62 http://www.model-engineer.co.uk Model Engineers’ Workshop
lathes is not an especially precision item
and usually wears to a certain extent
adding to any inherent inaccuracy. This is
oft en given as a reason not to worry about
slight pitch inaccuracies when cutting
metric threads on an imperial lathe as
such errors will be “absorbed within the
leadscrew wear”.
I am uncomfortable with this reasoning
as:
- The errors are very likely to add
together to produce greater than
expected inaccuracy - As model engineers we should try to
get things as accurate as possible
The only way to cut accurate metric
pitches using an imperial lead screw is to
use a 127 tooth “translator” gear.
One inch is set internationally as exactly
25.4 mm. (Take no notice of older books
that give slightly diff erent conversion
factors).
The 127 tooth gear derives from the
factorisation of 254 into 2 x 127
As 127 is a prime number this is the
lowest number of teeth that can be used
on a translator gear.
This brings a number of practical
problems (described below) so Drummond
and other manufacturers introduced gears
of 63 teeth which enabled the user to
get ratios that gave more accurate near
misses.
Table 1 shows my screw cutting chart
using standard Drummond gears and
quantifies the pitch errors which are quite
small and frankly quite good enough for
all practical purposes.
A college lecturer was very fond
of using the phrase “for all practical
purposes” The lack of precision in this
phrase puzzled one of the students who
asked the lecturer to explain what it
meant. After thinking for a few moments,
he said
Suppose all the young ladies lined up
on one side of an empty room and the
young gentlemen did the same on the
opposite side facing them. Then at my
signal they all moved forward enough
to exactly halve the distance between
them. No matter how many times this was
repeated theoretically they would never
meet, but after only a few moves they
would be close enough together “for all
practical purposes”.
If you do use a near-miss set-up it is
best to leave the thread fairly full and
then finish it with a die as this gives
some improvement over short lengths of
thread.
Problems with 127 tooth
wheels
The fi rst problem is fi nding one for your
lathe!
They may be available for current lathes
if you are very lucky, but it will be diffi cult
with older and obsolete machines.
The second problem is the diameter
3
4
The “wonderful” gear and its mate.
Gear train set up for metric screw cutting.