A metal working lathe is the most useful machine tool in any metal working shop. Also like all power equipment can be very dangerous if not operated with caution. The term used in cutting metal on an engine (metal) lathe is turning for obvious reasons as the piece of metal in the machine is being rotated and the cutting instrument (tool) is stationary.
Most engine lathes come equipped with a three (3) jaw chuck but some have a four (4) jaw, since the three-jaw is most common for this article I will base my instructions relating to a three-jaw chuck.
There are three most important elements of using an engine lathe to understand the use of the chuck, the tool post carriage and cross-slide, and the tailstock.
The chuck is directly attached to the drive mechanism of the engine lathe and rotates at variable speeds up to as much as 6500 rpm on some machines. To start, clamp the piece of metal to be turned in the chuck, the three-jaw chuck is self centering, however since pieces of metal are not always perfectly straight and level it is recommended that you use a dial indicator to check the location of the piece of metal in relationship to the machine.
This can be done by placing the indicator on top of the tool post with the dial stem touching the part and with the machine turned OFF and in neutral position rotate the chuck checking the trueness of the part within one or two thousand of one inch. Remember this is a precision machine that can make the cut to sizes within that tolerance or better. Once you are sure that the part is true, tighten the chuck as tight as you can place the chuck key in each key receptacle on the side of the chuck.
Check once more for true and then place the chuck key on the workbench away from all moving parts. Never, repeat Never, leave the key in the chuck except when using it to tighten the part in the chuck. The key can become a flying chunk of lethal steel if left in the chuck and the machine is started into motion.
The Tool Post Carriage and Cross-slide:
It is self explanatory that this is where the cutting tool will be located. The cutting tool, for safe and efficient cutting the tip of the tool must be located direct on center of the part in the chuck, too high and the base of the tool will drag on the part deflecting the depth of the cut and too low the tip will tend to gouge and/or cut too deep.
If you are not familiar with using an engine lathe, I would recommend that you use the lower speed of chuck rotation and take shallow cuts until slowly increasing the rpm’s until the tool cuts smoothly and does not appear to heat up, this can be seen by the tip discoloring to a blue tint when no longer cutting. The depth of the cut is regulated by the dial handle on the cross-slide of the carriage, the tool post is mounted here, each line on the dial of the handle of the tool post (small-wheeled handle) is equal to either 1 one thousands of an inch or 2 one thousands of an inch.
With the part rotating at a moderate speed touch off the tip of the cutting tool near the end of the rotating part, very slowly ease the tool towards the part until it just touches, as a witness there will be a very fine hair-like shaving on the cutting edge of the tool and back the carriage off from the part (using the large wheel on the carriage) without removing the tool from its position. The set the indicator marks on the dial to the Zero mark, now your ready dial in the depth of the cut from zero.
The reminder the carriage moves along the ways (the metal tracks) to and away from the chuck, the cross-slide move to and away from the center or the part. There are two other handles (lever like) on the carriage these are the power feed handles. One feeds the carriage toward the chuck at a predetermined speed and the other feeds the cross-slide. To understand the operation I recommend you test these operations without a part in the chuck at the different speeds of feed to get the feel of the action of the machine.
The higher the speed, within reason to the size and weight of the part, and slower the feed combined on most Chromoly steels, if you are not sure test speed and feed combinations on a scrap piece of the same metal you will be turning. Be absolutely cautious and be sure to stop the motion of the carriage before coming into contact between the chuck and tool post, this can damage either or both parts as well as dangerous to your safety.
When cutting metal it does heat up, heat can damage both the part and the cutting tool, the use of water-soluble oil mix with water or light cutting oil sprayed on the part is recommended to extend the life of the tool and keep the part from creating hard spot from overheating.
The Tail Stock is located on the opposite end of the engine lathe from the chuck, mounted on the ways of the machine. The tail-stock has limited uses, the most common is to drill out the center of the piece of steel in the sleeve of the unit you can insert a Jacobs Chuck, a chuck like on your hand-held power drill, and can be fed into the rotating part using the wheel at the outside end of the tail-stock.
The other general use is for a live center, the cone-shaped object with a tapered tang shaft that fits into the sleeve, and the cone portion spins on a ball bearing internal mechanism. This is used to fit into the center hole of the workpiece holding it firmly between the tailstock and chuck.
Before feeding either the drill chuck or live center into the moving part, be sure to lock down the tailstock into position by tightening the screws at the base of the unit or the tension lever, they come equipped either way.
On the carriage, there is one more handle that I am going to cover now. That is the thread cutting lever – it is referred to as the half nut. On the backside of the carriage you will see a dial with four numbers and marks, these are you engage marks. It is hard to explain fully but in generalities, for even-numbered threads as in ÂĽ 20 use even numbers for odd-numbered threads the odd numbers.
Now the first thing you will notice is using the half nut and the carriage will take off at a high rate of speed. So when threading, do not use high speed, the feed relationship is direct to the rpm’s the part is rotating. Reminder: this is just a general description of the threading process and it is recommended that you seek direct hands-on instructions.
Until you get used to the operation of the piece of machinery test at various speeds and feeds. If you get a long stringy cut instead of a chip increase feed until material comes off in chips. Keep the ways and carriage clear of chips from cutting, do not use your hands to use a small brush; the chips can be HOT and are extremely sharp and can cause injuries.