How to thread stainless

[FTlatheworks]

Your compound angle is off. Asian lathes don't have consistent markings. If your index says 29.5, try setting it to 61.5 and try again.

I did it at zero this time.

 

[FTlatheworks]

I did it at zero this time.

I’ll try that next. I chased it with a die and it helped, but I don’t have the money for a whole die set to chase every thread I try to cut.

500 RPM is what I use. Turning the tool upside down, running the spindle in reverse thus feeding away from the chuck will save your nerves when threading at higher RPMs. Also turning a relief near the shoulder will be very helpful.

Your part looks good. Nice job!

 

[FTlatheworks]

I did it at zero this time.

What makes you say that Mikey? Cause of a previous post? Or because you can tell by looking at the threads that I wasn’t setup properly?

 

[brino]

@FTlatheworks

What I see is uneven steps to your threads.
Ideally the angle on the left side is the same as the angle on the right side.

Here's a zoom of what you posted:


The angle to the left of the thread crest look good, but the angle to the right of the thread looks almost square to the bolt axis.
It looks like the thread I exaggerated in sketch in red in that picture.
We want something more like the green one.

When the compound angle is off your threads can look like that.
The problem is that some lathes call zero degrees as the compound travel being perpendicular to the axis of the work (same as the cross feed).
Other lathes call the compound zero degree setting as being parallel to the axis of the work (same as the longitudinal feed).

So what is 30 degrees on one lathe will be 60 degrees on the other lathe!

It is difficult to tell for sure if that's the problem you're seeing.
Usually when that is the problem, the left flank of the thread has multiple little steps in it and yours is one clean face.

It might just be the top of your threads are rolled over so much that it obscures the right flank.

Hopefully that makes sense.
Stick with us, we will help you get this sorted out!

-brino

 

[mikey]

What makes you say that Mikey? Cause of a previous post? Or because you can tell by looking at the threads that I wasn’t setup properly?

As Brino is pointing out, your thread form is distorted. If you look at the threads, your tool is cutting on the right side of the tool and leaving a torn edge on the left side of the thread. This suggests that the tool is not oriented properly. If, as you say, the compound was at zero and you fed straight in, then the tool itself is oriented to the right so that it is cutting on the right flank of the tool.

It is critical that the tool be ground properly with an accurate 60 degree included angle at the tip. The center of this 60 degree angle MUST be perpendicular to the work piece. The tip of the tool must also be on the spindle center height.

With the tool oriented correctly as above, if you feed straight in then both flanks of the tool will cut equally. If you angle the compound to 29.5 degrees then the left flank of the tool will do most of the cutting and the right flank will simply produce a clean up cut.

 

[savarin]

Did you use the fish gauge to set the 60' tool bit square with the work
I cant remember if the spring pass question was answered but thats just a couple of passes without advancing the tool to ensure the tool or work didnt deflect any and thus not cut to full depth.
Something I have done in the past with a crappy cut thread was to spin it reasonably fast and apply a wire wheel spinning the opposite direction for a second or two then reverse both thread and tool directions and do it again to sort of polish the thread flanks and tips.

 

[ttabbal]

The 'Fish Tail' 60 Degree Thread Gauge

I'll start with a simple tool. The 60 degree thread set up tool thats referred to as the fish tail. I've used the tool to set up my lathe bit to cut threads. I have successfully cut threads on the lathe & have compared my cut threads to some " professional" cut threads & could not see any...

www.hobby-machinist.com

I haven't seen anyone post a pic of the tool against the work. That thread shows the other use of the fishtail gauge, aligning the tool and the work. Regardless of how you advance the tool, compound or cross feed, you need to have the tool hitting the work properly.

You may have done it, but just covering the basics to be sure.

 

[NC Rick]

Ya it took a long time. I used a hss 3/8 blank. Now I know if I invest in fact tooling it will only get better. I calculate the sfm and came up with about 500 rpm. Can anyone confirm that’s correct?

on your project, do you have a tailstock die holder? I hear that’s a guaranteed way to do it.

I do have and used a tailstock die holder. I also turned the diameter down to less than the 75% size (i think i went to 0.295”). And while it cut clean, i had to turn the chuck by hand (scared to try power feed…) and break the chip every 1/4 turn where it would say “snap!”. I was attempting to make turn buckles and have left hand tap and die for 5/16”. My lathe is setup for metric threads. The project sounded easy. It felt hard so i did what i do and quit

 

[macardoso]

Great start. You are almost there.

It kind of looks like your thread is not formed correctly and maybe are cutting too much on the back side.. The top of the threads seem to be folded over away from the chuck. The bottom of the threads look to be well formed. It's hard to tell but the front and back side angle does not look symmetrical. Also looks like it is cut too deep. What might help you is to take a few passes with a file before you take the final couple of threading passes, that should eliminate the folded over sharp tops and give you a more visually pleasing thread form. It almost looks like your cutting edge is not exactly perpendicular to the work.

A 28 degree approach is maybe a little much, 29 or 30 degrees might be better. With a properly ground 60 degree tool, you can plunge straight in with the approach perpendicular to the work, but this does not always work well. If you swing your compound around to parallel to the work and step over 1 or 2 thou on each pass then feed in with the cross slide it works well also. A spring pass is a final pass without going deeper on the compound. It just cleans up the fuzz on the threads.

On a 1/2-20 thread, I turn the major diameter to 0.495 and thread to the proper minor diameter. This gives a better thread form with a bit of a flat on the thread peaks.

Here is an example of a 1/2-20 thread in 303 SS I did a few days ago. I'm not sure exactly where the ''chatter'' marks are coming from, but it may actually be chatter that I can't hear. Or it could be just the way the chip forms on 303 SS. 304 SS seems to be smoother on the thread faces, but we switched to 303 to save on tooling costs.

View attachment 365959

I have struggled with this on my lathe since the beginning. Added a bronze thrust washer to the leadscrew to take up end play and it made a significant difference.

 

[macardoso]

I would have concern that the tool was rotated with the compound and is no longer square to the work.

If you thread using the compound to advance the cut (I do not) then the tool should remain square to the work and be fed into the cut at the compound angle.