Thread pitches and change gears

[DLF]

Hello,

Some threads can not be cut on the lathe because the required change gear combination is not available.

But, most of the time there is a combination available which would result in a thread pitch very close to the ideal value.

My question is the following: what would be the maximum allowed thread pitch deviation (in percentage) that would result in a still usable thread?

The qustion is valid for both metric and imperial threads.

Can anybody give some info?

Thank you

Sent from my SM-G973U1 using Tapatalk

 

[benmychree]

You should be able to cut any necessary thread pitches with a full set of change gears, except some prime numbers such as 27 and 19 TPI in imperial, those are used in pipe threads, and they can be cut with a extra change gear of 27 and 19 teeth.

 

[WobblyHand]

You should be able to cut any necessary thread pitches with a full set of change gears, except some prime numbers such as 27 and 19 TPI in imperial, those are used in pipe threads, and they can be cut with a extra change gear of 27 and 19 teeth.

Perhaps you meant 29?

27 is not a prime number. 27 = 3 x 3 x 3. 19 is prime.

First 20 primes are:

2

3

5

7

11

13

17

19

23

29

31

37

41

43

47

53

59

61

67

71

You can find up to the first 1000 primes at https://en.wikipedia.org/wiki/List_of_prime_numbers

 

[Asm109]

It depends on two things. First how long is the engagement between male and female threads.
Second, what is the clearance (at the pitch diameter of the threads) between male and female.
The error in the pitch multiplied by the engagement length must be less than the slop between the threads.
Your homework is to dive into the thread standards to understand how much clearance is available at each thread class.

 

[benmychree]

Perhaps you meant 29?

27 is not a prime number. 27 = 3 x 3 x 3. 19 is prime.

First 20 primes are:

2

3

5

7

11

13

17

19

23

29

31

37

41

43

47

53

59

61

67

71

You can find up to the first 1000 primes at https://en.wikipedia.org/wiki/List_of_prime_numbers

My faux pas --- But 27 is a number that is used in our pipe thread series, and very few lathes can cut it, Axelson is the only one that I've seen, 19 TPI is used for pipe threads by the Brits.

 

[benmychree]

It depends on two things. First how long is the engagement between male and female threads.
Second, what is the clearance (at the pitch diameter of the threads) between male and female.
The error in the pitch multiplied by the engagement length must be less than the slop between the threads.
Your homework is to dive into the thread standards to understand how much clearance is available at each thread class.

To you goes the honor of actually answering the question!

 

[WobblyHand]

My faux pas --- But 27 is a number that is used in our pipe thread series, and very few lathes can cut it, Axelson is the only one that I've seen, 19 TPI is used for pipe threads by the Brits.

I was aware of the odd pipe threads. I'd agree that some lathes have trouble with some of these threads.

My unmodified lathe will cut 27 TPI, (not a tapered thread,) but it cannot do 19 TPI.

My ELS will do just about any weird thread, to the precision I need, with a little programming. Of course my little lathe will run out of grunt for the coarser threads like 4 TPI and under, but that's a limitation due to the lathe size and rigidity.

The TPI's that I could do at the selection of a button are: [ 2.5, 3, 4, 4.5, 6, 8, 10, 11, 11.5, 12, 13, 14, 16, 18, 19, 20, 22, 24, 27, 28, 32, 36, 40, 56, 72, 80, 96, 100]. I could add 31.27043 TPI if I wanted to, perhaps to drive someone stark raving mad, but the ELS is more limited by space to display the TPI's rather than the thread TPI value itself. It's one of the advantages of an ELS system, not to be limited by discrete integer gear ratios.

 

[RJSakowski]

I was aware of the odd pipe threads. I'd agree that some lathes have trouble with some of these threads.

My unmodified lathe will cut 27 TPI, (not a tapered thread,) but it cannot do 19 TPI.

My ELS will do just about any weird thread, to the precision I need, with a little programming. Of course my little lathe will run out of grunt for the coarser threads like 4 TPI and under, but that's a limitation due to the lathe size and rigidity.

The TPI's that I could do at the selection of a button are: [ 2.5, 3, 4, 4.5, 6, 8, 10, 11, 11.5, 12, 13, 14, 16, 18, 19, 20, 22, 24, 27, 28, 32, 36, 40, 56, 72, 80, 96, 100]. I could add 31.27043 TPI if I wanted to, perhaps to drive someone stark raving mad, but the ELS is more limited by space to display the TPI's rather than the thread TPI value itself. It's one of the advantages of an ELS system, not to be limited by discrete integer gear ratios.

You might want to add 9 tpi to your table. 7/8-9 tpi is a UNC thread.

 

[WobblyHand]

You might want to add 9 tpi to your table. 7/8-9 tpi is a UNC thread.

Thanks, sometimes you miss one!

I can fit 25 items on a page, so adding more threads is easy, but doing the graphics paging code is less so. If there's too many items on a page it's hard to select one vs the other. This is known as the fat finger problem. Will come up with a way to accomplish that adding another state to my state machine. The state machine handles the transitions between menus (states) to ensure operational integrity.

 

[RJSakowski]

Thanks, sometimes you miss one!

I can fit 25 items on a page, so adding more threads is easy, but doing the graphics paging code is less so. If there's too many items on a page it's hard to select one vs the other. This is known as the fat finger problem. Will come up with a way to accomplish that adding another state to my state machine. The state machine handles the transitions between menus (states) to ensure operational integrity.

I had made a request on github for a modification to the Clough42 code to include the ability for a user to enter a custom thread pitch. One of the contributors did just that, allowing entry of custom metric thread pitches. It will only allow for two decimal places, so .01mm resolution and won't hit inch pitches closely enough for my needs. My intention is to dig into his code and determine how he accomplished this. The stumbling block is the Clough42 display resolution which only allows two decimal places. There are ways to overcome this but it requires the coding knowledge to accomplish them.

If you permitted a user entry such as this in your code, it would allow creation of custom pitches without the need for anticipating some weird pitch requirement in the future and cluttering up your pitch table.