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I stand corrected -- revising my grind

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  • I stand corrected -- revising my grind

    Previously I had used 3/8 (10mm) HSS on almost-new larger (16x40) Taiwan lathes. I had been taught to just grind the nose of the tool, the side and the front angle as shown here: https://littlemachineshop.com/reference/rakerelief.php

    But leaving the top of the tool completely flat, with no back rake as shown in the side view. This grinding style worked OK on the larger machines with plenty of power and quick change tool posts. They were simple to grind and re-sharpen. I use oil stones after the initial grind.

    They worked OK on my South Bend but I could tell it was not ideal. The machine was laboring on shallow cuts, the chips were long and dangerous, and the parts got very hot. The South Bend manual shows a completely different grind style. After comments by some forum members, I decided to experiment.

    I took my favorite general-purpose bit that I use for facing and turning, and added some back rake, a very small amount. I do all my grinding free-hand, no rests, followed by stoning in oil. The difference for the lathe is night and day. The machine runs a LOT easier and smoother and leaves a finish as good as any modern machine. The chips are small instead of death strings, and very controllable. There is almost no heat in the part or the tool.

    Pics: Top view, looking right down on it:

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    ...and side view, hopefully showing the back rake and front clearance.
    This bit performed beautifully, and is exactly on center!
    This is the side that would be going towards the chuck (left)

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    It is now 1 AM here. By tomorrow morning all my bits will be re-ground.

    I'm stubborn, but I can learn!
    25 miles north of Buffalo NY, USA

  • #2
    I've done something like that on some of my lathe bits, and it does seem to help. However, it means that the top cutting edge of the tool will be a bit lower than the unground shank, and will not be on the center of the work, especially after several rounds of resharpening, although you can easily readjust the height. However, IMO, it is better to be a few thousandths below center, and also consider that, for turning, it is the left edge of the tool that does the cutting, making swarf that is like a Slinky. A facing cut is more like parting or grooving, where the top surface of the tool does the cutting, and the swarf is like a clock spring.
    Last edited by PStechPaul; 06-09-2020, 01:27 AM.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #3
      Originally posted by PStechPaul View Post
      I've done something like that on some of my lathe bits, and it does seem to help. However, it means that the top cutting edge of the tool will be a bit lower than the unground shank, and will not be on the center of the work, especially after several rounds of resharpening, although you can easily readjust the height. However, IMO, it is better to be a few thousandths below center, and also consider that, for turning, it is the left edge of the tool that does the cutting, making swarf that is like a Slinky. A facing cut is more like parting or grooving, where the top surface of the tool does the cutting, and the swarf is like a clock spring.
      Yes, I anticipated the height problems. But I'm also planning on a QCTP in the near future to take care of that.

      I don't generally use the grinder after the initial grind -- once they have their first shape, the oilstone does the rest and they never see the grinder again.

      I honestly don't understand the results I got with the chips. I had a chunk of 1020 TGP to test it out, and I got the little 6's and 9's with both manual feeds and power feeds. HOWEVER I did *not* grind in any chip breakers -- so why were my chips acting like that? The TGP stock is 1" dia at ~600 RPM.

      Either way, I do know the lathe is a lot happier, and performance is amazing with the new grind -- so its a keeper!
      Last edited by nickel-city-fab; 06-09-2020, 01:58 AM. Reason: typo, readability
      25 miles north of Buffalo NY, USA

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      • #4
        The rake on the top surface presents a cutting edge at a slight angle to the radius of the work, so it tends to pull chips away from the exposed surface under the cut, and that might cause them to break rather than form a long coil. That might explain it. I haven't done much machining lately, but when I do, I often experiment with different lathe bits and grinds. I've even done some turning with a tool that was ground for parting or grooving, but it had enough side clearance to cut along the axis of the work, to widen the groove. I have found that many different configurations will produce acceptable results, and sharpness is probably most important when cutting steel and aluminum. The last thing I machined was the rubber tire on a caster...

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        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

        Comment


        • #5
          Is it ok at 600 rpm ? What chip color ? By formula 1 inch steel using hss should be around 400rpm
          don't worry about back rake affecting tool height, check if tool is on centre with a shim or ruler, if it's not, shim it up..

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          • #6
            Originally posted by 754 View Post
            Is it ok at 600 rpm ? What chip color ? By formula 1 inch steel using hss should be around 400rpm
            don't worry about back rake affecting tool height, check if tool is on centre with a shim or ruler, if it's not, shim it up..
            Chips had no color at 600 RPM. I know I was pushing it just to see what happened.
            With the old grind, they were brown and blue, with the new grind no heat.

            I HATE shims! Not gonna happen, full stop. That's why I did this:
            Tools are dead-nuts on center. Will keep that setup for emergencies.
            Just turned up a fat thick bushing that puts the tool tip dead-nuts on.
            Getting a QCTP soon enough that it won't matter anyway.

