First Build 5 x 12 Steel Frame CNC router

[Scott Damman]

Very ambitious for a first build.

Some thoughts:

-the gantry legs dont seem adequate for the rest of the parts you will use on your machine

-the A may work for foam but i dont see it serious for anything more than wood

-the stepper motors , ok- the huge stepper motors, what drives will power them? 230VAC?

-that bracing may be was totally unnecessary. I dont see how that will strengthen anything. It could even make things worse, strengthening one point too much and not allowing the forces to spread uniformly and creating pressure points. So later all rings.


What will you be using this machine for? Remember- the weakest point is the criteria for how strong your machine would be. Again-the gantry legs.


PS:
That gantry legs will greatly benefit from bolting or welding on them 10mm steel plates in both directions, 2 per leg in total

Hi Scott,

This is how I see couple of not so little things.

Steppers:
Really Cannot comment if they will be good enough because you haven't said anything about what your using for linear motion on the X axis. I assume Rack n pinion but what ratio etc.? . . . But let me put it this way 550lbs or put anohter way 1/4 Ton is lot of Mass to Accel and stop so better get this correct or you'll seriously regret using steppers.


A Axis: You need to think again on this I'm afraid NO WAY will Stepper with such small ratio hold that Big spindle at angle without moving or vibrating.
Remember the Z axis is the MOST important area on machine so weaken this and all that OTT work you have done on the frame is pointless.

I think you are talking something along these lines for the sides of the gantry legs? I was wanting to do FEA on these legs but never got around to it, I did however do calculations (free body diagram) to try to calculate total deflection based on 1000lb force at the bit and keeping the total deflection ~3 thou at full load.

I could go higher with the bracing as well this is just a quick drawing.


The thinking behind the bracing was to connect the upper and lower gantry beams. I used box tubing between the beams. Now the rear bracing, the thought there was to reduce the twist in the gantry. Hopefully it doesnt cause issues...

So the A axis, this is exactly what I am worried about, holding it in place when at angle and also moving it. The A axis would really only be used when machining foam blanks for paddle boards. However since I want to be able to machine wood, and aluminum as well then I really need to make sure this thing is right.

Couple things I am thinking thoughts are greatly appreciated.
- Eliminate A Axis...
- greatly increase the belt ratio
- keep A Axis but have an air operated taper pin that holds it at 0 when machining wood/alum
- look into a harmonic drive rather than belt drive

X drive
I was planning on using rack and pinion with a pinion of 1 inch I was going for 4:1 ratio to the pinion.
so 3.14 inches per revolution of the pinion, 0.785 inches per motor revolution, works out to 0.0039 inches per step. I was thinking of micro stepping them x10 = 0.000395inch per micro step.

I am brand new to this and trying to learn what feed and rapid movements I need to shoot for to cut wood/aluminum properly. I know it depends on the number of flutes on the bit etc. Am I crazy to think I should shoot for 1000ipm?

The motors I have are chinese 1841 oz/in with MA860H drivers along with 60V drives. Based on the questions it sounds like these are not adequate?

Servos
So if scrap the steppers for X axis then im looking at servos. Something I am even more NEW to than steppers. I need to do more learning on how to size them please correct where I have errors!

Things to keep in mind when sizing?
Gear ratio - want to keep the servo motor spinning at higher speed to stay in its power range - unlike steppers which have the most power at 0 speed
Load to motor inertia, this one I have seen 10:1 ratio but I dont really understand how to use that

One thing I dont understand, when X is not moving what is holding if from moving? I had assumed that the steppers hold the load from moving at 0 speed (is this the case with steppers?). If that is the case what is doing the holding when using servos?

Can my router have both steppers and servo motors? I think the stepper would still be adequate for the Y Axis and could use one for the Z axis (as Im sure I can not send them back)... I plan to use ethernet smooth stepper with mach4.

Got the X axis steel drilled these needed to be through drilled as well since the linear rails are bolted from the back. 244 holes!!!!



Any help with servos for this size of gantry is greatly appreciated.