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Confusion with finding correct rake angle on lathe bits

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  • Confusion with finding correct rake angle on lathe bits

    Hi, I am new to this forum so sorry if I make any mistakes. So I am purchasing a lathe to learn on http://littlemachineshop.com/product...ory=1271799306. I was looking for bits for it and came across this pack https://www.grizzly.com/products/11-...ce=grizzly.com. I want to turn steel and brass, but it does not mention the rake angle. I am also getting this tooling package that includes bits but still does not mention rake angles http://littlemachineshop.com/product...5207&category=. Am I missing something?

  • #2
    You might consider a pre-ground tool-steel set to get going. Here's an example, but the price is far too high:

    http://www.grizzly.com/products/H5870


    Individual pre-ground bits from Little Machine Shop:

    http://littlemachineshop.com/product...4324&category=


    Otherwise, buying tool-steel blanks and grinding them yourself is probably the best option.

    Individual blanks from LMS:

    http://littlemachineshop.com/product...4322&category=


    A two-page discussion of grinding lathe tools from LMS:

    http://littlemachineshop.com/instruc...ngtoolbits.pdf


    An image-rich treatment of grinding lathe tools, including a discussion of rake angles (at the very bottom):

    http://www.mini-lathe.com/Mini_lathe...l_grinding.htm
    Last edited by tlfamm; 05-25-2017, 09:16 PM.

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    • #3
      Learn to grind your own..you need to to be efficient..

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      • #4
        Originally posted by 754 View Post
        Learn to grind your own..you need to to be efficient..
        I want to learn how to use a lathe first before i start grinding my own tools. I also have to get a new bench grinder. Are those bits with replaceable triangles, only for steel?

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        • #5
          Generally, a table-top, hobby-class machine will lack both the power and the rigidity necessary to use carbide tipped (or insert) tools effectively. Tool-steel bits are really the only viable option. Get a small selection of pre-ground bits to get going - and then get a grinder and tool blanks later.

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          • #6
            You can use carbide inserts if you wish but restrict yourself to positive rake finishing grades. They will work well. Brass works fine with neutral rake and no chip breakers. I'd skip that Grizz carbide set you linked... but the LMS TCMT 21.51 set will be ok.
            Last edited by lakeside53; 05-25-2017, 10:02 PM.

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            • #7
              I've been using carbide inserts on my 7x12 lathe for over 10 years. They work quite well. There are situations where steel tools work a little better but modern carbide works really, really well.

              From the original post, the first set of tools from Grizzly are brazed carbide onto a steel shank. There is no rake and no clearance. You have to grind the angles on them and sharpen them.

              The second set of tools from LMS use carbide inserts on a steel shaft, also known as indexable tooling. The insert designation is TCMT. That means T= Triangle, C=7 degree clearance, M=loose tolerance , T=40-60° double countersink hole in the middle with molded in chipbreaker. That's a good general purpose insert for steel and aluminum.

              The chipbreaker effectively provides positive rake.

              Inserts are ready to use. Brazed carbide needs sharpening and shaping before use, just as steel tools do.

              I recommend going with the inexpensive TCMT indexable tooling while you are learning so that you know what a good edge should feel like. Then you can grind some HSS (high speed stee) tools for special applications and you will know when you get it right.

              Dan
              At the end of the project, there is a profound difference between spare parts and extra parts.

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              • #8
                Originally posted by tlfamm View Post
                Generally, a table-top, hobby-class machine will lack both the power and the rigidity necessary to use carbide tipped (or insert) tools effectively. Tool-steel bits are really the only viable option. Get a small selection of pre-ground bits to get going - and then get a grinder and tool blanks later.
                When you see comments like this, keep in mind that the key is the term "effectively". In a busy machine shop, carbide is used a LOT and they have machines that make such a deep cut and do it so fast that they parts are made as fast as possible. Chips are glowing. The cost is that the inserts wear out in a very short time. Like hours.

                Using carbide effectively in a machine shop is to make parts as fast as possible without needing to change the inserts too often or in the middle of the job.

                With a small machine you can't push it that hard. You will not get the maximum benefit from carbide, but the modern carbide will generally work just as well as the HSS will, and it will last a lot longer before needing to be changed. HSS needs frequent sharpening.
                At the end of the project, there is a profound difference between spare parts and extra parts.

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                • #9
                  Originally posted by victor View Post
                  I want to learn how to use a lathe first before i start grinding my own tools. I also have to get a new bench grinder.
                  yeah you do need a grinder....but imo the first part is like wanting learn how to drive a car and then coming back to learn how the steering wheel works. Learning how to use a machine starts with understanding how the tool makes chips, it all ties together - tool geometry, speed, cutting force.....learn a little bit of that and so much of the rest of it becomes intuitive. First thing is to plough through a good basic book....it will help you so much vs stumbling around in the dark and you'll good results a lot faster
                  .

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                  • #10
                    I've been a machinist for over 50 years. Used lots of HSS and carbide tooling in all forms and combinations. If I was to distill this experience down to rules for grinding HSS tools for general lathe work it would be to keep the grind simple. Try to run the grinding striations in the direction of chip flow. Use the "Magic 7 degrees" tool angle (side clearance, end clearance, and top rake) but don't feel bound by it. Chip breakers are nice but not really necessary, and keep handy a little pocket or slip stone (for honing the cutting edge) is almost a necessity.

