Re: Look Ahead

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Scott Stephens wrote:

> -----Original Message-----
> From: Jon Elson <jmelson-at-artsci.wustl.edu>
> To: Multiple recipients of list <emc-at-nist.gov>
> Date: Monday, September 04, 2000 2:07 AM
> Subject: Re: Look Ahead
>
> >I use this when drilling PC boards with tiny drills, to make sure the XY
> >position has settled completely before starting to plunge the drill in, and
> that
> >the drill is fully retracted before moving X & Y to the next hole.
>
> Could you tell me what speeds you use to plunge the drill in and out? Do you
> ramp?

The inverter I'm using right now goes up to 400 Hz, which allows me to
get up to about 24000 RPM.  It is actually desirable to go faster, but
until I build a custom inverter, that is my max RPM.  You use about .001"
per revolution plunge on the bigger drills, around .035" and up.
Too fast a plunge will stall the drill motor with really large drills, so you
have to take that into account.  You get down to around .0002" / rev
on the very small ones, in the .021 to .018" range.  So, that works out
to 24 IPM for the larger drills, and down to about 5 IPM for the tiny ones.
It depends a bit on whether you are using entry and exit materials (and what
types), the board material, how thick the board stack is, etc.  I occasionally
use a stack of 3 .062" thick boards with larger drills only, .035" and up.
For drills down to .025" or so, you can do 2-high stacks.  With the smaller
drills, .021, .018 and down (if you dare) the chip binding is so bad that
you will likely break drills if you try to do a 2-high stack.  So, if there
will be small holes, I do a single board at a time.

I generally retract the drill at the same rate as the plunge.  You can go
twice as fast, but don't go faster.  Some experts say too rapid extraction
traps the chips that are often wrapped around the bit above the board
and jams it, breaking the bit.

I don't understand the ramp question.  Do you mean an acceleration ramp
of the spindle when plunging?  Yes, the machine ALWAYS ramps up and
down, but the way I set up the clearance plane .1" above the board, it
does the ramp up/down at the top of the stroke outside the board.
There is another ramp down/up at the bottom of the stroke, that is mostly
done in the exit material.  I use a piece of PCB scrap under the board
to keep from raising an edge around the bottom hole.  I have been
experimenting with using .010" aluminum for entry and exit material
with pretty good results.

Jon