Left hand thread cutting?

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LX Kid
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Left hand thread cutting?

Post by LX Kid » Sun Apr 26, 2015 7:08 pm

Newbie question about left hand threads. A gunsmith friend of mine told me it's possible to cut left hand threads on my lathe without having a back gear. He said that you turn the cutter upside down and run the chuck reverse, which also turns my lead screw backwards, relief cut and cutting right. Is it as simple as that? But of course the tool post would also have to be raised to catch center cutting. Curious minds want to know!

Torch
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Re: Left hand thread cutting?

Post by Torch » Sun Apr 26, 2015 7:51 pm

I think "back gear" refers to a low gear between the motor and the headstock, not between the headstock and the lead screw. In other words, it just slows things down and has nothing to do with changing feed direction.

Similarly, whether you turn the tool upside down (or mount the tool upside right on the back side) and cut in reverse makes no difference, you will still end up with a right hand thread because the relationship between the chuck and the lead screw has not changed. They are still rotating in lockstep.

Don't believe me? Chuck up a piece of material, mount a sharpie marker to the tool post, throw the lathe in reverse and see which direction the "threads" turn.

What you need to do is insert a gear into the change gear train between the headstock and the lead screw. So assuming that right now gear A (Clockwise) drives gear B (CCW) which in turn drives gear D (CW), inserting a gear between B and D changes things to A (CW) > B (CCW) > C (CW) > D (CCW).

The number of teeth of gear C does not matter, you can use any spare change gear that you can get in there. The final ratio between B and D will remain unchanged. EG: if B is 120 teeth and D is 48 teeth then the ratio of B:D is 2.5:1. Inserting a 12 tooth gear as C makes the ratio (120/12) / (12/48) = (10/1) / (0.25/1) = 2.5:1.

Without pictures, drawings, make and model, or something, I have no way of guessing if this is possible on your particular machine, but I know that others have made a reversing gear mod on other machines in the past.

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LX Kid
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Re: Left hand thread cutting?

Post by LX Kid » Mon Apr 27, 2015 6:27 am

Well that certainly clears it up for me. CW,CCW, then up over the rainbow and back down to the river for lunch! I think what your saying is, "No it won't work." LoL My lathe's lead screw only follows the direction of the chuck. (Fwd & Rev ) Thanks Torch.

Torch
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Re: Left hand thread cutting?

Post by Torch » Mon Apr 27, 2015 7:07 am

CW -- clockwise. Also known as right-hand rotation. CCW - counter-clockwise. Also known as left-hand rotation.

So when I wrote "A (CW) > B (CCW) > C (CW) > D (CCW)" I meant to indicate that if gear A is turning clockwise, gear B will turn counter-clockwise, driving gear C clockwise which finally turns D counter-clockwise.

Note that this only applies to gears which are meshed (IE: the teeth of one gear engages the teeth of the next). Many lathe change-gear trains include a compound gear. That is a gear with two diameters on a common axle; ie, face to face rather than teeth to teeth. They obviously turn in the same direction as each other at all times! So a compound gear changes the ratio without changing the direction. The change gear chart may label each diameter as a separate gear when calculating the final ratio, and may use A, B, C, D, etc. Please don't confuse my example above with your change gear chart.

Sorry for the confusion.

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Re: Left hand thread cutting?

Post by chucketn » Mon Apr 27, 2015 7:14 am

A 3-in-one machine can't reverse the lead screw separately from reversing the chuck rotation?

Chuck

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LX Kid
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Re: Left hand thread cutting?

Post by LX Kid » Mon Apr 27, 2015 9:12 am

Torch wrote:CW -- clockwise. Also known as right-hand rotation. CCW - counter-clockwise. Also known as left-hand rotation.

So when I wrote "A (CW) > B (CCW) > C (CW) > D (CCW)" I meant to indicate that if gear A is turning clockwise, gear B will turn counter-clockwise, driving gear C clockwise which finally turns D counter-clockwise.

Note that this only applies to gears which are meshed (IE: the teeth of one gear engages the teeth of the next). Many lathe change-gear trains include a compound gear. That is a gear with two diameters on a common axle; ie, face to face rather than teeth to teeth. They obviously turn in the same direction as each other at all times! So a compound gear changes the ratio without changing the direction. The change gear chart may label each diameter as a separate gear when calculating the final ratio, and may use A, B, C, D, etc. Please don't confuse my example above with your change gear chart.

