Used Metal Lathes
First you should know that the term metal lathe today is a bit of a misnomer. What some people refer to as a metal lathe can actually machine other things too, like plastic and composite materials. Some modifications may be necessary in order to properly cut materials other than metal. I have seen metal lathes used to cut wood too which seems like over kill to me.
There are variations on the types of lathes. A centering lathe uses dual heads and has the part being machined remaining fixed. These lathes can machine holes in the center of the work piece which then allow it to be mounted between centers (between the headstock and the tailstock) to allow other operations.
Today most lathes have variable speed motors which allows the operator to vary the speed of the material being machined to suite the task at hand. Some older metal lathes use pulleys to vary the speeds. These work okay but changing the speeds can be a hassle since it is a mechanical process of moving the drive belts from one pulley to another. Plus you do not get infinite speed variations with this type of system. An electronically controlled speed control motor makes adjusting the speed quick, easy and a lot more accurate than the older systems. If you are buying a used metal lathe, consider the advanatages of a electronically controlled speed.
Both wood and metal lathes have a lot in common. They all have:
- Bed
- Headstock
- Tailstock
The function of each of these parts is similar regardless of the type of lathe. If the lathe speed is adjustable (most lathes are adjustable speed types) the headstock contains the mechanism used to change the speed. This can be done via belts or gears depending on the lathe manufacturer and size.
Because a lot of pressure can be applied during the turning process the headstock should me made of heavy gauge material strong enough to withstand any flexing and with enough weight to reduce any vibrations. Even a small amount of vibration can cause the finish to be flawed in a metal lathe. When buying a used metal lathe pay close attention to the headstock.
The headstock will have a hole through the center to allow longer pieces of metal to be machined. This hole allows lathes to work on pieced longer than the actual bed length.
The next consideration when considering a used lathe is the bed. The bed is what connects the headstock and the tailstock. The bed will have ‘ways’ which is how the the carriage and the tailstock move on a set track parellel to the metal being turned. Although a lathe does not need to actually be level, the bed must be checked for level to be sure the lathe has not racked, or twisted. If the bed is twisted, the precision of the turning will be affected. Although any type of bed can twist, the beds in most industrial or large metal lathes are a flat bed type. Smaller lighter weight lathes may use V beds which are more prone to racking.
For metal lathes a feedscrew is used to precisely move the carriage mechanism. The feedscrew has a gear adjustment to allow for adjusting of the speed of the carriage.
For a metal lathe, the gears allow the correct ratio (movement parallel to the turning material) and the direction of the carriage. By properly setting the ratio, you can cut screw threads on a metal lathe instead of having to rely on a die. Some lathes use a single leadscrew to move the carriage.
To properly adjust the ratio on a used metal lathe you must know whether the the feedscrew was manufactured using standard or metric measures. Without knowing that, you will not be able to properly adjust the ratio to achieve the machining you want. Fortunately the gearboxes on most lathes are designed to correctly offset the proper ratios necessary for making various threaded stock.
The part of the metal lathe that holds the toolpost tools is the compound rest. The compound rest also moves parallel to the spindle and its’ movement is controlled via a feedscrew. The angle of movement can be adjusted to allow cutting of materials into a tapered shape and to control the depth of the cut when making screw threads. The cutting tool itself is mounted in the tool post. Some metal lathes come with a quick change tool For most people a quick change tool set up which allows the cutting tools to be changed rather easily. The limitation on this type of set up is the number of tool holders available. When looking for a used metal lathe you should try to find one that included the tool bits. These can be quite expensive to purchase so having them included in the purchase can save you a lot more money.
