Threading in reverse - upside down tool or tool at the rear?

[Bi11Hudson]

And remember, unless it's a special part, righty is tighty and lefty is loosy. All that upside down backside thread toward the tailstock with a 180-degree insert mumbo jumbo is gonna send somebody's part to the scrap bin if you're not watchful.

A good point. I too think you are "over-thinking" the issue. Just remember that you are still cutting right hand threads regardless of which way the machine is running. The only real issue is that the cutting point be square to the surface to be threaded. And is centered. Any other orientation is meaningless to you. Whatever works. . .

Left hand threads is a different issue and can be solved ONLY by reversing the relationship between the spindle and the feed screw.


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[Wheat.Millington]

Not sure what part of my post makes you think I'm confusing left and right handed threads. The only way you can make that error is by flipping the leadscrew direction, and I'm not sure why you'd do that.

 

[pontiac428]

Well, you are south of the equator, so there's the inverse Coriolis effect. Real quick: spindle forward, spindle reverse, infeed cut, outfeed cut, front side, back side, interior, exterior... that's 16 different viable setups, half of which are backward. That is all.

 

[mickri]

Like anything there are different ways to do things. Decide which way you want to thread and then practice until you have your preferred method down pat. I taught myself how to thread the conventional way towards the headstock. I practiced on pvc pipe until the motor skills were 2nd nature. A while back I tried a different method of internal threading from what I was used to. Things quickly got screwed up. I went back to my usual method. Was able to pick up the messed up thread and saved the part.

The most important thing is not how you do it but rather that you get consistent results from your chosen method.

 

[mmcmdl]

Most slant cnc lathes can't thread the " conventional" way a manual can , so you're always threading on the rear/upper of the part . This becomes simple after a while , just have to get used to it . The slants are limited to as to how far they can go past the x centerline is the reason why . Too far and the machine will throw an overtravel fault .

 

[timmeh]

Standard setup, front toolpost, tools mounted "right" way.
Right hand thread, external or internal, feed towards the chuck.
Left hand thread, external or internal, feed away from the chuck.
If however, the situation calls for feeding to a hard shoulder/face/stop, and you have no way to stop the feed fast enough, mount tool upside down and feed in the opposite direction.

 

[vtcnc]

If however, the situation calls for feeding to a hard shoulder/face/stop, and you have no way to stop the feed fast enough, mount tool upside down and feed in the opposite direction.

Joe Pie does a good video on threading away from the chuck. And he emphasizes in the video that threading to hardstops is the primary justification for setting up your single point threading in this manner.

His single point threading playlist on YT:

https://www.youtube.com/playlist?list=PL4wikbEbcE3Lkebdj7roUYG4RAr7MDhTj

 

[RJSakowski]

I believe the OP understands the consequences of flipping the sense of the lead screw. To cut a right hand thread, there are two options, either normal (ccw) spindle rotation and infeed or reverse (cw) spindle rotation and outfeed. In order to cut a r.h. thread with outfeed, either the tool must be flipped or the cut must be done from the back.

Cutting with outfeed has issues. My normal insert threading tool won't work flipped, at least for threading to a shoulder as I have almost a 1/2" of tool holder to the left of the cutting edge. I could use my internal threading tool but I would have to run the bar parallel to the spindle. An alternative would be to either make or purchase an tool holder designed to be used for cutting threads on outfeed or to grind a HSS tool.

Another issue is backlash in the gear train. If the lathe is worn or the gears are loosely meshed. This can amount to a fair degree of lost motion. In infeed threading, I would normally back the the cutting tool out by perhaps ten threads to allow the gear train to settle in. Threading with outfeed from a shoulder will require the backlash to be removed within the width of the gullet, presumably a thread or two at most.

All that said, the original question as to whether it is better to use an inverted tool from the front or a conventional tool from the rear would depend upon the individual setup. A restriction on cross slide travel is definitely a no go for rear cutting. The tool mounting method will also impact the choice. In my case, it would require moving the entire compound assembly to the rear position. If the machining first required turning to a diameter and then threading and multiple pieces were involved, this would get old very quickly. OTOH inverting the tool requires seating my QCTP holder a half an inch higher on the tool post and rather than having a solid floor on the tool holder to resist the cutting forces, I would be depending on the clamping screw for that.

Since I can thread conventionally to a shoulder by slowing the spindle to a crawl, I am not inclined to ever use outfeed. At 5 rpm, it takes 20 seconds to cut a single thread My reaction time is still less than a second so even cutting an 8 tpi thread, I can still stop at a shoulder within +/-.01". I also have a mechanism on the drawing board to disengage the half nuts to +/- a few thousandths.