MASTERING THE LATHE: II
IN THIS PART OF THE SERIES WE COVER: TURNING AND FACING, SPEEDS AND FEEDS, AND ‘CLOCKING UP’ IN A FOUR-JAW CHUCK
However, even when I was working at Prescott Engineering we still had applications for the HSS tool bits for the odd tricky job. It boils down to the funds, time, and patience you have available and, most importantly, your skill levels. For the purposes of this article, I will assume you have decided to use the indexable tungsten carbide tips. Prices vary from supplier to supplier and there are lower cost brands on the market. But the old adage that you get what you pay for holds true; none of us expects to get a Ferrari for the price of a Hyundai, but why buy a Ferrari if a Hyundai meets your needs?
In the illustrations, you can see how square and triangular tungsten carbide tips can be used for turning and facing. The square tip has the advantage that it can do both and is also handy for chamfering sharp corners. Unfortunately, it is not good for producing square corners and shoulders; we need the triangular tips for those. There are many other shapes of tips, but, as they all need their own special tool holder, I would suggest that the square and triangular tips are versatile enough for the home workshop.
fassttilclanhayovueruanppyoliucraltaitohne?s It wHouSlSd bteogorlebaittisf wfoe our cutting flat out. This would save a
oanvlayialasbhloert taimnde bylouuntr, asnkdiltlhleesvuerflas.ce finish on our project would look awful. Materials vary in ease of machining — generally, the tougher the material the slower we have to cut it. If the finish is not as good as you want, then try
gBuuidtet,htheeroeldare rawnhgeastofycouuttipngayspeeds aonfduwse lfeorrpmthferopmritcheosoef.
which is the rpm of the chuck — it is essential to grasp the idea of cutting speed. The cutting speed is how fast the surface of the job is moving past the tip of the cutting tool,
constant, the resulting cutting speed is greater. In other words, the speed is greater on a larger diameter job than on a smaller one because more surface area is being moved past the cutting tool in one revolution than the lesser surface area of the smaller job in the same turn of the spindle.
If that last sentence makes sense to you then you have got it. If not, then read the explanation again, as it is important to understand the logic. If you set a cutting speed that is suitable for the material and you know the circumference of the job, you can find out the required spindle rpm by transposing the equation thus:
RPM spindle = Cutting Speed Circumference of Job
See the side panel ‘Cutting speeds’ for the recommended ranges of cutting speeds (in metres per minute).
Broad range
The cutting speeds are dependent upon the type of material being cut because some materials cut more easily than others. Brass cuts easily and it is not difficult to produce a good finish on it. Personally, I’m not that keen on brass because its swarf is like very fine sharp needles, which have an annoying attraction to my fingers.
The table shows different ranges for high-speed steel and tungsten carbide cutting tools. Generally, when using tungsten carbide tips try using a faster speed first, as they tend to give better results at that end of the scale. It would be nice if we could be more exact and state one value for cutting speed for each material instead of what must seem like a broad range. The reason is that, apart from the material and cutting tool type, other factors also influence optimum speed. These include:
* rigidity of the lathe
* power of the lathe
* surface finish required
* coolant used or not
* depth of cut
Experience with your own lathe will tell you where in the range you achieve the best results.
Now, to get back to our formula, we will give an example of finding the spindle rpm that has to be set for our desired cutting speed and the diameter of the job.
RPM spindle =
Cutting speed
There we go. You have now conquered cutting speed calculations. Of course you may not be able to select this exact spindle speed on your lathe, but you will need to use the one closest to it.
Now, as a suggestion, it could be worth your while to make your own table of rpm vs cutting speed that you can refer to instead of working through the calculations each time. The table could relate to the material you work with most, be it steel, brass, or other material. The cutting speed used could be the actual value that you find suits your lathe and the cutting tools you use.
Feed rates
Feed rates refer to the use of the powerfeed function. The feed shaft is geared to the headstock spindle, and the saddle uses it to drive the motion of the cross slide or saddle for facing or turning, respectively.
