[biohazarus]

Hi, planning on building a gantry plasma table and looking at different options.

One thing I wonder is why are all plan I saw so far have the "double" guides on the long side.
I am thinking of putting my double guides on the short side and having 1 long cross guide(gantry).

In case, it is not clear:
Let say the table is 4x8.
Plans have 2 guides (1 each side) on the 8' and one rail on gantry of 4'. Which requires 20' of guides.
What if I put the 2 guides on the 4' and 1 on the gantry of 8'. Which requires 16' of guides!

Saving 4'!!!

Any reason I did not saw plan like that?
Thanks
Alex

[Torchhead]

It makes you gantry twice as heavy so you may have to spend more on bigger motors and/or drivers. to get the same level of performance
Acceleration is inversely proportional to the weight of the gantry

[NIC 77]

For a CNC router, the reason is obvious. The shorter beam is much stiffer. Deflection at the center is a function of the length of the gantry cubed. L^3. To make a tube of the same stiffness with a longer span requires much more weight and more powerful motors to accelerate.

On the build that I am currently working on, the gantry is on the longer axis. I did it this way so I could load and align long work pieces from the front of the machine without having a beam in the way.

If you look at some of the Biesse other other professional top end routers, there are ones with a cantilever design, which is the worst design for stiffness. But they have decided that other factors are more important, and these machines are still professional top end machines.

I've never owned a CNC plasma, but my understanding is that the Z axis weight is small, the same cutting forces that you would need for a router are not present, and the torch height control makes any deflection you might see in the center of the beam not an issue, so you can get away with a gantry beam that is less stiff. Someone with some more practical experience in CNC plasma can feel free to correct me if this is not the case.

But for a cutting length of 8 feet , you'd need a very long gantry! So your example is a bit at the extreme end of things!

Where are you in Canada? I'm in Southwestern Ontario, but I assume that you are in Quebec? I have some parts from a large router that I would like to sell to someone for a large plasma build but they are not the kinds of things that you can easily ship so it would have to be to someone local.

[biohazarus]

Thanks for you inputs, make sense!
Being a plasma table I don't see to much force applied on the "cutting head"
Will try to calculate and see if my logic holds up and I save in the final or end up the same, cost wise.

Where are you in Canada? I'm in Southwestern Ontario, but I assume that you are in Quebec? I have some parts from a large router that I would like to sell to someone for a large plasma build but they are not the kinds of things that you can easily ship so it would have to be to someone local.

I am in Gatineau, next to Ottawa.
Feel free to send an email.
If price is good, shipping could be doable, will see.

Also did not thought it was so extreme as sheet metal usually comes in 4x8 size.

Thanks
Alex

[Torchhead]

The single most important aspect in plasma cutting is not the gantry vertical deflection but the overall acceleration/deceleration of the gantry (not normally an issue with a router) Plasma cut rates are in the hundreds of IPM and with acceleration in range of 35 IPS/sec or greater. Example 14ga mild steel with a 45A setting cuts at 225IPM. HOW it cuts and makes tight turns at that speed is determined by the acceleration it can muster. So its basically Physics

A = F/M

Acceleration is equal to Force divided by Mass. So if you increase the Mass you have to increase the Force provided by the motors to get the same acceleration.

So you are going to save a few bucks by buying shorter rack and rails but the real cost for that may be anything from free to more than the parts savings, depending on what you chose for motors and the gantry construction.


.

[biohazarus]

The single most important aspect in plasma cutting is not the gantry vertical deflection but the overall acceleration/deceleration of the gantry (not normally an issue with a router) Plasma cut rates are in the hundreds of IPM and with acceleration in range of 35 IPS/sec or greater. Example 14ga mild steel with a 45A setting cuts at 225IPM. HOW it cuts and makes tight turns at that speed is determined by the acceleration it can muster. So its basically Physics

A = F/M

Acceleration is equal to Force divided by Mass. So if you increase the Mass you have to increase the Force provided by the motors to get the same acceleration.

So you are going to save a few bucks by buying shorter rack and rails but the real cost for that may be anything from free to more than the parts savings, depending on what you chose for motors and the gantry construction.


.

Pays to ask!!!
Going with 2 long guides and a "short" gantry!!!
So many details to take care of!!!

[BTA PLASMA]

You can build a machine this way you just have to size the components correctly. HVAC machines used to only be built with a wide gantry from some companies. This helped with fast loading of thin material. But time moved on and linear rail got much more economical (Even though some still don't use it or use it properly). So instead of flat bar steel with raised sides modern machines put the rail below the top of the work plane. This gives you an easy loading from 3 sides instead of 2. Depending on your works space an 8x4 may make more sense than a 4x8. But one is more efficient. Historically in the big router world once you get beyond 10' width of the gantry you would be using twin rails on BOTH sides of the long axis travel (Often 20 foot or more travel length).