Helix angles and HSS threading tools

[mikey]

There have been a few threads where guys ask about HSS tooling and how to grind them to account for the helix angles of a thread. The most recent one is here and I chose to open a new thread so as not to encroach on it: http://www.hobby-machinist.com/threads/threading-tool-helix-angle.58802/#post-484610

Note that I am not talking about inserted carbide threading tools with adjustable anvils to alter the helix angle/approach angle of the insert. I am referring to HSS threading tools for screwcutting on the lathe.

So, as a means to open the discussion, let me lead off with some questions that a hobby guy might have:

• what is this helix angle you're referring to? Can you measure it accurately and if so, how?
  
• assuming you can accurately measure it, how do you grind a HSS tool to account for the helix angle of a given thread, and do you need a different threading tool for each helix angle or group of helix angles?
  
• for the vast majority of threads we cut in a hobby shop, does trying to account for the helix angle of a thread make a demonstrable difference? I am not talking about Acme threads or large square threads here, just maybe up to 3/4 UNC or UNF or so.
• For these common threads, what role do the relief angles on the tool play? It is a given that the helix angle of a thread and the relief angles on a threading tool are totally different things.
  
• Does your tool holder that holds the turning tool on your QCTP allow you to alter the horizontal tilt or angle of your tool? If so, please show us a pic.
  
• Should I even bother about helix angles? Millions upon millions of threads have been cut with a standard HSS lathe tool so why change that?

These are questions that I've asked myself and I think I've come to an understanding that has influenced how I grind a threading tool but I could be wrong and am willing to learn. So, what do you guys say?

 

[Rustrp]

Yeah, forget snorkeling just go straight to suiting up and going deep.

I'm too busy today to allow to my brain to get wrapped into this, but my first guess would be; Should helix angle even be a consideration when we sharpen the tool, or..... is it inclusive when we sharpen the tool to the proper angle? See, now I'm in to far, gotta go.

PS. You were asking the questions about a single thread screw, correct?

 

[mikey]

Yeah, forget snorkeling just go straight to suiting up and going deep.

I'm too busy today to allow to my brain to get wrapped into this, but my first guess would be; Should helix angle even be a consideration when we sharpen the tool, or..... is it inclusive when we sharpen the tool to the proper angle? See, now I'm in to far, gotta go.

PS. You were asking the questions about a single thread screw, correct?

Yup, simple single lead/single start screw like the ones we all turn every day.

 

[Martin22250]

I like to learn, I'm eager to read some responses

 

[Bill Gruby]

You might want to start here

https://en.wikipedia.org/wiki/Helix_angle

"Billy G"

 

[mikey]

Okay, I'll begin and others can add on or correct my thoughts.

The Helix Angle we're talking about is the angle of the thread relative to a vertical line, as seen here:



A HSS threading tool typically has a flat approach angle, whereas some inserted carbide threading tools allow the insert to tilt (by using an angled anvil under the insert) so that the cutting edge is precisely angled to match the helix angle of the thread. You can see the difference here:



The HSS tool will look like the tool on the left and an inserted tool on the right. The angled tool allows for a more accurate thread that is going to be cleaner because there is reduced flank wear or rubbing occurring. This is the big deal about helix angles and threading tools - the geometry of the cutting edge and the potential for flank wear/rubbing. This helix or lead angle can, of course, be calculated; you can see the formula here: http://www.mitsubishicarbide.com/en...c_threading_formula_top/tec_threading_formula

Much is made about how we thread as well, either using a radial feed (straight in with the cross slide) or an angular approach (using the compound to infeed). It is this flank wear/rubbing thing that is the issue. In a hobby shop, either works. In a production situation where time is money, the approach used is typically an angular approach because of reduced wear on the insert. The angular approach also produces a cleaner thread, regardless of the type of tool involved, because only one edge is cutting - sorry, but its just true. Again, in a hobby shop, this is no big deal if you prefer the straight in radial approach.

If we are cutting the typical small threads that most of us cut, the helix angle is fairly small and the potential for flank damage from the tool is also small. It is only when we start to cut larger threads that it gets a bit hazy because the helix angle becomes large. A typical example is an Acme thread, where the helix angle of the thread is large and a lot of distance is covered by a single rotation of the thread. In such cases, a special tool needs to be ground or used.

