Re: [CAD_CAM_EDM_DRO] plasma bridge

Hi Chris,
I am afraid I can't take any pics at the moment as I have a lot of stuff
parked under it at the moment. If I remember, next time I use it I will take
a few photos.
I don't know how much the bridge weighs. It is basically a length of 50 x 25
x 3mm wall box section with a diagonal brace made out of 25 x 25 x 1.6mm box
section. The carriage is also made from 50 x 25 box section.
Les

> Hello Les
>
> Thinks for all the info, you dont have any photos? Im currently running my
> table in a similar way on a tube however it gets a fair amount of dust on
> it. I think 400 ipm would be the max speed. How much does your bridge way?
>
> -----Original Message-----
> From: Les Newell [mailto:les@...]
> Sent: Saturday, March 08, 2003 7:25 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] plasma bridge
>
>
> Hi Chris,
>
> The frame for the machine is made out of 80mm x 80mm steel box section.

This

> is probably a bit oversized but I happened to have a couple of straight
> lengths laying around. The frame also forms the track. The gantry is T
> shaped and the top of the T is a carriage that runs on the box section.
> There are two bearings running on the top of the track to take the weight.
> There are two more solidly mounted bearings that run on one side of the

box

> section and two spring loaded bearings directly opposite them. The springs
> are strong enough to still hold things square at maximum acceleration.

They

> will give a little in a crash to help absorb the shock loads. The far end

of

> the gantry just has one bearing that takes the weight. The areas where the
> bearings run are polished with abrasive paper to remove any bumps and high
> spots.
>
> If I was building the machine again I would be very tempted to use an
> inverted piece of angle iron as the track, with the bearings running in

the

> V. This way the weight of the gantry provides the preload and you only

need

> two bearings. Any dirt that falls into the track will sit in the bottom of
> the V, out of the way. Plasma is a very dirty process and I do find I need
> to clean the tracks on a fairly regular basis.
>
> The machine is crude but it manages positional accuracy within about
> +/-0.25mm over the whole travel of the bed, as close as I can measure

using

> my trusty tape measure. I can imagine plenty of subscribers to this list
> shuddering at the thought of checking the accuracy of a CNC machine with a
> tape measure :-) I use the cutter for cutting out parts for fabrication
> work (like Land Rover chassis bits) and heat distortion from welding is a
> far greater problem than a small inaccuracy in the cutting out of the

parts.

>
> > What do you think about making the bridge out of aluim.? Isnt aluim
> stiffer
> > than steel?
>
> Weight for weight, I am sure ally is stiffer than steel. However it is

more

> expensive and not as easy to work with (it is a pain to weld). If you are
> just plasma cutting, the bridge does not have to be all that stiff as long
> as the carriage that runs on it is is light. How fast will you be cutting?
> It does not matter if the bridge flexes very slightly during a high
> acceleration rapid as long as it is rigid enough to keep within your

desired

> tolerance at cutting speed and acceleration. If I cut a sharp right angle

at

> high speed I sometimes get a small amount of 'ringing' as the gantry
> suddenly changes speed. This shows up as a very slightly wobbly cut near

the

> corner. In practice I have never found it to be a problem though.
>
> > You build furniture dont you? I thought I saw a link to your web site a
> long
>
> Sorry, you are confusing me with Les Watts. His machines are in a

completely

> different league to mine. He sometimes uses a clever mechanism to

compensate

> for lead error in the ballscrews. I can't even measure the lead error in
> ballscrews!
>
> Les Newell
>
>
>
> >
> > Hello Les,
> >
> > In what form did you use mild steel as a track? Ive been going back and
> > forth with ideas...dual vee,or a round tgp shaft with 2 bearings oposing
> > each other riding at 45deg on it(I was told its an old esab design), or
> > pillow blocks...
> >
> > So am I understanding you are putting your bridge is intention by using
> the
> > spring loaded skate bearing?
> >
> > What do you think about making the bridge out of aluim.? Isnt aluim
> stiffer
> > than steel?
> >
> > You build furniture dont you? I thought I saw a link to your web site a
> long
> > time ago.. Do you do highpoint? We tried it many years ago but prooved

to

> be
> > too much work.
> >
> > Thanks again
> > Chris
> >
>
>
>
>
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