Model Engineers’ Workshop – July 2019

15

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Cross-slide Leadscrew

July 2019

Key

19.

15.

3.

3.

be cut: 12mm diameter x 2mm pitch
metric trapezoidal, and 10mm diameter x
1mm pitch metric. The latter pitch, whilst
not being a standard metric thread, is a
standard spark plug thread for which taps
and a die are available, and hence were
purchased and used. For the trapezoidal
thread I decided to make my own
screwcutting tool from an old fi le, photo 2.
Many readers will own a screwcutting
gauge: mine is by Moore & Wright, and
although it covers a variety of thread forms
and angles, it does not cover the metric
trapezoidal thread. I therefore decided to
make my own from a piece of 1mm thick
steel by marking two converging 75 degree
lines from one of the sides thus giving an
included angle of 30 degrees. The area
between the two lines was carefully fi led
away, and a short saw cut made at the
junction of the two lines in which the point
of the tool could sit, photo 3. Note that to
aid setting of the tool in the lathe, each pair
of opposite sides were made parallel to each
other and perpendicular to the other pair.
I could now carefully grind my tool until
the top of the tool exactly fi tted between
the jaws of the cutout. For this particular
size and pitch the end of the tool should be
ground to 0.598mm, that is, the width of
the root of the male thread. Measuring this
is diffi cult with only basic tools, so what I
did was to set the micrometer to 0.60mm,
then compared the tool tip to the gap
between the anvils. Possibly not the most
accurate way of setting the tool tip, but the
best I could do. I then did a trial cut on a
piece of scrap bar of the correct diameter;
and promptly broke the tool!
Following a regrind, I attempted another
trial cut by feeding the tool in very gently,

starting at 0.2mm infeed, then 0.1mm,

and fi nally 0.05mm as the cut deepened.

Another author suggested that the sides

of the cut should be shaved by the tool by

moving it very slightly to one side of the cut,

followed by moving across to the other side,

all by using the top-slide. This resulted in

yet another tool breakage, so that idea was

abandoned in favour of straight in and out.

Ultimately, I successfully cut the full

thread and kept deepening the cut until the

existing leadscrew nut could be run the full

length of the screw.

Order of work

Start with a length of 16mm diameter bar

slightly longer than required to allow for the

ends to be faced to the correct length. Place

the bar inside a 4-jaw independent chuck

with about 10mm showing, and using a dial

test indicator (dti) set the bar for minimum

runout, face the end and drill a centre.

Reverse the bar and repeat, but this time

reduce the bar to the correct overall length

whilst facing the end.

Now pull out the bar from the chuck

until about 150mm is projecting from the

chuck and using the dti set for minimum

runout adjacent to the chuck jaws. Note

that it may be advantageous to use the

tailstock with a dead centre to provide

some gentle support for the outlying end

of the bar whilst adjusting the runout. On

completion, lubricate the dead centre and

apply the tailstock to securely hold the bar

in place. The bar is now ready for turning

and threading.

First reduce a length of 138.25mm (as

measured from the end of the bar) to 12mm

diameter using a knife tool. Change the

knife tool for a narrow square ended tool

(I use my parting off tool) and create the

5.50mm wide x 9.30mm diameter thread

runout groove. Now replace the narrow

tool with the trapezoidal threading tool,

using the screwcutting gauge to ensure

correct orientation of the tool, engage

the appropriate changewheels, and start

cutting the thread.

The minimum speed of my lathe is

125rpm which is far too fast for cutting

short threads hence for short threads I

normally use a mandrel handle, however,

as using a mandrel handle on this length of

thread is very tiring, therefore I used power

drive. Unfortunately, this then introduced

another problem in that the inertia of the

lathe drive system means that the lathe

takes an appreciable time to come to a halt

once power is cut. Thus, it is necessary to

cut power at a point where the tool will just

run into the thread runout groove. Easier

said than done! So much so that I found

Fig.

2

Trapezoidal cutting tool.