Thread cutting. What is your opinion on this method.

In addition to my earlier post and setting the compound at 90° to the work would actually eliminate the scallop/step ladder cut effect on one side of the "V" because the tool would now be cutting straight in and would track the thread spiral at its root exactly on the same line every time instead of shifting its track ever so slightly with an angled compound during feed change.
However, the disadvantage in such a 90° plunge in feed is that both cutting sides of the tool and its tip would be simultaneously under stress and more power would be needed. That may not be a big deal when threading soft metal like brass or aluminum.
Of course, the cutting tool would have to be ground with a geometry that could allow it to cut simultaneously on both sides during its feed.
Isn't that how ACME threads are actually cut?
 
You can cut Acme threads that way, or you can use the same idea with them, but the angle is not 30° or 29.5° or whatever. The included angle of Acme and Stub Acme is 29°, so if you want to use the compound method, split that instead of the usual 60° for UN or 55° for Whitworth.

Or you can feed straight in. Generally there is a broader nose, and may tend to chatter on smaller, less rigid machines.
 
If your cutting using only the cross slide does this afect the shape of the root of the thread?

i've been using a single tool for a number of metric sizes and thought the angled compound made the shape of the leading edge but also helped give the (nearly) flat bit between threads.

Almost forgot my question , basicaly do you need a different ground tool for each thread size with a corect whidth end(square bit at the point)when using only the cross slide ?

Apologies for badly worded question ,

Stuart
 
From what I can see the shape of the thread root is predetermined by the way the tip of the cutting bit is formed.
At every step of the feeding/cutting process the tip of the cutting bit is always cutting a shape that conforms to its grounded profile.

The root shape/ bit tip roundness or flatness would become more pronounced or clearly seen specially when the number of threads per inch gets fewer, so it would follow that you have to use a different bit when cutting larger threads. For smaller threads (more threads per inch) a relatively more pointed bit would be appropriate.
 
Threading by feeding straight in using the compound results in more contact area between the tool and work. By feeding down the thread flank the contact area is halved, reducing the force on the tool by half - this is easier on the machine and also reduces the tendency to incite chatter. Setting the compound over to 29 or 30 degrees is one way to feed down the thread flank but Cleeve's suggestion of leaving the compound parallel to the bed and advancing the compound half the amount the cross slide is advanced will also feed down the thread flank. This is convenient because thread depth can be read directly from the cross slide dial. ( More details on this method here: http://www.gadgetbuilder.com/ThreadingTools.html#Threading )

And, as noted above, if the threading tool is ground for a finer thread than the one being cut, when depth is reached then the width of the bottom flat may be increased by moving the compound while leaving the cross slide at depth (not as easily done with the compound is at 30 degrees). When this method is used then the suggestion in the OP of moving the tool with the compound to clean up each thread flank makes sense.

John
 
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I went back and read your original post. Compounds are marked differently. On some 0 degrees is perpindicular to the workpiece onn others its parrallel. If his was parrallel I could see where it would give smooth cuts, only cutting one side of the thread at the time. But doing that raises 2 questions for me. Is the pitch diameter becomming too wide and if not is the root deep enough? I have cut acmes b4 using a no 5 acme insert to cut a no4 acme using this same technique. I have also cut double and triple lead threads the same way. Just using the compound to advance to the lead of the next thread. I have also advanced on the compound and the cross slide simultaniously to cut on the leading edge of the tool, until i get close. Acme's and stub acme's are a bit more complicated, especially when you get into multiple leads. Thats why I use Thread Measuring Wires so much.
 
Not to get caught up in the 'compound/cross slide controversy', but one thing to remember: the trailing edge of the thread is the surface that will bear the force of the torque. If that finish is rough or uneven, the strength of the thread may be compromised. My Southbend has so much slop that I don't think it matters how I feed the tool for the rough cuts, but I always sneak up on the final dimension and take at least one spring pass. And I keep a thread file handy.
 
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