Best rails position on frame in a dusty workspace

pit34 wrote:

Thanks, Andy, for the link. Interesting video with nice tricks to know !
I did not thunk wood routers need so much rigidity in gantry

He might be over-doing it, but better too stiff than too floppy.

Bear in mind that he is making it for a professional woodworking business.

I think that a heavyweight plasma might make an OK lightweight router. But it would be limited to thick metal and light cuts in wood.

Ok thanks. So as I really need to be able to cut thin mid steel sheets, I will not go for an heavy plasma.

For my personal knowledge and curiosity, why does a plasma cannot run more slowly to compensate the less rigidity and torque of the gantry and motors? Is it due to cutted materials that could glue on cutting tools at bad linear speed/cutting tool rpm ratio, like aluminium?

The plasma torch is "burning" material away. There is an optimum speed for a given material and thickness that you need to follow for optimum cut quality. Going too slow, burns more material away so the kerf is wider and the air supply/plasma stream is unable to blow the dross away. This leads to dross buildup on the lower side of the sheet which can be stuck on a lot tighter than if at correct speed. So you need a lot more clean up.

Also, if the accelleration is too slow (eg heavy machine), you can experience cut quality issues on corners because due to the laws of physics the torch must slow down to negotiate a change in direction. Whilst Linuxcnc can lock the torch height when it senses this velocity drop to prevent vioent crashes, you still end up with a wider kerf and more dross that affect corner cut quality. If you can get up over 3000 mm/s/s acceleration, the torch can negotiate tighter radii at desired cut speed so you get sharper corners. (I can do 5 m/sec/sec with a light machine).

Thanks for those details.
I did have an idea of these but not so precise.

So about acceleration, I deduce that there is no ideal acceleration value, the much, the higher the better, but a good start is around 3000mm/s/s. Good ?

About wood router, is there an ideal linear speed too? If so, what is it due to?
Intuitively I thunk there is not, and running a light gantry slowly could work on wood. I will make some search about that..

The problem with light cuts at slow speeds in wood is that you can end up burning the wood. You need to keep the chip load high enough to avoid rubbing.
But I am sure that there is an overlap between the two, so then the question is whether your work is in that overlap.

You could, potentially, have two gantries on the same rails.....

andypugh wrote:

The problem with light cuts at slow speeds in wood is that you can end up burning the wood. You need to keep the chip load high enough to avoid rubbing.
But I am sure that there is an overlap between the two, so then the question is whether your work is in that overlap.

You could, potentially, have two gantries on the same rails.....

 

 
 
Thanks, Andy, you oriented me in the right direction to search.

I have done some search to better understand what we are talking about (wood router). I have found this nice video that explain chip load, and how to calculate it with surface speed, bit's RPM, and bit's number of teeth. I now understand that having a correct chip load is the main aim.

I deduce of it that as chip load is a function of surface speed and tool RPM's, working at low spindle's RPM and at a not too low surface speed should permit to be in a correct chip load range. I still need to calculate the cutting forces of that setup on the gantry and look at the gantry's rigidity.
I remind here that the Spindle would be removable, that it is not mounted on the gantry when the plasma works, to allow having sufficient accelerations for the plasma. And that plasma would be used in my business, but the wood router only as a hobby.

Another constraint is that as the larger the diameter of the bit, the more powerful the spindle must be. And the powerful the spindle is, the more weighty the spindle be. As I cannot have a weighty and powerful spindle on a light gantry of a CNC plasma, I cannot have a large diameter of bit on it. But bits with little diameter on a light spindle could be OK.

I will try to do some calculation and simulation with those parameters :
Plasma max speed : 10 000mm/min (Hypertherm 45xp cut chart)
Plasma reasonably good acceleration : 3000mm/s/s (Rod)
Plasma top acceleration : 5000mm/s/s (Rod)