Very basic threading question

[mikeamick]

I was looking through an old book some call the bible ... the old southbend
book " how to run a lathe "

Anyways .. I was reading the area about thread cutting and have to
admit that I don't do it this way .. as far as tool setup goes.

a picture is worth a bunch of words so .. here is the diagrams in question



I never knew about the 29 degree for the compond part .. and the
associated angle of attack for the cutter.

I always just set it at 90 degree and went straight in ...

My threads have never been very good ... is this why ??
Does everybody do it the way in the book ?

I never ever even feed with the compound .. I always feed with the
cross slide wheel.

Mike A Mick

[Juergenwt]

Now you know why you are getting bad threads. 29 -29 1/2 deg. is best and feed in with the compound keeping the cross feed on "Zero". The book is correct.

[justanengineer]

Yup, I do it in a similar manner.

There is another common method of thread cutting which I do not prefer, which I believe is common in europe, so hopefully one of the members across the pond can explain it.

[Void]

Some people set their compound at 29, some 29.5 and some at 30. I usually set mine at 29.5. And some people go straight in. I go straight in sometimes on the last pass.

The reason for setting it at just under 30 is so the right hand side (tailstock side) of the threading tool tip does little or no cutting. The majority of metal removal is done on the left flank (headstock side) of the triangular tip.

It is pretty clear from the diagram (fig 223) Note that the cut (depth of chip) is to the left of the tool tip.

Your method of going straight in is cutting on both sides of the tool tip. The chip that forms is trying to squeeze in to a space that is too small for it.

There are many factors that can lead to poor finish on the threads. Some materials are bafflingly difficult to thread cleanly. You should chuck up some mild steel or aluminum, using a dead sharp HSS cutting tool set as exactly at center height as you can make it and practice the technique in that book.

Get some thread measuring wires if you don't yet have them so you can learn to make threads to exact class of fit.

I find threading on the lathe to be pretty enjoyable. It is one of the things the "modern" (since 19th century) that a lathe is the ideal tool to use.

-DU-

[Dr Stan]

Me 2, or 3 or whatever on feeding in on the slant depth. There are at least a couple of advantages advantages including 1) reducing tool pressure and 2) increasing accuracy (.001" on the slant depth = approx .00086" on the straight depth).

I do however tend to use the straight depth when threading 32TPI or finer as I've found for myself that cutting on the slant depth of very fine threads does not give me any advantage.

As others have pointed out setting the tool dead on center is critical for a good finish. Other key issues are grinding the tool to as perfect of a 60 degree included angle and proper alignment of the tool with a center gage. BTW, placing a piece of white paper under the tool helps to reflect light thus making it easier to get good alignment with the center gage.

If you ever cut Acme threads set your compound on 14 1/2 degrees and use an Acme thread gage. Cutting Acme threads on the straight depth will give you real fits.

[firbikrhd1]

I too use 29 degrees when I cut threads. The advantage compared to going straight in is that on one side to the 60 degree tool point the thread is shaved with a very light cut while the opposite side takes the brunt of the cut, resulting in a better finish. Exactly as stated in the SBL book.

[mikeamick]

wow .. can't wait till tomorrow to try the proper technique out ...

thanks

[Paul Alciatore]

It pays to read the instructions.

[Jaakko Fagerlund]

There is another common method of thread cutting which I do not prefer, which I believe is common in europe, so hopefully one of the members across the pond can explain it.

Care to share what that other method is? Using the proper size threading die/tap? Taking a roughing pass at the minor diameter with the proper pitch as a feed rate?

[EVguru]

Feeding straight in is not 'WRONG' but sometimes you won't get the required finish, so flank infeed is used (less than or equlat to half the thread angle). In terms of tool life, flank infeed is 'WRONG' becuase you're only fully using one cutting edge. CNC threading may use alternative flank infeed to get around both these issues.

To do flank infeed, you can just feed the compound a proportion of the infeed each time and leave the angle at 0/90 degrees (depending on how your scale is arranged). You could calculate the amount of compound feed per cross-slide feed to give you the same tool tip position as setting the compound angle to 29-30 degrees, but 50% works well enough.

I have my toolpost set up square to the spindle axis, so I prefer not to disturb it by messing with the compound angle. The other advantage is being able to read the infeed straight off the cross-slide. Particularly easy if you have a DRO.