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Harold Hall

Workshop Processes

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To illustrate my point that for a given thread there will be many combinations that produce the required value, there are 31 combinations that achieve 10 TPI. I should say that I am writing all these comments on the basis that the lathe has an eight TPI leadscrew and the popular range of  gear sizes of 20 to 75 teeth by five's plus one gear having 38 teeth.

 

However, I have stripped out those that use three drivers and three driven as there are still 12 combinations that use either 1 or two pairs of gears as table one shows.

 

TABLE ONE

 

 TPI   DR1 DR2   DN1 DN2

 

10.0000  20        25      

10.0000  20  60    30  50   

10.0000  20  70    35  50   

10.0000  30  40    20  75   

10.0000  30  40    25  60

10.0000  30  50    25  75   

10.0000  30  60    45  50   

10.0000  30  70    35  75   

10.0000  35  40    25  70  

10.0000  40        50      

10.0000  40  45    30  75   

10.0000  60        75     

 

The changewheel chart for my lathe list the 40/50 combination but it can be seen that if the 40 tooth wheel is missing there are still 7 combinations that do not use that wheel. Similarly, if the 50 tooth wheel is missing there are also 7 alternative combinations. Note that all these combinations give the exact results.

Metric Threads

Now, if attempting to produce a metric thread without the benefit of a 127 tooth gear, it has to be accepted that an exact result will not be possible. In this case, how much is the error?

 

What now if the requirement is to produce a thread with a metric pitch of 1.5 mm. Table two shows that there are 11 combinations that produce the lowest value and 11 combinations for the highest, the two mid values having only one combination each. The table also shows the value of the metric pitch achieved plus the error for each one.

 

TABLE TWO

 

Pitch mm  Error mm DR1 DR2 DR3   DN1 DN2 DN3

 

1.50090  + 0.00090  20  30  65    25  55  60  

1.50090  + 0.00090  20  35  65    25  55  70  

1.50090  + 0.00090  20  45  65    30  55  75  

1.50090  + 0.00090  20  60  65    40  55  75  

1.50090  + 0.00090  20  65        50  55      

1.50090  + 0.00090  25  60  65    50  55  75  

1.50090  + 0.00090  30  40  65    50  55  60  

1.50090  + 0.00090  30  65        55  75      

1.50090  + 0.00090  35  40  65    50  55  70  

1.50090  + 0.00090  35  60  65    55  70  75  

1.50090  + 0.00090  40  45  65    55  60  75

 

1.50053  + 0.00053  20  40  55    35  70  38

 

1.49992  - 0.00008  40  50  38    45  55  65

 

1.49967  - 0.00033  20  25  70    30  65  38

1.49967  - 0.00033  20  35  50    30  65  38

1.49967  - 0.00033  20  35  75    45  65  38

1.49967  - 0.00033  20  50  70    60  65  38

1.49967  - 0.00033  25  30  70    45  65  38

1.49967  - 0.00033  25  35  40    30  65  38

1.49967  - 0.00033  25  35  60    45  65  38

1.49967  - 0.00033  25  40  70    60  65  38

1.49967  - 0.00033  25  50  70    65  75  38

1.49967  - 0.00033  30  35  50    45  65  38

1.49967  - 0.00033  35  40  50    60  65  38