Very basic threading question

[Mike Amick]

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.

[Mike Amick]

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.

[PixMan]

It pays to read the instructions.

This may be true, but ONLY read instructions after you have tried it without first reading instructions. Extra points given if you screw things up first.

This is basic, standard operation procedure for manly men. Any man who reads instructions first may get his man card taken.

[Paul Alciatore]

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?

I do not know if it is what was referred to, but another method of achieving much the same result as the 29 degree method is as follows:

1. Set the compound to 90 degrees instead of 29. This makes the movement parallel to the long axis of the lathe. Take up any backlash in the compound feed.

2. For each pass, you must calculate a combination of infeed on the carriage and a smaller feed on the compound that will position the tool in the same place that a 29 degree feed would have accomplished. MATH is required.

The feed on the compound for this method is given by:

CompoundFeed = InFeed x tan(29deg)

The tangent of 29 degrees is 0.554 so you simply multiply the infeed by that amount. I suspect that many machinists simply use 0.5 or 1/2 the infeed amount as this is a lot easier and can be done in the head. 0.5 is the tangent of 26.5 degrees so this approximation would be the equivalent of using an angle of 26.5 degrees on the compound. The amount of shaving on the right hand face of the thread will be a bit more, but not enough to make a lot of difference.

In simple terms, you go in say 10/1000 and left half of that amount or 5/1000.

One advantage of this method is that if you know the depth of the thread, your infeed is simply that amount. With the 29 degree method, you must use math to find that infeed amount. However, if you start with only the value of the pitch of the thread, then the amount of infeed in both methods may take an equal amount of calculation.

I, personally use the 29 degree method.

[Paul Alciatore]

Oh yea! Well, you can't have my man card without a fight.

I have worked on many things where a screw up could easily cost tens of thousands of dollars. I believe in caution.

This may be true, but ONLY read instructions after you have tried it without first reading instructions. Extra points given if you screw things up first.

This is basic, standard operation procedure for manly men. Any man who reads instructions first may get his man card taken.

[David Powell]

Simple, easy and I do not forget where I am!

Most threading I do is relatively small , ie up to about 1/2" by 13 threads. While I am well aware of how to set up at 29/30 degrees I find it takes time, and If I get get interrupted I can have a bit of a puzzle to get going again( memory aint what it was!). I usually leave the top slide at 90 degrees , set the collar to zero but feed one thous forward with it for every two or three I feed inwards, If I get a " raggedy" edge I just go a couple forward without going inwards. I get reasonable results quickly. This scheme does NOT work well for threads with more than one start, unless you make sure you advance the same on the top slide ON ALL STARTS. Incidentally, I was taught this method by a fellow who had been a machinist for Napier, the Aero engine builders. Regards David Powell.

[Arthur.Marks]

1. Set the compound to 90 degrees instead of 29. This makes the movement parallel to the long axis of the lathe.

I have been reading Machine Shop Practice by Karl Moltrecht. He discusses thread cutting in Vol.1 and uses this method. One of the better tricks he discusses in relation to this method is cleaning up the thread flank. When he is getting very close to the final thread depth, he feeds in only using the cross-feed. The tool, then, only cuts on the back side of the thread form which is usually the most ragged from being cut in so many steps. He goes in until the tool just cuts both the leading and trailing edge of the thread. At this point, both have been skimmed clean, and you can finish to final depth returning to a 29〫in-feed. Moltrecht claims this produces a very clean cut thread form.

In fact, I really like the chapter Moltrecht gives to screw-cutting. It is not the same rote instructions. For example, he recommends using the screw-cutting gauge to confirm each side of the cutting tool rather than simply pushing in the tool until it centers on both sides of the "V" notch. That makes sense---if your angle is off, the tool will still center. Checking each side independently assures you are accurately at 30〫for each. While such accuracy may not always be needed, it is a sound practice none the less. There are a few really good points in the chapter which are unique.