Boxford Backplate Problem

I bought a second hand dividing head which is in great nick and has a Boxford 1.5" x 8 TPI thread on the spindle.

It came with a chuck backplate (but no chuck) however when I offer the backplate to the spindle it only goes on about half way and locks up. So I bought a new Boxford rotary table mount (needed one anyway) and tried the backplate on it - same result - it locks up.

I read on another forum that the issue is related to 55 degree verses 60 degree thread angle! The other forum also mentioned that a 1.5" x 8 TPI BSF tap, run through the backplate hole, will fix the problem. The cost of a 1-off 1.5" x 8 TPI BSF tap is £91+p&p = £116 which, as a pensioner, is way outside my budget.

So would any kind sole be prepared to lend me the BSF tap for a few days? I will cover expenses and return to the owner afterwards. I live in Co Antrim. My word is my bond.

Sounds like your backplate is a SouthBend one. Better to get the correct plate and sell the one you have to a SB owner who is finding that not so much SB stuff is not sold in the UK.

Just to throw a spanner in the works, are you sure that the backplate thread is 1.5 x 8 tpi?

If it goes on part way, it could be that the thread is slightly different, (possibly 38 x 3mm springs to mind, with a difference in diameter, pitch and thread form )

If the thread is 8 tpi, the thread depth would be 0.080", so the "bore" (internal diameter of the thread ) of the backplate should be 1.340", or maybe a few thou more for crest / root clearance.

Just a thought

Howard

Bazyle - I do intend to sell the backplate that came with the dividing head.

Could be the Southbend backplate which has the 60 degree thread angle and not the 55 degree of the Boxford - yes, you could tap it out BSF as its not the thread that locates the backplate, its the boss on the spindle / collar on the backplate that locates.

The Atlas also had 1 1/2" X 8 and that definitely had a 60 degree thread angle. I made one recently, and the register could also be different from Boxford. I am unable to check the Atlas sizes for you as the museum is not closed due to lockdown.

Edited By old mart on 05/11/2020 15:05:22

This is baffling me a little as I know a UNC Nut will screw onto a BSW bolt (60 goes on a 55 flank angle) BUT a BSW Nut will not fit a UNC bolt!

Therefore a 60 deg UNC style female threaded back plate should fit a 55 deg BSW male threaded spindle. Howards point about a 38mm x 3mm Pitch is more credible. I have an Aciera DH which is just this thread (38 x 3p). So my question to Alan is:- What is the make of the DH?

It may be the thread peaks that interfere but I have a SB backplate that doesn't go on my Boxford spindle.

A long time ago I did buy a ready made backplate from a major supplier at Ally Pally and initially it seemed not to fit. I then tried it backwards and it fitted the thread so it was just the register that needed easing. However the OP seems to be describing a thread interference nothe register.

You do have to be careful of second hand purchases being misdescribed as the seller might have been told a Boxford is a copy of a SB and not know the finer details.

Thanks Oily Rag - There are no markings on the Dividing Head so I don't know the maker. I will get the micrometer on iot and post here.

You could try degreasing the threads and blacken the spindle with felt tip. Then the tight parts will probably show up.

Have a look on ebay. I picked up a new tap for £13. Item number 224216785833 might be suitable.

Martin

Martin, many thanks. Followed up an eBay and have got one. Appreciate the tip off. Just hope that it works now!!

Thats a big tap! Just as well you will only be cleaning up an existing thread. It might be easier to clamp the tap in a vise and screw the plate onto it.

Edited By old mart on 05/11/2020 19:37:40

I've had something similar on my Boxford and on another lathe too.

I find that the crest and root shape of a thread is often overlooked. If for example your backplate has a really sharp pointed crest and the spindle has a slightly rounded root the two won't screw together. Even though the other dimensions are fine. Sometimes all you need to do is take the sharp edges off with a file.

I have a few different backplates that came with the Myford super 7. 1 goes half way & binds. Someone , cannot remember who said it may be a Boxford one.

I'll get my hat.

Steve.

Alan, can you post a picture of the DH to see if someone can identify it? If it is a continental unit (Walther, Hoffman, Rawler, Lucy, Aciera, or similar) it is credible that it is a 38 x 3p thread.

You'r right to get your hat, Steviegtr, A 1 1/2" Boxford backplate would fit a 1 1/8" Myford spindle like a -ric- in a bucket.

Alan, this is a good time to get yourself a set of thread gauges before going any further.

Edited By old mart on 10/11/2020 17:29:24

Dimensions of the thread as follows:

The numbers do not add up unless am mistaken and they are of 2 different items.

I stand to be corrected.

regards