Thursday, July 4, 2024

Never Machine What You Can Buy

Unless it is very simple, it is always cheaper to buy than make.

Sometimes,  that is not an option.   Big Bertha had an odd looking assembly on which DobStuff who rebuilt it for me had mounted my finderscope.  It clamped onto a 1.25" OD aluminum shaft.   Here you can see the replacement part I machined on thst shaft.

This is the original part:

Hw started with an oddly rounded piece of Baltic Birch, cut a 1 25" hole through it then sliced through one side to allow a 1/3"-20 carriage bolt and thumb knob to clamp down on the aluminum shaft .

I no longer remember what problem causes me to build a replacement.   As you can see, I cut a piece of acetal into a L shape into which I then milled a 1 25' diameter hole.  Then I used a .125" carbide mill to slice a slot across the circle border and drilled a 5/16 through hole to reuse the original carriage bolt and thumb knob.

What gobbled up lots of time.   Writing a C program to create the gCode for cutting a perfect circle.  (A Forstner bit might have worked but I have other applications for the circle cutting program.  Besides, I wanted a very tight fit.  Getting the difference between the post and the hotel to .01'm" seemed desirable.)

Figuring out how to cut a hole for that carriage bolt.  The width of the acetal part was long enough that my drill press would not have enough travel to make that hole.  So I did the first two inches on the mill.  Then I used my 1/4' drill on the drill press to continue the hole.  Still not enough travel.  If only I had a 3" long 1/4" drill!

 Solution: I have several .2010" drill bits of absurd length originally to speed up tapping 1/4-20 holes in ScooeRoller sleeves.  There were long enough to create an accurate pilot hole through the acetal for the 1/4" drill bit; half way through top and then bottom   Because the carriage bolt was still a tight fit, I used a  5/16" drill bit.  Because I still did not have enough travel in the drill press, I did as much as I could, then raised the table a few inches so the drill bit was submerged into the part when I started drilling.  Turn upside down, repeat.

In retrospect,  I should have contacted DobStuff to buy a replacement part.  As with most things, machining the part is not cost-effective until you count what you learned along the way.

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