Single point threading

[Bob Korves]

The 29.5 deg setting is recommended to allow for some potential error in the protractor on the lathe, since it is OK to be under 30 deg but never over. I think that it is a good idea to check the accuracy of your protractor for your own edification.

Also, if your protractor is set up like the one shown in the link that darkzero provided, I recommend that you establish a second index mark on the side of the cross slide. Set your compound at exactly 90 deg to the spindle axis and then scribe a mark on the cross slide opposite the 0 on the lathe's protractor. That way you will have a better time using the protractor for threading.

On most lathes owned by mere mortal hobby machinists, the compound degree scale cannot be expected to be accurate to within 1/2 degree. Either check that angle more accurately using a different method or simply choose a smaller number than the 29.5 degrees council of perfection, like 28 degrees or even less. Make sure it is less than 30 degrees in reality. Even zero degrees works...

 

[ewkearns]

There is a LOT of tearing (rather than clean cutting) in creating these threads. Make sure of your cutter height setting and tool geometry. Consider your cutting oil. Feeding in with the compound is recommended only for threading hot rolled steel and then using a cutter specifically ground for the task. Feeding in with the compound is specifically NOT recommended for carbide threading tools....

 

[darkzero]

Feeding in with the compound is recommended only for threading hot rolled steel and then using a cutter specifically ground for the task...

Feeding in with the compound is specifically NOT recommended for carbide threading tools....

That's interesting, I never heard that before. I don't agree but could you elaborate, I'm curious as to why?

 

[ewkearns]

When I was learning machine shop, I was taught, in part, by a bunch of guys that were making chips as early as just before the turn of the century. In their day, the material of choice was hot rolled steel and it WAS STRINGY. The technique was developed to grind the threading tool such that it cut on the leading edge only and sent the chip out of and away from the generated thread.... hence the use of the compound rest and the sainted and inviolable 29.5° setting. They were highly amused that the technique was applied to everything, rather than the task that it was specifically adapted to serve... They simply never used the compound to thread anything that would generate a non-destructive chip. (And, today, neither do I.)

When carbide tools came into wide acceptance, we found that the tips of those tools would fail using the compound rest. The carbide was simply not ground to accept a side load. So, abandoning the notion of the sanctity of the compound rest, threading was accomplished using the crossfeed, only, and the problem went away.

I realize that my position is anathema on this matter, but I was taught as much WHY as HOW and it has served me well. I believe that threading using the compound is so sacred, today, simply because it got a LOT more "press coverage."

My 2¢

 

[Tozguy]

When I was learning machine shop, I was taught, in part, by a bunch of guys that were making chips as early as just before the turn of the century.
When carbide tools came into wide acceptance, we found that the tips of those tools would fail using the compound rest. The carbide was simply not ground to accept a side load. So, abandoning the notion of the sanctity of the compound rest, threading was accomplished using the crossfeed, only, and the problem went away.


My 2¢

It seems to me that nowadays there is a carbide insert for just about any job imaginable.

 

[Bob Korves]

When I was learning machine shop, I was taught, in part, by a bunch of guys that were making chips as early as just before the turn of the century. In their day, the material of choice was hot rolled steel and it WAS STRINGY. The technique was developed to grind the threading tool such that it cut on the leading edge only and sent the chip out of and away from the generated thread.... hence the use of the compound rest and the sainted and inviolable 29.5° setting. They were highly amused that the technique was applied to everything, rather than the task that it was specifically adapted to serve... They simply never used the compound to thread anything that would generate a non-destructive chip. (And, today, neither do I.)

When carbide tools came into wide acceptance, we found that the tips of those tools would fail using the compound rest. The carbide was simply not ground to accept a side load. So, abandoning the notion of the sanctity of the compound rest, threading was accomplished using the crossfeed, only, and the problem went away.

I realize that my position is anathema on this matter, but I was taught as much WHY as HOW and it has served me well. I believe that threading using the compound is so sacred, today, simply because it got a LOT more "press coverage."

My 2¢

Another reason I prefer the cross slide method of threading most of the time is that the depth of cut dialed in is the depth of cut achieved, no calculating or trig involved, just read the dial and check the thread when you get close.

 

[Dave Paine]

I know how I made my first mistake when trying to use the compound set to the 29.5 deg angle. I read the scale on the compound and overlooked the axis which was the reference for the angle. I expect I am not the only person falling for this mistake. My Grizzly lathe made it easier to fall into this trap.

