Choosing a lathe
Be aware, as best as you can, of the size of your proposed work pieces. Distance between the centres should be sufficient to hold the longest. Swing over the bed will dictate the maximum diameter of a work piece – sometimes a gap bed will allow short pieces
of larger diameter to be turned. Don't forget the diameter of the hole through the mandrel can be quite restrictive if it is too small to accommodate the size of bar stock you need to use.
Some small lathes are equipped with flat beds, personally I prefer a vee bed because the flat bed, when worn will be inaccurate and trying to adjust for wear can lead to jamming up of the saddle where there is no wear. The vee bed is, to some degree, self
adjusting and remains free to move over the full length.
If you have the luxury of choice look for a machine graduated in imperial or metric and this may be dictated by the drawings which you will have available for your work and also by the available measuring equipment. Some machines have dual dials to work
in both metric and imperial but be aware that the dials must have a little compensating gearbox in side them. Just having both systems stamped on the bezel will not work! A dial on the handle which winds the saddle along is very useful as is a dial on the tailstock.
Before going to choose a metalworking lathe you should be aware that the accessories will be as costly as a second hand lathe if not more so. Make sure that you have all the chucks and tooling which you might need.
If you intend to make batches of items which are round or hexagonal and if speed of setting is important the make sure you have a three jaw self centreing chuck. If you intend machining awkward shapes such as castings or if you intend to re machine
old items, for example skimming brake discs or drums, then an independent four jaw chuck is a must. For really difficult mounting work such as re-machining the joint face of a motorcycle cylinder head the a face plate will be useful as will the clamps to hold jobs to it.
Tool posts need to be in good condition and if you need to achieve repeatability in batch work then look for a quick change tool post – the turret type tool posts are somewhat cumbersome in use but great if you are doing maintenance work and not production,
they are quite strong and rigid.
Steadies are useful for long work and the fixed, three point type are most common. The travelling steady is for long flexible jobs which might tend to vibrate causing chatter and they are attached to the saddle of the lathe so as to move along opposite the cutting
tool keeping the workpiece from moving relative to the tool.
Some other items to look out for if you are buying second hand :-
Do you have three phase electricity – many larger lathes require three phase. Don't forget the chuck keys. Maybe you will be lucky enough to get a dial test indicator on a magnetic base or a scribing block – both handy for setting up workpieces. Bed stops and
mandrel back stops are handy for repetition work. Cutting tools are expensive so if you can talk the vendor in to parting with some of those in the form of disposable tip tools and tool steel bits it will save you money later.
Making sure you get a machine in good condition.
Run the machine in all of the spindle speeds and listen for noises from the gear box, small machines tend to whine a bit but if you need to wear ear defenders – walk away. With the machine set to a fairly high spindle speed listen for noises from the gear train in
all of the feed speeds. Allow the saddle to travel along while resting a hand on the wheel and try to feel for even motion any jerkiness will indicate something bent in the gear train.
The most obvious signs of wear will be on the bed, a good way to check for this is to mount a dial gauge on the saddle and put the tip of the spindle on the flat slideway for the tail stock. The tail stock wears the bed most at the right hand end and the saddle wears
it at the chuck end so any deflection of the gauge will indicate wear and if it is more than 0.1mm. You should be wary, although I have worked on machines with much more wear on the bed; if you are aware of the problem, it can be compensated for.
The head stock bearings can be checked by putting the dial gauge on top of the head stock and resting the tip of the spindle on the top of the chuck. Then lever the chuck up and down with a length of wood resting on the bed – movement in the gauge will
indicate bearing wear. On a roller bearing mounted spindle the movement should be nil if the spindle is running in plain bearings then expect a few hundredths of a millimetre – plain bearings have to have clearance.
The tail stock quill can be checked for wear in the same way as the head stock bearings and you can also check the quill for wear by measuring the diameter, with a micrometer, at various points along its length.
Phil and Claire Bleazey,
The old Furness St. Stables,
Telephone:- 01524 849085
click below to :-