Single Point Threading Issue - might be gearing?

[brino]

@WobblyHand

I just gotta compliment you on the great close-up pictures you are posting.
Both the pictures of your threads and the tool tip.
The detail you've captured is like being there.....with a magnifying lamp and my good eye!

Obviously your hands are NOT too wobbly!

-brino

 

[WobblyHand]

Thanks. Used magnification (zoom) for the pictures AND rested my hands and phone on the desk. Oh yeah, the phone does help a bit with image stabilization. Otherwise, everything would all be a blur.

 

[Tozguy]

It looks like your tool could use some more side relief to accommodate the helix angle. I will try to take a picture to illustrate it.

 

[WobblyHand]

It looks like your tool could use some more side relief to accommodate the helix angle. I will try to take a picture to illustrate it.

Thanks. A picture, or figure would be awesome. This is a stock LMS threading bit. I just honed the faces of the bit. (And forgot to put on a flat on the tip) I haven't ground my own threading bit yet, just a key-stock example.

 

[Tozguy]

This might show what it looks like. There has to be more relief on one side which makes the front edge slope at an angle (of at least as great as the helix angle of the threads).

 

[WobblyHand]

Ok. I see what you mean now. The LMS tool has little or no relief. So does my example key-stock. How does one get the relief? Not sure how I'd grind the tool to get this better match to the helix. @mikey have any insight to get a little more "helix" correction?

 

[mikey]

Thanks. That chip off the tool is about 1/64" wide. So I'll try to flatten the whole edge. So this looks like a honing sort of thing, rather than setting up the belt sander again.

Yes, just use your diamond stone and hone it on.

 

[mikey]

Ok. I see what you mean now. The LMS tool has little or no relief. So does my example key-stock. How does one get the relief? Not sure how I'd grind the tool to get this better match to the helix. @mikey have any insight to get a little more "helix" correction?

First, the relief angles on a HSS threading tool makes a difference in how clean the threads are. Standard relief angles (the angles under the cutting edges) are between 5-8 degrees. That's okay, they work. However, my tests suggest that a 15 degree relief angle produces lower cutting forces and much cleaner threads. My threads have almost no burrs so it seems to work for me. If your tool is not working well, try increasing the relief angles and put that flat on. I bet it will cut better for you.

As far as helix angles go, I would worry about it if I was using carbide inserts to cut threads. The smaller the diameter of the work and the greater the number of TPI, the more helix angle matters with inserts because of the geometry of the insert itself. With HSS tools that have zero rake on top, this is much less of a concern so I personally do not alter the grind to compensate for helix angles. I've cut threads down to 4-40 with no issues at all so I don't worry about it.

 

[WobblyHand]

The LMS tool is not cutting clean threads. Or at least not for 32 TPI. That's after honing the three faces. Unfortunately, I forgot to flatten the tip. So, I'll try increasing the relief angle by grinding a new bit. I have a key-stock template I ground, (from your excellent tool grinding thread) but didn't get around to grinding it in HSS. Sounds like I need to set up the belt sander and vacuum cleaner again!

 

[Tozguy]

Helix angles will vary depending on diameter and threads per inch. When single point cutting threads it is important to be conscious of the helix angle if only to realize its a factor. If the relief angle on the tool is routinely generous then chances are you do not need to 'worry' about helix angles. But if your thread cutting tool does not have enough relief then it will rub or cut poorly. The reason to increase relief from 7-8 degrees to 15ish for cutting threads as mentioned above is largely to accommodate the helix angle and still have some relief at the cut.