Re: CNC flame/plasma cutter

rtr@... Ted Robbins

I've been trying to avoid writing this piece because I thought it would
just muddy the water, but there seems to be enough interest in the problems
of long, 5 to 20 thousandths accuracy drives to warrant this input. Flame
cutters, foam cutters, and wood routers are probably in this catagory.
Those who need more accuracy should avoid the method suggested here.

The Automotive industry often needs long positioning systems which get
their final positioning from the workpiece, ie; the engine or body. They
only need the positioning system to get them within this range of 5 to 20
thousandths. Typically, they use a bicycle chain for the relatively short,
lightweight positioning chores, and bigger chains built like bicycle chains
when more distance and force, but less accuracy is required.

The chain is anchored solidly at the start end and retained with a
tensioning device at the other end. Depending on forces, sometimes it is
used as a flat track for a single sprocket. Sometimes it has idler
sprockets on each side with the chain wrapped around it. this chain is not
used to guide the axis. A rail or another method must provide that
function.

Rack and pinion systems are normally installed with the rack teeth facing
down to minimize debris problems. That way only the pinion needs careful
debris shielding. When you must mount the rack with the teeth up, you must
shield the whole rack.

When the rack has only an encoder riding on it, I always use a reed
transfer device to hold the encoder. This protects the rack from excessive
shock, but it won't work to apply power through such a device.

When we developed this reed transfer device for rack driven encoders back
in '06, I mean 1966, we also developed covered racks which used spring
steel covers which rode in slots on the front of the reed transfer plate.
We rolled up Sandvic spring steel, put it in a pipe nipple, screwed caps on
the pipe to limit oxygen, and heat treated it in a small ceramic baking
oven.

We welded up boxes for the end of the rack holders for the spring steel
rack covers to wind themselves up in. They lasted a long time in machine
shop environments.

I am submitting this information now becaus it is relevant to the string in
question, but it will also be relevant to the DRO string which should be
starting when the boards for your new DRO's get in.
----------

> From: Dan Mauch <dmauch@...>
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] CNC flame/plasma cutter
> Date: Thursday, June 24, 1999 6:30 AM
>
> From: "Dan Mauch" <dmauch@...>
>
> Let me add a few thoughts about racks and pinions..
> They work very well in cnc applications but suffer from two problems. One

is

> the pitch diameter of the pinion. The second is debris that that clogs

the

> teeth and jam the mechanisms.
> The pitch diameter for the smallest pinion that works on a neam 23 motor

is

> .5"D. Thus it will travel .00785" per step at 200 steps per rev and half
> that for 400 steps prer rev. Generally a nema 34 motor is used and this
> would have to have a 1" pitch diamter so that there is enought meat for

the

> pinion to be bored for .3755. Then the resolution increades by pi*D or

3.14

> for 1 rev at 200 steps=.0157 per step and half that for the half step

mode.

> You will get very fast travel. I have one system that ran no load at over
> 600" per minute with 150 oz in nea 34 motors.
> An alternative is to then use a belt drive to drive the pinion then you

can

> increase the resolution by the pulley ratio. Some people just use timing
> belts cut and both ends secured for non critcal tolerances.
>
> Dan
> -----Original Message-----
> From: Tim Goldstein <timg@...>
> To: CAD_CAM_EDM_DRO@onelist.com <CAD_CAM_EDM_DRO@onelist.com>
> Date: Wednesday, June 23, 1999 9:15 PM
> Subject: Re: [CAD_CAM_EDM_DRO] CNC flame/plasma cutter
>
>
> >From: "Tim Goldstein" <timg@...>
> >
> >
> >My friend only got the table parts from Torchmate. He is using a

bi-polar

> >chopper drive stepper controller (better torque and rpm than a unipolar)
> >that he built from a kit he purchased from Dan at Camtronics. For the
> >software he is currently running DeskNC DOS version which does handle
> >g-code. But, we are soon going to move him over to EMC and Linux as it
> seems
> >to run a stepper machine much smoother.
> >
> >As far as ball screws v/s rack I think it is not a question of what is
> >better, but more a question of what is most appropriate for the job
> >considering the cost. A ball screw is more accurate, but is the rest of

the

> >machine such that the added accuracy will ever be noticed? If you are
> >thinking of a machine that can handle 4' x 8' and figure the cheapest
> >ballscrew material is $1/in and nuts are $20 you will have over $200 in
> >just the parts and I don't think you will see any difference.
> >
> >Tim
> >[Denver, CO]
> >
> >> -----Original Message-----
> >> From: smcdonal [mailto:smcdonal@...]
> >> Sent: Thursday, June 24, 1999 1:11 AM
> >> To: CAD_CAM_EDM_DRO@onelist.com
> >> Subject: Re: [CAD_CAM_EDM_DRO] CNC flame/plasma cutter (was Stepster)
> >>
> >>
> >> From: smcdonal <smcdonal@...>
> >>
> >> Tim Goldstein wrote:
> >> How does the torchmate handle files? Can it read standard g code or

does

> >> it use some other format?
> >>
> >> Also would like to know if rack and pinion is better then ballscrews?
> >>
> >> Thanks
> >> Stan
> >>
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