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Harold Hall

Workshop Processes

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TABLE SIX

 

PITCH GEARBOX TUMBLER PITCH    ERROR  

REQ.  SETTING   GEAR  RESULT     %    

0.75    48      34    0.7497  -0.046

0.90    40      34    0.8996  -0.046

1.00    36      34    0.9995  -0.046

1.25    28      33    1.2473  -0.214

 

1.50    24      34    1.4993  -0.046

1.75    20      33    1.7462  -0.214

2.00    18      34    1.9991  -0.046

2.50    14      33    2.4946  -0.214

3.00    12      34    2.9986  -0.046

 

Worm Gears

Cutting metric threads is not the only situation where it will be necessary to set up the changewheels to cut, what is in imperial terms, a complex value. Another requirement is when cutting a worm gear. Here though the similarity ends, as whilst there will be just one required value for a metric pitch this is not the case for a worm gear. Because of the difference turning worm gears and the changewheel combinations required are dealt with on separate pages.

 

When Cutting a Thread

The details in these pages have been given primarily to enable the viewer to better understand the process of choosing changewheel combinations for complex thread pitches.

I would though suggest that faced with the need to cut such a thread that he of she looks at my pages that provide the results for all possible combinations of available gears. These based on gears having 20 to 75 teeth by fives plus 21 teeth and 38 teeth gears.

 

If you are missing a few of these gears you can still select the combinations that use only the gears that you have.

 

There are two sets of lists one for a lathe having an imperial leadscrew and the other a metric leadscrew.

 

When cutting a metric thread on a lathe with an 8 TPI leadscrew use the Imperial list and convert the metric pitch required to an Imperial TPI. Typically, for a 1.5mm pitch divide 25.4 by 1.5 giving a value of 16.93333 TPI and look up this value. You will find that as the listing also uses 21 tooth gears, there are more, and more accurate, values than were listed in Table two.

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Drawings