Hi All,
I have recently purchased a pen turning mandrel and had a go at turning a few kits. Enjoyed it a lot and should have done this years ago.
I am now looking at kitless and would really appreciate some help with cap threads.
I am aware of triple-lead taps & dies, however, I am also aware of their cost (ouch!). So I started thinking about cheaper ways to cut a faster cap thread and figured out that a 7/16-12 ACME thread might fit my draft slim rollerball design quite well. The main theoretical reason being (please correct me if I am wrong): ACME thread height is half of it's pitch. That is, for a ~2mm (12tpi) pitch, the threads are only 1mm high, which, in my simple mind, should be roughly equivalent to a 1mm, 2-lead pitch.
For every turn, the cap shall advance 2mm onto the body, but the difference between minor and major is only 1mm, right?
Mr Google says I'm wrong. For example, here:
For 7/16-12 ACME, the smallest Major-minor difference listed as:
Minor: 0.3442 (8.7mm)
Major: 0.4325 (10.1mm)
Difference: 1.4mm
If 12TPI pitch is 2mm, and teeth are only 1mm high, such a big gap between minor & major leaves only a 0.6mm for the teeth to engage.
What's worse, if I take the max G2 tolerance values, I end up with
Minor: 0.3253 (8.3 mm)
Major: 0.4375 (11.1mm)
Difference: 2.8 mm
In which case 1mm-tall threads will not engage at all. There's a whopping 0.8mm gap between the threads with this spec.
I am clearly missing something, but I don't know what it is. Please help a newbie out.
PS I can get 7/16-12 ACME tap & die for about 1/10 of the cost of a 3-lead M10x1, which gives me some room to experiment and see if this thread size suits me, hence the whole thought process.
I have recently purchased a pen turning mandrel and had a go at turning a few kits. Enjoyed it a lot and should have done this years ago.
I am now looking at kitless and would really appreciate some help with cap threads.
I am aware of triple-lead taps & dies, however, I am also aware of their cost (ouch!). So I started thinking about cheaper ways to cut a faster cap thread and figured out that a 7/16-12 ACME thread might fit my draft slim rollerball design quite well. The main theoretical reason being (please correct me if I am wrong): ACME thread height is half of it's pitch. That is, for a ~2mm (12tpi) pitch, the threads are only 1mm high, which, in my simple mind, should be roughly equivalent to a 1mm, 2-lead pitch.
For every turn, the cap shall advance 2mm onto the body, but the difference between minor and major is only 1mm, right?
Mr Google says I'm wrong. For example, here:
External ACME Thread General Purpose Size Chart
The following table defines recommended industry standard sizes for External ACME General Purpose Threads per American National Standard ASME/ANSI B1.5. There are three classes of General Purpose ACME threads, 2G, 3G, and 4G each class provides some clearance on all standard size diameters for...
westportcorp.com
Minor: 0.3442 (8.7mm)
Major: 0.4325 (10.1mm)
Difference: 1.4mm
If 12TPI pitch is 2mm, and teeth are only 1mm high, such a big gap between minor & major leaves only a 0.6mm for the teeth to engage.
What's worse, if I take the max G2 tolerance values, I end up with
Minor: 0.3253 (8.3 mm)
Major: 0.4375 (11.1mm)
Difference: 2.8 mm
In which case 1mm-tall threads will not engage at all. There's a whopping 0.8mm gap between the threads with this spec.
I am clearly missing something, but I don't know what it is. Please help a newbie out.
PS I can get 7/16-12 ACME tap & die for about 1/10 of the cost of a 3-lead M10x1, which gives me some room to experiment and see if this thread size suits me, hence the whole thought process.
