Thread cutting dilema?

[brino]

i'm back!! set the compound at 61 degrees as per TechnicalTeds instructions. PERFECT threads.

I believe that's the same thing multiple people have said in this thread.....however it is a difficult thing to explain with words.
I am glad that @Technical Ted 's explanation made sense to you.

Did you notice this post by @Dave Paine: https://www.hobby-machinist.com/threads/threading-help.71189/post-596785
(it was in the thread that he pointed you to above.)
He shows a no-nonsense way to ensure you have the cross-slide set properly; independent of the angle markings on the lathe.

a little confused as to why everything that I have read states to use 29.5 degrees.

It depends how your lathe is marked; which end is 0 degrees and which end is 90 degrees. (90-29.5 =60.5)

The bottom line is that we got it figured out.

-brino

 

[Technical Ted]

Yep, I wasn't the first or only one. I got a sense that the point wasn't hitting home. Sometimes we need to hear things a few different ways before it clicks and the light bulb goes off. Pictures and drawings help too especially with those that are more of the visual leaning type.

Sometimes it is hard to explain things from a keyboard on these forums and at times I think I go overboard when trying to make a point and get it across the way it is intended. But, then again, sometimes that's what it takes.

Agreed, bottom line he got it working and learned! This forum is a team effort!

Ted

 

[MarkM]

It s also interesting that in north america we use the 29.5 or so setting and in asia and many other areas around the world they just plunge straight in with the compound 90 degrees to the work.

 

[brino]

It s also interesting that in north america we use the 29.5 or so setting and in asia and many other areas around the world they just plunge straight in with the compound 90 degrees to the work.

I believe that is more a function of how stiff the lathe is.
I have found that when plunging straight in with the cross-slide (and cutting on both faces of the tool) any slack in the dovetails causes chatter presumably due to higher cutting forces.
I use the compound set at 30 degrees to cut on only one tool face, it seems to chatter less, on my old lathe with some wear.

Some folks get rid of the compound to get rid of one more set of dovetails and provide a stiffer tool, then they have to plunge straight in.

-brino

 

[savarin]

I plunge straight in usually but I did use the 29.5' when I cut the 39 x 4mm thread for the lathe back plate test bar thread

 

[Boswell]

I use the 90-deg straight in plunge approach. Found it much less likely that I would make a error and screw it all up and also have not had any problems with that approach. However I don;t do that much single point thread cutting. If your cutting inside threads it may be the only way you can do it due to clearances.

 

[C-Bag]

I just wanted to thank the OP and everybody for this thread. Once again I was able to find exactly what I was going through and the fix. I had successfully avoided single point threading for all this time and when I finally got backed in a corner and HAD to do it I was getting the same results. Who knows when I'll next have to single point something so that was another reason I wanted to reply so I could look through the history and find it again

 

[Janderso]

On my old South Bend, (new screw and nut on the compound and cross slide) I feed at 29.5 with the compound. But, I find when I take out the play moving into the piece, I can see the load push the compound back to the opposite side of the free play in the screw. Am I explaining it right?
I thought you guys were going to suggest HSS cutting tools to improve finish. Threading slow with carbide is tricky no?

 

[C-Bag]

Threading slow with carbide is tricky no?

Is this true? My 9x20 cannot go slow enough for the super short threads I was cutting so I made a hand crank and it totally does the trick. But my cutter is carbide and freshly sharpened correctly(except for the tip, just a little off as I'm not sure of the exact spec). But I was using some gummy steel and I'm sure that was the biggest reason the threads were a little rough. But I'm still so new to this I could be committing all kinds of faux paus.

 

[Technical Ted]

As with most things, it depends. But, generally, HSS can be sharpened to a very sharp edge and it will hold up. Carbide, on the other hand, if sharpened to a very sharp edge has a tendency to chip and break down and the sharp edge will not last. A lot of times when I want a carbide edge to hold up I will lightly hone the edge at a 45 degree angle. Running carbide with edges like this performs best with faster surface speeds.

I get my best threading results on my slower running lathes with a good quality HSS tool bit, sharpened with a very sharp edge (with light honing) using a good quality cutting oil.

YMMV,
Ted