All the diagrams I have for proper tool shapes show the angle of attack to the stock mounted in the up angle of the tool holder mounted in a lantern tool post. Now with the QCTP the tool will be presented to the stock without a upward angle. The presentation of the tool is different .... do I change the tool grind?
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New Qctp Do I Need To Change Tool Grind Angles From Lantern Tp
- Thread starter jbmauser
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positive rake vs zero rake. I get by with zero rake. I know there are materials that benefit from positive rake, but I just usually just leave the top flat. You could adjust your bottom angle to match the "not pointing upward" tool.
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All machine shop lathe tool grinds (by "hand" and inserts), depend on the material being cut and yes, if the tool holder somehow changes the tool angle(s)... IE: "leadloy"?, CRS?, prehard toolsteels?, unhardened tool steels?, cast?, aluminum?, plastics?, yada yada...Very generally; The "softer" the work pc = all relief angles need to be more and the harder the work pc the relief angles need to be less (then we can, of course, get into chip breakers and also machine and set up regidity is a factor)....
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JB, some of the lantern tool holders held the bit at an angle, some didn't. Depends on what you have. The shape or profile of the tools may not change but the cutting angles that determine tool geometry do change because the 15 degree up-angle is no longer used. You must grind them into your tool when using a QCTP or any other tool post that holds the bit horizontal to the work. You can find these angles in almost any typical geometry table, like this one:
http://www.machinistblog.com/wp-content/uploads/2011/08/GrindingLathe10.gif
Many of the old tool shapes included roughing, facing and finishing shapes. These tools were intended for use with a lantern style post, where the post itself was left in place and the tool holder was changed to different tools, depending on the operation. The lead angle of the tool was determined by the grind - less lead for roughing, more lead for finishing. Nowadays, the QCTP allows you to change the lead angle almost instantly so tool shapes are less rigid as long as it provides enough strength and access to the work you're doing. In fact, many guys just use a single tool shape but vary the angles at the tip for different materials. Heck, I know one Bozo who uses one tool to cut almost everything, even though he knows better.
In my opinion, tool geometry is very important. It determines how much cutting force the tool generates and this affects sizing accuracy and finishes ... a LOT. While I seem to be in the minority insofar as the importance of tool geometry goes, I'm pretty convinced that it matters. The size of your lathe also matters; the smaller it is, the more important tool geometry becomes.
http://www.machinistblog.com/wp-content/uploads/2011/08/GrindingLathe10.gif
Many of the old tool shapes included roughing, facing and finishing shapes. These tools were intended for use with a lantern style post, where the post itself was left in place and the tool holder was changed to different tools, depending on the operation. The lead angle of the tool was determined by the grind - less lead for roughing, more lead for finishing. Nowadays, the QCTP allows you to change the lead angle almost instantly so tool shapes are less rigid as long as it provides enough strength and access to the work you're doing. In fact, many guys just use a single tool shape but vary the angles at the tip for different materials. Heck, I know one Bozo who uses one tool to cut almost everything, even though he knows better.
In my opinion, tool geometry is very important. It determines how much cutting force the tool generates and this affects sizing accuracy and finishes ... a LOT. While I seem to be in the minority insofar as the importance of tool geometry goes, I'm pretty convinced that it matters. The size of your lathe also matters; the smaller it is, the more important tool geometry becomes.
Gentlemen, you have all kind of confused me a bit. The info I have at hand tells me that the tool must attack the stock at an upward angle to "Shear" the metal from the stock. Granted this is taken from an old text using tool holders in a lantern tool post.
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That upward angle on a modern tool is called back rake. The tool bit is oriented horizontally and the angle is ground into the tool. It amounts to much the same thing.
It might be a good idea to update your reference material, and there are many good articles and books you can search for on line.
It might be a good idea to update your reference material, and there are many good articles and books you can search for on line.
I did not mention that I have used Armstrong tool holders in my Lantern tool post which has a up angle built in to present the tool to the stock at a angle to shear the metal from the surface.
I will look into the "Geometry" and work up a few tools for Steel, AL and Brass. These are materials I have to play with. I am looking at images of tool contours on the net. Info and images do not match. a flat top surface will offer a disruptive force to remove material and a angled top edge would offer a shearing cut but I can't back any of what I said with proof.
