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Boring Head Blues

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  • Boring Head Blues

    Arrrrgh! Just used a 2 1/2" boring head in a Bridgeport M-head to enlarge a 7/8" hole 2" deep to about 1.070". Material = old cast iron. Started out using a 1/2" shank boring bar that cuts well when used in my lathe. I lowered the quill rather than raising the table. First few shallow cuts went OK. Then the surface finish started getting ropey, and the vibration got worse until the tool just about stopped cutting. The hole was seriously tapered, too: .020" less at the bottom than at the top. Tried different feeds and speeds and several different boring bars and cutting tools trying to smooth the surface and remove the taper. No joy. Finally I tried a sharp V-shaped tool with no radius and lubricated with kerosene. Very slowly, I got things smooth and straight. Where did I go wrong?

  • #2
    you dont say what the boring bar is made of .was it tipped ..
    you need carbide with cast iron ...hss no good

    also the longer the bar the more deflection and resonance ..
    short fat ones are prefered.

    all the best..markj


    • #3
      Maybe? Loose, Or on their way out spindle bearings. Something for sure doesn't sound right. With my mill I have power down feed and I bought it with boreing in mind. I'm certainly not going to use the knee feed when I have this feature. So. If? your cutting tool was sharp and correct along with the spindle speeds then I'd look real close at the bearings, Or maybe the wear in the spindle sleeve. Definately repost what you find.



      • #4
        Id blame the quill if extending it made it worse.
        Yes, it might be tight enough to mill with the quill unlocked, but I bet draging the quill lock clamp might help some, also for those with power downfeed, id suspect the quill might be a little higher quality fit then a standard mill.

        And generaly, it will just allways become less rigid with more quill extended, so it just may be that for that loss of rigidity you NEED a tiny radius thick bar to do the boring with light cuts.
        What works fine in a super rigid lathe is not gonna work well on a floopy quill with a floopy boring head. Also, minimize the overhang of the boring bar out of the boring head if possable.
        Remember: All metal is flexable as a wet noodle on the scale we work at, even 3" diamiter steel quills.
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