Wednesday, November 7, 2018

Machining Lessons

Somehow, I knew that tapping should be done with oil, but forgot it, perhaps because I have tapping 1/8" wall aluminum tubing.  I had to do some 1/4" wall, and my luck ran out, breaking my 1/4"-20 drill/tap combo.  Of course, I could not turn it out of the workpiece.  So here's the question: is it the cooling effect of the oil or the lubrication that matters?  Would water be as effective as oil?

I mentioned a few days back my discovery that tapped holes are supposed to be countersunk to help guide the screw into the threads.  I have been using a countersink that I inherited from my father.  It seems to be doing the job, but I think it was really intended for wood, so I have just ordered a countersink set intended for metals.

I have been considering replacing the acetal inserts that hold the casters in the ScopeRoller products with aluminum.  Aluminum is half the cost.  Today I made my first set using aluminum inserts and discovered the additional labor and oil do not create any real savings.  The inserts are held in the aluminum sleeves with 8-32 socket head screws.  With acetal, I can just drill a .140" hole in the acetal, then use a power screwdriver to screw the screw in place; acetal allows it to self-tap.  When the insert is aluminum, I need to tap the hole first.  For a variety of reasons because of my setup, this is a very slow process.

If I can find a way to combine the drill and tap in process, then the aluminum makes sense.  Unfortunately my model of TapMatic does not go down to 8-32 taps.  I would worry about breaking a tap that small under power.

As I was trying to fall asleep, it occurred to me that part of the struggle I was having is that is a bit of a struggle drilling and tapping the inserts when they are in the sleeve and in the drill press vise.  These are not tight fits so if I turn the assembly from side to side, the insert moves and getting the tap into hole I drilled gets harder.  Solution: glue the insert into sleeve so it does not move when I drill the holes.  Epoxy alone is not enough to hold the insets in position.  I would have done that with the acetal inserts to make them easier to drill, bu there is no glue that sticks to acetal.


  1. If your Tapmatic can't do the full job, what about setting up a fixture to hold the inserts to either drill the hole with a hand drill, or with the mini-mill or the mini-lathe?
    Then you can do the same with a tap. I run taps with a hand drill when feasible. Don't use an impact driver in place of the hand drill for such small sizes. Or a hammer drill setting on the drill.

    Some aluminum alloys are easier to work dry than others. The temper seems to be a component of this. Sharpness of the cutting edge, profile, speeds, all have an impact. Plain water is almost never used for machining. Requires additives of various sorts, mostly.

    Countersinks come in a variety of tip angles and cutting edge numbers. 82* and 90* are the most commonly used. Fasteners (flathead) mostly come in one of those profiles.

  2. Drilling fixture (sleeve) should be set up with setscrew(s) to lock the insert in place so it doesn't spin. Bolts, thumbscrews, etc...

    If your sleeve is thin enough, you could slot it so it could be squeezed with a hose clamp instead of using screws. (cat, skinning, more than one way of)