Cutting 11.5 TPI threads

LROYSON said:
If I find a 23 tooth

How did you determine a 23 tooth gearwould work?
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23 teeth is twice the TPI of 11.5, and if the levers are set correctly and the stud gear is selected properly one should be able to make 11.5 TPI, again read my prior post regarding how to select the change gears, this process works on my lathe, it should work on other lathes with inch lead screws.
 
BGHansen said:
You're not alone on this topic. I'm following as I'd like to do 27 tpi with a gear box that goes from 26 to 28.

Bruce
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If your gearbox has an 18tpi you can slow up the leadscrew by inserting a 2 to 3 reduction in the gear train with change gears. My Grizzly calls for 40t spindle and 40t gearbox gears when cutting imperial threads. If I swap the 40t on the gearbox for a 60t and set the gearbox to to cut 18 tpi I get 27tpi. ---- 40/40 = 1. --- 60/40 = 1.5 --- 1.5 x 18 =27 Seems a little weird but as the leadsrew speed goes down the tpi goes up.
 
On my lathe, 27TPI is achieved with 32T gear on the stud and 27 on the QC input shaft, the QC is set for 32TPI. For 19TPI, the same 32 T on stud, and 38 (2X 19) and QC is set for 16TPI.
 
I made a spreadsheet of the threading table in manual and experimented with different gears and a 26 tooth gear will give 11.53 TPI in 5C, a 47 tooth gear using gear positions 1C gives 11.49 TPI, and 52 teeth gives a ratio of 11.53 using gear lever positions 5B. I attached the spreadsheet I used so you can check my math.

I used the table for the 30 tooth gear as the basis of the calculations, divided the gear selection in cell K2 by 30 and then multiplied by the entry in the table to get the TPI for each lever position. The interesting thing is with the 49 tooth gear, the manual gives the first row of figures for lever 1 as integer numbers, but when I put it in calculated values section in columns J through L, they are not whole numbers, so those are approximations and not precise TPI values if the numbers for the 30 tooth gear are correct.
 

Attachments

  • PM1228 Threading TPI calculator.xlsx
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Just wondering.............. If Anyone has seen a video on lantern gears, gears made out of wood with shafts instead of gear teeth. A lot of people have DRO's with bolt hole functions that would lay out the tooth spacing and enable a functioning gear for an one off/ emergency situation.
 
@LROYSON

I have a potential solution for you.

Did you tryout my Excel workbook to see if you can cut the thread with the gears that you have. A macro in the a version of the program will prepare a table of ALL possible TPI values for virtually any lathe, once the lathe gearing is inputted to a specific tab spread sheet for your lathe model.

B2 said:
Improved Version: TPI, Feeds, X-Feeds: Generalized Lathe TPI Excel file: "TPI_ManyLathesRev1 N502_1630"
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Many times there is a very close approximation to what you want to achieve without making or buying any new gears. I can help you set this up for your lathe if I know a little more about your lathe geometry, operation, and the external gears. I have made a pass at this already, but do not know enough about your machine geometry to finish it.

For example, I believe that if you are happy with a TPI error of less than 1%. You may have several options. So I have some questions for you:

1) Does the exchange gears 86/91 separate and do they have the same axle diameter etc as the upper and lower gear axles? If so you can sometimes swap these around.
2) What external gears did you get with the lathe. I know you got a 30T, 90T and the 86/91 exchange gear but what others?
3) What is the max size gear that well fit at the upper position. What is the max size gear that will fit at the bottom position.
4) Not as important and more difficult to figure out is: If you put a smaller gear at the exchange gear axle can you reach it with the other gears?

For example, I think you will make n 11.4667 TPI if you use the following set up:

Knobs set to : 3-C
Upper gear: 35T
Bottom gear: 86T (one of the two from the pair 86/91 pair that you would usually use for exchange to go to metric)
Exchange gear axle: Any gear that will fit, say the 90T or the 86T, or even the 60T gear. (since you contact this gear from both sides it does not change the over all gear ratio.)

Anyway, try this set up and make a scratch thread and see if you can tell that it not quite 11.5TPI. If I have gotten something wrong in my program then it may not even be close. Let me know.

There are other gear combinations which yield a TIP value equally close, but you may not be able to arrange the gears physically to reach them.

By the way, if I am correct ..... The 86/91 exchange gear is not exactly a metric conversion. It is very-very close, but not dead nuts on like the 120/127 that other lathes have. 91/86= 1.05814 127/120=1.05833. This difference in the later digits is the error. The key here is the 127T gear. 127*2= 254. Likewise, there are 25.4 mm to an inch, by definition. 127 is a prime number where as 91 is not (7*13=91). So there is no way to get exactly to the metric scale with gears which have an integer number of teeth!
The manual says that you can make a 1.5mm/T using the 2-C, 30/90 gears and the 86/91 exchange gear. The actual number you get is 1.500275. Extremely close, but not exact. However, if you can swap gears around you maybe able to get even closer. But who cares about this case, does anyone really make threads this accurate anyway!

Dave L.
 
B2 is on the right track. Best I can tell from the PM manual, the 11 tpi thread is about 11.02, not bad for many applications.
That is using a 49 tooth change gear and "normal" settings.
A 47 tooth change gear in place of the 49 will get you equally close to the 11.5 tpi.
I like the answer to use 2.25 mm pitch as it doesn't require a new gear.
It isn't quite as close, but.......
 
@Ischgl99

I could not get your spread sheet to work right for me. I think I can fix it later.

@LROYSON

I looked at the manual some more. It appears that the exchange gear is a different size axle and it is threaded. But it does also take a 56T.

So I ran my Excel work book based upon the info I could find and I generated the following possibilities for you.
In this table when letters appear in two columns it means that you can use any gear at this location. The Screw2C column is the bottom axle gear position and the Spindle column is the upper gear axle. Looks like you have several possibilities to get close to 11.5TPI. But I will try to take a closer look at this later. If you agree then I can post the entire spread sheet for you.


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Dave L.
 
B2 said:
I could not get your spread sheet to work right for me. I think I can fix it later.
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Which part wasn’t working? It’s a very simple calculation comparing the table of the 30 tooth gear to a different tooth count.
 
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