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  1. #1
    Join Date
    Feb 2014
    Location
    Newcastle NSW
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    78

    Default Parting and tool holder issues...

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    I have recently purchased this tool holder from ebay and a parting blade from Hare and Forbes.
    After setting the tool in the holder dead centre and trying it out on some round bar, it still grabbed and the chuck and headstock appeared to lift up while this occured! I have a 1959 Hercus C model and am wondering if there is damage underneath the headstock causing it to lift or is it the bed and headstock combination flexing?
    Maybe parting off anything other than brass or copper is simply too much to ask from the lathe?

    I have noticed that this site is very quiet with no updated posts but thought i'd ask here anyway and if anyone has links to a more active forum would be appreciated.

  2. #2
    Join Date
    Nov 2007
    Location
    melbourne australia
    Posts
    3,228

    Default

    You should be able to easily part-off mild steel with your lathe. You don't say what sort of "round bar" you have. I would start with free machining steel like 12L14 until you have a handle on parting-off. I would have gone for a thinner parting blade than 2.4mm. I use a 2mm blade on my 260 and would get a thinner one if I was buying again.

    Is the blade sharp? I regularly give mine a lick on the flat side of a CBN wheel. It can make the difference between awful chatter and smooth sailing. Leave the blade in the holder when sharpening.

    Keep at it. You'll get there.
    Chris

  3. #3
    Join Date
    Nov 2017
    Location
    Geelong, Australia
    Age
    57
    Posts
    2,651

    Default

    Have you got the carriage and compound locked up tight, with only the cross slide able to move?
    You could also try turning the compound in line with the cross slide that way if there IS any play in the dovetail it will be supported over a longer length rather than trying to roll the compound over sideways.

    One suggestion I've seen online (never needed to use it myself) is to put some form of support under the bottom of the blade to brace it against the cross slide. Another is to set the tool slightly low so if it bites it tends to move away from the axis of rotation rather than digging in deeper.

    Rigidity is everything in my limited parting experience.

    From distant Hercus memory the only way the headstock end of things could lift up is either the clamp bolt that holds it to the bed is loose, or if you've got significant wear in the spindle bearings. I think you'd be seeing other issues if either of those was the case but worthwhile checking.

    Steve

  4. #4
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
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    7,182

    Default

    The first thing I would suggest is practice at slower rpms than you would otherwise use for normal turning.

    When I first tried parting many moons ago I remember trying to start at too high an RPM and too fast a feed rate, then the cut speed became to slow for the RPM so one tended to feed too fast and that jammed the parting tool.
    Years later when I got back to turning the same thing happened but now one of the benefits of using a VFD is you can start out slowly and get a better feel for rpm V feed V radial position and change the speed on the fly to suit the conditions.
    Once I practiced this a few times I found I could really use the VFD to good effect, start slowly and use a constant feed and increase the RPMs periodically to maintain a decent cutting rate.

    I also switched to a T cross section parting blade from eccentric Engineering and that copes with a much wider range of speeds and feeds than a regular/striagh parting cutter.

  5. #5
    Join Date
    Nov 2018
    Location
    Tasmania
    Posts
    97

    Default

    I've always had problems with my model C (more me than the lathe)
    My biggest problem was that I had a scare early on and was too cautious, from then on running at the lowest speed etc and couldn't match the feed to the speed

    You don't want to part off at normal turning speed, but don't slow down too much. You want a speed that you can match with your feed. I try and get a chip forming and feed in to chase the chip and keep it curling. Your feed rate may adjust slightly depending on depth

    I always lock at least 1 gib screw on the comound and tighten a cross slide screw (but can still move with some effort)

  6. #6
    Join Date
    Oct 2011
    Location
    Norwood-ish, Adelaide
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    59
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    6,541

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    Hmm.
    This is not Hercus specific issue, so could be discussed in the general metal work area, but parting is not radically different enough to any other turning operation such that it needs 'special' speeds.

