CNC Frame Design Help

[routercnc]

I'll echo Jazz's comments. Plus it looks like there is no gantry as such, just a pair of rails which support the spindle?

This might be OK on a plotter or laser cutter but if you want to cut aluminium this won't be anywhere near stiff enough.

Combined with the unsupported rails on the sides of the bed and you will do more rubbing and vibrating rather than cutting aluminium.

[komatias]

Stick an extruder on it and you have yourself a wooping good 3D printer!

[Rufe0]

Hello


I'm looking at making a machine similar to yours with a moving bed only abit bigger. What your going to need is HIWIN linear rails or similar http://motioncontrolsystems.hiwin.co...es-Catalog.pdf these are available from several sources such as https://www.cnc4you.co.uk/. One of the main reasons this type of linear guide system is superior is that it is screwed down along its length to a (hopefully) solid base, meaning its basically as strong as whatever its mounted to. However they do have a rating, the lowest rating on HG25 is 26kn. A newton is basically 1kg so thats 26,000kg.


So aslong as you screw it down to something solid your good. Which brings me to my next point. Unless you really absolutely need it to be portable then it doesn't make sense to construct a machine with aluminium. Steel is actually stiffer for its weight, its cheaper and you can easily weld it, welding is significantly better than bolting it together. Per weight steel hollow box section is the stiffest strongest thing around for its cost. I don't know if you've ever played with it but if you can imagine a machine built with 100x100mm steel box section welded together, its going to be incredibly strong and stiff.


Why all the talk about stiffness? Well it comes down to deflection, you can get a good approximation of how much by calculating your cutting forces, then use an FEA program or there are some online calculators for simple beams.


Using some feed/speed numbers for aluminium cutters here http://www.endmill.com/pages/training/spdfeed.html you can calculate your cutting forces here http://www.kennametal.com/en/resourc...and-power.html I was surprised to see the forces for a small aluminium cutter are actually alot higher than a bigger cutter in steel, I guess its because you can run a much higher spindle speed in aluminium. I'm getting over 300n Tangential cutting force which means how hard you have to push the cutter sideways through the material. 300n is 300kg, so then you take that number and calculate the deflection in your steel box section and you might get a very low number. Lets say its 0.1mm, you might think thats OK but what you've got to think about is vibration. Vibration is a bad thing for a number of reasons and its really why you need to make the machine as stiff as possible.


Another thing you will want to factor in is the force your stepper motors can apply. Steppers are rated for holding torque or 0rpm. Often you can find a torque curve for your motor, you might be surprised to find how much power a stepper will loose once it's up to speed. You might not ever be able to make that 300kg of force at the feed rate the cutter wants. Search for something like ballscrew force calculator to find how to calculate it. I figure you'd need about 1.3nm torque to get your 300kg force assuming your using 2 ballscrews. Thats alot for a stepper motor turning at a decent rpm, plenty have 1.3nm at 0rpm but not many at speed.

This brings me to my final point you need to use the cutter manufacturers recommendations as a guide, reduce those numbers and you'll find you can build a machine strong enough.

[zhenning]

Some updates on v2. Any feedback is welcome. By the way, any consideration in sizing the gantry and linear profiled rails? Currently, I modeled it with HGH35.

[Rufe0]

Some updates on v2. Any feedback is welcome. By the way, any consideration in sizing the gantry and linear profiled rails? Currently, I modeled it with HGH35.

Kinda hard to see whats going on but you definitely need a wider gantry with 2 carriges per rail like this

[JAZZCNC]

Don't need HGH35 that's massive overkill. 20mm will be fine 25mm Max. After that your just wasting money and increasing weight for little to no benifit.

Also Like Rufe0 mentions you need 4 bearings per axis on all axis.

[zhenning]

So, I have changed the rails from HGH35 to HGH25. Added 4 bearings on the rails. The gantry consist of 3 8080 profile and hold by a 20mm plate that sits on the profiled linear guideway on both ends. For the endstops, i am thinking of some capacitive sensors and the motors, i guess a Nema23 2N.m. would do?