Internal threading a Whitworth cross slide nut

[mikey]

I have a new-to-me Emco Maximat Super 11 CD lathe. It has less than 100 hours on it and works great except for one little thing - the cross slide nut is the wrong part and binds. The original owner never addressed it and now I have to fix it. The stock nut is made of Zamack and is a 14mm X 10tpi BSWhitworth rolled thread and nobody stocks a tap like this anywhere in the world, at least not that I can find. A custom tap would cost well over $200.00 so I'm not inclined to go that route.

I think the lead screw itself is still okay, although I suspect there is at least some wear on it from that nut. Still, I want to try and make a new LH nut from leaded bronze but to do that I will have to screw-cut an internal thread on a new nut on the lathe. As you know, the Whitworth thread form has rounded profiles at the top and bottom of each thread and I suspect it is going to be a challenge to grind a tool like this.

My questions are:
1. Has anyone screw cut a BSW thread before and if so, how did you grind the tool?

2. Would you bother doing this or would you just switch over to an Acme thread? I know I can switch from the 14MM BSW lead screw to a purchased 1/2 X 10tpi LH Acme screw and machine the end to fit my lathe. Then I can make a nut by screw cutting and then chasing with a more readily available Acme tap to get the fit I want.

Any advice would be welcomed!

Mikey

 

[savarin]

If it were me I would go the acme route because I believe it would yield a better end product. but I'm not a machinist so take this with a pinch of salt

 

[Ebel440]

Whit worth originally had sharp angles without the radius it was changed due to problems with the tools wearing out too quickly and changing shape. At least that's what I read someplace. So I don't think the radius has any real effect besides easing production. I think you could use sharp corners and be ok. Except the sharp angle may interfere with the minor diameter on the screw. That may mean you would need to increase the minor diameter some to clear it. If it was me I would try single pointing a test piece and see how it fits it may work well enough

 

[mikey]

If it were me I would go the acme route because I believe it would yield a better end product. but I'm not a machinist so take this with a pinch of salt

Yeah, the more I look into this, the more I'm leaning toward an Acme changeover. I will contact Nook to see if they can give me a threaded nut that has enough meat on it to carve a nut from. If they have one then that is the route I will have to go.

 

[mikey]

Whit worth originally had sharp angles without the radius it was changed due to problems with the tools wearing out too quickly and changing shape. At least that's what I read someplace. So I don't think the radius has any real effect besides easing production. I think you could use sharp corners and be ok. Except the sharp angle may interfere with the minor diameter on the screw. That may mean you would need to increase the minor diameter some to clear it. If it was me I would try single pointing a test piece and see how it fits it may work well enough

Thanks for your thoughts. I would worry that a sharp profile might wear the leadscrew rather quickly because the load is on the flanks of the thread, right where that sharp corner is. Your point is well taken, though. I might be able to get away with simply rounding the tip of the cutter. The rounded profile does have a specific height and width but not having done this before I am not at all clear how critical that profile is. I'll give it some thought.

 

[kwilliam]

10mm Internal threading bars that take inserts
and they are cheap
then get your insert and bingo - do your own.

 

[xalky]

I like option 2 above. Except I would still single point thread the nut in the lathe. I did it for one of my lathes a while back. An acme tool is pretty simple to grind. I'm afraid that you might not like the backlash with a purchased acme tap , unless you can get a tighter fitting class tap.

 

[mikey]

10mm Internal threading bars that take inserts
and they are cheap
then get your insert and bingo - do your own.

I looked for inserts but this is a 10 tpi application, not a 12 tpi like a standard BSW thread would be. I can't find a small 10 tpi insert that would fit a 10mm/3/8" bar. This would have been a great option, though.

 

[mikey]

I like option 2 above. Except I would still single point thread the nut in the lathe. I did it for one of my lathes a while back. An acme tool is pretty simple to grind. I'm afraid that you might not like the backlash with a purchased acme tap , unless you can get a tighter fitting class tap.

Yup, its looking more and more like I'll have to go with an Acme screw. I am going with a Nook screw, which is a class C2 screw, and I'll have them supply a nut with the same class fit instead of messing with a tap. Hopefully I'll be able to machine the nut to fit my application. The OEM nut configuration has a slot into which a pointed screw can be inserted to eliminate backlash, a characteristic for which the Emco lathes are known. We'll see if I can get a nut that is big enough to allow me to trim it as required.

Thanks, guys, for all your input and help. It has helped me to realize that if I persist in chasing the BSW thing then my lathe will never be repaired. I'll give Nook a call and see what they can do for me. I'll do a follow up thread on how this all turned out.

Mikey

 

[RIMSPOKE]

WHY WOULD AN AUSTRIAN BUILT ( METRIC ) LATHE HAVE A BRITISH THREAD IN IT ?

DID EMCO SUB OUT THAT PART OR WAS IT CHANGED ?
I WANT TO KNOW BECAUSE I HAVE THE SAME LATHE .