Model Engineers’ Workshop – July 2019

16 http://www.model-engineer.co.uk Model Engineers’ Workshop

it necessary to be prepared to smartly
withdraw the cutting tool if it looked as if
it was going to overrun the runout groove.
Fortunately, most cutting runs stopped
short whereupon I used the mandrel handle
to fi nish the run.
Returning the tool to the beginning of
the thread was under power and the same
problem arose at the tailstock end.
There is another problem. The usual
method is to cut the thread, clean it up as
described below, and then try the nut. But
the only nut I have is inside the saddle,
and to get it out requires the removal of
the cross-slide. But the cross-slide is in
use cutting the thread. A truly chicken and
egg situation. Actually, it is not as bad as
it sounds because it is possible to remove
and replace the cross-slide without losing
that all-important registration between the
mandrel, changewheels and longitudinal
leadscrew, so the method becomes cut the
thread until the cutting tool just starts to
scratch the runout groove, clean up the
thread, remove the cross-slide, extract the
leadscrew nut, back off the tailstock and try
the nut. If it doesn’t fi t, then put it all back
together, take another very fi ne cut and
repeat as above.
Unfortunately, there are two other
problems. I fi nd that oft en the tool does
not cut to full depth as indicated by the
cross-slide dial. This is due to spring, that
is the work bending ever so slightly away
from the tool or slack in the compound slide
components, or both. Now for rapid stock
removal this relatively unimportant, but
when getting towards the fi nish, I fi nd that

I need to take a number of cuts all at the

same cross-slide setting. In fact, when I do

this, I repeat the cuts until the tool stops

removing material.

The second problem is that my

experience of lathe screwcutting is that the

action of cutting the thread always throws

up burrs and these can be a confounded

nuisance to say the least. Ideally, one

should clean the thread with something like

a chaser, but if, like me, you have not got

one, then the alternative is to gently use a

fi le against the thread to remove the burrs,

followed by applying the thread cutting tool

again just to clean the fl anks of the thread

as the fi le may have turned some of the

burr over into the thread. Messy I know, but

there is not much I can do about it.

Once the trapezoidal thread has been

completed, the embryo leadscrew can

be turned around and the other end

completed. To protect the already threaded

portion, I use a copper sleeve made from

a piece of surplus/scrap copper pipe. As

before, the leadscrew will need setting for

minimum runout adjacent to the chuck

jaws, and although it may be possible to do

the work without tailstock support, I would

strongly recommend it.

As before, the fi rst step will be to reduce

the 16mm down to say 10.5mm, then,

using the narrow bladed tool, turn the two

grooves paying particular attention to the

face of the fl ange as this becomes part of

the bearing for the leadscrew. The section

for the 10mm x 1mm pitch thread should

then be reduced to 9.87mm-9.88mm to

allow for the inevitable extrusion eff ect

when screwcutting, whilst the plain section

should be reduced to as near as possible

10.00mm paying particular attention to the

fi nish as this forms part of the leadscrew

bearing system.

The 10mm x 1.00mm section can be

either cut full depth in one pass by a die, or

as I prefer as it lessens the eff ort required,

partially screwcut in the lathe before

fi nishing off with the die.

The fi nal operation will be to cut the

keyslot slot using a 4mm slot drill. This

can be done either with a milling machine

or with a lathe vertical slide attachment.

For items as small in diameter as this, I

prefer to use the vertical slide because I

have diffi culty getting the milling machine

head close enough to the work, requiring

substantial packing to raise the work with

the consequent risk of something slipping.

Mistakes

Eagle eyed readers will have noticed the

strange marks on the circumference of the

fl ange. What happened here was that at

the end of one thread cutting run, having

backed the tool out prior to returning it

to the start of the thread, I inadvertantly

switched the lathe into normal forward

mode. Now whilst the tool was well clear

of the thread, it was not clear of the fl ange

and it promptly cut a thread along the top

of the fl ange before coming to a halt. I fi lled

the resulting groove with epoxy putty,

but I now think that soft solder may have

been better. As it happens, the top of the

fl ange plays no part in normal operations,

nevertheless it does look ugly. ■

3

Home-made screwcutting gauge.