Lasers::Comments & Questions fo the List

On Wed, 12 May 1999 11:28:48 -0700, you wrote:

>From: Don Hughes <pencad@...>
>
>John:
>
>Thanks for the reply, yes, you will be of great help as I go down this
>road. What I need to first know is either the best or safest way of
>mounting the laser to the base of the machine table. Should it be
>mounted stationary, beside/away from all the axis' and use mirrors to
>convey the beam, or mounted on the gantry and use the beam directly
>through the focal optics with no mirrors at all?
>
>Also John, what is the most commonly used software for controlling
>lasers in cutting operations. I am mostly wondering about the turning
>on/off and the percentages of power control. I will be cutting mostly
>lite ply up to 1/4" thick and balsa up to 3/8" thick for model aircraft.
>The software would ideally control both the laser and the motion, so I
>would need some insight along this road as well. I read in a previous
>thread here that IndexerLPT has a great range of uses, do you feel this
>may be useful as it uses a HPGL file directly to do the cutting profile?
>

>Thanks very much,
>

Firstly let me make it clear that I am a mechanical engineer and have no part
with the electronics of the lasers but I can ask and get info on that subject.
A bit of background first on the company. They have 6 lasers A to F from 1Kw to
6Kw. the first one [ C ] is in mothballs due to lack of room. This is a moving X
and Y table machine with the fixed laser mounted above the table firing down
through the head by one 45 degree mirror.
Machines A,B,D, and E are virtually identical with a moving X axis, the bed and
the head moves on linear rails to provide the Y axis. Again the laser is mounted
above to save space and protect it from fork trucks. The beam is taken through
three mirrors at 45 degrees to take the beam along, down and then along again to
the last mirror which travels on the top of the head on the Y axis.
The last machine F, was new at Christmas and is the big 6Kw model.
This one has a moving gantry that moves in the x axis and on the gantry is the
moving Y axis. The laser is floor mounted behind and fires through two 45
mirrors to the end of the gantry then to the top of the head. On all machines
the beam is contained within alloy tube 4" in diameter other than inside the
travelling Y axis and these are inside a metal enclosure. There is no way that
any stray beams can be seen.
All machines are on ball screws except the latest one which is moved by
precision rack and pinion, Max certified error is 0.002mm over 1000mm.
probably why this machine cost nearly 1/2 million pounds UK.
All machines are on AC servo's with encoder feedback.
As regards software anything that can move motors under say G code instruction
will work. Note though that the main enemy of laser cutters is having software
that pauses between moves, this will burn the work. I use Ahha controllers on
milling machines and although I am a big fan of Ahha I feel that it's not up to
commercial laser applications, note I said commercial.
Programming can be done by any software that can handle canned cycles or macros
as you need to start to cut off the work and then move into the cut, similar to
the approach depart moves like in Bobcad V17.
You mention controlling the power, I must admit this is a bit of a grey area of
my knowledge but I will ask tomorrow when I am there how power is controlled.
Possibly it is done by altering to input voltage / amperage to the laser.
Some controllers can alter spindle speeds by using analog output to inverters,
perhaps this is a way?
Some laser cam switch off and on rapidly, others have to startup so they stop
the beam by swinging a thermal block into the beams path. if this is done close
to the laser output the beam is not concentrated and the excess heat is carried
away by having the block water cooled.
HPGL would be one way to go. A laser doesn't need a true Z move, it just needs
to be about 1/2 to 1mm above the work. 'My' machines do this by using a LVTD to
track the work and control a servo motor on a Z ballscrew.
It could also be done manually by lowering it on to the work with a slipper
giving the correct hight, be careful though not to drop down into any holes.
The pen up / pen down signal from the HPGL file could be used to fire the laser.

>What steppers have you found to give the ideal results in controlling
>gantry's, 130 oz./in. or higher motors? The effective cutting table size
>I desire is 36"(Y) x 48"(X). I am still toying with the fact that do I
>want this to be just a laser cutter, and a mill if I can do this, or
>just have two separate machines? I will need to machine plugs for
>cowlings and gear pants, canopies, etc. from 3D files for vacuum forming
>of these parts.
>

Sorry I can't answer this the first part of this but remember laser cutting is
much different from milling as it is a non contact process and there are no tool
or side forces to contend with. and the last part precludes the use of HPGL
files as you will need a Z axis for milling.
If you haven't already I would suggest you look at Ahha's web site at
http://www.ahha.com download the demo and read the manedit.txt file in the demo.
Even if you don't want to use their controller the information is good
groundwork.

Sorry if this has been a bit long but like most things the learning curve is
rather steep to start

Regards,
John Stevenson
Nottingham, England