Why Do My Threads Look So Rough?

[Jon In Tucson]

First, let me say that I am a novice when it comes to machining and thread cutting, unless it has to do with using taps and dies. I have been using some aluminum scraps from a friends work to practice on. The pitch and diameter of my practice pieces has been satisfactory, although the finish not so much. Here is what I'm doing:
Spindle speed: 230 RPM, HSS threading tool, Marvel Mystery Oil.

I tried the threads out and that is the reason for the smooth first few threads. Thanks for the advice. God bless.
Jon In Tucson

 

[eeler1]

Might be the material, some stuff is kinda gummy. Or maybe you're just not done cleaning them up. Some folks run a triangle file thru the threads after to clean them up, or a flat file to break the burrs off of the v. I often chase threads with a die after to get them nice looking. Are you feeding in with the compound?

 

[Franko]

I'm no expert on cutting threads on a lathe, but it looks like the tool holder may have slipped. The threads look good on the first few turns, but the buggered up threads are much too thin on the high part of the last 3/4ths of the cut.

I've had similar results when the bit is too high. It will hit a snag and torque down if the dovetail ways are a little loose, causing it to dig in too deep on the last pass. That can happen right in the middle of a pass.

230 rpm is a little fast for threading. Many people just turn the chuck by hand when single pointing threads.

 

[planeflyer21]

Hey Jon...I'm in Tucson too if you want me to come by and help you out some.

Send me a PM and we can set something up.

 

[pdentrem]

I have seen this on steel as well. Most Aluminium alloys threads like the stuff out of a cow. Have to have a really sharp tool, not the point, just over all and a LOT lube is important. Get the specific cutting lube if Marvel is not the right one. Even WD40 will work in this case. Have the tool set right on center and do not take too deep a cut per pass.
Pierre

 

[Quai_Oui]

Jon,

When you cut the thread you should make a series of cuts going deeper, with each pass. I work on the basis for threads under about one inch of the first pass being say 1/3 total depth, then reducing this quickly as I go deeper. say 1/6th and halving each pass. As you set the cross slide in, then also advance (in the direction of tool travel) the compound slide about 1/2 the amount of the cross slide. This means the tool is then cutting the front face only. As you come near the correct depth make those advances on the cross slide small, and slightly less than 1/2 on the compound slide advance. If you get this correct, then the back edge of the tool will "chase" the back side of the tread on the last couple of passes. Near the correct depth check the nut each pass, don't remove to much and make the nut lose on the thread. Run a triangular file (with handled) to put a slight radius on the peaks. You can also use a thread file, but the first will give an excellent outcome with some attention to detail.

Also look at which cutting oil you use. I would recommend something like Rocol RTM Metal cutting fluid. Apply with a brush to the work before each pass. Providing you have you tool angles correct, this should give a good outcome regardless of the metal being machined.

Good luck, let me know if I can be of any other assistance

 

[Jon In Tucson]

Thanks for all the input! I believe that everyone's critique holds merit.

Eeler1: the material is aluminium. What type or alloy I do not know as it is from my friends scrap pile at his shop.

Franko: It is entirely possible that my tool is too high. I eyeballed it but we all know that isn't the best way to do it.

Planeflyer21: Thanks for the offer, see your message box.

Pierre: I am also a novice at grinding tools as well as lathe operation. It is very possible that I have a 60* angle ground on my tool but not sharp enough to cut cleanly.

Quai_Oui: The lube is more important than I thought. I shall look for a more suitable cutting oil than just what I have on hand. As to the depths of the cuts, I made a series of cuts the same depth. In the future, I shall make increasingly shallower cuts.

This is why this is the best forum on the 'net for those wanting to learn how to operate lathes and shop machines. Good advice offered in a friendly and non judgmental way. Thanks guys! God Bless.
Jon In Tucson

 

[pdentrem]

Do you have a Lathe Threading 60 degree "fish tail" gauge? Next a lathe center height gauge? Look up the MrPete YouTube videos, they are an excellent resource. Do not use hot rolled steel like rebar. It is no good to practice on either. The threads will look just like the one in your photo. Use some mild steel, leaded steel or plastic pipe (like the grey conduit 1" stuff) it is cheap and cuts just fine.
Pierre

 

[brino]

Hi Jon,

Forgive me if it is obvious, but you did not reply to this part of eeler1's advice.....

When you advance the tool into the work, are you doing it straight in with the cross feed, or with the compound feed set at 29 degrees?

With the first method the tool is advanced at a right angle to the work and cuts on both faces...it can chatter, especially on a light-duty machine.
With the second method it cuts only on the left face(towards the headstock)

That fish-tail gauge can be used not only to check that you sharpened the cutter to 60 degrees, but also that you have set it at the proper angle to the work.

-brino

 

[markknx]

+1 brino's post the threads look like they were forced. could well be a dull bit. Also That fish tail gauge can be used to set the tool bit at center.
Mark