Steel sizes for threading.

[Seth]

Hi there,Can anyone help me with what size I need to turn down a length of steel to put a number 13 thread from a screw plate on??I am rather confused on how this sizing works and would be most grateful if anyone can help me with any information on sizes of steel or brass before threading.Regards, Seth.

[rodabod]

Is this a Martin screw plate? There are many types, even amongst the ones made by Martin. 

[MechanicMike]

face a length?

or turn down a diameter?

two different things. which one? for diameter, look up "thread pitch", "thread pitch major diameter" and "thread pitch minor diameter." or I just totally misunderstood your question.

[Seth]

Hi there the screwplate I have is the one I enclose a photo of I got this in a set with all the taps from Cousins UK.The reason I want to find out the correct size of material is I have to replace a couple of case screws on an old pocket watch that have the heads broken in half and I have managed to get them out.I have tried threading a piece of 1.10 steel with a number 13 on the screwplate but the threadform  seems rather flat so before I go ahead and turn blank screws on my Lorch I want to find out the correct size of material I need to use.The origina broken case screws fit the number 13 screwplate fine and a number 13 tap screws into the watch plate perfectly.Regards Seth.

[canthus]

To tap a hole for a screw to fit in it, then you use the 'thread pitch minor diam' for the drill diameter.  This will drill a hole the size of the bottom of the threads of the screw.  The tap will then take out the metal between this diam and the top of the thread in form of the thread.  A larger diameter drill will mean the thread depth is not full and so may strip easy if the screw is tightened too much or just be too loose a fit.  A smaller diameter drill will mean the tap is cutting into more metal and if care is not taken will jam the tap and snap it.  Normally most screw threads will have tables that show the tap drill diameter.  The smaller the screw size then the closer the tap needs to match the pitch minor diameter.

For making a screw, then you use the 'thread pitch major diam' for the blank rod/wire diameter.  The rest is similar to above, but too big diam will mean more metal removal and possible breaking of the blank in the die, and too small diam will result in a loose fit and possible stripping when tightened.

There are basically 2 forms of tap/die for each size.  One is tapered to allow easier starting of the tapping stage but will not tap all the way to the bottom of a hole or all the way to a shoulder on a screw.  The second will do this and is normally used after the tapered one.

The secret is to use some oil or grease on the tap/die when working steel and tougher metals.and ensure the tap/die is fully square to the work-piece. If you don't have tapping paste/oil then try and use one with EP (ie motor EP gear oil or Moebius EP 1300 etc,) even engine oil will do as these have additives that work to reduce friction at the cutting edge of the tap/die and only come into effect at those local high temps.  Brass, copper and aluminium can mainly be cut without a lube or just use paraffin oil.

Take it slow and just cut a very small bit at a time, backing off every now and then to clear out any swarf, don't force the tap/die.  When following a tapered tap with an end tap always clear out the swarf so you have a clean start with the next tap.

The pitch is simply the distance between adjacent thread peaks and the form/size based on several industry standards.

 

Edited July 9, 2020 by canthus

[nickelsilver]

That screwplate appears to be metric. So the sizes listed are the thread size, 20 is 2mm, 13 would be 1.3mm. So you would want to start with 1.3mm as your diameter. But- it might (probably) not cut the thread so much as cut and form the thread by displacing metal. I say that because it looks a little dubious in quality. So try a little smaller and see if you get a good thread form. You can turn a taper from say 1.15 to 1.3, and then thread that, and see where along the taper you get a full thread. Make a note of it for the next time you need to thread that size.

 

The old Martin screwplates had a screwy (haha) numbering system. There were two main designations from Martin, L and B; both seem to have the same thread diameters regarding their numbering, the difference is the pitch of the thread. Martin G plates were for left hand threads. Funnily enough, the "backward" numbering system for watch stems and crowns corresponds to the Martin sizing, I guess it was taken up at a time when Martin screwplates were the primary threading tool for watchmakers. 99% of the time I use industrially made metric taps and dies for threading, but sometimes for an old piece I will use a Martin plate. The threads are a little more rounded on their crests. I find that for a given size I need to turn a little undersize, as mentioned above. With new "real" metric stuff I turn to the nominal diameter.

 

 

[Seth]

1 minute ago, nickelsilver said:

That screwplate appears to be metric. So the sizes listed are the thread size, 20 is 2mm, 13 would be 1.3mm. So you would want to start with 1.3mm as your diameter. But- it might (probably) not cut the thread so much as cut and form the thread by displacing metal. I say that because it looks a little dubious in quality. So try a little smaller and see if you get a good thread form. You can turn a taper from say 1.15 to 1.3, and then thread that, and see where along the taper you get a full thread. Make a note of it for the next time you need to thread that size.

 

The old Martin screwplates had a screwy (haha) numbering system. There were two main designations from Martin, L and B; both seem to have the same thread diameters regarding their numbering, the difference is the pitch of the thread. Martin G plates were for left hand threads. Funnily enough, the "backward" numbering system for watch stems and crowns corresponds to the Martin sizing, I guess it was taken up at a time when Martin screwplates were the primary threading tool for watchmakers. 99% of the time I use industrially made metric taps and dies for threading, but sometimes for an old piece I will use a Martin plate. The threads are a little more rounded on their crests. I find that for a given size I need to turn a little undersize, as mentioned above. With new "real" metric stuff I turn to the nominal diameter.

 

 

Brilliant, many many thanks for your reply and help I will now have an afternoon tinkering with my lathes to see how it works.I have a Lorch lathe with lots of accessories and I have just bought myself a brand new Proxxon lathe as I'm retired and have lots of time (no pun meant) to tinker with old watches.Take care,keep safe and many thanks ,Regards,Seth.

[rodabod]

I hate saying this, but that looks like an Indian screw plate and if so, I would consider chucking it in the bin unless you just want to use it for mucking about. The taps they come with are horrendous - they just snap. I’m not sure what thread they are supposed to conform to, but I’d experiment using ISO/metric Swiss stem threads and inserting them into the plate to see if they bind. I do own one of these plate and tap sets. 
 

For sizes down to 1.0mm, the common SE Asian set made from HSS is excellent.

For individual sizes, WIRU Germany sell high quality taps and dies. I used mine the other week to make a shouldered 0.8mm screw for a JLC hack lever.

In terms of rod size for a particular thread, I usually experiment. The WIRU set gives a hole size for the taps which may give you a feeling about the difference in size required. In practice, I find the rod size is very close to the major diameter. I always taper the start of the rod (and cut off later). For tapping holes, a broach to adjust can be a good idea, not only because it gets the diameter just right, but because it creates a slight taper to start.

 

Edited July 12, 2020 by rodabod