Collision detection, linear rails, servo motors

[edclayton1]

Hi all,

I have been dreaming of buying a new Plasma table lately or more likely upgrading mine to some newer technology features.
I see a lot of machines now seem to be coming with collision detection torch holders now rather than simply magnetic breakaway torch holders.
Is that something that can be easily integrated onto a CANDCNC system? It looks like a good idea but I am not sure how one would go about adding one to a table if the table does not already come with the system included.

Another feature that seems to be gaining in popularity are linear rails. I have the old steel rails with skate board bearings. It works but its not the smoothest motion and the rails and bearings do wear out from time to time. Is it worth the expense for linear rails. It seems like it would be a nicer smoother motion and require a bit less motor torque but maybe more prone to getting contaminated and damaged from plasma dust. Any thoughts or experiences out there?

The final question I have is a rehash of an old question but it keeps coming back up. Steppers VS. servo motors. Now it seems that the medium to higher price tables are moing to servos like Plasmacam has been doing for years. I really don't quite understand how they always know where they are without talking to the controller and the software. Is this really a big deal? I have a fairly heavy (approx. 100 lb.'s) gantry for my 620 oz. stepper motors and I think I may lose steps some times on thinner metals when thing are going at fast FPM settings. I went pretty big on the 8020 beam thinking maybe I might want to make it also function as a router table but I have since built a CNC router table instead. I used 8020 3060.

[Plasma-art]

The colision detection is often 3 normally closed switches wired in series.
As long as you have a free i/o pin available you can set it up.
It is however advised to also run a magnetic break away at the same time but have it setup so the collision detection toggles first.
You can put that sensor in series with the ones of the collision detection sensors.

Regarding the linear rails they are great and if you care for them properly (grease them and keep them clean) they last a long time.
If its properly designed the rails should be protected so that dust does not settle on them.
That way you get the most life out of them.

[acourtjester]

These things are like many, how much money are you willing to spend. One factor is are you in an industrial operation or hobby type.
One of the reasons for servos is they have feed back as to steps moved, steppers will position as well are servos. IMHO servos will only give you an error if movement stops, not correct for a problem on the fly. Missing steps is a maintenance problem, software keeps very good control of the location while an operation is under way. The software also will protect using home/limit switches for the perimeter of the movement and torch break away will handle torch crashes. A break away switch wired into the limit circuit is a simple solution for torch crashes.
Linear vs roller bearings there still is the maintenance issue.

[adbuch]

The final question I have is a rehash of an old question but it keeps coming back up. Steppers VS. servo motors. Now it seems that the medium to higher price tables are moing to servos like Plasmacam has been doing for years. I really don't quite understand how they always know where they are without talking to the controller and the software.

Ed, the servo motors such as the ones on my Plasmacam table have encoders on each servo motor. The encoders send signals to the controller to let the controller know when the motor rotates, and how much. So in this manner, the problem with stepper motors loosing steps (stalling, etc.) is avoided. I do have stepper motors on both of my cnc router tables - one running from Mach3 and the other Centroid Acorn/cnc12, and have yet to experience any problems with lost steps due to the fact that they are running open loop (no feedback). Both of these tables are from Avid CNC, and use linear rails. I suspect that the possible downside to the linear rails for cnc plasma cutting is contamination from the plasma dust getting into the bearings. I like the system Plasmacam uses with the rollers, which are much less prone to contamination issues.

David

[edclayton1]

The collision detection is often 3 normally closed switches wired in series.
As long as you have a free i/o pin available you can set it up.
It is however advised to also run a magnetic break away at the same time but have it setup so the collision detection toggles first.
You can put that sensor in series with the ones of the collision detection sensors.

Regarding the linear rails they are great and if you care for them properly (grease them and keep them clean) they last a long time.
If its properly designed the rails should be protected so that dust does not settle on them.
That way you get the most life out of them.

Thanks for the info. I apricate it.

[edclayton1]

These things are like many, how much money are you willing to spend. One factor is are you in an industrial operation or hobby type.
One of the reasons for servos is they have feed back as to steps moved, steppers will position as well are servos. IMHO servos will only give you an error if movement stops, not correct for a problem on the fly. Missing steps is a maintenance problem, software keeps very good control of the location while an operation is under way. The software also will protect using home/limit switches for the perimeter of the movement and torch break away will handle torch crashes. A break away switch wired into the limit circuit is a simple solution for torch crashes.
Linear vs roller bearings there still is the maintenance issue.

Hi,
Thanks for the response. As for the linear rails I was thinking of building a frame out of 8020 and putting the gear rack and the linear rails on the outside edge of the Y axis. I also thought about buying the dust seals from Hiwin and then adding another tighter fitting secondary layer of wiper on the ends of the blocks using felt and oiling it routinely. The X axis linear rails will be harder to protect and much closer to the sparks and what not. So they might prove to be harder to maintain, but I see lots of tables now being built with them so I thought I will give it a try.
As for the servos vs. steppers there must be some reason PlasmaCam, Shop Sabre, Boss tables, etc.. are moving towards them these days. I am just not sure why.
Cheers,
Ed