Tuesday, August 1, 2023

Today's Machining Lessons and Boo-boos (Pretty Much the Same Thing)

I needed to make some spacers to fit a 3.5" x 1.5" inside rectangular tube to a 3" x 1.5" leg.  Adding to the fun, there are a couple of screw heads sticking above the top of the leg, so I need to mill a slot in the spacer allowing it to slide over those heads.  The outer leg which this sleeve slides over has a 3/8"-16 thumbscrew that locks the inner legs in place.  My goal is to reuse that thumbscrew to clamp the sleeve onto the outer leg.  The inner leg will not move because it will rest in the sleeve.

The spacer thus also provides a clamping surface that distributes the pressure of the thumbscrew over an area of plastic to prevent marring the surface of the outer leg.

To make these spacers was a bit of a struggle.  They are roughly 1.45" by 1.85" by 0.5" (which likely needs to be reduced to 0.4" to make the fit loose enough to slide in easily, along with milling that channel for the screwheads).  

These are too small to safely cut on my chopsaw.  (I like my fingers.). So I cut a 6" x 1.5" or so piece of acetal on the chopsaw, clamped to the fence with a carpenters vise.  The long things with reversible jaws.

To cut these into 1.45" long pieces with minimal waste, I pulled out a .125" end mill that I bought several months ago for another project. With this I was able to cut across at 1.85" with only .125" of acetal lost.  The end mill is 0.5" cutting length, so it just barely did the job.
i was worried at first while falling asleep last night counting thousandths instead of sheep that I would not have enough length to do all three spacers from that one piece.  For that reason, I was thinking of how to attach this 6" x 6" sheet of acetal to the mill.  It is clearly too large to put in the mill vise.  I think if this ever comes up, I will use the hold down screw and T-nut assemblies to hold the sheet on top of parallels (they are all the same width) directly to the table.  The parallels will add enough space to cut off with the .125" end mill.

Lesson learned: When the end mill is substantially smaller than the end mill holder, it is surprisingly easy to forget this and run the holder into your mill vise, which makes a horrible sound and mars the vise.  It did no functional damage (except to my ego).  I rather doubt any machinist uses a mill vise with any regularity without inflicting some cosmetic defects on it.  At least that is today's rationalization.

The milling of the spacers worked pretty well.
it is not a press fit but there is no real slop.  Two 8-32 screws will hold the spacer in place.  Clearance hole in the aluminum and tapped holes in the acetal.
In between those 8-32 screws goes the 3/8"-16 hole for the thumbscrew.

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