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  1. #1
    Join Date
    Feb 2013
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    Laidley, SE Qld
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    Default Turning shallow domes on the ends of 316 SS round stock

    I need to turn the ends of between 8 to 12 pieces of 22mm dia 316 SS into a shallow dome, dome radius will be 30mm, dome height is around 4.5mm.
    This is purely cosmetic no accuracy required.
    The domes need to have a polished finish so I want to turn them as smoothly as possible in the first place.
    This will be done on a Hafpos AL340 lathe.

    For those with experience in this I guess the questions are.........
    The quick option of course is to make a form tool, will that give me a smooth surface or am I going to have serious chatter? I'd be working right at the chuck and I'd step turn the domes in .5mm? steps before using the form tool.
    Or should I spend more time upfront and make a quick and dirty 30mm radius turner?

  2. #2
    Join Date
    Apr 2012
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    Healesville
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    2,129

    Default

    do you have a ball turner?

  3. #3
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    Default

    Hi Bob,

    Don't mess about, grab a woodwork router rounding bit and have at it...

    28012015-007.jpg
    Obviously you have to get the cutting edge on centre hight. In my case it had to align with the bar end.
    Best Regards:
    Baron J.

  4. #4
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    59
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    6,540

    Default

    I doubt you will find a router bit with a R30 radius on it though John.

    A ball turner will do the job, but for a small quantity, you might be better turning to a rough profile and then smoothing with a file. A cardboard template will be accurate enough or you could go 'high tech' and use a profile gauge normally associated with laying tiles.
    P1040488.JPG

    If you do decide to rough turn then file I'd suggest a large radius cutter (this one is R3/8") rather than a something pointy, as it means that you are less likely to get those annoying circumferential grooves in the part if it digs. It is surprising how quickly you can get the coordination with both hands to almost free hand a curve on a part.

    Michael

  5. #5
    Join Date
    Sep 2011
    Location
    Ballarat
    Age
    65
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    3,103

    Default

    Hi Bob
    I also have an AL 340 and I would use a form tool and plunge cut but with that grade of stainless I would run the lathe as slow as possible and plunge fairly hard using cutting oil not coolant.

    Phil

  6. #6
    Join Date
    Nov 2010
    Location
    Gippsland Victoria
    Posts
    733

    Default Chatter with form tool

    In one of his books L H Sparey gave the tip that running a form tool on a rear mounted toolpost can significantly decrease chatter, and that he ran 1" radius form tools on a little myford lathe with no chatter. Similar principle to rear mounted parting.

    Might be worth a try if you get too much chatter on a conventionally mounted tool doing the 30mm radius.

    Bill

  7. #7
    BobL is online now Member: Blue and white apron brigade
    Join Date
    Feb 2006
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    Perth
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    7,182

    Default

    Quote Originally Posted by Michael G View Post
    I doubt you will find a router bit with a R30 radius on it though John.
    If its cosmetic and no accuracy required maybe a 1 1/4" (31.7mm) or1 1/8" (28.6mm) router rounder bit would do?
    Local prices will be expensive (~$100) but amazon has them a bit cheaper for about $30 plus $35 shipping!

  8. #8
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
    Posts
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    Default

    Hi Guys,

    My router cutters came as part of one of those Chinese £10 boxes that Aldi and Lidl sold a while ago. Most of them have 1/4" shanks, a blessing and a curse is some ways. I also have a heavy duty JCB router that came with a box of 1/2" shank tools. There are 25 and 30 mm radius router cutters on Banggood.
    Best Regards:
    Baron J.

  9. #9
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    Default

    Quote Originally Posted by steamingbill View Post
    ... running a form tool on a rear mounted toolpost can significantly decrease chatter, and that he ran 1" radius form tools on a little myford lathe with no chatter. Similar principle to rear mounted parting.
    Rear mounted tools 'do better' because they use a section of screw (and slide?) that is normally less worn; on the down side they cause forces on the carriage that a lathe is not designed to deal with. Chatter is a spring action - the forces cause the tool post etc to deflect, this deflection reduces the forces, so it bounces back and so on. If your lathe is not very rigid it will be more prevalent. The solutions are to scrape gibs/ slides in, minimise overhang, make sure screws and nuts do not have excessive backlash and don't overload tools.

    If your driver's seat was broken you wouldn't drive your car from the passenger's seat, so why do something similar with a lathe?

    Michael

  10. #10
    Join Date
    Nov 2010
    Location
    Gippsland Victoria
    Posts
    733

    Default Myford rear mounted accessory table

    Quote Originally Posted by Michael G View Post
    Rear mounted tools 'do better' because they use a section of screw (and slide?) that is normally less worn; on the down side they cause forces on the carriage that a lathe is not designed to deal with. Chatter is a spring action - the forces cause the tool post etc to deflect, this deflection reduces the forces, so it bounces back and so on. If your lathe is not very rigid it will be more prevalent. The solutions are to scrape gibs/ slides in, minimise overhang, make sure screws and nuts do not have excessive backlash and don't overload tools.

    If your driver's seat was broken you wouldn't drive your car from the passenger's seat, so why do something similar with a lathe?

    Michael
    Thanks Michael,

    Have been pondering your reply for a while now.

    The way I see it ........ if I can get some benefit by managing to use the relatively underutilised and therefore unworn parts of the lathe then there is no harm in trying. If the downside is that this causes harm to the lathe then that's something I need to bear in mind. I simply do not have the skills to scrape in the gibs and slides but I can do the other things suggested above.

    Myford made a very solid accessory for rear mounted tools, maybe that addresses some of the concerns raised above, see attached, from "A Man and His Lathe" by L H Sparey. It doesn't address the 180 degree change in the direction of the forces. The attached short description might be of interest to some people.

    Bill
    Attached Images Attached Images
    Attached Files Attached Files

  11. #11
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    Default

    Bill, at the end of the day it is your lathe and you decide how you use it. I did not mean to cause you 3 month's worth of reflection, so I apologise if my comments came across too strongly.

    Rear mounted tool posts are a pet hate with me as too many small lathe owners see them as a cure for all manner of problems, without putting any thought into why the problems are there in the first place and/or taking steps to address the underlying issues. Some people want to use that space, and I can accept that as a reason - however, if they make a massively solid tool post for that rear position and then claim it has cured another problem they had (that was really because their front tool post was on the soft side), I would much prefer to read about the efforts that were made to fix the front post. (Tubal Cain's Gibraltar tool post springs to mind)

    Michael

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