glass-cleaning fluid and is sold under the brand name “Windex.” After emptying out the glass cleaner, rinse thoroughly with water followed by methylated spirits (it absorbs both aqueous and some non-aqueous solutions) and dry before use. The spray bottle currently being used in my shed has at least six years of service behind it and is still going strong.


Removing mill scale from black mild steel.
Any black (hot-rolled) steel that is to be used for fabrication purposes on a model, whether it be for loco frames, stretchers or just general plate work, should have the mill-scale removed by acid pickling. Failure to do so will result in poor adhesion of or peeling paint on the finished job.

The requirements for pickling are a plastic container large enough to hold the job and a quantity of hydrochloric acid. The latter is available in four litre quantities from builder’s merchants and suppliers of swimming pool consumables. Ensure that the material is fully covered by acid and allow to pickle, turning every five or ten minutes. When all the mill-scale has been removed, rinse thoroughly under running water, dry immediately, and apply a film of oil mixed with turpentine in the ratio of 1:4. If one has the mix in a spray bottle it’ll be that much easier. When ready to deal with the work-piece, remove the oily film with tissue paper. Recoat if it’s to be left for a prolonged period.

The acid thus used can be saved for future de-scaling since it will still be reactive. Store in a separate container and when next used, add a little fresh acid if necessary. Rinse the pickling dish after use to minimise hardening of the plastic and store away from direct sunlight.
IMPORTANT: Hydrochloric acid is a dangerous reagent and its fumes are highly corrosive. Keep well away from anything that may be affected, especially machinery. Preferably, work in the open. Eye protection and rubber gloves should be used at all times. Avoid breathing in the fumes. Any splashes on the skin and clothing must be rinsed off immediately. 
When the acid is finally so weak as to be ineffective, neutralise with bicarbonate of soda using litmus paper (available from pharmacies) as an indicator. Elementary chemistry tells us that:

                    Base + acid = salt + water

so apart from discolouration the resulting solution is safe for disposal. Dispose of responsibly.