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
>