I will look into the "Geometry" and work up a few tools for Steel, AL and Brass. These are materials I have to play with. I am looking at images of tool contours on the net. Info and images do not match. a flat top surface will offer a disruptive force to remove material and a angled top edge would offer a shearing cut but I can't back any of what I said with proof.
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I normally avoid referencing my own stuff but I will in this case because you seem to be looking for the basics and this will avoid having to type stuff out. This is an introduction to tool geometry:
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander/
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander-part-2/#more-5888
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander-part-3/#more-5897
We also had a LONG discussion about tool geometry here: http://hobby-machinist.com/threads/turning-tool-and-facing-tool-questions.36687/
What kind of lathe do you have and what do you use to grind tools with?
Let me clear one more thing up. Lantern type tool holders, Armstrong and others, canted the tool bit at an angle. That angle varied but it required the machinist to alter the grind of the tool to suit the application. For example, a tool for brass needs zero back rake and maybe up to 5 degrees of side rake. An operator would have to grind a flat on the top of the tool that would be perpendicular to the work and then lock it into the Armstrong tool holder. That flat would then be at right angles to the work. For other materials like steel, he could just grind in a few degrees of back rake, a little side rake and the standard relief angles and he would be set. All of this changes with a QCTP.
A QCTP will hold the tool bit perpendicular to the work. Any required tool angles must be ground into the tool since there is no cant provided by the tool holder. For example, that tool for brass that requires zero back rake now simply needs some relief angles ground and the tool is ready for use; the top is already perpendicular to the work so no top angles are needed.
Almost all modern tooling used in the last 75 years is like this. Is it better? I think so; it is certainly a more rigid way to hold a lathe tool and it is far simpler to get the tool on center. You have to sort of change your paradigm of what a tool tip should look like and I hope the articles above will give you a head start on that.
Keep in mind that when you speak of shear, you are speaking of shear planes and sheer lines. That is a whole other discussion that involves the physics at the point of cut and how the various angles affect chip formation. While this is important if you want to put in the time to study it, it isn't necessary to grind a good tool. Sort of like not understanding how a modern TV works but still having the ability to turn it on and watch it.
Look this stuff over and ask questions. One of us will be able to sort things out for you.
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander/
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander-part-2/#more-5888
http://www.machinistblog.com/grinding-lathe-tools-on-a-belt-sander-part-3/#more-5897
We also had a LONG discussion about tool geometry here: http://hobby-machinist.com/threads/turning-tool-and-facing-tool-questions.36687/
What kind of lathe do you have and what do you use to grind tools with?
Let me clear one more thing up. Lantern type tool holders, Armstrong and others, canted the tool bit at an angle. That angle varied but it required the machinist to alter the grind of the tool to suit the application. For example, a tool for brass needs zero back rake and maybe up to 5 degrees of side rake. An operator would have to grind a flat on the top of the tool that would be perpendicular to the work and then lock it into the Armstrong tool holder. That flat would then be at right angles to the work. For other materials like steel, he could just grind in a few degrees of back rake, a little side rake and the standard relief angles and he would be set. All of this changes with a QCTP.
A QCTP will hold the tool bit perpendicular to the work. Any required tool angles must be ground into the tool since there is no cant provided by the tool holder. For example, that tool for brass that requires zero back rake now simply needs some relief angles ground and the tool is ready for use; the top is already perpendicular to the work so no top angles are needed.
Almost all modern tooling used in the last 75 years is like this. Is it better? I think so; it is certainly a more rigid way to hold a lathe tool and it is far simpler to get the tool on center. You have to sort of change your paradigm of what a tool tip should look like and I hope the articles above will give you a head start on that.
Keep in mind that when you speak of shear, you are speaking of shear planes and sheer lines. That is a whole other discussion that involves the physics at the point of cut and how the various angles affect chip formation. While this is important if you want to put in the time to study it, it isn't necessary to grind a good tool. Sort of like not understanding how a modern TV works but still having the ability to turn it on and watch it.
Look this stuff over and ask questions. One of us will be able to sort things out for you.
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The main thing you must do is to reduce the front clearance of the tool to make up for the lack of the 15 degree angle that the Armstrong type holders have; side clearance stays the same, and side rake would be the same. Using back rake is not very practical, and it greatly reduces the strength of the tool, especially with repeated sharpening on top of the tool.