13 The Lathe
The lathe is probably the most used machine tool in engineering workshops and, with the exception of the drilling machine, the oldest.
In principle the lathe is the same today as it was hundreds of years ago. There is evidence, in the form of turned bowls found at Glastonbury Lake Dwellings, that the early Iron Age people had learnt the art of turning.*
* Everyday Life in Prehistoric Times by C. H. B. and M. Qucnnell (Batsford)
The simplest form of lathe still used today is the pole lathe as used by the chair leg turners in the Chilterns (fig. 1). We believe because of its simplicity, this could have been the kind of lathe used by the Iron Age turners. This lathe is operated by a treadle which pulls on a thong wound round the work and attached at the other end to a springy branch of a tree. The work revolves in one direction under foot pressure and in the other direction when the branch pulls the thong. The operator cuts as the work turns towards him and withdraws his chisel as it returns.
The modern engineers' lathe differs only slightly in principle from the pole lathe in that the tool is held in a tool post and is moved mechanically.
Henry Maudslay, an English engineer, is reputed to have made the first lathe of this kind in 1800.
A good modern lathe suitable for small workshop use is shown in figure 2. Figure 3 shows many of the hidden details.
In England the size of a lathe is given as the height from the centre of the headstock spindle to the bed and the maximum distance between the centres. In America it is stated as the maximum diameter of the work that can be turned, i.e. the swing and the overall length of the bed.
HINTS ON USING THE LATHE
The beginner usually starts by doing a piece of simple turning rather than by first learning the names of all the parts. Let us then start by assuming he is given a piece of 1" diameter mild steel 6" long.
Start by putting the workpiece in the 3-jaw chuck so that about 3" protrudes from the jaws. Tighten the chuck with the key and return the key to its rack.
For convenience only, we are starting with the work protruding 3". In this case it is quite safe. However, the rule, when turning in a chuck is to keep the work protruding only just far enough to complete the operations you have planned, thus making it rigid.
First bring the tool up to the work by using the apron and cross slide hand wheels. We are assuming that the correctlysharpened tool is set up as shown in figure 4. Switch on the machine at about 400 R.P.M. and, using the cross slide, start taking a facing cut by feeding the tool in gently. Try to keep the tool cutting and the feed as constant as you can right to the centre, then stop the machine. If the work is not properly faced take more cuts (1/32" at a time is enough) until it is properly faced.
The important thing to remember with all turning is: keep the tool cutting and do not let it rub. By rubbing we mean the act of leaving the tool against the work whilst the lathe is spinning. This blunts the tool more quickly than taking a cut.
Without altering the setting of the tool (this is the reason for having the tool this shape and at this angle) take a sliding cut as shown by using the apron handwheel, not the top slide, and try to keep the cut going smoothly. Stop the machine or withdraw the tool as soon as you get to the end of the cut.
The machine in figure 2 is sturdy enough to reduce a 1" bar of mild steel to nothing in one cut, i.e. a 1/2 " deep cut. But for this first trial a cut of about 1/16 ", i.e. 1/8 " on diameter is sufficient.
Never let the tool or any part of the lathe hit the chuck whilst it is spinning. This can cause serious damage.
Having got the "feel" of the machine, try the same facing and sliding cuts by using the self act or automatic feed. Since lathes vary in the method of feed engagement, the Operator's Hand Book must be consulted, or ask the teacher.
Generally in turning we use a wide range of cutting tools (fig. 5) but it is common practice to take the first heavy cuts with a sturdy roughing tool then to finish with a tool accurately sharpened for the purpose.
A deep cut and a light regular feed arc better than a light cut and a coarse feed. A coarse feed causes more abrasion at the cutting edge of the tool thus it becomes blunt more quickly.
Before starting on a piece of turning you must plan your operations carefully, otherwise you might find yourself with a partly finished component which you arc unable to hold in the chuck, for further work, without damaging it.
