My circa 1930 SB 9" lathe w/single lever quick change gear box says that to
determine longitudinal feed rate to multiply the tpi by 3.25. When I
multiply 3.25 times 96 tpi I get 312. What does this number mean? As a
check, I multiplied 48 tpi by 3.25 and the number is 156, which is smaller
than for 96 tpi. Seems to me that 48 tpi should feed faster. Note: I am a
hobbyist and so far I just experiment to find the best feed rate, and have
found that 96 tpi works fine for mild steel. Thanks.

Jim,
There is a gear reduction in the apron when engaging the lead screw with the
feed clutch of 3.25 to 1 in comparison to directly engaging the lead screw
with the half nut. South Bend means 1" / 96TPI / 3.25 = .003205 inches per
spindle revolution. It's easy. You just have to learn to think South Bend.
Good luck,
Steve
> My circa 1930 SB 9" lathe w/single lever quick change gear box says that
> to determine longitudinal feed rate to multiply the tpi by 3.25. When I
> multiply 3.25 times 96 tpi I get 312. What does this number mean? As a
> check, I multiplied 48 tpi by 3.25 and the number is 156, which is smaller
> than for 96 tpi. Seems to me that 48 tpi should feed faster. Note: I am a
> hobbyist and so far I just experiment to find the best feed rate, and have
> found that 96 tpi works fine for mild steel. Thanks.
>
>
>

TPI corresponds to turns/inch, feed is specified in inches/turn. The plate
stops short of telling you that you need to take the reciprocal of TPI x 3.25
to get the carriage advance in inches per revolution of the spindle.
1 / 48 x 3.25 = .0064 in / rev
1 / 96 x 3.25 = .0032 in / rev
Ned Simmons

> Jim,
> There is a gear reduction in the apron when engaging the lead screw with
> the feed clutch of 3.25 to 1 in comparison to directly engaging the lead
> screw with the half nut. South Bend means 1" / 96TPI / 3.25 = .003205
> inches per spindle revolution. It's easy. You just have to learn to think > South Bend.
> Good luck,
> Steve
>
>> My circa 1930 SB 9" lathe w/single lever quick change gear box says that
>> to determine longitudinal feed rate to multiply the tpi by 3.25. When I
>> multiply 3.25 times 96 tpi I get 312. What does this number mean? As a
>> check, I multiplied 48 tpi by 3.25 and the number is 156, which is
>> smaller than for 96 tpi. Seems to me that 48 tpi should feed faster.
>> Note: I am a hobbyist and so far I just experiment to find the best feed
>> rate, and have found that 96 tpi works fine for mild steel. Thanks. >>
>>
>>

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