Need advice for cutting #12-20 threads

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mwebster51

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I refurbish pre-war hand planes as a hobby and occasionally need to replace the posts for the wooden knob and tote/handle. The posts have #12-20 threads on the ends. When I look up the Major diameter in the tables for the #12 it says it is 0.216”. I have tried different sizes of rod stock and it seems it is either too thick and difficult to cut the thread (using a #12-20 die) or too thin and wants to pull out of the female thread in the plane body. I assume the ideal rod material would be 0.216” in diameter. I have tried rod material from McMaster-Carr, (tight-tolerance multipurpose 01 tool steel rod) 0.2120 and 0.2188 since they don’t sell 0.216. The 0.2188 is REALLY hard to get the threads on but will fit once completed. Interestingly the 0.212 also seems hard to get the threads started, but when completed fit well enough. Rod material less than 0.2120 produces a less satisfactory fit. I have tried to bevel the ends of the rods using my grinder to enable the die to grab with some success, but it still seems difficult to get the threads started. So… if the major diameter for the #12 is 0.216 why is it still difficult to start the threads on a 0.212 rod? I assume it would be even more difficult to use a 0.216 diameter rod, even if I wanted to try it I can’t seem to find any to buy. Any suggestions on the size of rod, thread cutting technique (mainly how to get the thread started) that would make things easier?

Thanks for the help.

I am a woodworker not a machinist so please excuse my ignorance.
 
I'm guessing it is because you're using tool steel, which is particularly hard. You likely want to use a softer steel instead. As far as .216" steel, I'd suggest finding someone in you area with a lathe that can just make the part out of some mild steel instead of trying O1.
 
Again excuse my ignorance.... I thought the O1 tool steel rod was annealed and therefore relatively soft. In the annealed state is it still harder than a mild steel rod and hard enough to make hand threading difficult? Thanks
 
Relative compared to in a hardened state, but still pretty hard compared to a normal mild steel. O1 is a machinability index of 42% it seems. See http://www.carbidedepot.com/formulas-machinability.htm for a chart. Lower % is 'harder', higher % is 'easier'. Still possible to do with harder metals (so carbide tools should be able to do it), but a normal HSS die would have a heck of a time.
 
Make sure you are using a quality die. Not the miserable chasing dies they sell at the hardware store made from powdered metal. A good sharp HSS die. Years ago I was manufacturing parts from stainless and thought I'd try a less expensive die that MSC called Interstate (import). It lasted for six parts. The more expensive die, which I purchased at the same time for twice as much money ($50 or so), has done over 300 parts. I still have the die and use it on occasion. Not sure what the difference was but the quality tool makers clearly make better dies. The cheap die was not a value at all.

Another thing to consider is to use a split die which you can loosen a bit for the first pass and then adjust it to your final dimension. Taking two passes will make it easier. If your doing a lot of these you may want to consider having two dies. One to rough and another to finish.

Annealed O1 tool steel should cut easily with a quality HSS die and cutting oil. You can get dies from cobalt or chromium steel but they are not common. Carbide is likely too brittle to be useful as a die. ;)
 
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forhire said:
Make sure you are using a quality die. Not the miserable chasing dies they sell at the hardware store made from powdered metal. A good sharp HSS die. Years ago I was manufacturing parts from stainless and thought I'd try a less expensive die that MSC called Interstate (import). It lasted for six parts. The more expensive die, which I purchased at the same time for twice as much money ($50 or so), has done over 300 parts. I still have the die and use it on occasion. Not sure what the difference was but the quality tool makers clearly make better dies. The cheap die was not a value at all.

Another thing to consider is to use a split die which you can loosen a bit for the first pass and then adjust it to your final dimension. Taking two passes will make it easier. If your doing a lot of these you may want to consider having two dies. One to rough and another to finish.
Click to expand...
I hadn't thought of that, that could definitely do it too! A die that isn't explicitly and correctly made for thread cutting (rather than thread repair) will be more difficult to use and will not cut as easily.
 
ErichKeane said:
Relative compared to in a hardened state, but still pretty hard compared to a normal mild steel. O1 is a machinability index of 42% it seems. See http://www.carbidedepot.com/formulas-machinability.htm for a chart. Lower % is 'harder', higher % is 'easier'. Still possible to do with harder metals (so carbide tools should be able to do it), but a normal HSS die would have a heck of a time.
Click to expand...
The die is HSS and yes "a heck of a time" .....It is from Victor Machinery. Not too many places sell the #12-20 die.
 
A #12 thread has a Dmaj of 0.216 and the OD required for a class 3 thread is 0.2088 to 0.216"; for a class 2, OD is 0.2078 to 0.215". At best, a die will probably produce a class 2 fit unless you use a split button die, in which case you might get a class 3 if the OD is right. Basically, your best bet is to have your repair posts screw cut on a lathe if the fit is important to you.
 
forhire said:
Make sure you are using a quality die. Not the miserable chasing dies they sell at the hardware store made from powdered metal. A good sharp HSS die. Years ago I was manufacturing parts from stainless and thought I'd try a less expensive die that MSC called Interstate (import). It lasted for six parts. The more expensive die, which I purchased at the same time for twice as much money ($50 or so), has done over 300 parts. I still have the die and use it on occasion. Not sure what the difference was but the quality tool makers clearly make better dies. The cheap die was not a value at all.

Another thing to consider is to use a split die which you can loosen a bit for the first pass and then adjust it to your final dimension. Taking two passes will make it easier. If your doing a lot of these you may want to consider having two dies. One to rough and another to finish.

Annealed O1 tool steel should cut easily with a quality HSS die and cutting oil. You can get dies from cobalt or chromium steel but they are not common. Carbide is likely too brittle to be useful as a die. ;)
Click to expand...
Thanks for your response. I am using a split die and have tried to open it a little. Helps a bit. I will try to get a second one for the finish cut. Once I get the thread started I can do pretty well... it is mainly the starting that is the problem. As far as finding a "quality die", being a #12-20 makes it difficult to find a range of choices. As far as I know Victor Machinery is one of the only places to find that size.
If I went to a softer steel rod, can you give me a name of a material that I can use to see what McMaster-Carr offers?
 
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