How to cut internal Acme threads in a finished piece?

[tetrode]

And why will this not work or something similar?

Bushing

Many reasons:

1. The diameter of the bushing is greater than the outside diameter of the piece into which it would need to be inserted.

2. Instead of cutting 3/4-5 internal Acme threads, I'd still have to cut internal threads that match the external threads of the bushing.

3. The cost of the bushing is roughly seven times the cost of the work piece (if purchased in moderate quantities).

Cutting internal Acme threads in the existing castings is the ONLY way this is going to work.

Here's the latest disappointment: I received a reply from Tracy Tool in the UK last night regarding availability of the 3/4-5 RH Acme tap I need. Unfortunately, the taps will not be back in stock for at least two months. That's truly a pity as Tracy has the taps listed at a very reasonable 22 GBP. I haven't been able to find any other source for the taps at such low prices.

 

[RandyM]

Many reasons:

1. The diameter of the bushing is greater than the outside diameter of the piece into which it would need to be inserted.

2. Instead of cutting 3/4-5 internal Acme threads, I'd still have to cut internal threads that match the external threads of the bushing.

3. The cost of the bushing is roughly seven times the cost of the work piece (if purchased in moderate quantities).

Cutting internal Acme threads in the existing castings is the ONLY way this is going to work.

Here's the latest disappointment: I received a reply from Tracy Tool in the UK last night regarding availability of the 3/4-5 RH Acme tap I need. Unfortunately, the taps will not be back in stock for at least two months. That's truly a pity as Tracy has the taps listed at a very reasonable 22 GBP. I haven't been able to find any other source for the taps at such low prices.

Obviously you aren't telling us all of the design and manufacturing criteria that you are adhering to. Without that information our suggestions are pointless.

 

[tetrode]

Obviously you aren't telling us all of the design and manufacturing criteria that you are adhering to. Without that information our suggestions are pointless.

On the contrary, Randy. I've provided all of the criteria along with visual aids. My issue and question were very straightforward and presented in the first post in this thread:

"I need to have 3/4" x 5 Acme threads cut into the mounting socket on a piece very much like this one: (image followed)"

Followed by:

"Depth of the socket is approximately 1-1/8". Material is cast aluminum."

and:

"Is this a DIY task for the non-mill owning person or something better farmed out to a machine shop"

Reiterated in my second post in the thread:

"The object of this exercise is to determine if photographic grip accessories with standard 5/8" "baby" pin receptors can be successfully modified with internal Acme threads so that they can also be attached directly to standard painter's poles which all have the 3/4-5 (nominally) Acme thread"

Nowhere were alternative mounting solutions solicited. For this proof-of-concept exercise cutting 3/4-5 Acme threads into existing pieces is the goal and the only solution.

That said, I do very much appreciate the guidance forum members have offered concerning fabrication of a DIY tap.

 

[Tony Wells]

I'm surprised no one has pointed this out yet, but tapping a hole with a slot already cut in it can be problematic. Presumably, this hole is the correct size to be tapped in the first place and doesn't need to be opened up, but you still need a good bit of depth to allow for the lead on the tap. It's common practice to specify a bottoming tap when length is at a premium, but for an acme tap, bottoming taps are probably custom only. I don't believe I've ever seen one myself. If that is not a thru-hole, then I'd say that tapping is not a viable option. That leaves machining the thread in a lathe, or thread milling in a CNC machining center. Not possible for the owner, if I read the OP correctly. In a lathe, a fixture of some sort would likely be needed, and add to the cost of a single piece job.

And as an aside only, I've never actually measured a standard broom handle to confirm that it is a standard 3/4"-5 TPI full acme thread. That would be the first step, IMO.

 

[pdentrem]

I think that a custom tap, using a spiral thread form would be the only method to tap the socket with the groove that is already there. Possibly use two taps, one that has standard V-thread form to rough out the threads and the second to finish the job.
Crazy idea, I was also thinking that making the finishing tap with an expander to start small and expanding to a larger size would make it easier to work. Due to the shortness of the socket there is not much room to work with such a fast thread, therefore my idea of the expanding tap. Crazy?
Pierre

 

[tetrode]

Some serious food for thought, here. I have a bunch of bottoming taps in common sizes and, truthfully, it did not occur to me that an Acme bottoming tap might not be an off-the-shelf item. The bore of the sockets on the photo accessories I'm hopping to tap is close enough to correct for a 3/4-5 tap that it shouldn't be a problem.

Regarding the actual size of painter's pole/broom handle threads, there is a pretty wide variation. Most seem to be at least a bit undersized. "Officially", the thread is supposed to be 3/4-5. However, my caliper has yet to clamp onto a painter's pole tip that conforms to that spec exactly.