Very basic threading question

[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]

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.

[Dr Stan]

go to: http://www.instructables.com/files/o...K7EP282OTX.pdf and download a copy of the Navy's Machinery Repairman manual. Starting on pg 6-46 you'll get more info on single point threading than you ever thought was available.

Stan

formerly MR2 Lightner