Subject: Re: Fly Cutter Discussion Thread (1/4-20 NC)

From: [email-masked] (murr rhame)
Date: 16 Feb 1995 20:20:30 -0500

[email-masked] (Mike Rehmus) writes:

<much deletia for space>

>In eagar anticipation of more discussion, I remain,

As a practical matter I've seen a lot of miles put on fly cutters.  I have
not seen one damaged spindle bearing that's been attributed to the use of
a fly cutter.  All of the fly cutting I've done or seen was on Bridgeport
or heavier class machines. 

The rate of removal is not too bad.  From memory, a 4" wide cut can be run
at something like .020" deep and .020" per rev. at 1200 RPM cutting mild
steel on a Bridgeport.  This works out to .48 cubic inches per minute. 
For heavy cuts on short production run it's common practice to setup with
a barrel strategicly placed to catch the chips streaming from the machine. 
This sort of cut does require a very ridged setup and a good heavy duty 
fly cutter (at least a 3/4" fly cutter shank and a 3/8" cutting tool).

The finish you can get with a carbide tipped tool is very impressive.  In
free machining materials a 32 Ra finish or better is not uncommon on a
finish cut.  The key to a great finish is tramming the head to near
perfectly flat.  

Fly cutters are often the only practical way to make form cutters without
spending a fortune on custom tooling.  I've made radius, bevel, V groove
and other form cuts with fly cutters.  A few minutes with a steady hand at
the bench grinder can save hours of work on a cutter grinder. 

Ball bearings are very fragile in some ways.  They are very sensitive to
overheating, overspeeding, contamination, lack-o-lube, misalignment, bad
preloading and mishandling when not mounted.  One of the most common ways
to ruin a bearing is to "spin dry" it with an air gun after cleaning.  The
air spin gives you zero shock but VERY high speed.  To the best of my
knowledge they deal with modest shock loads very well.  As I said once
before, the spindle does not see that much shock when fly cutting.  Much
of the shock is absorbed by the rotating mass of the fly cutter head. 

All of this is based on my personal experience and opinion.  

BTW: The "RCM" units referred to earlier were "rec.crafts.metalworking
units" ie. hobbyist machines. 

-- 
         murr rhame       /\/\ |_| |~ |~      [email-masked]
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