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Help, British threads !!!

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Darren:
I have found a chap on Ebay that may have a backplate for my lathe, he's saying it's a BSF 2" x 7TPI thread.

I guess he's wrong on two counts, lathes as far as I know use BSW not BSF, and I've never heard of a 7TPI being used?

He says he's measured it with a BSF guage. Could that mislead him, if he had a BSW guage would it tell him it was 8 TPI?

I have neither guages so I'm hoping one of you good fellows out there has....fingers crossed, these backplates are hard to get hold of...!!

bogstandard:
Darren

What is the spindle thread on yours?

This site might give you a bit of info on your particular lathe. It looks like S&B were 1.75" x 8TPI

http://www.lathes.co.uk/smartbrown/index.html

You shouldn't really jump to conclusions, almost any thread can be used on the spindle. And is usually determined by diameter and thread pitch, plus angle of thread, I suspect that US made machines were 60 degrees, and UK machines 55 degrees. Boxford backplates will fit American Atlas but I don't know if it works the other way around if there is an angle difference. Thread gauges don't normally come for a particular thread, just a range of pitches. If he says it is 7 TPI when measured with a pitch gauge, I would tend to say he is right, 8 TPI will go nowhere near.

Here is a link to a BSF threading chart, which shows 1.75" and 2" do in fact have 7 TPI, which makes it a nono for your lathe.

http://shopswarf.orconhosting.net.nz/bsf.html

If you can get the cast iron, it is a fairly easy job to make your own backplates in any size and pitch you desire by single point cutting. But you really need the spindle nose off the machine so that you can check it whilst machining.

John

Darren:
Hi John,

The S&B's are normally 1.75x8tpi with their own design collets, but mine unusually has a 5C spindle. For this they upgraded the nose to a 2" x 8tpi thread.

That's why the backplates are hard to find, cos it's hard to find a 5C S&B.


The chap says it's a BSF thread, which I understand is a fine thread system. In the picture I have of his backplate it doesn't look like a fine thread. So I guessed that it is really a BSW thread, but I am guessing of course.

I would cut one, but would the mini be able ? That's a big thread...for a little lathe, even turning by hand.

Darren

bogstandard:
Darren,

I think you are getting confused by a name. BSF means what? There is nothing to compare it with except BSW. It is just a size, pitch and angle. The chart I showed proves it.

The metric threads you normally use are called metric coarse, only because they are more coarse than metric fine, but I don't consider them very coarse at all. It is just a name.

Your mini lathe wouldn't stand a chance, not only wouldn't it fit diameter wise, it would most probably burn it out just trying to turn a lump of metal that size.

I can make backplates for you, but it will have to wait until my workshop has been cleared of all jobs and tidied up, it could be a few months. But it would mean you removing your lathe spindle, I wouldn't even attempt it without. Just a couple of thou out and it wouldn't fit or be too sloppy a fit. Cast iron is the only material I would use, again, I wouldn't attempt it with anything else. Just get a quote for material, and be sitting down when you read it. Expect around £30+ per inch for 8", around £10+ per inch for 5".
It all depends on what sized chucks you want to fit, 5" would be reasonable, 8" might be a second mortgage type of job, especially as it seems the S&B has a longer than normal thread, so would require a longer length of material. It would need a length from the back datum face to the end of the threads, plus 3/8" to allow for machining and thread front recess.

John

Darren:
Hi John,

Yes I probably am confused, VERY !!.
I'm really lost around British threads and measurements, only just getting my head around what a thou is. Well I know what it is, but relating to it when machining is another matter. I try to convert it to metric all the time. One thou = roughly 0.4 of a mm...

A bit like when someone around here speaks to me in Welsh, I have to convert it to English to be able to speak back...

I've prob got this wrong, but I understood BSW was British Standard Whithworth and BSF was British Standard Fine (or a fine Whitworth)

TPI should just be Threads per Inch no matter what the standard type is. Even metric can be measured this way but we tend not to as they have a standardised TPI. And because it would be mixing metric with imperial which we tend not to do.

6mm is 26 tpi I believe.


I'll do some studying on the matter.....oh and look at your links which I have yet to do.......thanks

Darren

PS, I have one backplate, would that help in making another? 6" chucks seem to be about right for this lathe.

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