Remember too when using a 3-jaw chuck that once the turned work has been taken out of the chuck it can seldom be put back to run absolutely true again however good the chuck. For this reason always try to do as many operations as you can at one setting.
The beginner now needs to know the different ways in which the work can be held and turned in the lathe.
METHODS OF HOLDING AND TURNING WORK
The 3-jaw Self Centring Chuck
This is shown in part section in figure 6. These chucks are provided with inside and outside jaws. The sets of jaws are numbered 1, 2, 3, and so is the body of the chuck.
It is most important that each jaw goes into its proper place, i.e. number 1 in slot number 1 and so on.
When inserting the jaws turn the chuck so that number 1 on the chuck is in the 12 o'clock position. With the chuck key turn the scroll so that the tail of the scroll just begins to show in slot No. 1. Now turn the scroll back slightly so that the tail just disappears and insert jaw No. 1, then turn the scroll so that the tail just engages the first tooth on the jaw. This can be tested by pulling the jaw: if it is properly engaged it will be held. Now turn the whole chuck so that No. 2 on the chuck is at 12 o'clock and insert jaw No. 2. Press the jaw against the scroll and rotate the scroll until the tail just engages the first tooth.
Repeat this for jaw No. 3.
Note that apart from the number 1, 2 or 3 on each jaw there is also a long serial number which is the same as on the body of the chuck. The two sets of jaws should be kept with their proper chuck. They are not interchangeable with other chucks.
Four Jaw Independent Chuck
Each jaw can be moved independently and is reversible. By adjusting each jaw separately irregular work can be held. Also round work can be set up to run quite true with the aid of a Dial Test Indicator (D.T.I.). Any slight movement of the plunger or arm, which bears on the work, is shown on the dial of this instrument (fig. 6A).
Method of Setting up Round Work. Switch off the isolator. Set the work central by using the concentric rings on the face of the chuck (fig. 7). Turn the chuck by hand. With a D.T.I, check the work close to the jaws. By tightening or slackening the jaws opposite to each other—No. 1 opposite No. 3 and No. 2 opposite No. 4—it is possible to get the work to run true.
Now withdraw the D.T.I, and move it to check the end of the work furthest from the chuck. Any error at this end is removed by tapping the work with a hide mallet, but remember to withdraw the D.T.I., whilst actually tapping the work, otherwise it will be damaged. By using the mallet only, this end is made to run true. Next check the end of the work nearest the chuck. This will now be slightly out of true. True up by using the chuck key. Now check the end furthest from the chuck: use a mallet as before and make it run true. Repeat as often as necessary.
Remember: True the work near the chuck by using the chuck key only and the part furthest from the chuck with the mallet only.
By this method it is possible to get the work quite true over its whole length when checked with a D.T.I.
LATHE TOOLS
The grinding and setting up of a lathe tool is of primary importance and it takes not a little practice to become proficient in grinding these tools. Figure 8 shows a typical general purpose tool.
The lathe tools we use in the school workshop are mostly of high speed steel (H.S.S.) but we sometimes use carbon steel tools, particularly when we need to make form tools or special boring tools. Carbon steel tools are excellent but they are not as tough or long-lasting as H.S.S. and must be run at a slower speed. Tipped tools are also available. These consist usually of a small piece of tungsten carbide brazed to a carbon steel shank (fig. 9). These tools arc so hard that they need special diamond wheels to grind them. For this reason they are seldom used in the small workshop.
However, from whatever steel the tools are made the cutting and clearance angles must be made to suit the material being cut. The following table can be used as a guide when grinding tools:
Material |
Top Rake |
Side and Front |
Aluminium |
25°-35° |
|
TABLE OF TOOL ANGLES
(Notice that the softer the material the greater the top rake.)
Lathe tools can be either in the form of tool bits which are held in a tool holder (fig. 10) or large solid tools (fig. 5) held directly in the tool post.
