Saturday, September 26, 2015

Shop Time - A Little Trick on the Lathe

Chances are most real machinists know this and do this, but I'm still learning.  The first parts I'm making for my Grizzly G0704 CNC conversion are threaded standoffs.  The lengths vary, but they're all tapped 10-32 threads 3/4" deep on axis on both ends.  This is a natural job for the lathe. 

I did the first four with a standard hand tap, the kind I've used manually many times.  These have straight relief slots along the length of the threads, like this, and are usually driven with a small, special wrench - they're square drive.  As part of the tapping, you typically drill a hole to a set size (depending on diameter and material), then screw the tap into this hole, advancing a little and then retracting the tap to break chips and get them out of the hole. On the lathe, I put the standoff into the lathe chuck, the tap in the drill chuck held in the tailstock.  While I tried using power, that didn't work well, so I ended up manually turning the lathe chuck.  It was slow and tedious.  We're talking maybe 15 minutes to do one hole and I have many holes to tap.

I did some research and the issue with power tapping is the reciprocating action; driving the tap forward a turn or so, then backing it out half.  There are machine attachments available, but they cost much more than the project.  I thought that perhaps a spiral flute tap would work better because they're designed to clear out your chips faster.  

The spiral flute taps arrived yesterday and I got to try them out.  It increased the speed many times.  I wouldn't doubt I went from 15 minutes (or more) per hole to about two minutes.  This video is 2:12 and shows the whole process.  You can see how it works: I start the lathe in its lowest speed and bring the tap up to the pre-drilled hole.  The tap is held in the chuck in the tailstock with a drop of cutting fluid on it, but the tailstock isn't locked down, allowing it to move freely.  The tap literally screws itself in while cutting threads, perhaps a half inch, pulling the tailstock with it.  At some point, it's too much load on the tap and it will start to spin in the chuck.  At that point I put the lathe into reverse, backing the tap out.  Once the tap is backed out and cleaned of caked on aluminum (swarf), I put the lathe in forward and repeat.  Most of these took two plunges of the tap, but some took three.  Not quite sure why.

Hope this is useful to someone.


  1. Wow. I am going to put this one into the permanent part of my brain because sooner or later this is going to save me a lot of work. (although sometimes I think the permanent part of my brain is eroding faster then a sandcastle at high tide)

    I also looked up some other thoughts on power tapping.

    And I took a glance at thread forming taps.

    And the only thought that wrinkled my brow, do you need to have that much length of thread engagement?

    Thank you for posting the video.

  2. "... do you need to have that much length of thread engagement?"
    Probably not. The drawings call out 3/4" deep threads and 3/4" cap screws but another piece of metal is going on the end of that spacer, and I'll bet 1/4 to 3/8" gets engaged.

    The next pieces are about 1" long with 10-32 threaded from both ends. If I remember correctly.

    Also, good to know it's useful to someone else. The last time I tried this, I did four standoffs, that's 8 holes tapped, and took the entire afternoon. This time four standoffs didn't take an hour - including setting up to take the videos!

  3. "The next pieces are about 1" long with 10-32 threaded from both ends. If I remember correctly."

    A quick trip down to the machine shop/secret laboratory/wine cellar (smells great right now as three five gallon pails of red juice are in the final phase of the primary ferment) shows that a 10-32 tap will most likely not thread all the way through the 1" piece. So, is it time for a redesign? might be a lot easier to use a longer fastener and to drill the pieces for 10-32 body size and then use a self locking nut or nut/lockwasher/washer?

    This week's shop project was to take the cross slide and the apron apart to clean and tighten gibbs. I removed some grease and dirt that had not seen the light of day since mainland China.

  4. Were you doing those dry?

    If not, what lube were you using?

  5. Dr.Jim - I was using Tap Magic. Just one drop before the first plunge cut. I think enough stays in there to do the entire tapping.

    WD40 works well on aluminum, as does paraffin.

  6. I've used all those, too, in addition to others when tapping steel.

    I pretty much stick to TapMagic these days as I bought a gallon of it sometime ago, and have plenty left!

  7. I machine mine from delirn block, or layered acrylic blocks.



  8. Hi Graybeard

    Try roll form taps..( they use a larger diameter root drill) and are stronger
    as well as not creating any chips. It does take a little more torque though.

    and Alumtap for aluminum


  9. I see you have the quick change toolpost-I just got one and like it a lot, except for doing cut off. These always seem to be a problem. This is when I replace the original steel toolpost. With the q/c, I can watch the entire toolpost flex when it's getting ready to grab. I get the impression that a lot of hobby workers turn a guide groove and finish the cut with an Armstrong cutter (hacksaw).

  10. Ritchie, if there's anything I don't like about this setup, that's it. The holder doesn't hold the cutoff blade securely and the whole QCTP wants to rotate. I was thinking it's just defective but I don't know.