backplate threading

[John Rudd]

I'm struggling to thread the new backplates I purchased for my 9*20....

I've bored the hole to the tap size (35mm) but I'm struggling to hold the backplate without it turning in the chuck....

M39 taps sure are big!!! I'm having to turn the tap with a 12 in shifter and that really aint big enough....scaff tube on the end!

I can lock the chuck by inserting a brass bar thru the jaws  and catching on the lathe bed, but Im not convinced this is the right approach.......I may end up

damaging something......

Guess I need access to a bigger machine?

Im thinking if I held the backplate in a vice but then I'd lose the relationship between it being centred in the lathe for machining the register in the back of the

plate?

Any ideas guys, on how to proceed?

[Ugluk]

Sorry if this is a stupid question, but why use a tap at all?
I've always cut such threads using a hss ground bit with good results.

[awemawson]

Can you not thread the backplate on the lathe to a 'partial thread' - 80% r so, and then finish it with the tap.

[Pete W.]

Hi there, John,

I've never actually threaded a back-plate, though there are a couple on the horizon.

I'd screw-cut using the lathe change-wheels or QC gearbox to remove most of the meat, the profile of the screw-cutting tool doesn't need to be ever so accurate.  Then use the tap or taps to finish the thread to the correct profile.

That way, the initial screw-cutting establishes the axial alignment and pitch accuracy of the thread, which would probably not be achieved while wrestling with a tap as you've described.  Then, the tap or taps will follow the screw-cut thread with much less effort (a smear of tapping lubricant would help) and just remove enough material to bring the thread to its correct shape.

Then carefully bore out the section that fits the mandrel register.

I see that someone else has posted while I was typing - probably beaten me to it!    Hi there, Andrew!!

(Returns to arm-chair!!           )

[John Rudd]

I'm thinking that cutting an initial thread is the way to go........if I can get a suitable tool of sufficient length to cut the thread.....and then run the taps through .....
A 4mm pitch isn't a thread pitch generally catered for on the 9*20......

[NormanV]

I made a couple of chuck backplates using a 1 1/4" BSW tap. I bored it out to tapping size and also bored a register. I started the tap in the lathe and then transferred it to a vice and finished tapping the hole using a big spanner. Then, with the backplate screwed onto the spindle I was able to machine the front and the register for the chuck. It worked fine and the chuck located accurately each time I used it.
P.S. Does anyone want a hardly used 1 1/4 BSW tap, I no longer have that lathe.

[John Rudd]

Norman,
Your approach is great.....the critical part is ensuring that the spindle register and the threads are concentric which as my main concern.....
So, while the plate is still lathe mounted, I'll machine the rear register  before proceeding further......

Thnx for the advice....

[chipenter]

I cut a 4mm backplate thread on a 920 , the gearbox only went to 3mm so cut a 20 tooth 1 module gear for the leadscrew , had to turn the spindle with a handle wich took a while .

[John Rudd]

Jeff,
The 9*20 will cut a 4 mm thread using the 40/80 change wheels and the qcgb selected to position1....
I threaded a shaft when I made my indexer....

[chipenter]

It may not be a 920 but the next size down and it is the clubs , max size was 3mm pitch anyway it's done now , used a home made thread cutting tool from one of Bogstadard's posts on hear .