Figure 8 shows the tool ground for efficient performance with the cutting edge horizontal. However, it is not always possible to keep the cutting edge horizontal when the tool is set up, especially since manufacturers make tool holders which hold the tool at an angle.
The tool should always be set up so that the tip is at centre height. Figure 11 shows a tipped tool being adjusted to centre height against the point of the lathe centre, and figure 12 shows a parting tool set by the same method. Another good method is to lightly hold a thin 6" rule against the work with the tip of the tool (fig. 13). If the rule is upright the tool is central.
Figure 14 shows a roughing tool which is used for taking heavy cuts. The finishing tool has a radius on the tip. If this is used in conjunction with a fine feed the resultant finish will be smooth. The round nose tool can also be used for finishing or for making radii. The knife tool is used as shown.
A tool is either right hand or left hand. A right hand tool is used on the right of the piece being turned and the left hand tool on the left.
TOOL POSTS
Figure 15 shows three tool posts: on the left is the swivel type tool post; in the centre is shown a rear tool post (this is useful for parting off, the parting tool being held upside down); the third is a pillar type tool post (by means of the rocker or boat the height of the tool can be adjusted).
Figure 16 shows a quick change tool post with its set of tool holders and wrenches. These are useful where various tools are being used on a component. Once the centre height of the tool is set they can be taken out and returned in a second and will always return to centre height. Figure 17 shows the swivel type four-way tool post.
CUTTING SPEED
The speed at which the work revolves when being turned is determined by I. the diameter of the work, 2. the material being turned, 3. the kind of tool, i.e. H.S.S., carbon or tungsten tipped, 4. the rigidity of the machine, and 5. whether a coolant is being used or not.
It is difficult to lay down hard and fast rules because there are all these factors to be considered.
The following tabic of speeds will be found useful, but common sense based on experience must be applied.
TABLE OF CUTTING SPEEDS
Cutting speeds in feet per minute
|
Aluminium |
Brass |
Bronze |
M/S |
Highcarbon steel |
Cast iron |
Dry |
400 |
300 |
75 |
90 |
50 |
70 |
With coolant |
500 |
- |
80 |
110 |
65 |
- |
Screw cutting |
60 |
60 |
60 |
40 |
30 |
30 |
The above table is for H.S.S. tools. For carbon steel tools these speeds must be halved. For tipped tools they can be trebled. Notice that brass and cast iron are machined without lubricant. To calculate the speed at which a piece of work is being cut: given the diameter of the work and the speed in revs, per minute
Cutting Speed = (Diam. of work in inches X 35 X R.P.M.)/12
e.g. 3" diameter work revolving at 140 R.P.M. Find the cutting speed.
(3 X 3-1/7 x 140 )/12 = no ft. per mm.
This speed of no feet per minute is also known as the surface or peripheral speed.
If the lathe when cutting makes an excessive noise, chatters or overheats, stop the machine and check the speed, the sharpness of the tool and the rigidity of the setting, i.e. the tighthess of the chuck, the amount of play if a centre is being used and consider, if applicable, the use of a coolant. Check also the feed. Generally a lighter cut and a finer feed combined with a sharp tool and coolant (if applicable) will improve the work.
Coolants lubricate the tip of the tool and make the passage of the swarf over it easier; they also take away the heat from the work and the tool thus reducing the wear. Brass and cast iron form small chips and for this reason they do not abrade the top of the tool in the same way.
Soluble oil is the coolant most used in school for steel but there are many others available but they tend to be more
expensive (see Chapter 16). Here are some recommended coolants:
TABLE OF COOLANTS
Metal Coolant Aluminium Paraffin Duralumin Paraffin Mild steel Soluble oil Carbon steel Soluble oil Cast iron Dry Brass Dry
PROCESSES
Parting Off
This process causes some difficulty.
For successful parting off the lathe bearing should be good and the top slide and cross slide properly adjusted so that they are not too loose.
The tool must be accurately ground as shown in figure 13 and figure 18 with little top rake and front clearance: this is to keep the tip as strong as possible.
