Gantry CNC mill capable of mild steel? (warning: heavy flaming inside)

[lukewarm]

Let me just start off by thanking you in advance for any input, negative or positive, that you can supply. I am preparing myself for a fire storm of negative comments, lol.

We're building a gantry style CNC mill right now, the thread for it can be found in the wood machine section. In the beginning we were hoping for a wood capable table that might see some light duty aluminum cutting. But now we're wondering if our rigidity might not suffice for light steel work? It was never our intent to build a gantry style mill for steel so the design is probably all wrong for precision milling. I've attached some photos of the construction so far. I would be thankful to anyone who could point out any problems with our design.

For those interested in the design specifics the bed is made out of 6" I-Beam and weighs close to 1000 pounds as a very rough guess. The gantry is a piece of 6" by 10.5" I-Beam capped by end plates of 1/2" plate metal. The gantry weight is currently around 350 pounds without the Z-Axis carriage which we expect to add another 200-250 pounds including the X2 milling head and motor (it will be counter balanced). The z-axis will be mounted on 2" shaft running through Rulon bearings. Both the x and y carriages currently are mounted on 4" sawmill carriage wheels that temporarily run on angle iron (until we finish casting and scraping the cast iron replacements)

Thanks again,

[RICHARD ZASTROW]

There must be something I can't see. What's keeping the gantry from jumping up? All I see is wheels rolling on the X-axis beams.

Z-axis on one 2" round rod? 2 would work a whole lot better. Still, light milling only.

Just curious.

Dick Z

[lukewarm]

Richard - the only thing keeping the carriage down is gravity. It was designed for wood routing originally and we thought the carriage weight (600 pounds now) would keep it from jumping up. Is this not the case for milling steel? Do we need to redesign our long axis linear guides?


The z-axis does run on two 2" shafts. I wasn't very clear in my writeup.

Are there any improvements we can make that would increase our ability to do heavier milling?

[Geof]

Your 6 x 10.5 I-beam may not be stiff enough under a torsional load. I-beams are intended for loads acting parallel to their long axis and in this direction yours will be very stiff, but I-beams are not stiff under a twisting load and that is the type of load that is applied by a cutting tool that is located many inches away from the centerline of the I-beam. If you are comitted to using an I-beam and cannot switch to a length of 6" x 10" tube you may find it necessary to box in one side of the I-beam to creat a tube and enhance the torsional stiffness.

[lukewarm]

Great suggestion Geof we already have come to that conclusion but it's nice to hear it confirmed before going ahead and boxing in the backside!

The reason we never went with box tubing is with our current carriage wheel configuration we need the open front face of the ibeam to place the ball screw in. Perhaps we might have been better off going with conventional linear guides and saving ourselves the headache of engineering around something that wasn't intended for our use.

[dertsap]

what is your plan for a spindle , to reliably cut steel , aluminum , and wood then your going to need a spindle that's quite versatile (fast , slow , and torque) , probably heavy and most likely expensive

also what drive motors do you plan to use , starting and stopping motion with that kind of weight may prove interesting if the motors are small , the accel/decel can be adjusted to compensate for this but with too small of motors then by the time your axis is up to speed , it will be decelerating

[lukewarm]

The spindle will be an X2 R8 spindle driven by a large (we have motors anyway from 0.5HP to 20HP) AC induction motor controlled by a 3 phase VFD. We already have the motors and the VFD we're just waiting for the X2 spindle to be restocked. For wood cutting we have a separate Makita plunge router.

To start out with we've got some tiny Nema 34 973 Oz-In stepper motors. The calculations we've done suggest these will suffice but in the case that they don't we will most likely switch to AC servo motors.

[lukewarm]

I also believe we might be able to lighten the preload on the X2 spindle bearings and cut up to 10,000 RPM with stock bearings. However maybe this is false information.

[RICHARD ZASTROW]

Geof's remarks on the boxing of the "I" beam are spot on.

As to using the weight of the gantry to hold it down, I wouldn't do it. You could use opposing rollers as on the gantry cross axis (Y?). If the top and bottom are not exactly parallel, a counter-acting load could hold the rollers tight against the top rail (X axis?).

The load could come from a spring, hydraulic cylinder or air cylinder.

Milling steel takes force and remember all that high school stuff about equal and opposite forces. LOL

If you intend to plunge a milling cutter or drill into steel, it will lift the gantry without arresting the X rollers from below.

Dick Z

[hanermo]

The steppers are actually much bigger than you need !
From the pics, you are using a transmission with belts, and a moving nut, which is an excellent choice.

What is the transmission ratio ?

FYI, using an X2 spindle, you will never use more than about 60-90 kg force (600-900 N). This would be with about a 8-10 mm end mill.
A bridgeport used to use 600-800 oz-in steppers, and was a much more rigid machine and a better spindle.
These were the original Bp BOSS cnc machines.

A 3Nm nema 23 stepper at 3:1 will easily move a gantry of 300 kg if using ballscrews. It accelerates to top speed in 0.2-0.5 secconds depending on voltage and steppers.
My millling machine is like that, and it works well in steel.

At 3:1, 425 Ozin or 3Nm is about 240 kg push, or about twice what you really need.
With a 240 kg gantry you would have 1G of acceleration which is very high, about on a par with industrial production machines from a few years back. Today they might do 2G at most.
The 3:1 ratio is in the sweet spot not because of power (torque converted to push, at speed) but acceleration and accuracy (to 0.01mm, so build to 0.005 at +/-1 step).
I am using Nema 23s because they accelerate a lot faster than the 34s, and run a lot faster, they are just a better option.
However, I dont really see a need for max speed, and think the rapid speed people look at is often misguided.

You wont have any problems with your current motors.
Depending on voltage ? and drivers ? and transmission ratio you should get about 300-600 rpm usable max speed on the steppers.

You may find that the roller bearings are not rigid enough.

I use 20-25 mm THK rails in a steel frame. My mill is 2.4 x 1.8 m in size, and about 2000 kg.
I am using a 50 cm x 160 cm subtable at the moment (moving table in a gantry/portal design, very rigid).

Quote:
To start out with we've got some tiny Nema 34 973 Oz-In stepper motors. The calculations we've done suggest these will suffice but in the case that they don't we will most likely switch to AC servo motors.[/QUOTE]

[lukewarm]

Very informative reply Hanermo, thanks!

You're confirming what we suspected according to our calculations but doubted when seeing the motor next to the machine.. If that makes any sense?

Our transmission ratio is 3:1. You may be misguided by the pictures though, that shaft connected to the stepper on the gantry is actually a piece of 5/8" shaft with 10 tooth 1/2" pitch sprockets on the end. The long axis is so long we decided to try out chain drive as we've heard good things.

The z-axis and x-axis will both be driven by ball screws however, and we're planning on using no transmission.

[leepi]

hello can anyone advise me of which nema motors to use on my new mill router table
x=1000mm y=1500mm z=450mm 16mm ballscrews are fitted,
cheers,