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RussZHC
09-21-2010, 05:56 PM
I had hoped all the reading would help but in some ways, "...a little knowledge is a dangerous thing...".

In this particular case it involves the uses for right hand vs. left hand screws.

Lead screws seem to be largely right hand though some are left.

Every lathe cross slide I have seen referenced or seen is left hand and lathe compounds right hand and the tailstock I have is left hand.

The main table on the horizontal mill is left hand thread, are some right hand?

My impression was it has to do with acting against prevalent forces or a particularly handed thread for a particular reason [the bottom bracket and pedals on bicycles springs to mind as with the pedaling motion it is possible to unscrew parts and there was a reason given as one direction draws water (i.e. wet conditions) in whereas the opposite pushes it away]

What is the logical/science behind which hand of thread is used where? Does speed play a role? Or does force of gravity in the case, say of mills

strokersix
09-21-2010, 06:02 PM
Turn the handle clockwise and the machine element moves away from you.

Ever run a turret mill with two right hand screws? Not easy to get used to.

Lew Hartswick
09-21-2010, 06:06 PM
I think most of the idea that on a manual machine the "action" of
whatever is being controlled is AWAY from you for CLOCKWISE
motion of the crank/knob. That is for all the lathes and mills we have
at school.
...Lew...

TGTool
09-21-2010, 06:50 PM
The actual hand of the screw could vary depending on which feature anchors the screw and which holds the nut. So, for instance, the compound has a right hand screw provided the upper casting holds the screw and handle and the nut is in the base. But it would also be feasible to design it like the crosslide where the handle would remain stationary with the lower casting and the nut would be in the upper. In that case it would need a left hand screw to provide the same relative motion moving the compound inward with clockwise rotation.

KiddZimaHater
09-21-2010, 07:02 PM
The Tailstock on your lathe is a perfect example of the need for a Left-Hand thread.
When you crank the handle clockwise (away from you), the spindle advances out of the tailstock.
If it were a Right-hand thread, the cranking motion would be backwards. You'd have to spin the handle towards yourself to advance the spindle. Not a very practical thing to do.

darryl
09-21-2010, 08:26 PM
You mentioned bicycle cranks- anytime a shaft is turning and a pulley or a crank or whatever has to be held to it with a nut, the direction of rotation has to be such that the nut would tend to tighten. A cordless drill for example- the chuck screws on the normal way, since it's turning CCW and would normally screw onto the chuck in that direction. For reverse rotation however, the chuck would tend to unscrew. That's why there's a left-hand threaded bolt in the center, keeping the chuck from coming off.

Some table saws, where the arbor is to the right side, have left hand threads on the arbor. Both of ours at work are like that, and on one of them, the nut is all but ready to strip. It will be my job to either find a replacement nut, or make one. Good thing I added the idler gear to my lathes gear train so I can make left hand threads-