So, what is the hobby guy to do about this helix angle thing if he prefers to thread with HSS tools? There are really only two options: One, grind the top of the tool to the proper helix angle and provide adequate relief angles under the tool or two, leave it flat and increase the relief angles to minimize flank contact. The issue with the first option is that every single thread has a helix angle and if we have to grind a tool for every one we are going to be grinding a lot of tools. The issue with the second option is that as threads get coarser, we have more potential for flank contact.

In an effort to test this, I ground a HSS tool with an angled top. This was about 12-15 years ago and I don't recall the exact angle but it had the typical 7 degree relief angles on it. To compare, I also ground a flat topped threading tool with 15 degrees of relief and then cut 3/4-16 threads with both tools. Under 10X magnification, I could see no difference between the cut threads. Fit was the same with my test nut. I will grant that this is a very crude test and I'm sure if I took macro shots and compared the helix angles I might have seen differences but for my purposes, I was satisfied that a flat topped 60 degree threading tool with adequate relief angles will cut a decent thread and that grinding a tool for helix angles was not necessary in the home shop setting. Well, at least in my home shop.

So, for HSS tooling, I just use a flat top rake 60 degree tool with 15 degree relief angles on both sides. This tool cuts cleanly, without excessive flank wear on the tool in the vast majority of threads I cut and without excessive tearing of the threads themselves. It is very likely that my threads are not perfect 60 degree threads with this tool but it is close enough not to matter for my purposes.

If you want perfect threads then the best option is to buy an adjustable inserted carbide threading tool, calculate the helix angle and use the recommended anvil for that thread.

Okay, that's my take on this subject. I would welcome corrections to my understanding of this subject since I am by no means an expert on this.

 

[thomas s]

Mike thank you for posting about threading. I don't know much about threading but will be reading and learning.

 

[francist]

I wish I was in a better position to speak in a more experienced manner about this topic because I do find it quite fascinating. However, the amount of single point threading I have done is really not going to provide much insight.

I do remember though that on my first couple threading tools I ground taking the helix angle into account, at least on the flanks anyway. It was a minor bother to calculate, and even more of a bother to try to measure if I was on the mark or not. I mean, the angles are often quite small -- like a couple degrees -- which can be tough to measure on a small lathe tool. I have no idea if it made a difference or not, my thread was still pretty ugly!

The last tool I ground just a month or so ago I did not grind for helix. Same relief on both flanks, didn't want to be bothered, and the tool works just fine. My threads even look better, but I'm sure that has nothing to do with the angles and more to do with the execution.

But oddly enough, I don't like the tool. I look at the end of it and think "it's wrong, there's no angle there and there should be one". So while I can offer that the tool works fine without the helix being taken into account, I suspect the next time I will put the angle back just because.

Maybe I'm one of those odd persons who thinks it's valuable to understand and know things even if you don't always choose or need to employ them. Like knowing how to cut a dovetail or square a board by hand. Maybe for me, the helix angle is one of those things. I don't really need to worry about it (now anyways), but maybe down the line I will so I should continue to pay attention to it. Even if I don't always use it.

Nice write-up Mikey. It's a pretty big subject, but worth thinking about. I have one question though, and that is do you think the helix angle is more important to consider on an internal thread? Seems to me it might be, but again I'm not sure that it would really make any difference on anything but larger thread sizes.

-frank

 

[mikey]

I have one question though, and that is do you think the helix angle is more important to consider on an internal thread? Seems to me it might be, but again I'm not sure that it would really make any difference on anything but larger thread sizes.

For the vast majority of threads, no, I don't think the helix angle is worth worrying about for typical threads in the under 3/4" category. Here I am referring to single start helical threads, not multi-start threads or acme threads. I also don't think it makes any more difference if it is an internal or external thread.

What happens when we use a tool with a flat approach angle is that our threads don't come out to exactly 60 degrees; they come out a fraction of a degree more or less. Does this matter in a home shop? I don't think so. More important, to me anyway, is the pitch diameter and major diameter when fits are a concern.

 

[Rustrp]

Okay, that's my take on this subject. I would welcome corrections to my understanding of this subject since I am by no means an expert on this.

I knew if I waited long enough you you would draw.

My first thought when you posted was that the helix angle was more important when machining gears and this still may be true, but after looking through the Machinery's Handbook I realized they didn't print that many pages for something unimportant. I've done little single point threading and after reading I realized I had placed no emphasis on the helix angle............because I didn't know. The satisfactory results I got when I have threaded was proportional to the accuracy I found acceptable. Thanks for the post with the additional info. I think this short video may add a little clarity, or should I say it brings some of the points together.