This all revolves around a Sherline mill, so it's tiny.

Mine came with some small collets and a 3/8" endmill holder that threads
onto the spindle with a 3/4-16 thread.

The holder itself is just a threaded cylinder with a 3/8" bore, grub screw
and holes for the tommy bars to tighten it onto the spindle, which also
has a hole drilled though it for another tommy bar. No wrenches or
anything like that are needed.

These were all made by Sherline and run true, no matter how I measure it.
The OD of the holder wobbles less than 0.001 and if I try to measure the
3/8" bore with an indicator on the shank of tightest endmill, it's barely
wiggles the indicator pointer. This matches on the lathe or mill, so it's
pretty consistent that is all lines up.

Good stuff.

I just got some "A2Z CNC" stuff which as far as I can tell, is just
junk. I have a quick change tool post from them which is iffy as well. It
shipped with the wrong length screw, and the tool holders are quasi
threaded through, where you need to chase the treads with a full out
handle hex driver for the first use. It's just cosmetic, but a couple have
that made from the end of a piece barstock look.


Both of the A2Z endmills holders wobble about 0.005" or more. The runout
is clearly visible. The OD is better, but the bore is not concentric at
all.

The threads are loose and the things wobble around on the spindle thread
until you tighten them down. The Sherline holder has a firm fit, but never
grabs or jams up.

One has deeper threads than the other too.

Anyways, this stuff is going back since it seems useless if it can't even
match the accuracy of a drill chuck.

The question is are there rules of thumb for how much runout is
acceptable for a small machine like this?

On another rant, I have a new no counry of origin marked 1/4" Jacobs drill
chuck. It's horrible as well, it feels rough and can't be broken in. I'm
guessing it's chinese and can't even be opened to polish the rough parts.

Every Rohm chuck I have is from and seems like a German machined product.

I have a real small 5/32 Yukiwa chuck which is also really nice.

Are all Jacobs drill chucks junk now?