Advice on lathe Threading tools

Hi all.
I own a Sabel lathe and one part I am struggling to source is the back plate with a 1-3/4x8 thread.
with this in mind my long term goal is to be able to cut that thread onto a blank but I’ve never attempted threading on the lathe.
I am now looking at carbide tooling for internal and external threading but need some advice.
How does a boring bar differ from an internal threading bar? Is it just the inserts required?
What is would the thread form be for the back plate and do I need inserts appropriate to the thread form?

Is it reasonable to purchase some Glanze tooling now so I can make a start?

Can I get some Top Tips for my first lathe threading Exercises?

sorry for the long post, and thanks for any responses,

Best, Jim

Hi Jim,

FWIW I buy and use carbide inserts for threading, only because it a right royal pain to grind HSS tool bits accurately.

The only difference between insert tooling boring bars and threading bars is the shape and set angle of the carbide insert. Internal and external inserts have the cutting tip on opposite sides.

Having said that, I also thread external threads using an internal threading insert on the rear of the work and run the lathe in reverse cutting away from the chuck. No danger of crashing into the chuck.

For internal threads such as your face plate where you can thread right through, I use the same internal threading tool but thread in the conventional manner. For blind holes it is safer to use a lathe handle and wind the lathe backwards and using a normal external threading insert cutting from the inside away from the chuck.

I also just plunge, no messing about setting the top slide at 29 degrees. One other recommendation I can add, is to make an accurate test thread so you can gauge the thread without having to remove the chuck to test the fit.

Last thing make sure you use the correct thread angle, some are 60 degrees, some are 55 degrees and there are some others. Metric threads are usually 60 degrees. I don't know your lathe, but I would get some thread gauges and check it.

HTH.

Smart and Browne being American, most likely used a 60 degree thread form. BUT don't assume it, CHECK the angle.

Howard

Smart & Brown are British (Biggleswade), The thread is Whitworth form, I have a 1" bsw (8 tpi) tap that I ground all but 1 tooth off to use for cutting this thread on backplates. I did pass one of my backplates on to someone else for his lathe but that may have been on the Smart & Brown Yahoo group. He had no chuck or backplate so could not even get started with his newly bought used lathe.

Martin C

Thanks for all the responses so far.

S&B were indeed British as Martin says. The Sabel and derivatives are close to the South Bend where I guess the confusion lies.

Martin, I'm intrigued as to how you use the 1" tap for this process. Can you give me any more details?
Being Whitworth, its going to be 55' right?

I bought (at some expense) a backplate for a Southbend from the US which seems to go on the spindle perfectly - do you think this could actually be cut to a 60' thread?

Jim

I think it is on the lathes.co.uk website that it mentions chuck registration on spindles being simpler than you would think and that a loose fitting backplate will still register on the spindle satisfactorily. It's in the fitting a chuck and making a backplate page. So as long as the plate you have screws on ok then it may be suitable.

I only recommend this if you have a suitable tap you no longer want!

Martin C

Edited By Martin Connelly on 19/05/2020 19:19:30

I have made several S & B backplates with the 1 3/4 X 8 W threads. When first plunging headfirst into the scary world of threading, it will be best to perform the actual threading part with the lathe switched off. Make a winding handle which fits into the left hand end of the spindle bore. You then have time to think about what you are doing without worrying about crashing the lathe.

I recently saw a couple of backplates for sale on ebay with the thread, a 5" and a 6". I bought the 6".

APT sell threading inserts and holders, the 16mm diameter will fit a Sabel (SNR0016 M16 ), and they sell threading inserts singly (16NR 8W T300). VAT and postage have to be added to the prices. You need to shim the height to the centre line.

You haven't mentioned what diameter plate you are after. The Southbend plate will work well enough.

**LINK**

Edited By old mart on 19/05/2020 19:30:46

Edited By old mart on 19/05/2020 19:34:22

Edited By old mart on 19/05/2020 19:35:44

Jim, it may help to point out that martin's modified tap is held in the tool post and used like an internal threading tool, not wound into the hole. The advantage of using a tap of the desired tpi is that it will give the correct crest and valley radius for the thread which are not so easy to accurately grind onto an HSS tool

Like Jason says. I forgot to answer that bit.

Martin C

The most vesatile bar for internal boring or threading is a plain steel bar with square broached holes at the ends. One is at right angles to the axis of the bar. The second is at 30* or 45* to the axis. HSS bits are used and are ground to the profile needed. Use the largest bar that will fit.

Why go to the expense of a 1" Whitworth tap and then destroy it?

Thanks Jason and Martin. That makes sense.

Thanks for that John. I’m hoping to use carbide at the beginning so I don’t have to be concerned with my shoddy grinding but I get the principle of what you’re suggesting.

Thanks for the link Old Mart. I’ll check oUT APT tomorrow. I have used Korloy up to now as I found Cutwel very helpful but their threading holders are over £50+Vat.

I wish I’d spotted those back plates!
I’m after a 5” as the one I have is knackered as is the chuck and they both need replacing.

I’ve now measured the thread and it’s definitely 55’ so there we go.

Does anyone have anything against Glanze holders?

thanks all for the great tips so far.

Old Mart, as I said, only do it if you have the tap and no longer want it. I think from memory this was a salvage from a shattered end.

Martin C

I've got several Glanze and they work for me.

I expect a damaged tap with a 8 tpi thread would be as rare as hens teeth. I have made a sketch of the thread which shows the main dimensions. The starting bore of the thread is not critical, it could range between 1.59" and 1.614". I have always gone for the smaller size using carbide inserts. The size is right when the first sign of the thread shows in the 1.75" register. It would take multiple passes from just touching the bore to be threaded. The register size needs to be no less than 0.001" greater than the measured spindle size, or it will be very difficult to screw the plate on. 

I would go 12,10,10,8,8,7,6,5,4,3,2, and 1 in thousandths of an inch, keeping an eye out when close to the end for the witness mark on the register.

Edited By old mart on 19/05/2020 21:31:08

Old Mart, when I was at work we had carousels for routine consumable issues such a ppe and regularly used items like tapping size drills, clearance drills and taps. Anything unusual I sourced and ordered or else designed and arranged manufacture. Anything that broke in use and was not in the carousels I ordered replacements. As a result I had all sorts of things brought into the office with the request to get a replacement. I binned most stuff as scrap but the occasional item was kept as it might come in handy. Hence the 1" bsw tap that I could butcher to make an internal threading tool for 8 tpi whitworth. As you say, rare as hen's teeth.

Martin C