August 2019 61›
8tpi Screwcutting
True metric pitches using
an 8 tpi leadscrew or the
story of a wonderful gear
T
his is the story of how I have
convinced my 1902 Drummond fl at
bed treadle lathe to cut true metric
threads using a “wonderful” gear. I do
not claim any originality for the method
described as I am sure others have done
similar things before but to paraphrase
Newton we all “stand on giant’s
shoulders”....
Whilst doing some research for this
article I bought a copy of “Screwcutting in
the Lathe” by the late Martin Cleeve which
is quite the best and most comprehensive
book on the subject that I have ever
encountered. And you have guessed it- there is one short paragraph on “my”
method.
The Drummond lathe has an 8 TPI lead
screw which is very common on small
lathes used by model engineers so the
principle described can easily be adapted
to a wide variety of machines.
At Amberley Chalk Pits museum, I once
saw a very similar Drummond lathe on
display which has a small information sign
casting doubts on the ability of anyone to
carry out screw cutting on such a machine.
Eric Clark describes how
he cut accurate metric
threads on a small
imperial lathe
1
Let me assure everyone that a treadle
powered Drummond fl atbed lathe is an
excellent screw cutting machine for the
model engineer due to its very good back-
gear arrangement, ease of reversing, and
the fact that speed can easily be adjusted
to suit the job in hand, giving control over
the process in total with no fear of crash-
ups. It also provides good leg exercise!
When I fi rst obtained this lathe, it had
no graduations on any of the movements
so I began a quest for suitable micrometer
dials with suitable graduations going in
the correct direction. Aft er a long search
I found a lovely one suitable for the cross
slide (100 graduations) but never located
one with 125 for the lead screw. I decided
to make one and pondered the method
to be used to graduate it. Dividing is also
very easy on this lathe as it has no tumbler
reverse so suitable gears can be mounted
directly on the end of the mandrel.
The snag was that I did not have a 125
toothed change wheel. It is possible to
obtain 125 divisions using a compound
train with string and weights to maintain
tension, but this was considered too
indirect for ease of working.Enter the “wonderful” gear.
At a Model Engineer exhibition a few years
ago one of the major machine suppliers
had a bucket of odd change wheels priced
at a very reasonable £2 each. Being a bornmagpie, I searched this wonderful bucket
and found a gear which had a double track
of teeth 125 and 127. This was obviously a
modern 1.0 mod gear and therefore much
fi ner that the old 14 DP gears used on
the Drummond but all I wanted it for at
this stage was as an indexing wheel, so
the tooth pitch did not matter. I bushed
the bore of this wheel to 5/8” diameter
and drilled an appropriate hole for the
Drummond driving peg.
Aft er successfully making and fi tting the
resettable lead screw dial, photo 1, and
a similar resettable dial for the tailstock,
photo 2, my thoughts turned to the 127
tooth part of this useful gear wheel and
could it be used to produce true pitch
metric threads?
Cutting “near miss” metric threads.
It is possible to cut slightly inaccurate
metric threads on an imperial lathe using
a standard set of change wheels i.e.
progressing in steps of 5 teeth or together
with a 21 tooth wheel (Myford’s answer)
or a 63 tooth wheel or several other
combinations of translator gears detailed in
Martin Cleeve’s book.
The errors produced are quite small
and may not be too signifi cant for short
threads for attaching nuts say up to one
diameter long. However, the errors quickly
accumulate so this method is not always so
suitable for longer threaded items.
The leadscrew on ordinary workshopResettable lead screw dial. Resettable tailstock dial.2