Unusual Lathe Backplate

**electrosteam** · 23rd January 2015, 09:53 AM

I have a 4-jaw to mount on a D1-3 backplate.

The chuck came with a threaded backplate, that seems a little unusual.
It appears to be 'about' 2 1/4" x 3 mm.
All the threaded holes are 3/8" x 16 tpi.

To test the measurement I made up a dummy cast iron spindle register and mounted the backplate.

I started with OD 57.1 mm (2.249 "), 3 mm pitch and a carbide tool.
Progressively reduced diameter, recutting the thread as I went, until the backplate slipped on nicely and tightened up on the (very small) shoulder.
The feel of the fit is close to what I remember when mounting chucks on my Sheraton 9A.

The final values were:
- OD 55.4 mm 2.18",
- Thread pitch 3.0 mm,
- Thread depth 1.9 mm 0.0748".
The TIR is difficult to measure as there is no circular track available for the DTI, but my limited observation is that it is quite good.

The photos show the situation.
Note the threaded holes breaking into the periphery.
Now that I have done the job, it is tempting to make up a steel version to use this backplate as a mini-faceplate mounted on the 4-jaw.
(I cannot use this CI component because it shows the typical longitudinal cracks of stock sourced from the TAFE scrap bin)

Backplate 4-jaw.JPG Test Register 4-Jaw.JPG

If someone can identify the true thread details, and can make use of the backplate, happy to dispose of it at mates rates.

John.

**mcostello** · 24th January 2015, 10:05 AM

Any chance the threads are 2 3/8" X 6 tpi? Don't know if You are familiar with Imperial threads.

**Big Shed** · 24th January 2015, 10:54 AM

More likely to be 2 1/4 x 8tpi, which is a fairly common lathe spindle thread, particularly in the US, Southbend used this thread IIRC.

8 tpi is very close to a 3mm pitch (25.4/8 = 3.175mm)

**Michael G** · 24th January 2015, 11:49 AM

In the true spirit of bastardisation, my dividing head thread was M55x8tpi. It threw me for months trying to work out what it was - finally I made up some samples and tried them. (early Japanese production in the US - 8tpi was probably easier to find on a (pre-CNC) lathe in the US)

Michael

**electrosteam** · 26th January 2015, 02:31 PM

Guys,
I can assure you the backplate is not 6 tpi or 8 tpi.
I have thread gauges and the 8 tpi does not sit into the thread correctly, but the 3 mm fits exactly.
The fit as the backplate engages the dummy spindle with 3 mm pitch over 7 turns is perfectly normal, a little firm because it was screwcut to fit.

The difference between 3 mm pitch and 8 tpi over 7 turns is 1.2 mm.
The backplate could be 2 1/4' diameter, just a little looser than normal.
Perhaps, with the looseness the 3 mm pitch error can be accommodated.

Is M56 x 3.0 regarded as interchangeable with 2 1/4" x 8 tpi ?

John

**Steamwhisperer** · 26th January 2015, 08:46 PM

Would it be possible that someone has mucked up the backing plate by cutting a 3.0mm pitch in a bore machined for a 2 3/16 " 8 TPI thread.
2.18" is 0.075" short of 2 3/16

Phil

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