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            Last edited by nickel-city-fab; 06-09-2020, 02:07 AM. Reason: clarity
            25 miles north of Buffalo NY, USA

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            • #7
              Next on nostalgia trip you need to try angled hss bit holders. Either tangential lathe toolbit holder or classic american? Armstrong? Style.

              saves you buttload (metric ton) of grinding and solves the height problem.
              Location: Helsinki, Finland, Europe

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              • #8
                Originally posted by MattiJ View Post
                Next on nostalgia trip you need to try angled hss bit holders. Either tangential lathe toolbit holder or classic american? Armstrong? Style.

                saves you buttload (metric ton) of grinding and solves the height problem.
                No thanks. Absolutely not. I'm trying to get away from that altogether. I actually do have some of the old holders, the lantern tool post and rocker/wedge. ALL of that is going in the bin as far as I'm concerned. It was good back in it's day, but it's day was a LONG time ago. Maybe save it for emergencies or something but that's all they will ever be used for. I'm saving for a modern QCTP, and I do know these HSS bits will work just fine in that.
                25 miles north of Buffalo NY, USA

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                • #9
                  Don't bin or sell the lantern or the Armstrong holders.
                  You will one day find a use for them on a job where a lantern with an Armstrong is handier than a QCTP.
                  I use my QCTP 99% of the time, but a couple of years ago i made a lantern to suit my Armstrong for the other 1%.
                  I also have a genuine, original, ratcheting every 45 degrees, 4 way toolpost and i despise using the thing.
                  I think i've relegated it to daft stuff like single wheel straight knurling or something equally rare.
                  If it was worth any money i'd sell it, but they are not worth more than $30-35 in the UK.

                  Concur with MattiJ above, the Lantern is designed really to work hand in hand with an Armstrong.
                  You've got rid of the rocker, replaced with a solid ring, try an Armstrong in there.

                  ‚Äč

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                  They all have their advantages and disadvantages.
                  Last edited by thaiguzzi; 06-09-2020, 05:42 AM.

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                  • #10
                    The grind on the cutter bit depends on the type of material you intend to cut with it. Some material likes the positive rake you ground into the bit, some like no rake, some like an negative rake. If you want to get fancy, grind in a chip breaker using a fine cutting wheel on a Dremel.

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                    • #11
                      I have a few older hand ground tool bits that are ground just like that. Apparently that was the old school way of grinding them. The guy that gave them to me had an old 9" SB lathe. The bits are 1/4" and 5/16" sq. Some of them are ground flat on the top and some of them have the top ground for back rake. But they were all used with a lantern tool post so it was easy to tip the bit up or down. With a QC tool post you can't really do that?

                      JL....................

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                      • #12
                        Originally posted by nickel-city-fab View Post

                        No thanks. Absolutely not. I'm trying to get away from that altogether. I actually do have some of the old holders, the lantern tool post and rocker/wedge. ALL of that is going in the bin as far as I'm concerned. It was good back in it's day, but it's day was a LONG time ago. Maybe save it for emergencies or something but that's all they will ever be used for. I'm saving for a modern QCTP, and I do know these HSS bits will work just fine in that.
                        Even with a QCTP, the Diamond Tool Holder is the ultimate answer for HSS bits. Only one face to grind and that is very easy with the included jig. I have a 13x36 lathe with QCTP and use inserts for 95% of my work. I do have one HSS bit in my tool tray and it is mounted in a Diamond Tool Holder. It does everything needed except internal boring and cut-off. Try it, you will like it.

                        RWO

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                        • #13
                          Don't bin ...the Armstrong holders.
                          A rocker type is 1000% easier to adjust the futzing about with shims on a 4-way, and even if you have a QCTP you may keep some special infrequently used tool in it.
                          "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                          • #14
                            Originally posted by nickel-city-fab View Post

                            No thanks. Absolutely not. I'm trying to get away from that altogether. I actually do have some of the old holders, the lantern tool post and rocker/wedge. ALL of that is going in the bin as far as I'm concerned. It was good back in it's day, but it's day was a LONG time ago. Maybe save it for emergencies or something but that's all they will ever be used for. I'm saving for a modern QCTP, and I do know these HSS bits will work just fine in that.
                            Don't throw them out! Some of the Armstrong & Williams holders go for $25-$35, particularly the T-series (holds the tool straight, not at 16 degrees). The threading, spring-tool, and shaper ones are worth waaayyy more.


                            If you happen to have a #T0-R or some unusual ones you want to get rid of, I and I'm sure others here would be interested!

                            Location: Northern WI

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                            • #15
                              lately I been grinding HSS without the siderake at all. I never understood side rake when you can turn your tool post for any angle at all without affecting height or other rakes.
                              Unless you getting really really close to the chuck, or the tail stock, I go without side rake and cant the TP to suit.
                              Basically been grinding HSS to imitate the brazed carbide when used in QCTP.

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