tapping drill

what size tapping drill do i need for a southbend back plate?

its 1.5" and net is giving dif info from 32,33,34 or 35 mm drill.

ta.

About twice the thread depth less than the fixing bolts. What bolts/cap screws are you using? 1.5'' is 38mm.

Assuming its 8tpi then about 1.375" will give you a reasonable thread engagement of about 75%

NDIY think he is talking spindle nose thread not bolts

Edited By JasonB on 24/07/2016 16:56:26

Naughty Boy,

Bear in mind the spindle thread is the American standard 60 degree thread form, NOT Whitworth as would be the case on the Boxford clone.

Brian

yes spindle nose thread is what i am talking about.

need to make some backplates.

thanks brian where i got lathe from actually wrote and stuck label to spindle.

got set of taps from tracy tools.

It would be better to screw cut the thread and tidy (if necessary) with a tap thus keeping the thread and locating surfaces coaxial and true.

Naughty Boy,

You can more easily bore out the back plate on your Clarke lathe than try drilling it; it would be a big drill and I think rather difficult to control.

Do the taps not quote a hole size for tapping?

Brian

never done internal screwcutting so was going to thread hole then fit it and face of front of backplate up.

will end up boring out last bit but needed to know sizes.

my biggest drill is only 31 mm

I assume there is a register diameter as well as a thread? If so take KWIL's advice and screwcut it at least close to size, then clean it up with the tap. This will ensure concentricity and squareness to abutting face. I would make a dummy nose first as a gauge. Winding a 1.5" * 8 tpi tap straight into a tapping hole will give you muscles on your muscles. See also recent discussion on thread angle, if it should be 60 degrees and your tap is 55, then it is reported that it can bind.

as to actually screwcutting, put the tool in upside down and cut on the back face with the lathe going forward. This has at least 2 advantages, the set up and sequence of handle twiddling is exactly the same as external screwcutting, and the chips fall clear of the tool which gives better finish. Practice on some scrap first, then you can record dial settings for the final attempt and it doesn't cost a mint if you overshoot.

It doesn't need to be fancy if you are going to finish with a tap. Just get rid of most of the material and give the tap something to follow. Along the way you'll find out something about internal screwcutting.

If you do decide to tap it from scratch, I hope you have a lot of muscle

Edited By Bandersnatch on 25/07/2016 01:12:52

Arc sell Boxford backplates, less than 20 quid.

As do RDG, who also sell a tap and die.

Me, i would'nt even think of using a 1.5" die, let alone a tap, apart from cleaning up an already cut thread.

A lot of muscles required, I reckon, if starting to tap after drilling a 32mm hole, unless! in possession of a complete set of taps.

This link gives the size for Whitworth theads. Wrong thread angle, but gives a good idea of amount of material to be removed.

http://www.engineersedge.com/hardware/whitworth-screw-threads-chart.htm

Boxford back plates will be threaded as 55 degree Whitworth, not American National Coarse [ANC] at 60 degree and if fitted to a Southbend nose are likely to bind before the register diameters meet.

Any chuck on that back plate will then show variable holding accuracy as all that locates the combination is poorly fitting threads.

I would not recommend trying to recut a Boxford back plate with the 60 degree form either as the geometry of the finished thread will be poor. Best by far to do the job correctly as a lathe cut thread right from the start and use the tap as a finishing tool.

Brian

Lathes.co.uk has a section on fitting a chuck and making a backplate for a screwed nose spindle. The part about register accuracy is interesting to read.

**LINK**

Martin

To further the discussion on registers, my Mellor lathe has its register at the same diameter as the thread, much the same as this Myford MF74, proving it was pretty common back in the day:

There may well be additional benefits to having a useable register, but I have not noticed them on my lathe yet.

I will freely admit that there are much better methods of mounting a chuck than with a spindle thread, but this is what I have.

In all the tests I have performed, the chucks and faceplates locate repeatably against the abutment face using just the threads.

The rear of the backplates, or the abutment face, would need to wear significantly between fittings to cause any real difference and even then so long as it is still concentric and I am not attempting to re-cut a thread between fittings am I ever going to notice?

There is no register on the backplates or faceplates, in fact there is a significant gap where the register would be expected to exist, to clear the threads when initially mounting the chuck/faceplate before engaging the threads.

These lathes were good enough for all sorts of precision engineering jobs back in the 1930s and 40s.

Food for thought?

Edited By Zebethyal on 26/07/2016 15:05:31

I have made back plates for a Churchill Cub, 1 1/4" Whit + 1 1/4"register. I have the required taps but there is no way that you can hold the back plate while tapping. I reckon you need a 4' long tapwrench to cut the thread. Screw cutting is an adventure but it can be done. The main thing is to get the register -.000 +.001 inch diameter right, The thread just pulls the back faces together.

Mount the casting face onto a face plate with a bit of hardboard packing. Bore and turn internal diameters, cut thread. Remount the casting directly onto the nose of the spindle then turn all the other features using jury rigged long reach tools. This should get everything concentric.

Biggest problem I had was facing a 9 1/2" faceplate on automatic facing with a very fine feed was tool tip wear due to the very long tool path( 1000 ft?) but some how got the flatness down to .002" across its face. And much to my surprised it was still that 10 years later.

Tomorrow I'll measure it to see how its doing (30 years on).

Frank