Models for grinding HSS Lathe Tools

[ddickey]

Tool E is a finishing tool as it can turn to a shoulder and face the shoulder. To be fair the angle of the cutting face means the vector forces want to pulll the work into the tool. However the small positive lead angle, of usually only a degree or two aren't as strong as the negative lead angles on roughing tool that usually set in the 10 to 20 degree range.

I thought the more negative lead angle the better the resulting finish. That's a question.

 

[Dan_S]

I thought the more negative lead angle the better the resulting finish. That's a question.

In the texts i have read, and based on my own observation it has no real effect on the finish. It does have a chip thinning effect though, that allows you to increase the depth of cut, or the federate, thus allowing you to remove more material.

In my honest opinion unless you are trying to squeeze every last bit of performance out of your machine, just about anything will work. from what i have seen , read, and personally experienced i would prioritize things in this order.

1. Honing the tool bit after grinding.
2. getting a tool bit radius that coincides with the feed rate you are using, and surface finish you want to achieve.
3. get the SFPM into the right ballpark for the material and diameter you are turning.
   1. If you are working with a material that work hardens make sure the depth of cut, feed, and sfpm are correct.
4. make sure you have enough side and front clearance. 8 degrees works for almost anything.
5. get the side and back rake into the ball bark you will find in old texts or online.

 

[mikey]

View attachment 245921

In my oinion part of the issue comes down to how you have your lathe set up, and how you prefer to use it. I use a QCTP and it is aligned as close to perfectly square to the spindle as I can get it. This is so that threading tools don't require any fiddling to get aligned. In general I just drop the holder on and go, because all my angles are preset by how the tool is mounted in the holder.

In my opinion tool B above is a roughing tool and tool E is a finishing tool.

I call tool B a roughing tool for two reasons. It can't turn to a shoulder and face a shoulder with out being reoriented, and to me that's a no go. Also because of how the cutting face is oriented to the direction of travel the force vectors want to push the work away from the tool, making it hard to accurately dimension a part.

Tool E is a finishing tool as it can turn to a shoulder and face the shoulder. To be fair the angle of the cutting face means the vector forces want to pulll the work into the tool. However the small positive lead angle, of usually only a degree or two aren't as strong as the negative lead angles on roughing tool that usually set in the 10 to 20 degree range.

I know guys who either don't use A QCTP or have no qualms about reorienting and use nothing more than a tool shaped like A.


In my eyes side rake and back rake depend on the material being cut, so I have different tools for different materials. I've personally found the envelope of acceptable angles to pretty wide so as long as you are in the ballpark of what's quoted in a lot of old texts you are good to go.

With regards to tool bit geometry, tip radius is the most important thing to get correct. The main thing to remember is that you are cutting a fine pitch screw, so the finish is directly related to the tip radius and feed rate. A higher feed means you need a bigger radius to get the same finish as a smaller radius and a slower feed. Calculators like this one are good tool to help think about it. http://www.custompartnet.com/calculator/turning-surface-roughness

the other thing to consider is that back and side rake effects the effective tip radius. The more back or side rake you have the bigger the radius needs to be. This can best be seen by looking at the cross section of a plane and a cylinder at different angles. When cut though at a perpendicular angle the cross section is a circle. At any angle other than a 90 you get an ellipse, and the further than angle is from 90 the more pointy the small end of the ellipse becomes. This image kind of shows it, but its easier to visualize buy just cutting though some round stock at an angle other than perpendicular.

In my opinion honing had the greatest effect on surface finish and cutting performance. It was a night and day difference when I started honing tools many years ago.

Dan, I've been pondering the best way to respond to your posts. I want to preface this by saying that I mean no offense but am concerned that some of your statements may be a bit misleading.

Fixing a QCTP is often done by carbide insert users. It allows you to index your tooling and then use the tool with the orientation for which it was designed. This is not how a contemporary HSS tool user generally uses a QCTP. I know that you have stated in the past that you use mainly inserted carbide tooling so it makes sense to use your post this way.

The tool shapes you posted have been around for over a hundred years. All were intended for use in the old-style lantern or 4-way tool posts and were oriented perpendicular to the work. They were all typically ground with the conventional tool angles found in most tool angle tables. Nowadays, we can grind tools to whatever shape we want because the QCTP makes changing lead angles simple, and that greatly expands the capabilities of those tools. Modern hobby guys have moved on with regard to shapes and how the tools are oriented. I, at least, have long since moved on from conventional tool angles as well.

