Thread lenght in hole?

[Luis Van Damme]

I have 20mm deep hole that I need to tap with 1/2-13 thread. Will the thread go to the bottom of the hole or will there be short part that will be unthreaded at the end? I'm ordering these pieces from professional machinist shop.

I'm also getting pieces that have 20mm long 1/2-13 male threads on the sides. How about the thread lenght on those?

[Geof]

If you specify that the thread has to be 20mm deep the hole has to be drilled slightly deeper to allow for the taper that is on the end of the tap. Probably the hole will need to be about 1-1/2 to 2 threads deeper which is about 4mm. If it is not possible to drill the hole 24mm deep you will not get 20mm of full thread.

On the male threads it is possible to get very close to the full length threaded; only the last thread would be incomplete. However on these it is a simple matter to specify an undercut at the bottom of the thread so that the mating part can screw tight against the end.

You could also specify an undercut at the bottom of the internal thread to make it possible to screww something right to the bottom, but this is finicky to do and will likely increase your cost.

[Luis Van Damme]

Thanks for the reply. 4mm is too much in this case. 1-2mm is doable. I'm not familiar with the undercuts. The problems is not as important in male thread, but more important in female thread. Could you give me a link for more info or how the undercut is done?

[Geof]

How is an undercut done? For undercutting a 1/2"-13 internal thread on a mill I would probably use a small Tee-slot cutter. On a lathe a small internal grooving tool.

I think you should discuss what you want with the machinist who will be doing the job, if possible. Explain what you would like to get under ideal conditions and let them tell you what is feasible.

[Luis Van Damme]

Yeah I wasn't familiar what that english term meant, but now I understand. Thanks.

One more question about threads. How do I know what the internal diameter of female thread is? I mean the total diameter what the female thread takes. This:

[Geof]

Over here you find that information in Machinery's Handbook. There are probably equivalent publications over there. But the easiest way will be to Google something like: "internal thread major diameter".

[Luis Van Damme]

I didn't find any charts, but one site says that 1/2-13 internal thread major diameter is 0.500 inches. So it's same as the listed diameter of 1/2-13 screw. The screws are always a little bit smaller than the listed dimensions though, so it could be that 0.500 is the correct major diameter.

[Luis Van Damme]

I was told that first they would make 17mm long thread with standard tool and the remaining 3mm would be made with special threading tool that would need to be machined for this purpose. So it is possible to make thread that goes to the bottom, but special tool is needed.

[Luis Van Damme]

I got more information about the full lenght thread:

full thread for a 20mm deep hole, we just do it as two steps:

1. tap the thread to 17mm (maybe a little longer) with the regular tool
2. grind tapered head of the tapping tool to plane and then use this tool to tap the thread to the bottom of the hole

it's only needed for female threads