Have the tool protruding from the holder just enough to part through the work (and no more), and set it up perfectly central and at right angles to the axis of the work.
Try to plan the work so that the parting off cut is done as close to the chunk, i.e. the point of support, as possible.
Start the lathe and commence parting off. If it starts chattering do not withdraw but increase the speed of your feed. Keep the tool cutting but do not force it. Figure 19 shows pieces that have been parted off a 1" bar of M.S. without any coolant, for demonstration purposes, but a coolant will prolong the life of the tool and give a cleaner cut.
Parting off can be done at speeds just below those for ordinary turning.
Knurling
This can be either straight knurling or diamond. Figure 20 shows a diamond knurl: this is done with double wheels as shown.
Knurling imposes a considerable strain on the work and the lathe so be sure the work is held tightly in the chuck and that the knurling wheels are oiled.
For smaller lathes the knurling tool shown in figure 21 is recommended because the tool itself takes much of the strain.
Start with the wheels partly off the work as in figure 22 and apply enough pressure to start the pattern. Feed the knurling tool along using the apron hand wheel and return to the starting point, feed in the cross slide about 0.004". Repeat as required.
The knurling is not complete until the diamonds are properly formed. Beware of over knurling. This causes pieces to be broken away from the knurled surface.
The edge of the knurling can be finished either with a chamfer or with a turned step as in figure 22.
The speed for knurling a 1" diameter piece of mild steel is about 75 R.P.M. A coolant may be used according to the type of material being knurled. The speed for other materials and diameters can be judged from the speed for mild steel.
Drilling
This is done with the drill held in the tail-stock chuck. Always start by using a slocombe centre drill (figs. 23 and 24) Be sure to run the lathe fast enough. Centre drills are often broken because the speed is not high enough.
Start centre drilling gently with the quill clamping nut partly tightened, then slacken it off as the drilling proceeds. It is important to keep the centre drilling true, particularly if it is to be followed by other drills which must be concentric.
Drills can be kept steady by holding a piece of steel against the tip as shown in figure 25.
A large taper shank drill may be held in the Morse taper of the tailstock quill. A Morse taper sleeve may be necessary for this.
Boring
Boring can be done either with a solid boring bar as in figure 26 or a boring bar and tool bit as in figure 27.
The solid boring bar can be of H.S.S. steel or it can be forged in the workshop from carbon steel.
When setting up a boring tool keep it as short as possible, i.e. rigid. The tip of the tool can be set slightly above centre when boring parallel holes. By setting it high the tendency to "dig in" is reduced because as the tool "whips" downwards it will tend to clear itself (fig. 26). It is often necessary to increase the front clearance angle on the boring tool as shown.
Another method of boring, less used in school, is to have the boring tool revolving in the chuck and the work clamped on the cross slide.
Turning between Centres
Figures 28 and 29 show work set up between centres.
Before this can be done each end of the work to be turned must be properly centre drilled with a slocombe centre drill. The large cutting edges of a centre drill are ground to 6o° to suit the lathe centres (fig. 28).
The work is driven by a catch plate on which there is a catch pin, which turns the carrier (fig. 30). A bent tail carrier (fig. 31) can be used with a catch plate which has a slot instead of a catch pin. The tail of the carrier fits into the slot.
Lathe centres are shown in figure 32: the right hand one is cut away. The centre one is a revolving centre and the left hand one is the standard model.
Before the centres are put in the lathe they must be thoroughly wiped with a cloth and the holes in the headstock and tailstock cleaned, also the sleeves if they are being used.
Often the centre which fits into the headstock spindle is soft so that it can be turned when it becomes out of true. It is important that the headstock centres run true. This can be checked with a D.T.I.
The tailstock or dead centres are hard. The cut away variety are also known as half centres and are useful for facing work with a right hand knife tool (fig. 14).
