Need Help!- to OPTMIZE my build

[jestah]

I would like to ask you guys who have actually done this before to make sure my thinking is on the right track as this will be my first build. I would like to choose the right parts that work well together rather than just trying to get the biggest motors and screws i can find.

I have started to get a clearer idea of what my cutter is going to look like. We have focused on keeping weight down in the gantry by building it from carbon sections we are estimating the total mass for the gantry with the z and the spindle will be around 60-70kg and have a total cutting size of 2.4x1.2x0.25m

There will be recessed pockets within the gantry beam that the ballscrew will be tucked into to keep them out of the way and also very close to the Z mounting plate to max out the stiffness. I have 300mm seperation of the end of my cars on the x, 250-300 on my y and 200-250 in the z.

We plan to have two 25mm 10mm pitch screws down each side (about 2700 between the end mounts) that i have calculated to be able to spin up to 470rpm and i am happy to drop that back to 400rpm and get rapids of 150ipm without risk of whipping. The spread sheet i use (on the zone called cnc_calc ) gave me a required force of 175 oz /1.23Nm to move my gantry. This seem very low and im also worried about the mass of the larger screws could lead to a lot of strain/whipp. Could you help me select motors that best suit my setup? I plan to do light cuts in foam, mdf, thin carbon and glass fibre and also some cutting of al. but dont think that i will be driving the tool much harder than 100N (this is a guess number thinking about how hard i drive my hand router)

I would be really stoked if you could help me understand how to calculate what force i need to accelerate and brake my gantry quickly. Im assuming i want it to accelerate at 1g up to 400rpm so as a stab in the dark would i need work out F = mass X acceleration. Is it safe to accelerate a gantry faster than 1g as i can see that if i got bigger motors i would be able to get more overall speed by accelerating. I am guessing that is more to do with the structure?

My only assumtion about getting larger motors was that i could accelerate faster but at one G i would reach my max of 400rpm after much under a second and at many g it was just less of a second but not enough that i think i would see a huge difference.

Do i trust the above calculations and find a motor that can push 1.23Nm at 400rpm?

[jestah]

After a few hours behind my calculator I have come up with the following:

my longest screw is 2.8m long, about 10kg with 25mm diameter and has a 10mm pitch

according to Nook i should be able to spin this at 470 rpm but dropping this down to 400rpm give a speed of a little over 150ipm which is fast enough for what i need

i have also calculated that i should only need a 275oz at 400rpm to accelerate my table to 1g

So it seems that i should be fine with old stepper but then i ran the issue of rotational inertia. The 10kg ballscrews have 8.4kg/cm that i need to cover. I have been told a safety margin of a 1:3 ratio is good practise? so that would mean i need to find motors with specs above 2.8kg correct?

thanks in advance for the help!

[ger21]

Can't help you with the calculations, but here's some advice. You're going to find that 150ipm will be very slow for a machine that size. It appears you'll be spending quite a bit of money. My recommendation would be to use screws with a pitch of 25mm or more, and possibly using servos to drive them. And if screw whip will be limiting your speed, perhaps a different drive system may be needed. For 2.4m, I'd be looking for 400-500ipm rapids. For cutting large pieces of foam, you even want to cut that fast.

[jestah]

Originally Posted by ger21 Can't help you with the calculations, but here's some advice. You're going to find that 150ipm will be very slow for a machine that size. It appears you'll be spending quite a bit of money. My recommendation would be to use screws with a pitch of 25mm or more, and possibly using servos to drive them. And if screw whip will be limiting your speed, perhaps a different drive system may be needed. For 2.4m, I'd be looking for 400-500ipm rapids. For cutting large pieces of foam, you even want to cut that fast.

thanks for your advice but we are locked into the 10mm pitch ballscrew and are happy to go slow for the sake of accuracy as a lot of the work we will be doing will need a high resolution.

one option i am starting to look at will be to spin the nuts that will not only reduce the inertia of the system but also allow them to safely reach 700rpm (or 275 ipm) with out any sacrifice to our resolution.

so far i have been looking for nema 34 steppers around the 600 oz mark as they have decent inertia values.