Re: [CAD_CAM_EDM_DRO] Re: small spindle ?

On 06/09/2012 05:30 PM, Larry Guthrie wrote:

> The runout would depend only on the quality of the bearings.

Not true. The shaft also affects runout. Let's start with a simple
shaft with a hole and setscrew for 1/8" bits (like a Weldon shank
<http://www.tools-n-gizmos.com/specs/Tapers.html>.) If the hole is
oversized, the tool will shift during operation and will run out more
than the bearings, even if initially the spindle is initially adjusted
to zero. If the shaft deflects (bends) due to cutting loads, that shows
up as runout, especially if the deflection is permanent. If the shaft
wears loose in the bearings, or the bearings wear loose in the bore,
that is another static form of runout.

Then we get to dynamic runout. If the bearing has zero runout when not
moving, the hydrodynamic effects on the lubricant film can change the
center. This can be a few millionths to many thousandths. Add in out
of round balls or rollers and runout can get pretty bad.

> By the way, why aren't spindles made with the outside rectangular? They would be much easier to mount.

Actually, some spindles are made in rectangular form factors, but they
are more expensive. DOM tubing is generally round (or is supposed to be
round =-O ) and is cheap for what it is. Making round holes in mounts
is easier than making arbitrary shapes. Machining costs money, take a
round spindle blank, chuck it in a CNC machine and round bores are
easy. Features which are not symmetrical to the main bore are always
harder (hence more expensive) to machine.

Let's start with two spindles, one hexagonal, one round. Either the hex
spindle must be cut from a round blank or the hex blank must be bored
out. The round blank can be a simple hollow piece of pipe (or tubing.)
Machine the bearing mounts, both need similar amounts of work until
internal stresses in the metal do their work. The stresses in the hex
are different from those in the round, aging to allow those stresses to
be relieved (this can be time or thermal processing or both), costs
money. Once the internal stresses are relieved, the spindle must be
machined again to final dimensions. Machine any feature and the process
may need to be repeated. Metrology is not really a science, it is a
black art! And grinding bores is not a trivial task. The issue gets
bad quickly if the metal has internal stresses. Check out the bore
requirements for precision bearings.

Dave 8{)

--
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media, which holds forth the proposition that it is entirely possible to
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