I agree that the rake angles must change for each material; even the tables take this into account but you say the envelope for these angles is pretty wide and I am unclear as to the basis for your statement. What is your understanding about the function of these angles? Even an increase of 2 degrees of side rake can allow a turning tool for stainless steel to produce chips instead of stringers so the envelope is not wide, not really.

With regard to tip or nose radius, you say, "The more back or side rake you have the bigger the radius needs to be." I'm not sure where this came from. It turns out that side rake has little influence on the nose radius. Back rake on the other hand, does. As back rake increases, the cutting forces are focused more and more at the tip of the tool and this greatly aids finishes because the force is focused in a smaller and smaller area as the angle increases. Of interest is that this smaller area is not so much at the nose; it is primarily on the side cutting edge, up near the tip where a properly oriented HSS tool actually finishes. So, the nose radius need not be larger as the rake angles increase; the reality is that the opposite is true. I say this based on years of experience and experimentation, not something I read, and I'm making an issue of it because I don't want the guys to misunderstand something that is very important. Large nose radii on a HSS tool increases radial cutting forces and this leads to more deflection and reduced accuracy. Especially on a finishing tool, it is better to increase back rake and keep the nose radius small. The tool will finish well but it will also be more accurate.

In post 341, @ddickey said, "I thought the more negative lead angle the better the resulting finish. That's a question." Your response was, "In the texts i have read, and based on my own observation it has no real effect on the finish. It does have a chip thinning effect though, that allows you to increase the depth of cut, or the federate, thus allowing you to remove more material."

What you are saying is that increasing the lead angle so that the tool is turned more toward the tailstock does not have a real effect on the finish. This is curious because Machinery's Handbook says otherwise, as does my own experience. This orientation actually brings more of the side cutting edge into contact with the work so cutting forces increase; rather than increase your depth of cut or feed rate as you suggest, you must actually reduce your depth of cut or the tool will chatter. The purpose of turning the tool toward the tailstock is to enhance finishes, and it does. As previously stated, you are finishing primarily with the side cutting edge up near the tip. Furthermore, you may be surprised at how much better some inserts finish when the lead angle is increased; this is not standard practice but it can significantly improve finishes. Ask @Bamban.

In item number 2 of the same post you recommend "getting a tool bit radius that coincides with the feed rate you are using, and surface finish you want to achieve." Are we still talking about HSS tools or does this apply to carbide tools? The nose radius of a carbide insert influences all of your cutting conditions, perhaps more than any other part of the insert's geometry. On a HSS, not so much and definitely not if you alter your lead angles to finish.

In item 4, you said, "make sure you have enough side and front clearance. 8 degrees works for almost anything." In my experience, this is true only if you are grinding tools to conventional angles. If you wish to use tools that do not finish as well and cut with higher cutting forces then this recommendation makes sense. Otherwise, I would have to disagree. Some materials like aluminum and stainless respond really well to increases in the relief angles; typically you have better finishes and reduced work hardening. Brass also likes larger relief angles than a table calls for; the tool will cut more easily, be more accurate and will finish much better.

In item 5, you said, "get the side and back rake into the ball bark you will find in old texts or online." This might be true if you want a conventional tool. However, if you wish to have a tool that cuts with lower cutting forces, finishes better and dimensions more accurately then no, I disagree. Again, the rake angles are really important, more so than any other tool angle, and this is especially true if the lathe is a smaller one because they have such a huge impact on the cutting forces the tool produces. They also allow us to reduce cutting temperatures enough to reduce work hardening in stainless to a noticeable degree.

I realize that this post may seem to be offensive to you, Dan, but that isn't my intention. You have made statements that you believe to be true and I understand that. I have taken issue only where there is misleading information and have sought to clarify, not argue or denigrate. If you would like to take this further, we can start another thread or take it off line. I understand tip geometry well enough that I can support my position.

 

[Dan_S]

Fixing a QCTP is often done by carbide insert users. It allows you to index your tooling and then use the tool with the orientation for which it was designed. This is not how a contemporary HSS tool user generally uses a QCTP. I know that you have stated in the past that you use mainly inserted carbide tooling so it makes sense to use your post this way.

I set up my tooling this way even before I had insert tooling. I've been at it for almost a decade and a half now, and the first 3 or 4 years, all I had was HSS tooling. When I first started out, all I had was a 4 way tool post, and it was a pita to get tools set up as you had the shim tools each time you swapped to a different one.

