is it the arbor, or the mill?

The arbor for My First Shell Mill (new from Fisher-Price!) arrived today, and it seems to be a thing of beauty and a joy forever:

But experience has taught me not to be a trusting soul, so I thought I'd put the thing in the mill and put an indicator on it to check the runout.

No doubt those more experienced than I are already chuckling at my expense.

 I got a real shock when the indicator showed me a solid .001" TIR (± .0005) of runout, measured barely an inch below the R8 spindle nose!  I would have thought that would be unlikely given the precision of modern automated grinding machines, and given the ease of maintaining good dimensions and geometry on a cylindrical feature.

Indicating the inside taper of the spindle was going to require me to bend over more than I wanted to, so I tabled that for the moment and took the arbor over to the surface plate.

I set up the arbor on two vee blocks...

Oh bother; the two features I'll be rotating on are two different diameters... I need to shim up one of the vee blocks... well, paper is very consistent on its thickness... our printer paper is... hey, that's convenient, it's exactly .005 thick and I need one vee .120 higher than the other... so I need 24 sheets of this particular flavor of printer paper. Sweet, done.

Ahem. I set up the arbor on two vee blocks, fastened an indicator to some 1-2-3 blocks as an improvised height gauge, and measured the by-god-actual runout of the arbor... which was zero.

We now scowl at Mr. Bridgeport in a very disapproving manner.

Indicator goes onto the (rather rough and beat-up) outside nose of the spindle.

One thou of runout.

Hmph.

Indicator goes inside spindle to measure actual taper that touches arbors and collets.

One thou of runout.

>> Outrage face <<

I stomps myself into the house (no computer in the shop, yet, still, some day, need to find the space) and I searches YouTube and DDG and Google and I got learnt!

What I found surprised me: there is a lot more spindle runout tolerated and expected from general purpose mill spindle bearings than I had expected.

In all likelihood, the spindle on this machine had a max runout of .0005 when it was new.

Everyone agrees that more than .004 or .005 is unacceptable and time to replace or rebuild the spindle.

.001 now is actually quite good for this machine.

Well, if the spindle is free to flop around so much, how is this machine and others like it holding tolerances as tight as .001?

A moment's reflection, and something I had never thought of before this hour, gave me the answer.  It took me ten minutes of searching on the internet before I knew I was right, though. Yay, good intuition... it's the machining reaction forces!

The moment that cutter touches work, quite strong forces - ranging from ounces to hundreds of pounds - pushes against the spindle bearings, taking all the slack out of all the moving surfaces, and so long as that cutting force remains fairly constant, no sloppy movement should occur.

And then one of those cartoon light bulbs went on over my head as I realized: I never could get good surface finishes the few times I tried non-helical cutters... well, helical cutters tend to keep the cutting force constant if the cut is over the side face of an end mill.

Learn something new every day.

Side note: if I'm still learning, then I am still "swimming upstream", therefore I am not a complete loss to society, yet. Just remember though: the only fish swimming with the current... are dead.

Sadly, it will be another week at least - if I can trust the tracking information from China's Post, which is roughly as accurate as British Rail timetables in the 1980s - before I receive the shell mill itself, and am able to evaluate the real, fundamental Quality of my "bargain".