Need help finalizing plan for CNC Plasma table

[David Bord]

I have spent the last couple of weeks pouring over this site and the manufactures of CNC tables trying to get ideas on how I will contruct my CNC Plasma Table

Ive decided to use linear slides on harden rods unless it makes more sense to do something else. The gantry will be made with 4x4 aluminum tube in the shape of a T. The table size will be approx 4'x4'

The drive will be gear and rack. Ive seen a few on this site that have created their tables with direct drive using bigger motors 400oz/in plus.

My questions are:

1. Will linear slides on solid rod work well for a plasma table?(both X and Y) Im thinking about 20mm.

2. Does it make more sense to use a big stepper 400 oz/in + and a quality stepper driver like gecko, or smaller stepper around 200 oz/in with a 2:1 to 3:1 reduction and belts to get the torque and increase resolution?

If I used the Gecko drivers with big drives direct drive, will running it in mega microstep mode to get the resolution I need? Will running in microstep increase resolution, or will it just get rid of the harmonics?

If I run the belts and reduction will the hobbycnc board and 200 oz/in steppers give me plenty of power, smoothness, and resolution, or is it still best to go Gecko? Im not super concerned about rapids, but want to be able to cut up to 300 ipm if need be.

The table will be used for cutting suspension brackets and aluminum bits out of .125 - .250" I want it to be as accurate as possible but build the machine on a budget..

As soon as I get a few more bucks together I will upgrade to THC from Cambell designs and cut some thinner stuff.

Im going to use Mach2 and SheetCam for software.

Im super excited about the project... any help would be greatly appreciated!!

David

[anthony]

your post reads like I could've written it .. I'm at the exact same stage.

from my own design backgroud i tend to think a 2:1 reduction, geared or belted, would be better than putting a bigger stepper directly on the rack. the belt will also serve to keep unwanted mechanical loads off the stepper if your brackets/assmebly aren't perfectly aligned (budget machine) .. no offense of course.

i'd imagine that chromed (hydraulic) rod would work well and cost less though they aren't hardened. if your gantry isn't too heavy, that is; otherwise the bearings will wear grooves on the surface.

i started off with the "T" gantry design and moved it under the table (over the sides) so i have as few obstacles as possible when loading the table. i quickly moved to an "I" shaped (make sense?) gantry with shorter (closer) supports. my 4x4 table actually needs to be 4x5 to give me a 4x4 cutting surface. i'm considering putting the stepper in the center, below the table.

-anthony

[David Bord]

Thanks Anthony. I agree with an I gantry being a bit more stable and honestly, thats what i was thinking of doing.

While the bearing carrier and belt adjustment will ad complexity it would probably be better.

My gantry will be made with alum and lightness and being rigid are the first orders of the day.

Im hoping some of the die hards on the site (like Al) will chime in. There is so much knowledge on this side!

Do folks think a steel rod with linear bearings are the better way to go, or roller bearings straddling cold rolled steel?

David

[Al_The_Man]

Here is a site of Routers and Plasma tables that I have installed in the past,
http://www.larkencnc.com/index.shtml , they are an exellent product and they use hardened rod and rack & pinion, so this method works well, with rack & pinion it pays to use the largest reduction you can within the range that will give you the required rapid rate.
Al

[David Bord]

Al,

Thanks for the link. Do I need to know the exact weight of my Y gantry to calc rapid rate before I can select my steppers?

Do you have a guesitmate of what size steppers I will need for the table described above?

David

[Al_The_Man]

Technically yes, there is an inertia calculation requred and a factor that is used to keep a certain inertia ratio between motor and load and is calculated for a certain feed rate. I think most people here wing it to some degree or go by others experience, unfortunatly Steppers are not my forte in the design area, as I am mainly involved with servo's, but I am sure people like Gerry would give you and idea.
Al.

[David Bord]

Couple more questions:

If I am using a Gantry of approx 36 inches, and its designed like a triangulated L for rigidity, will driving it from the long side be sufficient? Cutting speeds wont be over 200 ipm however, I want it to be at least as accurate as the plasma torch can take advantage of. Hopefully better than .01"

I keep bouncing between gear racks, belts and rolled ball screws.. but trending towards the gear racks.

I think I am going to have to gear down the spur gear to motor a bit and am having a hard time determining how to implement a lower cost but tight bearing/bushing for the pulley and spur gear... and how I would interface them. It appears a pull with a spur gear pressed or fitted together running on a spindle/bearing would work great but dont know how I would do it without considerable effort.

Any suggestions for the drive mechanism, or ways to implement the reduction using pulleys would be greatly appreciated!

David