what external diameter when die screw cutting?

[Numb Nut]

This is no doubt a very basic question but I haven't been able to google an answer...

I want to make a drawbar that is threaded 3/8" BSW 16 tpi. My mill came with a M12 drawbar. The thing is ... I'm really not sure what diameter to machine the mild steel bar down to prior to threading it using a die?

The obvious answer would be 3/8" ... but most external threads that I've measured don't seem to be the full diameter. Is there some rule of thumb for the external diameter that is run through a die?

By the way - I'm not too fussed about making the die last a long time - more concerned with making an accurate thread for the drawbar.

Thanks for any advice!

[SCzEngrgGroup]

This is no doubt a very basic question but I haven't been able to google an answer...

I want to make a drawbar that is threaded 3/8" BSW 16 tpi. My mill came with a M12 drawbar. The thing is ... I'm really not sure what diameter to machine the mild steel bar down to prior to threading it using a die?

The obvious answer would be 3/8" ... but most external threads that I've measured don't seem to be the full diameter. Is there some rule of thumb for the external diameter that is run through a die?

By the way - I'm not too fussed about making the die last a long time - more concerned with making an accurate thread for the drawbar.

Thanks for any advice!

If you really want to be anal about it, use Google to look up one of the billions of thread dimension tables on the web, and see what the major diameter is. But, it will be close enough to 3/8" as to be silly to worry about the difference.

Regards,
Ray L.

[Numb Nut]

Are you sure?

The tables I've seen all say that the major diameter is the thread size. E.g. a 3/8" BSW thread will have a 3/8" major diameter.

http://mdmetric.com/tech/thddat8.htm

The thing is that none of the threads I've measured comply with that. For example I measured a bolt that screws into the back of a jacobs drill chuck - threaded 3/8" BSW 24 tpi and it was 0.366" not 0.375". It did felt slack though.

Perhaps some allowance should be made for having some dirt on the threads?

On the other hand a drawbar takes a tensile load on the threads so maybe closer to the maximum.

Anyone got a 3/8" BSW drawbar and a set of calipers to hand....?

[Superman]

You will find items that go together usually have a fit tolerance
a shaft diameter will be controlled by a max. and min. size and the mating part will also be controlled, but not necessarily by the same values

as a starting point
--if you want two items to slide onto each other, then there has to be a gap. We'll take 3/8" as a size basis

shaft max. size= 0.375"
shaft min. size=0.370"
this means someone has a size range to be able to make this part.

now for the hole
for that shaft to "slide" into a hole, the smallest hole you can make to fit the largest possible shaft is 0.376"
therefore
hole min. size = 0.376"
hole max. size = 0.380"

This is to give a little understanding towards "limits and fits (Holes and Shafts)"
Threads also have their own tables for size specs. plus there is a few different types of tolerence specs ( ISO , BS etc -all you have to do is look in the "Machinist's Handbook" )

Generally speaking for threads, the designated thread size is the maximum shaft ( or OD ) size for the thread, nuts have their own specs.
If you get a 1" BSW bolt and the OD measures Ø1.01", then it will never fit an off the shelf nut

[Numb Nut]

Thanks for the reply - I don't think the tolerance data you are suggesting is widely available however. Perhaps post a link and prove me wrong?

I've found a reference to it here (see post #33)
http://elsmar.com/Forums/showthread.php?t=29369&page=5

Looks like BS84 appendix B is the specification to refer to - if only a copy was available somewhere on the web.

Oh well - the plan is looking like that I will size the bar to 0.374" and then see if the HSS die can chew on it....

[Journey_Man]

I have single pointed waaay too many thds. If its not in the zipped files/Then use The Machinist Handbook...It has 27tons of info on thds...
Use PeeDee Thd. wires if you Really want them 100%

[beege]

Close Tolerance Software's ThreadPal has the answers. If you want a close fit, Ø.3695 - Ø.3750. For a medium fit, Ø.3666 - Ø.3736. "Free" fit is Ø.3643 - Ø.3736. MRainey wrote the software, VERY worthwhile.

[Numb Nut]

Thanks to everyone for the helpful replies!

I hope to make a start on the drawbar very soon and then test it for fit in a MT3 fly cutter tool and a pair of direct collets all of which are threaded 3/8" - 16 BSW.

The die that I bought is a HSS split die and has a tiny little screw incorporated to adjust the gap. Never seen that before - my other split dies are adjusted using the 3 screws in the die holder, the centre of which has a cone point and can be used to wedge the die open.

[mactec54]

Hi Numb Nut

You will not get a very good thread with your button die To get it good enough for draw bar use, You will have to cut the thread on your lathe & .004 under .375 is ideal this will make a good thread you may also need a better steel than the mild steel as well 4140 or something similar would be better/ideal

[escott76]

Hi Numb Nut

You will not get a very good thread with your button die To get it good enough for draw bar use, You will have to cut the thread on your lathe & .004 under .375 is ideal this will make a good thread you may also need a better steel than the mild steel as well 4140 or something similar would be better/ideal

Why specifically do you say that you can't get a good thread with a button die? It certainly isn't the ideal solution, but to say you can't get a "good enough" thread is kind of silly. I suppose you'd tell me the threads I cut on the ends of the ball screws I turned with a die aren't good enough. Seems both the thread gauge and the zero measurable movement in the screw (with a tenths indicator) would disagree though.
Making the drawbar out of mild steel may actually be better than making it hard, as you won't chew up collets or tools with a mild steel draw bar. Just making the drawbar out of 4140 (or any other heat treatable steel) is the exact same thing as making it out of mild steel without a heat treatment step. The only difference is that it's more expensive. A different alloy only offers you better properties if you actually treat it the way it needs to be to attain those properties.

[Numb Nut]

This is going to be a learning exercise for me so I welcome all opinions on the right way to cut the drawbar threads and the material to use.

I have tried cutting threads on the lathe using a single point cutter (and the reverse method) but the threads looked very rough indeed - I need to troubleshoot this further when I get time - it might have been the DIY ground HSS cutter, the cross slide gib screws which were slightly too loose, the steel bar (which was an unknown grade of steel), depth of cut per pass, surface speed, choice of cutting paste (trefolex) - who knows!?

I've used screw cutting taps in alu and brass and had what I thought was good enough results. I don't have any experience of using dies nor threading steel.

Will post some pics when I make progress.

[M100]

Forget about using a die, use a cutoff bit of a bolt into a tapped hole. Retain the bolt with a high strength retainer like Loctite 601 or 638. Then when the thread wears you heat it up, use some mole/vice grips to remove the cut down bolt and then fit a new one.