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  1. #1
    Join Date
    Oct 2011
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    Default On parting and rear mounted parting tools

    Now the subject of rear mounted parting tools has come up I thought that a separate discussion might be of benefit. I did consider posting this in Ironbark's parting tool thread but decided that as it was a distinct sub topic another thread may be better.

    Parting is one of those operations that causes a lot of difficulty. Because of the width of the blade far more reaction force can be generated than from normal turning. This can be an issue for smaller lathes. To part successfully calls for a rigidly mounted work piece and a rigidly mounted tool that is both perpendicular to the work piece and properly at centre height. Power also comes into it too – a small lathe is not going to be able to part through material as quickly and easily as a large one. Carbide will probably only make things worse as it usually requires more power and greater rigidity. My sub 1 HP lathe struggled with a carbide parting tip. The current lathe (3HP) doesn't have a problem – to the extent that I part using power feed most times
    The idea that rear mounted parting (RMP) is better because of tool deflection is not the whole truth because how well a lathe parts depends on a raft of factors. A turning tool is going to deflect away from the cut. On a front tool post this is down towards the bed, with a RMP tool this is up. The tool is a cantilever, so it will always pivot away from the work - front or back. How much is of degrees - if the front tool post rigidity is very poor the centre of rotation will be low and there will be less tendency for the tool to swing away and lessen the depth of cut, if the tool is far less rigid than the post there will be a greater tendency – this is the principle that goose neck tools work on
    Quite simply, a RMP tool only conveys two 'benefits'

    • When applied it uses a portion of the cross slide screw that is normally relatively unworn while the spindle reaction force pushes the spindle down and
    • Allows a tool to be permanently mounted on the cross slide and not take up space on a front tool post - although when you consider that the cross slide travel of most lathes is not large this is only useful if work diameter is limited to a fraction of the swing of the lathe and/ or you don't use the compound slide.

    Rear tool posts are credited with miraculous results and almost every time the subject of parting came up in ME or MEW, the advice from others would be the same – slow down and use a RMP tool. Slowing down should not be necessary. The material being cut does not know that you are using a parting tool and so does not magically allow the recommended surface speed to reduce.
    As for a RMP tool, most newer lathes these days use rolling element bearings so there is no reason that there should be spindle play. I freely concede that if you have plain bearings there may be wear – but then that will affect everything and should be fixed anyway. On a correctly set up lathe there should not be play in the jibs and while there may be wear on the screw when parting the force on the screw is all one way (does not reverse) so should not pull the tool in.

    So in summary, unless you have a worn lathe, a RMP tool should not magically make your parting better. Effort would be better spent working on the lathe to improve rigidity and parting tool set up.

    Michael

  2. #2
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    Default Just the thoughts and observations of a book machinist

    Hello Michael,

    I use a rear tool post mounted cut off blade because when I replaced the standard cross slide on my little '69 Hercus with the extended tee slotted slide from the later Hercus 260, the height of the slide increased by about 4mm. This height increase prevented my use of the the only cut off tool I had, a Lock holder that required mounting in a lantern tool post. The Lock certainly does not offer the rigidity of the rear mounted post. When I did use the Lock 'n Lantern on my older plain bearing '58 Hercus it was not an overly relaxing experience. Wear and the lack of rigidity came into play. It was less stressful but more strenuous to resort to the use of a hacksaw as a parting off tool. And that was back in the pre 4140 days.

    One of the often touted advantages of the rear mounted tool is that cutting forces raise the cross slide tightening the dovetails thereby increasing rigidity. I have found that using the tailstock centre for support is essential for most of my parting off.

    Another bonus of my "RMP" is assured squareness of the blade to the work. And my post is secured with a single fixing making its removal quick and easy.

    BT

  3. #3
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    Cool

    I built a rear mounted tool post with a cut off blade and it worked great. Then i started grooving the pulleys i make and i do it from the front like 90% of most people would. Once i got the bugs worked out it worked great also, never went back to the rear post. I found that the cross slide gibs need to be a tad tighter to keep the chatter down and running in back gear on larger dia parts works better too. The smaller dia the part the faster spindle speed you can run. Just my thoughts...Bob

  4. #4
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    Default

    It's all good Bob(s).
    Re-reading the post I sound a little heavy handed (my excuse is that it was 3am and I was having trouble sleeping). However, I was just trying to point out that for most people a rear mounted parting blade should not be needed. There seems to be a bit of a myth about how wonderful they are and how they will solve all parting problems. In reality the problems should be fixed first before going that way as otherwise the underlying issues are just being hidden.

    Michael
    (Book, magazine and DVD machinist)

  5. #5
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    Default

    What puts me off is the thought of not being able to see the chip flow - in the case of an upside down one. Also, whichever way up it is, your feed direction is reversed. I can see myself flinching the wrong way when the cut starts to grumble. But people seem to like them so I guess it's something you get used to.

  6. #6
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    Default

    So from reading the above post weather or not front or rear is better, would not the same result as a rear parting off tool be acheived by turning the parting tool upside down and runing in reverse.

  7. #7
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    Default On parting and rear mounted parting tools

    Correct. It would do the same thing
    As long as it wasn't a screwed spindle of course.

    Phil

  8. #8
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    Default

    I agree with you Michael, despite never using a rear toolpost. When i first tried to part i had a hell of a time, chatter, catches to the point of smashing the insert, etc. Locking may saddle does very little, i still do it, but the big change was when i put a pair of locking bolts on the compound slide. Just 2 M5 adjustable handled screws, the indexable type. It made a huge difference straight away, to the point of being able to get nice long shavings that only break when you pause the feed. I still have to use the tailstock for support, even if cutting right at the chuck, but my bearings are needing attention.
    Parting is now no longer the scarey proposition it once was.
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

  9. #9
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    Default

    unless you have a worn lathe, a RMP tool should not magically make your parting better.
    Finally!
    Someone who puts an end to the ferry-tale.
    In fact, there is only the advantage noted re the worn spindle. The rest are only disadvantages:
    With a RMP, the cross-slide is lifted upwards. In normal operation, it is pressed downwards, following gravity (and thus already pre-loaded). This only increases chatter for the RMP.
    Also, if you look at the rear lower way of the saddle, it often is neglected. Not properly adjusted, or even completely missing. Because there is no need for that way if the bed is wide enough. That's just an other way that has play and adds to the chatter with a RMP.

    Adjust your spindle, adjust your ways and be happy.

    What most people do wrong with parting off, is that they don't feed steady and fast enough.

    On my CNC, I part off with 100 m/min surface speed (constant surface speed, so RPM increases up to 2000 RPM) and 0.1 mm / revolution feed. That IS scary, but it works great. I always close the door and search for cover.


    Nick

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