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# Thread: Gantry - Rigidity V. Weight

1. ## Gantry - Rigidity V. Weight

Early stages of designing a machine that will handle 4X8 material so the approx dims I'm playing with are 6X10.
In pursuit of a rigid 6' gantry, I've noticed it's probably becoming fairly heavy and am wondering what the limits are
for a couple of large nema 34's. I haven't got anything on paper but just licking my thumb, checking the wind, I'd
say I'm between 400-500 lbs. Trying to use a lot of material I already have.... 1" T6 plate may be aluminum but it's
still heavy as hell and structural steel not much better. Would appreciate your thoughts on what the limits are and
what to watch out for.
thanks
cncneon

2. ## Re: Gantry - Rigidity V. Weight

At that point, you might want to make it a fixed bridge and move the table instead.

3. ## Re: Gantry - Rigidity V. Weight

Originally Posted by awerby
At that point, you might want to make it a fixed bridge and move the table instead.
10' moving table the machine would be near or over 20' long

4. ## Re: Gantry - Rigidity V. Weight

Hi Neon - The gantry is not being lifted so it can be moved by small motors. Its the same as you can push your 1 tonne car around in a flat carpark. The issue is what acceleration do you want to achieve? If a hobby machine then modest accelerations can be achieved using nema23s and a fine ballscrew pitch. So theres a Q? is this ballscrew or R&P? The usual design workflow is to complete your structural design. Then you know the weight of everything. Then you can design your motion systems. Hiwin publish all the formulas needed for this... Peter

5. ## Re: Gantry - Rigidity V. Weight

Hi,
as an example of what Peteeng is talking about: I have cast iron axis beds of 115kg each, moved by 32mm diameter 5mm pitch screws with 750W servos.

You'd think that the main inertia would be the 115kg piece of cast iron, in fact I allowed a bit extra for the vice etc, say 150kg......but you'd be wrong. When you do the calculation 80% of the
inertia is in the rotating steel ballscrew, 12.5% in the armature of the servo and only 7.5% in the 150kg axis mass. The acceleration of the axis is then determined by the ballscrew rather than the
weight of the axis. It sounds counter-intuitive but the calculations don't lie. If you use fine pitch, say 5mm or less, ballscrews then the acceleration will be determined largely by the ratio of the ballscrew
inertia to the torque of the stepper....and the mass of the gantry probably quite small influence.

Craig

6. ## Re: Gantry - Rigidity V. Weight

Hi All - seems the Zone has hiccups at the moment. Lots of repeat entries... Peter