When setting up work using a dead centre there must be a slight amount of play between the work and the centre, i.e. it must be able to revolve freely. For this reason the back centre must be kept greased and great care taken to avoid the work overheating at the centre owing to friction caused by the work expanding as a cut is taken.
This friction can be avoided by using a revolving centre but care must be taken with long work which is likely to expand more than short work.
Revolving centres turn at the same speed as the work.
When starting to turn between centres first take a light cut along the full length of the work and check both ends with a micrometer. If the work is tapered the tailstock must be adjusted; this is done by first slackening the clamping screws and then setting the top part of the tailstock over by using the side screws. This varies from lathe to lathe.
Use of Mandrels
Figure 33 shows a mandrel. These are made from hardened tempered and ground steel. They have a slight taper of about 0.003" over a six-inch length. Each end is centre drilled and recessed to protect the edges of the centre from damage. A flat is provided for the screw of the carrier. Mandrels can be bought in sizes from 3/16 " up to 1 1/2 " diameter.
Work which has been previously bored or reamed is pressed on to these and held by friction then turned between centres.
Gear wheel blanks arc often done in this way.
Use of Steadies
Long work between centres or long work which cannot be supported with the tailstock centre can be supported with a steady.
There are two kinds of steady. Figure 34 shows a fixed steady. Figures 35 and 36 show travelling steadies.
When setting up a steady adjust the points of support for the work, which are made of phosphor bronze, so that they just bear against the work and then lock them with the screws provided. Lubricate the points of support and turn the lathe by hand to be sure they are not too tight.
The work must be running true before the steady is adjusted to it. Never force work to run true with a steady as this strains the steady, causes the work to be turned out of round and can cause other parts to be eccentric if turned, when the steady is removed.
The travelling steady is particularly useful when cutting long slender threads.
Taper Turning
The top slide is used for most short tapers as in figures 37 and 38
If the angle is not given on the drawing it can be calculated as in the example in figure 38.
Tan Θ = (opposite/adjacent) = (3/4)/3 = 0.75/3 = 0.25
Tan 0.25 = 14° approx.
The tool itself set at an angle can be used for very short tapers as in figure 39.
Long slight tapers can be made with the work between centres and the tailstock centre set over as shown in figure 40.
Figure 41 shows a method of calculating the amount of "set over" or "off-set".
If we assume the component at the top of figure 41 is to be made from 1" diameter bright drawn mild steel an accurate way to set over the tailstock is shown. First set the work up between centres, then by calculation reduce the base of the triangle to a length which can be accommodated on the top slide. Be sure the top slide is set to cut parallel. With a D.T.I. fixed in the tool post wind the top slide over to the left and return it slightly to eliminate the back lash and set the micrometer collar to zero. Wind the cross slide in so that the D.T.I. starts reading, then zero it. Now wind the top slide back 2" (carefully counting the turns). Check the reading on the D.T.I. If it is less than 0.0625" the tailstock must be set over more and vice versa. Repeat until the D.T.I, reads exactly 0.0625 when it is traversed 2".
Always check beforehand that the D.T.I, is well able to accommodate the range of reading that you require.
A better way to turn long tapers is to use a taper cutting attachment as shown in figure 42. Greater angles can be turned by this means and turning and boring can be done. The cross slide nut must be disengaged and the extension piece clamped by the nut over the taper cutting attachment. The top slide must be turned through 90° (not shown in fig. 42) so that the tool can be "fed in". When this attachment is set over at an angle and the saddle is traversed the tool will run parallel to the slide of the taper cutting attachment and so cause the tool to cut a taper. (The cross slide nut need not be disengaged on lathes with special splined cross slides.)
The taper turning attachment is marked at one end in degrees inclusive and at the other inches per foot inclusive.
Spiggots
These are generally slightly tapered and the work is pressed on and held by friction (figs. 43 and 44). They are used for holding work which would otherwise be difficult to hold. In Figure 44 the spiggot is being used for turning a small amount off a gilding metal bezel. In figure 43 the cover of a small circular box is being held for turning.