When I finally got a QCTP, what convinced me to set it and and not change it was when I broke a parting blade. I couldn't afford a lot of tooling at the time, so it was ground out of 1/2" square blank. It took several hours to grind, because it was long, and my grinder was weak. When it broke because the tool post wasn't square enough, a lot of cursing and yelling could be heard in my shop, because not only did i need to regrind a new tool, I had scraped a part that took about 20 hours to make.

What is your understanding about the function of these angles? Even an increase of 2 degrees of side rake can allow a turning tool for stainless steel to produce chips instead of stringers so the envelope is not wide, not really.

My father started off his career as a tool maker, and when he was visiting one time, he set me strait so to speak. I was trained as a physicist, so I had always been exact with my angles (notes and all). My dad came out to the shop while I was regrinding some tools, and after a few minutes I got the "what are you doing line", because I was setting my tool rest to the proper angles with a protractor/level. I gave him the bit, and he just went at it free hand, not even using the rest, other than to support his hand. he bared down so hard he'd bog the grinder down.

his "words of wisdom" where something close to just get it close and stop wasting time, if your within a couple of degrees it will work fine. If it doesn't just change the feed rate, depth of cut or surface footage. My personal experiences since then support this.

I say this based on years of experience and experimentation, not something I read, and I'm making an issue of it because I don't want the guys to misunderstand something that is very important. Large nose radii on a HSS tool increases radial cutting forces and this leads to more deflection and reduced accuracy. Especially on a finishing tool, it is better to increase back rake and keep the nose radius small. The tool will finish well but it will also be more accurate.

I think you miss understand what I mean by larger. When I say larger I'm meaning give it a few more passes on the hone. In other words the radius will get bigger by a few thousands.

What you are saying is that increasing the lead angle so that the tool is turned more toward the tailstock does not have a real effect on the finish. This is curious because Machinery's Handbook says otherwise, as does my own experience. This orientation actually brings more of the side cutting edge into contact with the work so cutting forces increase; rather than increase your depth of cut or feed rate as you suggest, you must actually reduce your depth of cut or the tool will chatter.

We will have to agree to disagree on this point. Machine Shop Practice and my father have both referenced the chip thinning effect. Page 183 in Machine Shop Practice has a diagram explaining this. It is worth noting, that the power and rigidity of you lathe as well as the rigidity of the work piece must be taken into account.

In item number 2 of the same post you recommend "getting a tool bit radius that coincides with the feed rate you are using, and surface finish you want to achieve." Are we still talking about HSS tools or does this apply to carbide tools?

It applies to both HSS and carbide tools in my opinion.

I realize that this post may seem to be offensive to you, Dan, but that isn't my intention. You have made statements that you believe to be true and I understand that. I have taken issue only where there is misleading information and have sought to clarify, not argue or denigrate. If you would like to take this further, we can start another thread or take it off line. I understand tip geometry well enough that I can support my position.

With respect, if your intent was not to denigrate, then you probably should have just skipped this whole paragraph. It gives your entire post a dismissive tone. It also makes the bolded read as don't post in "my thread" if you don't agree with me, and you can be assured i won't again.

 

[mikey]

With respect, if your intent was not to denigrate, then you probably should have just skipped this whole paragraph. It gives your entire post a dismissive tone. It also makes the bolded read as don't post in "my thread" if you don't agree with me, and you can be assured i won't again.

It was not my intent to upset you, Dan. I actually worried about how to word my post so as to avoid that. However, it is difficult to portray intent with words and I cannot help how you perceive things. With that said, I was not being dismissive in the least. I am more than willing to discuss tool geometry with you here or elsewhere - I was simply giving you the choice.

You may not believe me but I understand how you feel. You contributed and I called you out on multiple points - who wouldn't be upset? And yet, the intent was to clarify; it had nothing to do with me being right. So, participate or don't participate, the choice is yours.

Oh, just so its clear, this is not "my thread". It is our thread, me and all the guys who have posted to it, and that includes you. The intent of the thread was to help guys understand how (and what and why) to grind a good tool and then let the tools and the results prove themselves.

 

[Aukai]

Got my models when I got home last night, hope to start on them tonight.

 

[Aukai]

Well my ego got a check tonight. My finess skills need some tuning. The grinding will take a couple of tries to get exact, and the honeing is an animal.

 

[mikey]

Don't forget - we need pics!

 

[Aukai]

I took a couple, but not ready to share yet.

 

[mikey]

No pressure, Aukai. If you care to share at some point, fine. If not, that's okay, too.

Believe me, I know how hard it is to grind a tool in the beginning but take your time, it will come.

If you're ever in Honolulu, let me know and I'll give you a hand-on demo on tool grinding.