Sorry for the confusion.
I was just making fun of my ignorance. I don't think my gunsmith friend has it thought thru or he didn't realize that my lathe chuck followed direction of lead screw. I was just making fun of the situation. Thanks

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Re: Left hand thread cutting?

Post by Torch » Mon Apr 27, 2015 10:45 am

chucketn wrote:A 3-in-one machine can't reverse the lead screw separately from reversing the chuck rotation?
Apparently his cannot. Some of the smaller, cheaper machines don't even have a lead screw. (eg: http://www.northerntool.com/shop/tools/ ... _7171_7171)

However, AFAIK, the current crop of "better" machines sold by Grizzly, Busy Bee, Shoptask, etc. all have a reversible lead screw.

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LX Kid
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Re: Left hand thread cutting?

Post by LX Kid » Mon Apr 27, 2015 11:01 am

Torch wrote:
chucketn wrote:A 3-in-one machine can't reverse the lead screw separately from reversing the chuck rotation?
Apparently his cannot. Some of the smaller, cheaper machines don't even have a lead screw. (eg: http://www.northerntool.com/shop/tools/ ... _7171_7171)

However, AFAIK, the current crop of "better" machines sold by Grizzly, Busy Bee, Shoptask, etc. all have a reversible lead screw.
I really don't know why I need to make left hand threads anyway. Just an idle curiosity of a passing conversation I had with a friend. Are there other situations were a person might want lead screw to reverse direction of chuck?

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murgie
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Re: Left hand thread cutting?

Post by murgie » Mon Apr 27, 2015 11:12 am

Hello everyone. New here. I used to cut a lot of left-handed threads back in the day. I cut a lot of pipe threads for the oilfield supply houses and we would cut both the collar and the pipe threads as a set. Up to 13 3/8ths 8 round threads, on a 3/4" taper. What they would be used for is to back off a string of pipe resting in a well. To release, all they would have to do is tighten the pipe. All the right threads were already tight so that left only the left thread to loosen up. Once it was de-coupled, it pipe would be removed from the well.

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Re: Left hand thread cutting?

Post by Torch » Mon Apr 27, 2015 11:23 am

LX Kid wrote:I really don't know why I need to make left hand threads anyway. Just an idle curiosity of a passing conversation I had with a friend. Are there other situations were a person might want lead screw to reverse direction of chuck?
If cutting away from the chuck -- for example, to a shoulder on the right -- and using the lead screw for feeding (bigger lathes usually have two separate screws, but the 3-in-1 machines usually only have the one).

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Re: Left hand thread cutting?

Post by Harold_V » Mon Apr 27, 2015 2:59 pm

Torch wrote:If cutting away from the chuck -- for example, to a shoulder on the right -- and using the lead screw for feeding (bigger lathes usually have two separate screws, but the 3-in-1 machines usually only have the one).
That tends to send the wrong message. In my experience (commercial), industrial machines don't have two screws, but a second rod for feed, which does not have threads. In such a case, the rod will be a hex, or splined, even just slotted, and is used exclusively for feeds. With such machines, both the feed rod and the screw are readily reversed by selecting the desired lever position.

In order to preserve the level of precision generated in the screw, the thread is not used for anything but threading. Machines with just a screw often have a linear slot in the screw, which drives the feed mechanism in the carriage. This mechanism drives both the longitudinal feed as well as the cross feed. Lesser machines lack the cross feed drive. Bottom of the heap machines use the half nuts for feed, which is not a good setup, as the half nuts as well as the screw are worn prematurely.

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Re: Left hand thread cutting?

Post by Torch » Mon Apr 27, 2015 5:53 pm

Well, even the better 3-in-1 machines fall into the latter class: a single slotted lead screw is the only provision for feed, be it threading, carriage feed or cross-slide feed. Carriage feed is via the half-nut on the threads, cross-slide feed is the hand crank assembly driven via a keyed gear in the apron that engages the slot.

I didn't realize that the second, feed, rod was not threaded (which shows just how limited my experience with commercial-class machines really is). How is the carriage feed accomplished, then? Is the rod just driving the hand crank so it's pulled along the rack?

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