Face Plate Work
A face plate is shown in position on the lathe in figure 45. Because the weight is not evenly distributed balance weights have been attached.
The face plate is often used for work which would be difficult or impossible to hold by other means. It is frequently used on castings where one machined face is clamped flat on to the face plate by means of the holes or slots in the plate.
When setting work on the face plate it is often easier to set it up in its approximate position with the face plate on a bench before it is put on the machine. Then it can be trued with the aid of a D.T.I, or a pointer as in figure 46.
Paper between the work and the face plate improves the grip. Care must be taken not to distort work when the clamps or holding screws are tightened.
Screwcutting
Screw threads have been mentioned in Chapter 3. Here we are concerned with cutting a thread on the lathe using the lead screw to move the cutting tool in a certain relationship to the revolutions of the headstock. This relationship or ratio is obtained by gearing the headstock spindle to the leadscrew. On most modern lathes this is done by selecting the gears on what is known as a Norton gearbox.
The lathe in figure 2 has a quick-change gearbox. However, many lathes still use change wheels. Whatever sort of lathe is used you should understand how to calculate the gears or change wheels. Figure 47 shows a lathe with the guard removed to reveal the gears which connect the headstock spindle with the leadscrew.
On a simple gear train (fig. 48) if the gear on the headstock spindle or stud wheel is the same size as the gear on the lead-screw then the ratio between them will be 1:1, i.e. the head-stock spindle will revolve at the same speed as the leadscrew.
The formula used for calculating change wheels is as follows:
t.p.i. of leadscrew / t.p.i. to be cut = Drivers/Driven
(t.p.i. is short for threads per inch).
If we want to cut a thread with a quarter of the pitch of the lead screw then the ratio is 114 and we should use a driving wheel a quarter of the size of the leadscrew gear. The gear train would be as in figure 48. The intermediate or idler gearwheel merely transmits motion and keeps the driver and driven wheels going in the same direction.
English lathes are usually supplied with wheels ranging from 20 to 120 teeth in steps of 5 and with a 127 wheel for metric threads. The 40 and 60 toothed wheels are duplicated.
The direction of the leadscrew can be reversed for cutting left hand threads, by tumbler gears (fig. 49). These gears are not taken into account when calculating the change wheels because having a ratio of 1:1 they cause the stud and headstock spindle to move in unison.
Example 1
To cut a thread of 12 t.p.i. on a lathe with a 6 t.p.i. lead-screw.
Driver/Driven = t.p.i. of leadscrew/t.p.i. to be cut = 6/12
Multiply top and bottom by 5.
(6/12) X (5/5) = (30/60)
i.e. a 30 tooth gear on the spindle or stud and a 60 tooth gear on the leadscrew meshed with any suitable intermediate gear.
Example 2
To cut a thread of 25 t.p.i. on a lathe with an 8 t.p.i. lead-screw.
Driver/Driven = t.p.i. of leadscrew/t.p.i. to be cut = 8/25 = 40/125
However, there is no 125 wheel in the set so we write down 8/25 as 4/5 X 2/5 or 40/50 X 20/50, but there is only one 50 tooth wheel in the set so we multiply 2/5 X15/15 = 30/75, this gives us (Drivers 40/Driven 50) X 30/75
These gears are available but must be set up in a compound train (fig. 50).
Metric Threads Cut on an English Lathe
There are approximately 25.4mm in 1". A large lathe having a leadscrew of 1" pitch with a 1 : 1 ratio set up on the gears would cut a thread with a pitch of 25.4mm. A 10:1 ratio would give a pitch of 25.4mm. This would be 10 Driver/1 Driven.
To cut a pitch of 1 mm the ratio would be: 10/1 X 1/254 = 5/27 But this is for a lathe with a 1" pitch leadscrew. A 4 t.p.i. leadscrew would have to turn 4 times as fast to cut the same thread.