                    There is no convenient way to separate learning tool geometry from learning to operate a lathe. You can study them separately as topics but you soon learn that feeds and speeds, tool geometry, chip control, size control, use of coolants, order of operations, etc all interact. Learning this simultaneously is not hard but it does take a little time.

                    Good machinists have been made from the most unlikely people; I had a 40 year old divorced housewife as an apprentice who turned out to be a real crackerjack of a machinist after a little skill building. Don't expect miracles overnight. Practice a few basics then make something simple. Build your skills and knowledge on that

                    If you can, find a local mentor. Feed him beer as he sits on a stool and spins yarns. He can instruct you in the fine arts and the dumb-ass simplicities of working with machine tools. The cultural background of the machinist's trade is almost as important as knowing the knobs and levers.
                    Last edited by Forrest Addy; 05-26-2017, 12:42 AM.

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                    • #11
                      Originally posted by Mcgyver View Post
                      yeah you do need a grinder....but imo the first part is like wanting learn how to drive a car and then coming back to learn how the steering wheel works. Learning how to use a machine starts with understanding how the tool makes chips, it all ties together - tool geometry, speed, cutting force.....learn a little bit of that and so much of the rest of it becomes intuitive. First thing is to plough through a good basic book....it will help you so much vs stumbling around in the dark and you'll good results a lot faster
                      Well I have a grinder but its not very stable. I just ordered a new bench grinder with a dressing stick. When I order the lathe I will order some hss blanks. Also should I get hss blanks with 5% cobalt?

                      Thanks for the help! I'm just trying a shot at metal working. I used to woodwork but my father complained about sawdust so metal chips and shavings are the next best thing. I'm only 15 with a job learning machining. And yes I'm pretty young for machining.

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                      • #12
                        Well written Forrest.., I tire of people that want to learn to run a machine tool, but not put in any effort.

                        When I was in trade school, we got 3 in long pieces of 1/2 square cold roll. Then we started to grind basic cutting tool profiles, with proper clearance....over and over till we got it right..and then we understood it. Then they let us on the lathes.
                        Gotta ask yourself why they did it in that order.

                        Its not really that hard, and a bit if theory is just part of learning...

                        I will say right now, trying to make a small lathe remove metal, everything better be close to right....or you will be wasting time...

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                        • #13
                          Hi,

                          While I am of the camp of learn to grind a tool bit for beginners, there are now some decent insert tools available for small machines and home shops.

                          This set could be all you might need for quite a while - https://www.banggood.com/7pcs-10mm-S...rmmds=category

                          The 10mm, (~3/8") shanks are a good fit for most benchtop lathes. The quality of the tools and inserts are quite a bargain for the money. I actually own and use these in my home shop on a 8x14 lathe. I am very satisfied with them.

                          I would stress that Forrest Addy makes some good points. You really need to learn HSS tool grinding and shaping. Even top shelf HSS is cheap, versatile, and forgiving of mistakes. A dollars worth of it be any tool you need, just need to shape it.

                          Dalee
                          If you think you understand what is going on, you haven't been paying attention.

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                          • #14
                            Why is it that the older folks who had the benefit of trade schools and apprenticeships and large commercial grade machinery keep saying that small lathes need some magic to work well?

                            I'm an old fogy who had to learn the hard way through reading manuals, tutorials and then testing what I learned by doing. One of the impediments to learning was improperly grinding the first tools that I used. The other was the constant outcry of 7x10 bashers who said that poor performance was to be expected. That stream of vitriol kept me from realizing that the grind was wrong and the lathe was OK. I spent weeks taking incredibly small cuts because that's all I could do, just like the "experts" said. I almost gave up.

                            I had missed the need for front clearance.

                            Then I stumbled across a cheap set of indexable turning tools at Harbor Freight. $15 for 5 1/4 inch tools complete with TCMT inserts. It was like night and day. I could take decent cuts and get a great finish. It turned out my lathe really was capable of removing metal! My work began to match what I was led to expect by the Army training manual.

                            Eventually I revisited the grinding of tools. I knew what to expect from a tool by that point. Investigating the features of a TCMT insert drove home the need for clearances. I reground the original HSS tool bit using 7 degrees for all surfaces and it worked quite well. Coincidentally, that's the same angle as Forrest mentioned.

                            And that's why I suggest that a beginner who lacks a mentor/trainer should buy an inexpensive indexable turning set to start out. It's so that they will have a reference point for what they should expect when using the machine. Then they can learn the basics of using the machine safely before they get into the esoterica of grinding tool bits.

                            Dan
                            At the end of the project, there is a profound difference between spare parts and extra parts.

                            Comment


                            • #15
                              Its not magic, its just common sense. My 9 inch Southbend could take off 1/8 on steel...1/16 DOC, with HSS I ground. But I understand, feed, speed, cutter geometry, and setting a tool at center..
                              Books like Southbend How to run a lathe, walk you thru a large part of it.
                              I have no problem helping anyone that genuinely wants to learn, but people who only go at it with minimal effort....have burnt me out.
                              Had one buddy I tried to help, went thru my Southbend book, noted pages and paragraphs of stuff that mattered the most... But he would not buy the book. Finally I asked him why..and he said..and I kid you not..
                              The book is SouthBend, but my lathe is an Atlas..

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