On 2010-04-30, Andrew VK3BFA wrote:
On Apr 29, 10:57*am, "DoN. Nichols" wrote:
* * * * Another trick to keep it from chattering -- works with
countersinks as well -- is to bring the tip into contact with the
workpiece with the motor turned off, apply a reasonable pressure, and
flip on the switch. *This keeps it from getting room to bounce around
before it bites.
* * * * Enjoy,
* * * * * * * * DoN.
Thanks Don for the tip re countersinks - I have had grave trouble
with the things chattering, even after locking the table down - always
looked crap with the chatter marks in the metal - will give it a try.
(found it works better with the lowest speed I can get - possible
similar to what you were describing?)
Related. My trick makes sure that when they start, they are
already in contact through the slowest part of the speed-up cycle. If
you start them in firm contact, they will usually stay in firm contact,
unless the workpiece has a lot of flexibility.
[ ... ]
Countersinks? - seems to be two types, the "usual" hardware store/
woodworking types with multiple flutes, and the ones with but a single
shaped and sharp hole in them - they seem to go a bit better, but only
marginally. Can you explain why they are different? - is it different
applications, or for different metals?...
There are actually more than that. The wood ones are usually a
pair of flutes 180 degrees apart. And they may be available in
different included angles, depending on the availability of screws with
similar angles.
The metalworking ones come in quite a few styles, and in
included angles of at least 82 degrees, 90 degrees, and 100 degrees.
The 100 degrees is for relatively thin sheet metal so the entire length
of the head can be within the metal to be mounted.
Aside from the different angles, they also come with two flutes,
three flutes, six (irregularly space) flutes (made by Severance), and
supposed to minimize chatter -- better than equally spaced flutes, but
still chatter under certain circumstances. Start them as I described
above and they are pretty good. Back away quickly when deep enough so
they don't get a chance to start chatter. If you see a six fluted
countersink, look carefully at the end, and you will see that some
flutes are closer together than others, and you will likely see the name
"Severance" on the side of the bit -- though I think that the patent has
expired, and others are now making them as well.
Then there are the M. A. Ford single-flute ones which seem to be
quite good at not chattering if started well.
And the ones with the angled hole in a single face are also
pretty good. They are harder to resharpen. There is a special fixture
for resharpening the M.A. Ford ones.
Also -- there are spotting drills (single flute going to center,
full cone) used for making a starting point for a drilled hole working
from either a center punch or crossed scribe lines. These typically
have the shank the same diameter as the cutting end and are short so
they are stiff and won't deflect easily.
And -- there are the combination drill/countersink (double
ended, 60 degree countersink with a very small very short drill in the
end, used for drilling center holes for use when turning between centers
on a lathe. These are often used in place of spotting drills, but are
not quite as nice.
Oh yes -- then there are the Micro-Stop countersinks (usually
100 degree angle for metal to be riveted as skin in aircraft building).
The countersink has a pilot on one end for a specific size hole to guide
it in and minimize chatter, and a 1/4-28 male thread on the other end
which screws into a shaft in bearings in a cage which can be adjusted in
0.001" increments to control the depth of the countersink. You can use
these in a drill press, or in an electric or air powered drill. Very
nice for consistent countersink depth if you are doing many. The depth
stop on the drill press is usually not precise enough for this kind of
control.
Unless you really *need* these, don't buy them new. But you can
often get the adjustable cage and a number of the countersink bits
(along with facing bits to finish the already set rivet flush with the
surface) in a lot on eBay.
Enjoy,
DoN.
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