Guys,
I'm following this subject with great interest. One of the reasons I
subscribe to this group, is to learn enough to convert my mill to CNC. I
have been fabricating some parts on a mill, using the DRO, and have run into
problems drilling holes. Using certain drill bits, many of the holes have
come out oversize, and some completely out-of-round. Changing bits has
helped, especially if I use a shorter length bit, but I'm going to
investigate using the 135 degree, or other similar drills. I'm looking
forward to a CNC conversion, when I intend to center drill the holes before
using the correct drill bit.
Marv Frankel
Los Angeles
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----- Original Message -----
From: "Alan Marconett KM6VV" <KM6VV@...>
To: <CAD_CAM_EDM_DRO@...>
Sent: Tuesday, October 22, 2002 12:05 PM
Subject: Re: [CAD_CAM_EDM_DRO] Dumb question on the drilling of holes
Hi Alan,
I've read that you DON'T want to center punch the holes for CNC,
although you might want to make light "witness" crosshairs to mark their
location. Best would be to always start with a center drill, then
follow with the appropriate drill. The trouble with a punch mark if not
EXACTLY where you want it, is that it will pull a drill bit off. If the
CNC has moved to the proper location, then you just want to drill where
you are!
To position the mill to a hole's location, set up a "wiggler" in the
spindle, and, using a magnifying glass if necessary, move it tho the
crosshairs. Or, you might prefer to locate everything relative to a
pair of edges (LL corner is often good), and let the CNC work from
that. I like using the edges if it's a new part, and then let the CNC
find the other points.
I'm using Vector CAD/CAM, and if I put a point at each hole location,
down at the depth I need for the center drill, Vector will do a move to
a location ABOVE Z=0, and then a slow Z down to the required depth,
center drilling it for me. It will also generate a following G81 with
the coordinates, which I modify to G83 to actually drill the hole (I
like the "peck" and "dwell"). I "comment out" the G83's with a '/' in
the part program, and turn on "block deletes" on the controller program
(DON'T FORGET!). With the center bit "touched off" in Z, I run the
program with block deletes on. This way, the center drill drills all
the holes I want. Then I replace the center drill with a drill bit of
the required size (you may want to repeat the process with two or more
drill bits), disable block deletes, and run the part program again.
With block deletes disabled, the G83's will be run, and the holes
drilled. OK, this may not work for a "PRO" shop with tool changers and
the like, but it works fine for my Sherline's, and I assume it will work
as well on my RF-31 mill (when I finally get it converted) and possible
a lathe in the future.
HTH
Alan KM6VV
P.S. You can set the Z depth to just "kiss" the stock with the center
drill, and thus run a "verify pass" first, to check your program! Also,
a "pen holder" in the spindle, and a piece of paper can give you
re-assurances that you've programmed the Gcode correctly. I've taken to
printing a 1:1 of the part and the cuts on paper (HP laser is not far
off), and this also helps to verify setup, CLAMPS, etc.
alan@... wrote:
Forgive me this seemingly dumb question, but how does a person "drill"
accurately placed holes under CNC control without first center-punching
them.
I currently use a center drill to start the hole ( for deep holes ) or
just mill the hole via G02/03 command for shallow ones.
But it occurs to me that there's got to be a better way. Are "spotting
drills" that better way ?
Thanks in advance for any thoughts.
Alan
--
Alan Rothenbush | The Spartans do not ask the number of
the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta
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