Imperial stock vs metric thread cutting

[MagnetoSound]

I’m a novice when it comes to metalwork and could use some help from the experts with a basic question on tolerances.

I plan to build a microphone using 1” (25.4mm) aluminium tube for the external housing. (I chose this size partly for aesthetic reasons, but also because I was able to order it with the exact internal diameter I require.)

However, the connector I plan to use has a metric M25 x 1.0 thread, so I would like to be able to thread the outside of the aluminium tube using the corresponding die. I’m confident that the tube has enough wall thickness to support the thread, but before I order the tools I would like to be sure that the die is not going to bind on the tube, or maybe even refuse to fit at all.

So my question is, will I be able to do this without first milling the tube, or will that 0.4mm be enough to cause problems?

TIA for any insight.

 

[Rusan]

The extra .4mm will be enough that the die won't even start onto the tubing. The basic major diameter of a given male thread size is usually spec'd as being the nominally stated diameter, which will achieve standard 75% thread depth. You can go very slightly larger on the outside diameter and achieve a higher percentage of thread, which will *usually* still be okay. Complete thread tables will specify nominal, absolute minimum and absolute major diameters, and thread tolerance class. Permissible maximum diameter on something in the 1" range is usually only about .010"/.25mm above nominal at most, which will yield a sufficiently high percentage of thread as to be problematic in some cases.

There's also another issue. Even if the tube's wall thickness will support the thread depth after it's been turned down to the correct diamater, it'll be extremely difficult to start a die onto it, then it'll be extremely difficult to hold while threading. Despite it being soft aluminum with lots of tapping lube (WD-40 is one of the best for aluminum), it'll take a LOT of torque to turn that die. Even if you have a lathe chuck to hold it in, there's a very high risk of deforming the tubing.

Alas, you'll need the services of a machine shop to lathe an area down to the correct diameter. But, you could also have them single-point thread it in the same setup, with extremely clean threads and no danger of deformation. A win-win, except for the expense.

 

[zamproject]

Hi

Not sure to get it completely
If your thread is 1mm pitch, the thread height is 0.87mm
I think half that (your 4/10) is way to much tolerance ?

Maybe have a look at threads and filet tolerance chart.

 

[MagnetoSound]

Thanks. One thing that is confusing is all the different numbers.
For example, the chart below shows the male thread height as 0.613 … obviously, the 0.4 becomes even more extreme by comparison, but I find parsing the numbers quite difficult.

 

[MagnetoSound]

@Rusan thanks for that in-depth explanation. I did have my doubts about the feasibility of my idea, it’s good to have it put into clear terms.

I am trying to decide whether to invest in a mini lathe for this kind of stuff. Perhaps for continued project work it might be worthwhile, and a better option than pestering my local fabricators on a regular basis.

 

[zamproject]

For example, the chart below shows the male thread height as 0.613 …

This is h3
See technical drawing below

 

[Rusan]

Thanks. One thing that is confusing is all the different numbers.
For example, the chart below shows the male thread height as 0.613 … obviously, the 0.4 becomes even more extreme by comparison, but I find parsing the numbers quite difficult.

It all has to do with thread percentage and thread pitch diameter, which determine two things:
1. Will the threads I'm cutting screw into/onto threads of certain dimensions?
2. Then, wIll they have proper fitment? Will they bind and/or be difficult to assemble on the one extreme, or on the other be too loose or weak for the application?

Thread percentage is the ratio of the maximum attainable thread height (which would be to a sharp "V" at top and bottom) and the actual, physically existing thread height. The optimal percentage varies according to thread tolerance class, thread pitch, diameter and simply how tight the threads need to be for a given application, but generically it's about 68 to 75%. Pitch diameter is the distance from the midpoint (the pitch line) between the major and minor diameters. Perfectly mating male and female threads will have equidistant pitch lines.

Single-point threading is among the most difficult and unforgiving of common lathe operations to set up and execute successfully. It also demands at least a fair quality small lathe, that's decently rigid and has minimal backlash in its mechanisms. But, it's definitely doable on a hobby-size lathe with some practice.

Where it becomes exponentially far more difficult is if you don't have the actual mating piece to the one you're threading, to be used as a gage. That requires measuring the threads as you progress, with a thread micrometer or thread wires, to determine how much deeper you need to cut them. Just a couple thousandths of an inch can make the difference between perfectly fitting threads and sloppy-floppy ones. On small-diameter threads, say less than about 1/2" or 14mm, it can mean the difference between perfection and scrap.

 

[MagnetoSound]

Thanks again Rusan, you explain things very clearly. I’m fairly keen to get into metalwork as a hobby/accessory to my electronics work. But I am fully aware of my limitations, and a rather steep learning curve. These posts of yours are very enlightening, and I appreciate the detail.

I think for this project I should aim to mill the tube and die cut the thread. I have a friend that may have a lathe. I will ask him if he can help with the milling.

@zam thanks for the diagram, also very helpful.