1. ## Equivalent Thread Chart - Inch to Metric

I'm looking to make a piece that requires a 20 tpi, but I want to keep the whole design metric, is there such a chart that can help me out or maybe someone can show me a simple how to calculation ?

I'm turning a 40 mm stock making two parts. 1 male (40 OD) and 1 female (40 ID ) part. Not really sure if the size of the stock will make a difference in my threads on the part, or will it ?

Ex.

Inch Metric
20 tpi - 2.5 mm

2. If you are trying to make these two parts fit then the thread doesn't have to standard. But if one part 40mm od and the other is 40mm id then there is no stock for threads. example if 40mm is your id then your od would have to be 41.376 so with a pitch of 1.27 (20tpi) your minor would be 40mm.

3. "example if 40mm is your id then your od would have to be 41.376 so with a pitch of 1.27 (20tpi) your minor would be 40mm."

Hmm see I didn't even know that. I'm still very new to cutting threads, and still need to learn alot. Maybe this is why some of my pieces seem a bit wobbly and don't tighten until I screw it all of the way in..... :0

So there's always a 1.376 male clearance when threading ?

Which gear needs to be switched on my HF 9 x 20 for something like this ? I understand how to read the charts, and have the 120T gear in place, but don't know how to determine the conversion of inch to mm.

4. Your clearance is always going to be different depending on pitch. your conversion is simple inch to metric conversion. 1" = 25.4 mm so 20 tpi is .05 inch per thread (1/20)
and then .05 X 25.4=1.27mm. But the problem is on an inch machine such as you are using will not always be exact to metric so 20 tpi is 1.27mm but commonly it would be a 1.25 pitch there is always a little variance. most machines will have a metric chart but if they don't then you will have to do the math witch like i said 1=25.4.

5. "Your clearance is always going to be different depending on pitch."

Hmmm I'm a little lost now. So how would the pitch change the clearance (depth) ?

I thought the pitch is just referring to the length from one crest to another, not the depth ?

6. That's correct but a thread is a included angle of 60 deg. So the more tpi the shallower the thread will be for example: 12 tpi .045" deep, 16 tpi is .0388" deep,
20 tpi is .027" deep, and 24 tpi is .0225".

7. Try efunda.com it is an engineering website with all kinds of specs and tolerances and formulas. All ways a good place to start to make sense of things.

8. Well I do appreciate your time and thank you for your help. I'd like to make this a helpful thread, so maybe I'll post a few of the basics here for others to reference.

It's to bad someone hasn't made a simple 9 x 20 basics DVD, especially on how to thread....

9. ## Equivalent Thread Chart - Inch to Metric

TO IDENTIFY AN UNKNOWN THREAD, CERTAIN TOOLS AND DATA ARE NECESSARY. INCLUDED IN THESE CHARTS ARE A HELPFUL COMPILATION OF THE DATA NEEDED FOR THE IDENTIFICATION OF MOST EXTERNAL (MALE) THREADS FROM THE THREE MOST POPULAR THREAD SYSTEMS IN THE WORLD, METRIC STANDARDS, U.S.A.(INCH) STANDARDS, AND THE BRITISH (INCH) STANDARDS. ALSO THIS CHART WILL OUTLINE THE USEFUL TOOLS AND PROCEDURES IN ATTEMPTING TO IDENTIFY AN UNKNOWN THREAD.

THESE ARE THE THREE BASIC STEPS TO IDENTIFY A THREAD:

1. MEASURE THE MAJOR DIAMETER OF THE MALE THREAD. {THE MAJOR DIAMETER IS THE OUTSIDE DIAMETER (OD) OF THE MALE THREAD}. IF YOU ARE TRYING TO IDENTIFY AN INTERNAL (FEMALE) THREAD THESE DATA CHARTS WILL NOT CONTAIN SUFFICIENT INFORMATION. FOR ADDITIONAL INFORMATION ON MEASURING INTERNAL (FEMALE) THREADS; SEE THE INSTRUCTIONS IN CHART VIII.

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10. twocik - Advise. As mentioned before. Pick up some books and start reading.
If you want to keep everything solid metric you will have to use a standard metric thread. I do not know the OD of your thread, but depending on the OD needed you will need to stick to the metric thread that is close to it. The same goes for the pitch.
There is no table since the two systems are completely different.
If you need 20 treads per inch you will have to stick with standard inch thread.

11. Originally Posted by juergenwt
twocik - Advise. As mentioned before. Pick up some books and start reading.
If you want to keep everything solid metric you will have to use a standard metric thread. I do not know the OD of your thread, but depending on the OD needed you will need to stick to the metric thread that is close to it. The same goes for the pitch.
There is no table since the two systems are completely different.
If you need 20 treads per inch you will have to stick with standard inch thread.
True, but what a mess.
Imagine someone years later having to identify this thread, nightmare.
Not sure if the 20 tpi has to be all that precise tho.
If it dose then I would use a OD in imperial units allso.
40mm / 25.4 = 1.5748 inch
maybe make a 1 9/16 x 20 thread or 1.562 x 20

if the 20 tpi could vary slightly, as mentioned before a
40mm x 1.25 thread would be suitable as well.

Both can be cut acurately on the 9 x 20 Lathe.
Good Luck whatever you decide.

12. ## Equivalent Thread Chart - Inch to Metric

The tap size chart provides a list of standard size taps, specifying the diameter and thread spacing, for fractional, metric, and screw sizes. The decimal equivalents of the diameters are shown in both English and Metric units. Fractional sizes are listed in inches, while metric sizes are listed in millimeters following the letter "M". A screw size number corresponds to a diameter which is larger for a higher screw size. The thread spacing, which may be coarse or fine, is listed after the diameter. In the fractional and screw size systems, the thread count is used, measured in threads per inch. The metric system uses the thread pitch, which is the distance between threads, measured in millimeters. For each thread count, the equivalent thread pitch is provided and for metric taps, the approximate thread count is shown based on the pitch. Lastly, the recommended tap drill size is provided for each standard tap size. This size drill bit should be used for drilling the initial hole that will then be tapped.
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