Model Engineers’ Workshop – July 2019

July 2019 23

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Denford Synchro

drawings for the quadrant, the gears
and all the fittings. I set to making a
pattern for the quadrant, and while
that was away at the foundry, I started
gear cutting. My Centec mill and Vertex
dividing head handled the 18DP gears
comfortably, although for the 127T gear,
I bought a division plate with 127holes
in it from the late John Stevenson. I had
to do some work on the dividing head
to make the division plate fit as it was
larger diameter than the standard Vertex
plates, and I had to fit a spacer under the
dividing head to raise it for the larger
diameter of the 127 and 135 T gears,
photo 12. My Centec had the facility to
use a rapid rack feed on the X axis, and I
used this when cutting the gears. A few
seconds saved per tooth adds up when
you have about 600 teeth to cut!
The translation set as supplied by
Denford would enable a metric lathe
to cut 21 imperial threads from 4 to
28TPI, but I made an additional 3 gears
extending the range to 28 imperial
threads from 4 to 56TPI, which covers
the majority of standard UK and
American threads, the main exception
being 19TPI. It takes around 10 minutes
to fit the attachment, photo 13, less with
practice. The chart I drew up for gearbox

settings is fig. 1.

Thread Dial Indicator

One diff erence between an imperial

leadscrew and a metric leadscrew is that

the thread dial indicator (TDI) on an imperial

machine covers all threads, but on a metric

machine several diff erent gears are needed

for the bottom of the TDI. In the case of the

Denford 280, two gears are needed to cover

the full range of metric threads, a 28T and

a 30T. I only had the 30T, which was on the

TDI fi tted to the machine. A search of the

Denford forum came up with a drawing for

the gears, so, having bought a spare TDI on

the internet, quite cheaply, I cut a 28T gear,

for it, photo 14, inclining the Vertex head to

get the correct skew to match the leadscrew

pitch. In truth, I usually keep the half nuts

closed, and simply withdraw the tool at the

end of a pass, and reverse back, something

that’s quite safe to do with a D1-3 spindle

...the original factory

drawings for a component,

showing dimensions,

tolerances, material specs

etc, are oft en available...

Screw cutting chart for imperial threads.

Fig.1

nose. To date, all my screw cutting has been

into a relief groove, so speedy withdrawal of

the tool hasn’t been an issue. In any event,

cutting imperial threads on a metric lathe,

leaving the half nuts closed is the only way.

Still, its nice to have both TDIs, just in case.

Taper Turning Attachment

My lathe didn’t have a taper turning

attachment, but Denford had offered

one, of slightly unusual design, in that it

relied on a stylus screwed into the back

of the standard cross slide, bearing on

the edge of a flat steel plate, which could

be skewed to the desired taper angle. The

cross slide and stylus was pressed against

the plate by two strong springs fixed at

one end to the saddle, and at the other to

the cross slide, one each side. The plate

could be removed, and a pair of centres

substituted, which if a master workpiece

13

The completed imperial screw cutting
attachment with all the gears. The red item is
the foundry pattern for the quadrant.