I have a piece of 6061-T6 aluminum (4" by 12" by 1/2") that I want to
cut to a uniform arbitrary thickness but a close tolerance to flat (+
or - .001"). The surface finish should look "professional". I have a
small horizontal mill, a small vertical mill, a surface grinder and a
belt sander available. All are big enough to handle the piece without a
Which machine would you use and why?
Either the vertical mill with a big flycutter or the horizontal mill
with slab cutter.
You say small on both mills, so making the wild assumption that implies
maybe a bench mill, I would suggest the horizontal mill would be
better. I say this is better because a fly cutter is an off balance
load on your spindle. If you don't have enough rigidity in the total
machine you may get some chatter. A 4"+ slab cutter is going to be
much more expensive than an equivilent fly cutter.
As others have mentioned, a fly cutter makes a very nice finish on aluminum. If
you can't swing a 4" cutter, make two passes with a 2" flycutter. You will need
to be trammed in very well to assure that there will not be a step.
Whatever you do, you should take equal cuts on both sides to allow for slight
warping of the slab.
The answer is obvious: Whichever is your best machine. Holding .001 over
that large of an area demands a machine with essentially no wear in the
ways. Most older mills are a few thou "swaybacked," which you don't notice
on small stuff, but it really shows up over long pieces...
While it likely doesn't matter in your case, you are unlikely to
achieve the tolerance you have specified with that piece of material by
simply flycutting. Actually hitting +/-.001" flatness over 12" is not
trivial, especially with what is probably extruded aluminum.
To answer your question (and to agree with everyone else) flycutting is
the way to go. With a sharp cutter you should be able to take a very
light finishing pass which will make the effect of the unbalanced
nature of the flycutter negligible. As another aside, you should
consider investing in a facemill if you're going to be doing a lot of
that type of work. Much more productive.
Which one has a power feed (of the mills)?
Is one of the mills in better nick than the other?
Forget about the sander.
A flycutter, or a "sticky sharp" end mill, on the more accurate of
One direction will cut a little nicer than the other. Recognise and
use this to advantage. A nice pattern of evenly spaced cuts, with a
shiny rainbow refraction pattern, look good, feels nice under the hand,
and makes the crows drool (crows, the guys that stop dead in their
tracks when they walk by and see shiny stuff onthe bench).
I chose the word to be "A nice pattern of evenly spaced cuts, with a shiny rainbow refraction
pattern, look good, feels nice under the hand, and makes the crows
drool (crows, the guys that stop dead in their tracks when they walk by
and see shiny stuff onthe bench)."
THAT is as good a definition as I've run across.
In response to other questions:
Both mills are benchtop models but heavy for their size. Each has a 5"
by 20" table. Neither has a power feed. The vertical mill is in better
shape than the other one.
Thanks for the information. I'm off to find a face mill.
If you're really serious about the +/- 0.001 inch tolerance, invest in a
knife-edge straightedge and test the raw material first. Use in subdued
light with a backlight to highlight the contact surfaces. Chucking the work
to the table can easily warp the item much more than 0.001. On occasion, I
have used a castiron sub-table with a fresh finish cut, then hold down the
work with contact cement that will quickly release with moderate heat, say
200-deg F. The contact cement will be more than strong enough to withstand
stress from the flycutter with a sharp bit and light cut, and will not
likely warp the work piece.
One of the "off-the-wall" thoughts I had about "lite cuts" was to mount
the aluminum on my surface grinder and replace the grinding wheel with
a 1/2" wide milling cutter. Since the grinder is controlled by a VFD, I
can reduce the RPMs to just about any speed I want.
What do you think about that approach?
We do some things in aluminum sheet stock. I have looked for a
"professional" finish also. Engine turning is out of the question
for the size, patience, tooling, and time involved. Buffing to a
mirror finish falls in the same category, though we have done it
on a few small items. To date I have ended up using 180 sand
paper disks on an orbital sander to provide a uniform matte
Some WAGs (as I've never heard of anything like this being done...)
A grinder spindle is a precision tool. Interrupted cuts (such as you're
describing) are *not* what the spindle is made to deal with. Look up
the price of new spindle bearings.
Manual/hydraulic grinders have no provision to controlling the table
feed to the degree required to prevent grabbing and jamming in
aluminum. You'd probably end up burning/breaking your cutter and/or
tossing your workpiece.
Mag chucks don't have the same holding power has screw clamping devices
(vices and hold down clamps) do. Again, potential for grabbing and
tossing your part, no matter how you got it to actually stick in the
Mills are made for this job. Grinders are not...
Although not what you're wanting, a nice finish can be had by tumbling
aluminum in a vibratory tumbler (burr box). We've finished an entire
machine that way but things to watch for are the rock size. To small
and they get in the tapped holes, to large and yo end up with a pitted
surface. 15-30 minutes seems to work well. Sheet stock does NOT work
well unless it's small enough to fit in the box