Bolton bt 800

@Ray Smiley

"a b c d" refers to the positions of the gears of the change gear train.

Gear "A" is usually the first gear below the "spur" or main shaft gear, gear "D" is usually the gear mounted to the lead scew, or otherwise connected to it in the gear train.

Each position can hold a different gear and in combination a series of gears can be used to reach the desired TPI (imperial) or Pitch (Metric).

You should have a chart on the lathe somewhere that shows an image representing the gear-train (similar to the image below) and a list of gears in a table which references the gears by tooth count and in which order they need to be placed to get to the desired TPI or Pitch.

Sometimes the chart will direct you to use "A" and "D", "A" + "B" + "D" or "A"+"B"+"C"+"D" depending on desired TPI or Pitch.

Is the image you posted on your lathe? you did not clarify.

ABCD.png

*** Edited to add info ***
 
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In the case of 28 tpi the A gear is 72 teeth and the D gear is 42 teeth. The 127 tooth gear is used to transfer power from A to D.
For metric threadcutting the 127/120 compound gear is used, but for imperial only one of those two is used as an "idler"
 
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A is one one gear or I can’t get it apart
 

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markba633csi said:
In the case of 28 tpi the A gear is 72 teeth and the D gear is 42 teeth. The 127 tooth gear is used to transfer power from A to D.
For metric threadcutting the 127/120 compound gear is used, but for imperial only one of those two is used as an "idler"
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Thank you that made sense
 
The 120 tooth could be used to bridge the gap instead of the 127 if it will reach- idlers don't affect the ratio at all so it doesn't matter the tooth count
Only the first and last gears set the ratio if there are no compound gears in the chain
 
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