This page records the steps in the development of a
MC-KIT application, in this case a lathe. Similar principles would
to the conversion of any machine tool, basically involving the
of manual handwheels and gears with electric motors and shaft-encoders.
on the design of the machine, some imagination may be needed to devise
suitable way of mounting these components. The solutions shown here are
necessarily the only possible way, but these have been found to work
The following pictures show the process of converting the mechanical
parts. The electrical and computer parts form the basis of the MC-KIT
system, which are intended for use on a variety of machine tools where
conversion is done in a similar way to that shown: namely, replacing
handwheels and gears with electric motors and shaft encoders. In each
the main problem is finding a way of mounting the motor / encoder. This
will vary from one machine to another, but the following illustrates
View of the headstock showing the gear-train and changewheels of the
longitudinal leadscrew drive. The first task is to replace these gears
by an electric
motor, which will allow the computer to control the leadscrew feed, and
mount shaft encoders both on the headstock spindle, and on the
spindle. This will allow the computer to measure the position of both
and replace all the mechanical gearing with the much more flexible
Here, all the mechanical leadscrew gearing has been removed. The lathe is still functional, only now the leadscrew has to be turned by hand, using the graduated handwheel at the tail end (right-hand end) of the lathe. This is important in this case - since we do not have another lathe available, we will go on using this one to make some of the parts (fortunately fairly simple) of the new system. Thus the development will have to take place by well-planned steps so as to always have the basic lathe functions available (a bit like modifying an aircraft while it is still flying!). Of course, workshops equipped with more than one lathe can use one to make the parts for the other one being converted - but not every workshop has more than one, so it is useful to see if it is possible for one lathe to make its own parts for its own conversion.
This is done simply by pulling with a spring balance on a steel ruler (or bar of some kind) attached to the handwheel, thus measuring the force necessary to make the handwheel start to turn, at a certain distance from the centre of rotation - thus giving the torque (force x distance). This is not an accurate measurement, rather it is to get an idea of the maximum torque needed, so as to choose an adequate motor type from the motor manufacturer's performance tables.
This is done in exactly the same way as in the case of the cross-feed handwheel, but this time the ruler is attached to the leadscrew handwheel. The force was a little greater in this case, suggesting that a larger type of motor be used for the longitudinal leadscrew.
To give a rough idea for this lathe, the typical torques to start
handwheels turning under normal conditions were measured as: 8.85
lbs.ins (1 Nm - Newton-metre)
for the cross-feed, approx. 18 lbs.ins (2 Nm) for the longitudinal
leadscrew. These values seem
typical for a small lathe, perhaps larger values would be expected for
large machine, but not very much greater - otherwise it would mean the
were difficult to turn by hand.
In practice, even before the motor was mounted, it was tested at half the nominal voltage, and the leadscrew turned easily even though the motor was held by hand so as to maintain tension in the drive belt, thus giving reason to be optimistic that the final system would work. In fact, as a rough guide, after mounting the motor as shown below it proved impossible to stop the leadscrew turning by using hand pressure on the manual handwheel, corresponding to a much greater pressure than would ever be used in practice even for the heaviest job, and just about at the limit of human hand strength! Basically, the above approximate calculations are useful as a starting-point, but the final aim is simply to have enough motor torque to keep the leadscrew turning easily under any normal conditions.
The starting point is simply an aluminium plate 1/4 inch (6.35 mm) thick, and an electric jigsaw. (the plate could have been mild steel, this does not matter - this plate just happened to be in stock). It will form a sufficiently rigid base for this application. It was cut a little larger than the final dimensions, to allow for finishing after mounting.
In order to mount the plate, it is necessary to make various holes of different diameters to fit onto the available mounting points - the studs and spindles of the headstock. Due to the difficulty of measuring exact positions, even with a vernier calliper, all at the same time, it was decided to start with the large hole that fits over the bearing of the headstock spindle. This being done with precision, it would serve as a reference for the other holes. Really, a milling machine would have been best to make this large hole. Not having a milling machine available, the improvisation shown was used - the plate bolted onto the lathe carriage, using the Myford vertical slide accessory. The boring tool was fixed into the 4-jaw chuck (which allowed the radial position of the tool to be adjusted exactly, with much patience) - thus inverting the normal arrangement of machining with a lathe. The hole, previously roughed out with the jigsaw, was finally machined to the desired tolerance of 0.010" (0.25 mm) larger than the external diameter of the headstock bearing shell.
place the plate, with the previously machined hole fitting over the headstock spindle bearing shell, and using studs in holes already made, then give a light tap over the pointed screw shown above (rubber hammer, please!) - the pointed screw makes a centre mark in the new position, which serves as a guide for drilling (naturally, starting with a small diameter drill and increasing the drill diameter to enlarge the hole diameter up to the final value). In this way, with care, the plate is made to fit exactly over all the fixing holes, with only small tolerances being necessary (of the order of 0.008" (0.2 mm) drill size over stud size). Packing washers are used between the lathe body and the mounting plate, to adjust this plate exactly at right angles to the headstock axis (visual alignment with setsquare is sufficient, extreme accuracy not required).
This encoder, which serves to measure the angular position and speed
of the headstock spindle, must be of a form which leaves free the
hollow centre of the spindle (which is used when feeding through
material in the form
of long rods). This requirement is not met by most commercially
encoders, which couple to a small shaft which would block the
through the spindle. Therefore, a codewheel is made to fit onto the
of the spindle. This could be in the form of a thin disc mounted
a hub, but in this case it appeared simpler to machine the whole thing
a solid disc of aluminium alloy. After turning the disc, leaving a thin
standing out, 48 slots were machined in the circumference, to be used
interrupt the light beams of optical sensors, thus generating the
pulses for measuring the rotation of the spindle. Why 48? Because it so
that 48 slots of a convenient size for an easily available optical
(and for which the milling cutter was already available in the
fitted exactly into the circumference of the disc. Any number could be
since the computer will perform the necessary calculations, but 48 also
the advantage of being exactly divisible by 2,3,4,6,8,12 which is very
useful to allow exact positioning at commonly met angles. These slots
are being machined in the picture, using the Myford angular divider
The encoder disc, machined from a solid aluminium alloy block such
that on the left. Note the 2 grub-screw holes in the hub, for mounting
the headstock spindle in the position previously occupied by the gear
The centre hole is machined to exactly fit the heastock spindle