Back plate modification

[Martin of WickParticipant @martinofwick]

The lathe for which the the ex Boxford backplates are being modified for is a Murad Bormilathe. The work is being carried out on a Myford. I have extracted the Bormilathe spindle and will use to check the re-worked backplates.

[duncan webster 1Participant @duncanwebster1]

I think the advice that the register diameter doesn’t matter and it is the thread that centralised everything is exactly wrong. The register should be a good fit and the thread slightly slack (not a lot)

The topslide parallel advance by half of cross slide technique is how I was taught by a time served craftsman many years ago. The angled infeed works equally well but is a faff to set up. If you want to use this method on an internal thread mount the tool upside down and cut on the far side with the lathe rotating normally. Then the topslide is angled the same way as for cutting an external thread. Second advantage is the dial numbers go up rather than down, which is more intuitive. Third advantage the chips fall away from the tool better.

[Martin of WickParticipant @martinofwick]

Thanks,

A lot of very useful advice from forum members has enabled me to synthesise a method! I would aim for a good fit to the register (which is surprisingly narrow on the Bormilathe). I will also bore back the existing threads to core diameter plus 20 thou to avoid the situation where the thread depth cut is correct but the crest height interferes with the fit, so thanks  to Hopper for that.

It is only cast iron, and as I am effectively chasing existing threads, will keep it as simple as possible by only using only the cross slide to advance tool. If it takes 50 passes cranking by hand, so what I need the exercise!

[old martParticipant @oldmart]

I have direct experiance of varying register, unlike the theorists among us. We have about 12 backplates which screw onto the Smart & Brown model A with registers varying from 0.0005″ to over 0.020″ oversize. They all repeat perfectly axially and radially and are checked for runout every time a chuck gets dismantled for cleaning. I have never had to reskim one in the last 15 years. The 0.0005″ one is difficult to fit and remove, but the chuck it holds does not get much use (P. B. 6 1/2″ serrated jaw) so I have never bothered to increase the bore.

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