In this picture I have moved the compound to have the scribe mark at the 30 deg mark. On my G9249 lathe, the only number shown is 0 (zero). The marks go up to 55 deg either side of 0.



After making some bad threads I also changed to using the carriage. I later realised my mistake was setting the angle to the wrong 30 deg, as in 30 deg from parallel to the lathe bed. It needed to be 30 deg from perpendicular to the lathe bed, or the face of the chuck. Easy mistake for a woodworker just starting out on a metal lathe.

I have a decent mitre gauge for the table saw so I cut a block of wood to 29.5 deg.


I glued a small magnet into the wood so it holds to the side of the compound.



I do not understand why Grizzly did not have the scale going to 60 deg. My lathe needs an angle block in order to set angles less than 35 deg (or 55 deg) depending on how the angle is defined.



Interesting post earlier discussing the potential origin of 29.5 deg angle. It is easier to use the carriage.

 

[Bob Korves]

(snip)After making some bad threads I also changed to using the carriage. I later realised my mistake was setting the angle to the wrong 30 deg, as in 30 deg from parallel to the lathe bed. It needed to be 30 deg from perpendicular to the lathe bed, or the face of the chuck. Easy mistake for a woodworker just starting out on a metal lathe.(snip)

Bingo! Thanks, Dave. You and MANY others have made the same mistake, and a very easy to make mistake, I might add. Thanks for putting up photos to help explain the problem and the solution. Everyone, please do not trust those compound scale quadrants on lathes to be accurate or to show the quadrants in the orientation that you expect. Stand back, get the big picture, and make sure that you are seeing something that looks like the angle of the block of wood Dave has posted. If you are too lazy to measure the actual angle of the compound accurately, then set the angle to somewhat less than 29.5 degrees, to make damn sure that you are not at more than 30 degrees in reality. 25 degrees will give you no visible difference in your threads, in fact they will probably look better, and even zero degrees will work just fine...

 

[Tozguy]

Dave, the Grizzly configuration is typical and my guess is that it's handier for setting to cut short tapers by feeding with the compound. Good idea to have made a gauge for setting the compound to 29.5 for threading.

My compound is usually accurately set at 29,5 deg. and left that way for any kind of turning unless the job requires otherwise. For most of my threading jobs I start by advancing with the cross slide and finish off by advancing the compound. I like being able to sneak up on final thread fit with the finer adjustments resulting from having the compound at that angle..

For really coarse threads like 8 or 10 tpi, cutting both flanks of the almost finished thread can get heavy in some materials. Personally I prefer to avoid such a wide cut so will likely do much of the coarse thread by advancing the compound.

 

[willthedancer]

When I was learning machine shop, I was taught, in part, by a bunch of guys that were making chips as early as just before the turn of the century. In their day, the material of choice was hot rolled steel and it WAS STRINGY. The technique was developed to grind the threading tool such that it cut on the leading edge only and sent the chip out of and away from the generated thread.... hence the use of the compound rest and the sainted and inviolable 29.5° setting. They were highly amused that the technique was applied to everything, rather than the task that it was specifically adapted to serve... They simply never used the compound to thread anything that would generate a non-destructive chip. (And, today, neither do I.)

When carbide tools came into wide acceptance, we found that the tips of those tools would fail using the compound rest. The carbide was simply not ground to accept a side load. So, abandoning the notion of the sanctity of the compound rest, threading was accomplished using the crossfeed, only, and the problem went away.

I realize that my position is anathema on this matter, but I was taught as much WHY as HOW and it has served me well. I believe that threading using the compound is so sacred, today, simply because it got a LOT more "press coverage."

My 2¢

I respectfully disagree here.

The issue with threading is certainly chip flow across the top of the tool.

When you plunge thread with the cross slide, there are conflicting vectors on each flank of the tool, resulting in a complex collision at the center line of the tool. This leads to stacking problems and resultant tearing for one and a difficult to predict curling of the chip. Swarf is subject to cold deformation and intense work hardening, so it will damage the workpiece on contact.

Using the compound allows the chip to flow freely across the tool. The little bit of back flank interference is just to keep the form true, and the tiny bit of swarf generated is easily swept along with the majority.

That said, the use of coated carbide makes for slick surface for the chip to flow across, and much higher surface speeds, allowing for a high finish that is more scratch and tear resistant when in contact with the curled chip. It's a cheat that buys some grace when plunge threading, and often good enough.

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