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  1. 14th June 2015, 06:28 PM #1
    sacc51
    sacc51 is offline SENIOR MEMBER
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    Default Trapezoidal/acme

    I’m trying to cut an internal 14mm x .2mm x 30mm long trapezoidal thread. First I tried grinding a blank and using that in a tool holder but the bit ended up so thin it just flexed. Next I tried a very short cutting tool in a boring bar but the boring bar at 10mm round and the hole for the thread at around 11.2mm just doesn’t leave room for advancing or backing off. I’ve never seen a boring bar smaller than 10mm and at that size it would probably flex too much anyway. I've cut trapezoidal/acme threads before without problem, just none this small, any ideas on a suitable tool.
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  3. 14th June 2015, 06:45 PM #2
    pipeclay
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    Grind the back of the bar to suit your needs or make a bar to suit,I had to do this for a 1/2" square thread,or you could try a tap if you have sufficient rear clearance.
  4. 14th June 2015, 06:59 PM #3
    .RC.
    .RC. is offline Pink 10EE owner
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    Make a tap and use it....

    When I rebuilt the 10EE, I needed a new cross slide screw.. So I used 4140 steel, made one longer then I needed, cut the screw to length and with the extra, made a tap out of it to thread the nut...

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  5. 14th June 2015, 07:39 PM #4
    Steamwhisperer
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    When I have that problem I mount the nut in the tool post and place the tool in a bar held between the chuck and a live centre in the tailstock.
    Basically a boring bar that you make yourself held in the chuck and supported by the tailstock.
    I also use this when boring out long brass bushing.
    To take a deeper cut I set up a dial indicator on the tip of the tool and tap it out the required distance.

    Phil
  6. 14th June 2015, 09:15 PM #5
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    Oldneweng
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    Quote Originally Posted by Steamwhisperer View Post
    When I have that problem I mount the nut in the tool post and place the tool in a bar held between the chuck and a live centre in the tailstock.
    Basically a boring bar that you make yourself held in the chuck and supported by the tailstock.
    I also use this when boring out long brass bushing.
    To take a deeper cut I set up a dial indicator on the tip of the tool and tap it out the required distance.

    Phil
    For threading you could just advance the cross slide tho?

    Dean
  7. 14th June 2015, 09:52 PM #6
    Michael G
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    Won't work Dean. Moving the cross slide will move the centre of rotation across, where as what Phil is doing is leaving the centre where it is and enlarging the rotation of the tool tip.

    Michael
  8. 14th June 2015, 10:31 PM #7
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    Oldneweng
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    Of course.

    Dean
  9. 14th June 2015, 10:38 PM #8
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    BaronJ
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    Quote Originally Posted by Michael G View Post
    Won't work Dean. Moving the cross slide will move the centre of rotation across, where as what Phil is doing is leaving the centre where it is and enlarging the rotation of the tool tip.

    Michael
    I'm glad that you spotted that as well ! I was going to say that it wouldn't work.

    Somewhere kicking about I have a thin (8 mm diameter) boring bar that has a 100 mm length of cut. It is used between centres and the blade is adjustable in a similar way to an adjustable reamer. From memory it was made to cut a square thread, though I don't recall the bit size. It was one of the tools given to me many years ago. I do confess that I've never actually used it, its probably rusted up by now.
    Best Regards:
    BaronJ.
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