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Questionable method for parting...

Discussion in 'MINI-LATHE & MINI-MILL INFORMATION' started by The_Apprentice, Jul 16, 2017.

  1. The_Apprentice

    The_Apprentice Canada Active Member Active Member

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    Came across a video of a different way to part off for mini-lathes. It involves reversing the spindle, and inverting the tool. But the tool is not on the back-side.

    It is interesting, but a few others have commented the this is not advisable because lathes are not built to have a load push upward on the tool-bit, and this can cause major issues with the dove-tails, etc?

    Thoughts?

     
  2. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Based on the various comments, parting seems to be more of an art than a science, It seems that those who have claimed to have mastered the art have tried the different method s and their professed method is the very best way. Curiously, they don't seem to converge on any one method and some, like the one above, are radically different.
    There are a few things that seem to favor successful parting. One is rigidity and another is power.

    A rigid setup is less likely to chatter, chatter being an alternating gouging and backing out of the parting tool. This includes a rigid setup for the part, as close to the chuck as practical, a rigid tool setup. heavy tool post, and a rigid frame (= big lathe).

    Power is self explanatory. Heavier cuts seem to work better than light cuts and they are favored by a lot of torque.

    Unfortunately, hobby class lathes don't offer either. Also, it unfortunately seems that all the videos teaching us how to part are runnin big honkin' machines.

    As to why it should work better to invert the parting tool, the theory is that if it catches, it will be pulled down and out of the work. That should prevent stalling type grabs. However, it is essentially the same condition as chatter. It does throw the swarf down though and lessen the likelihood of jamming the parting tool.

    As to cutting from the reverse side, to the part, the tool looks the same in either configuration. There are some geometry differences regarding the carriage, the cross slide and the compound which might play a part in the efficacy though. I would expect that it varies quite a bit from lathe to lathe which might explain why it works well for some and not others.

    Good lubrication seems to be somewhat universal but I have seen videos of parting steel without lubrication that looked like they were parting butter. I have notice in parting that there often seems to be a sweet spot in the cut where it is cutting flawlessly. Inward of that diameter it goes south again. Unless you have a variable speed and power cross feed, there isn't much you can do about this though.

    Super sharp cutting tools with good geometry are also essential as are proper tool setup. Out of the box, the parting tools come with zero rake. If you come in on center, that is what the part sees. If the tool is raised slightly, the part sees a tool with positive rake. Clearance has to be enough that the bottom of the tool doesn't rub though. Again, the lathe geometry and rigidity come into play here.

    Bottom line, don't be afraid to try the various methods. Find the one that works the best for you. I'm still looking for the best one for me.
     
    brino, higgite, Tozguy and 2 others like this.
  3. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    I have no comment on parting out while running the spindle in reverse , but the method I found on LMS site worked for me although it is more time consuming but every thing with mini lathes is time consuming ,lol.
    Here's a copy and paste of how to part out with HSS parting tool:
    http://littlemachineshop.com/products/product_view.php?ProductID=3864&category=

    [​IMG]
    Chris' Tips
    Here's how this cut-off tool holder is used.



    [​IMG]
    P-type cut-off (or parting) blades are T-shaped (don't ask). Because the body of the tool is narrower than the cutting tip, friction is reduced making the difficult task of parting off somewhat easier.

    Size P1 blades are 1/16" wide and are the most commonly-used size. P1N blades are 0.040" wide. These thinner blades are good for small work. P1X blades are 5/64" wide. Use these thicker blades for parting off larger diameters where you need additional stiffness.

    It is easier to part off large diameters if you cut a groove that is wider than the cut-off blade. First, make a cut about 0.100" deep. Now back out and move over at least 0.010". Cut in again, widening the slot. Continue with this cut until you are 0.100" deeper than the first slot. Repeat this process, moving back and forth, until you are through.
     
  4. mikey

    mikey Active User H-M Supporter-Premium

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    I've been using a rear mounted inverted parting tool (P1-N) on my Sherline lathe for decades and have not had an issue parting anything on that lathe. No chatter, no digs, not problems at all. It doesn't matter if my parting tool is extended too far or just right, it still works fine. My typical cutting speed is about 2-3 times faster than typical turning speeds for the material; 1200 rpm for mild steel, for example.

    My parting tool sets the tip of the tool at the exact center height of my lathe. The tool registers on the edge of the cross slide so it auto-aligns the blade. It takes me all of 30 seconds to mount the tool, lock the blade in place and I can cut.

    Parting from the front with the lathe in reverse allows the tool to meet the material in the same way but a rear mounted tool is far more rigid because it is sitting on the cross slide, not the compound. My 11" lathe can part easily from the front with any parting tool I own, including an Aloris tool with a GTN-3 insert, but I much prefer a rear mounted tool and will be making one as soon as time allows.

    Guys have been using rear mounted parting tools for a very long time, especially the Brits. The smaller the lathe, the greater the benefit. If your lathe has T-slots, consider a rear mounted tool on the cross slide - bet you won't regret it.
     
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  5. mikey

    mikey Active User H-M Supporter-Premium

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    Last edited: Jul 17, 2017
    The_Apprentice likes this.
  6. The_Apprentice

    The_Apprentice Canada Active Member Active Member

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    Thanks Mikey. I bookmarked that for now and will re-read over it later on. For now I'll just stick to the hack-saw method... and once I get more familiar, add this to my base... I just don't have a Milling Machine yet, and would like to master my lathe first before getting into too many big-toys at once here :)
     
  7. mikey

    mikey Active User H-M Supporter-Premium

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    If your lathe can run in reverse and the chuck bolts on then you might consider this tool. A parting tool is not just used for parting. If you need to groove something or cut a thread relief before screw cutting something on the lathe then the parting tool will come out. Sometimes you will want to cut an accurate shoulder and the parting tool will allow you to do that. Think about it.

    Better yet, if your cross slide has T-slots, make a rear mounted parting tool holder - it is far better than that tool from EE.
     
  8. Ken from ontario

    Ken from ontario Canada Active Member Active Member

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    I was curious to see how I could part 3/4" Aluminum with my parting tool inverted in the QCTP holder, I soon found out it is not possible to install the cutting tool upside down in the old Tormach OXA parting tool holder because it would not be possible to center the cutting edge any more.it'll be too low and could not be adjusted without having half the holder sick out of the tool post.

    The only possible way other than machining one is to buy a specially made tool holder that Miky linked .but I'm not even sure it will fit an oxa qctp.
     
  9. mikey

    mikey Active User H-M Supporter-Premium

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    You can also just make a parting tool holder out of aluminum to fit on your OXA and it would work fine.
     
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