This can be set down as(Drivers 5x4)/(Driven 127) cuts 1 mm pitch thread.
If we wanted to cut a 5mm pitch thread the leadscrew would need to turn 5 times as fast, so we would multiply the drivers
by 5.
(5x4x5)/127 = Drivers/V = 100/127
The rule then is:
(5 X t.p.i. of leadscrew X pitch to be cut in mm)/127 = Drivers/Driven
Note: When setting up any train of gears adjust them so that there is a little play between the teeth of the meshing gears.
Screw Cutting Method
The work must first be turned to the correct diameter and for ease of cutting the thread it is best, though not always possible, if it is turned down to the core diameter at one end and relieved with a groove at the other end as in figure 51. Long work must be supported with a centre.
The tool must be set up not only centrally but also square to the work. The tool for vee threads is set up with a thread angle or screwcutting gauge as in figure 52.
The tools are ground to the correct angle using the screw-cutting gauge and the radius at the tip of the tool can be stoned to suit the radius on the screw pitch gauge (see Chapter 3, fig. 16). Clearance and top rake as for other lathe tools is required depending on the material being cut. Remember that the rake alters the flank angles of the thread.
Assuming that the correct change wheels arc set up and that the top slide is parallel to the bed of the lathe, wind the cross slide in until the tip of the tool just touches the work. Now carefully set the micrometer collar on the cross slide to zero. Move the saddle to the right so that the tool is just clear of the end of the work. This is assuming we are cutting a right hand thread. Start the machine on a slow speed—the slower the better for a beginner—but the speed can be increased later (see table of cutting speeds).
Adjust the chasing dial (fig. 53) to the leadscrew so that it revolves when the leadscrew revolves. This indicates the relative position of the carriage, the leadscrew and the work. It has a pinion which engages on the leadscrew and indicates when the half nut (fig. 54) can be meshed with the leadscrew.
The number at which to close the half nut can be found as shown in the following examples.
Example 1
This represents a lathe set up to cut 5 t.p.i. having a lead-screw of 4 t.p.i. and a chasing dial with 8 divisions and a pinion with 16 teeth (fig. 55).
Every fourth thread of the leadscrew is coincident with every fifth thread being cut.
Each revolution of the leadscrew moves the chasing dial pinion one tooth. Therefore the pinion moves 4 teeth from one point of coincidence to the next. The pinion has 16 teeth so it will move 1* of a revolution or J turn, i.e. the half nut can be closed on any number.
The rule is:
smallest number of t.p.i. of leadscrew coincident revs of with t.p.i. to be cut/teeth on chasing dial wormwheel dial = revs of chasing dial
Example 2
A thread having 8 t.p.i. is to be cut on the same lathe. Each thread of the leadscrew is equal to 2 threads to be cut (fig. 56). Using the rule this gives us 1/16 revolution of the chasing dial. This means that the half nut can be meshed at any division on the chasing dial. Ffgure 56A shows the principle of screwcutting in diagrammatic form.
To continue with the screwcutting engage the half nut at the correct division of the chasing dial. The carriage will now start moving and the tool will start to cut a fine helical groove (the thread). Lift the half nut smartly when the tool has traversed the length of the work and is in the groove. Wind the tool away from the work with the cross slide handwheel and with the apron handwheel return the carriage to its starting point with the tool just to the right of the work. Now wind in the cross slide carefully to take a light cut, 0.002"-0.003" is general.
Remember, always wind in to the number you want to eliminate the play between the cross slide nut and thread (backlash). Drop the half nut at the correct division and take this cut. Repeat as before. After a number of cuts the top slide can be moved about 0.001" forward or back in conjunction with the cross slide. This is known as "side cutting" and is done to relieve the pressure on the tool and to get a better finish.
When the tool touches the core diameter the thread is deep enough. This thread now has a radius at the root of the threads but not at the crest. These are often left for general work with the crests unradiused. Where greater accuracy is required the tops of the threads are finished with a chaser (fig. 57). The thread chasers can either be set up in the tool post or can be used by hand on a suitable rest.