    There two factors that complicate parting
    • You are cutting in a narrow groove, and the chips need to be extracted. Carbide parting tips usually have some shape to them, to try and 'fold' the material into the centre of the tool. As soon as it binds up between the side of the cut and the tool, you are screwed. Therefore, make sure chips can get out easily. This may mean putting a groove on your HSS tool. Coolant/ lubricant also helps as it stops material building up on your cutting tool. Especially important for Aluminium. I part using power feed but particularly when parting larger diameter material (say steel over 25mm diameter) I will stop the feed occasionally and allow a little bit of time for the chips to clear before re-engaging.



    • Rigidity. Here, a worn lathe is not your friend. Any movement that means the tool can 'dig in' has potential to upset your parting. This could be due to
      • a flexible work piece
      • worn headstock bearings
      • loose gibs
      • worn gibs
      • worn lead screws
      • tool not exactly on centre
      • short material that can twist in the chuck
      • poorly ground tool
      • metal with hard spots in it
      • too much stick out


    The list goes on. Especially if your lathe has plain bearings, check for wear. If they are rolling element bearings, check that they are correctly pre-loaded. Gibs should be snug and apply even pressure across the whole traverse. A 'normal' lathe tool is relatively rigid and has plenty of space for the chips to go. A parting tool can flex so a change of cutting forces can make it either twist or spring. Neither is good as they both allow the tool to dig in, increasing the load on the tool and so increasing the risk of breakage. Make sure that the parting tool is only as long as it needs to be, and very important, is set to cut on centre, otherwise it will either rub or dig. A worn leadscrew is not a killer, but if the tool does grab, it allows the tool to pull into the work. Don't feel so quickly that the material is springing away from the tool. Once again it will grab, especially if that screw is worn.

    • Ensure that the parting tool feeds in square to the spindle axis. If it does not, you run the risk of the blade jamming in the cut. If you get a QCTP, centre height becomes a non issue but if you swivel it around, get into a routine of squaring it back up every time you move it from square. I'm of the school that believes it is better to grind a funny angled tool than move a QCTP away from square. Again, your call.


    Some people advocate rear mounted parting tools. All that does is potentially move the cutting forces to a less worn part of the lathe, while doing strange things against the way a lathe normally cuts. Your call, but like slowing down the speed, does not really fix the problem. The only reason to slow down is chatter. Slow down your feed in if the lathe is bogging down.

    I've broken parting tools because of pretty much every reason I've given, wear and movement of the lathe/ material being the main culprit. It is not hard to scrape gibs back to flat, and if your spindle bearings are not right then fix them anyway as they contribute to poor surface finish as well as other issues. Fit a QCTP (best improvement you can make on a lathe) and control your stick-out, thinking a bit about how to get the chips flowing out.

    A few thoughts,
    Michael

  7. #7
    Join Date
    Oct 2004
    Location
    Southern Highlands NSW
    Posts
    1,894

    Default

    Maybe you should check that the bolts clamping the headstock to the bed are tight.

    Toolholding rigidity should prevent the cutting tip from being pulled into the job.
    But there is another approach, that doesn't rely on rigidity but deliberate designed in flexibility.
    The scheme guarantees the tip will always move away from the job, if it grabs.
    It is a very smooth working tool, and is what was supplied with Hercus lathes, I think.
    Attached Images Attached Images

  8. #8
    Join Date
    Aug 2016
    Location
    Adelaide
    Posts
    80

    Default

    I had a similar jam when parting on my Hercus 260. When it happened it appeared the headstock moved up but I think the tool nose just dipped and material moved up in the chuck. I checked the 2 bolts holding the headstock to the bed, which were ok and the run-out between headstock and tailstock which was about the same as before. I think one of the causes for my jam-up was the blade was set a bit low. When the material diameter reduced to a point, the tool grabbed (exacerbated by the built in rake of the tool holder) causing the material to ride up onto the blade, break the tip and jam. That my theory anyway. I also found my compound slide to move due to chatter when parting. Last few times I've clamped or held the compound which as helped but I still need to find the source of the chatter.

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