By altering the gearing, we can have either coarse or fine feed rates. The lathe will have a plate on the side of the headstock showing the feed rates according to various positions of gearbox levers. To decide what feed rate you want, first you will need to decide if this is a roughing or finishing cut. This could be a good time to talk about tungsten
carbide tips for roughing and finishing. Tips can be purchased with different sizes of nose radius, i.e., the radius at the actual cutting point.
Larger radiuses are used for roughing cuts because they are more robust and less likely to break under heavy loads. Smaller radiuses are used for finishing cuts because the shorter length cutting surface minimizes the vibrations that create a bad finish. My preference is to use a 0.8mm radius for roughing and a 0.4mm radius for finishing. Feed rate is stated in millimetres per rev. That is how far the saddle travels along the bed during one revolution of the spindle.
As a general rule, the maximum feed rate for a tungsten carbide tool should be no greater than 80 per cent of the nose radius. To save you the trouble of calculating, the maximum feed for a 0.8 radius tungsten carbide tool is 0.64mm/rev and for a 0.4 radius is 0.32mm/rev. Again, factors such as lathe condition have some say about what feed rate suits best, and you will develop your own experience.
Another key factor is how deep your cut is. A deep cut with a coarse feed removes a bigger volume of metal from the job than a shallow depth of cut and a fine feed rate does in the same time.
This means that more power is needed to remove the larger volume of metal, so you must set these values within the limitations of your machine. Hopefully, you now have an appreciation of what sort of feeds and speeds to start off with on your lathe. Have fun determining what works best on your machine.
Centre height
If you are having trouble achieving a good finish on your job, the first thing to check is that the tip of the cutting tool is at the right height. It is absolutely vital that it is at the same centre height as the axis of the spindle. You can adjust the tool to the dead centre in the tailstock. As a further check, face the end of a bar to see if it leaves a ‘pip’ in the middle. This indicates that the tool is too low. You will also see from the machining lines exactly where the centre point is and can adjust the cutting tool to that.
‘Clocking up’
Sometimes it is necessary to grip in the lathe a square or irregularly shaped job that does not suit the three-jaw chuck. That’s when the four-jaw comes into its own. Each jaw of the four-jaw chuck can be adjusted in or out independent of the other three. This allows the chuck to hold different-shaped objects or let the workpiece be positioned offcentre.
While the three-jaw chuck is quick and easy to use and, by virtue of its design, centres the workpiece on the spindle axis, it does not do so with absolute accuracy. When it is vital that a job is running true — such as when new cutting is to be done which must be concentric with existing diameters — we need to ‘clock’ it up in a four-jaw chuck. The clock referred to here won’t tell you the time, but will show run out very accurately and sensitively. In workshop language we call it a ‘clock’, but strictly speaking it is a dial test
indicator (DTI) and these come in two main types, plunger and lever.
Plunger types tend to be less expensive while the lever types are more versatile. They are used with a magnetic base that has adjustable arms for positioning the DTI where it is needed, just like a sky hook. When ‘clocking up’ a job, it is best to slip the spindle out of gear so that it can be easily rotated by hand.
Position the stylus of the DTI so that it just contacts the surface you want to have running true, then slowly turn the chuck by hand through one full rev. The needle on the dial of the DTI will move to show the highest and lowest positions around the diameter of the job. Loosen one jaw to allow the job to move towards the jaw, then tighten the jaw directly opposite to push it over.
Be careful to move the jaws only a small amount each time and to adjust only one opposing pair at a time. If you loosen jaws next to each other, the job can, of course, fall out. By repeating this process over and over, you will eventually reach a stage at which the run-out or waver of the needle each time the chuck is rotated is extremely small. At that time, make sure all jaws are tight and recheck for run-out.
Get the DTI and its mounting base well out of the way before starting up the lathe. Being a sensitive instrument, the DTI is also a delicate one. Take care of it and it will serve you well for many years.
If you are setting up a round job in a three-jaw chuck, it is sometimes useful to ‘clock’ that job to see how true it is running. While you cannot adjust the jaws independently to correct any error, often a tap with a soft hammer on the ‘high’ side of the job will bring it closer to true. However, take the DTI out of contact before doing this as the shock from even a small tap can harm its internals. Happy ‘clocking’.