Another method of cutting a thread is to set the top slide at an angle of half the thread angle or one degree less for a better finish, i.e. 26.5° for Whit (fig. 58). At the end of each cut the cross slide must be returned to zero or to a stop and the amount of cut taken with the top slide only.
When cutting threads use the appropriate coolant as this will improve the finish.
Dieing and Tapping in the Lathe
Threads can be cut on the lathe by using a die holder lightly supported by the tailstock quill whilst the lathe is turned by hand (always switch off the machine at the isolator).
A very reliable way to die threads is to hold the die in a tailstock die holder (fig. 59). These are reversible and can hold a large die at one end and a small one at the other. The stem is parallel and the body part slides over it. The Morse taper fits into the tailstock quill. The work is held in the chuck and the tailstock with the die holder is brought up to the work. The chuck key can be put in the chuck to rotate it with the left hand whilst the handle of the die holder is rotated against it with the right hand. Slight pressure is needed to start the die cutting, but once it starts cutting it will move down the thread being kept quite true by the stem. Use the proper coolant for best results.
Tapping in the Lathe. This can be done providing the tap has a centre hole in the shank end.
The lathe is turned by hand with the tap supported by the tailstock centre and turned with a spanner or a tap wrench. As the tap starts cutting the thread so the tailstock centre is wound in to keep it supported. Care must be taken not to force the tailstock centre against the tap.
Smaller taps can be held in the tailstock chuck especially for tapping brass. However, this method is not recommended, except for starting off a tapped hole, because it is difficult to judge how much force is being applied to the tap.
Spinning. Spinning is a method of making hollow vessels on a lathe using a former over which the work is pressed, whilst rotating at high speed, with a smooth, hard and highly polished tool.
In factories spinning is done on high speed spinning lathes.
However, small work can be spun on any small lathe, but generally the larger lathes are better because the bearings stand the thrust imposed on them better.
Spinning is often said to be dangerous, but in fact it is no more dangerous than wood turning providing it is supervised.
Copper and aluminium are two metals which, because of their softhess, can be readily spun. 20 SWG is a good thickness for copper and 16, 18 or 20 SWG for aluminium. Brass and gilding metal can also be spun but need more frequent annealing.
Deep shapes are best done in two stages, i.e. two spinning chucks are used. This reduces the tendency for the metal to wrinkle or become too thin. Figure 60 shows two chucks which can be held in a 3-jaw chuck. These can be made from close grained wood such as maple or beech. Often boards have to be glued together to make up the required thickness.
The disc to be spun must be properly annealed and clamped centrally against the end of the former or chuck with a pressure pad (fig. 61) held by a revolving centre (fig. 62). (The size of the disc is calculated as for raising.) The pressure pad should be only slightly smaller than the small diameter of the chuck and should be reinforced with a piece of metal which is centre-drilled as in figure 61.
The tool (fig. 63) is supported on a tool rest (fig. 61) which can be held in the tool holder. The tool is levered against the fulcrum pin which can be moved into any of the holes provided. To counteract the thrust the saddle should be tightened. The disc must be lubricated with grease, soap or tallow to prevent it becoming scored.
Start spinning by pressing the rounded side of the tool against the revolving metal close to the pressure pad. Keep the tool moving from the centre outwards and then back towards the centre over a small area at first. As the metal is spun down to the chuck the fulcrum pin should be moved to the left so that better leverage can be obtained. If the metal starts to wrinkle press the spinning disc between the tool and the back stick (figs. 63 and 64). Always "coax" the metal with smooth strokes of the tool. If too much force is used the metal becomes grooved.
As the spinning progresses the work will need annealing from time to time.
Finish the work with the planishing tool. This is used in the same way as the spinning tool but a smoother surface can be obtained by its use.
The speed for spinning can be about iooo R.P.M., although in industry much higher speeds are used.
