Cutting Low Tpi Threads

B

BillWood

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I often see warnings regarding cutting low tpi threads, for example a 3 tpi thread off an 8 tpi leadscrew.

I also see that cutting these threads with a manual crank on the leadscrew or on the lathe headstock is an acceptable workaround.

What precisely is the problem please ?

Is it a bit like going up a steep hill in a small car in a high gear ?

Where will "failure" occur ie broken plastic spur gears on a chinese lathe, broken metal gears on a SB9" or equivalent or better, jammed spur gears, broken/jammed half nuts, stalled motor, broken leadscrew ?

Or would the belts simply slip if the load was too great ? Assume a South Bend 9" 60 years old.

Bill
 
I don't think the caution has to do so much with load on the lathe. You can take as light a cut as you want. It’s more about tool speed and operator reaction time. Imagine cutting a 12 tpi thread at your lathe’s slowest speed. Now imagine cutting a 3 tpi thread at the same spindle speed. The carriage will be moving 4x faster. That full second that you had to react to stop the spindle or retract the tool or disengage the leadscrew at the end of the 12 tpi thread is only 1/4 of a second at 3 tpi. Hence, the hand crank to prevent crashes. Or cut the threads from left to right. Others may have a different take on it.

Tom
 
Large depth of cut 3/16" or more, that is a lot of tool in the work, also the pitch is larger then the spindle on most micrometers and requires the use of a gauge block + wires to measure.
Good Luck
 
brino said:
At first blush that looks like a great solution.........unfortunately you end up with left-handed threads!!!

You'd hafta run the lathe backwards with the tool upside-down.....but make sure nothing would unscrew first.....

-brino
Click to expand...

Yep, that is correct. I just figured how to set up for left to right cut was a different topic.

Tom
 
I did cut a odd pitch thread close to a 1/2 pitch many years ago on my 9 South Bend lathe. It was about .090 deep and about .240 root width. That thread was about 1 1/2 turns long about 1 1/8 diameter. Ran it in back gear low as it would go. Came out nice !


Sent from my LG-V410 using Tapatalk
 
I had a gaul start while threading 4tpi and realized that by measuring the degree of rotation of the gauled section I could determine my reaction time.
It's surprising how fast one can move.
 
tin foil to catch those chips....good idea!
-brino
 
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