gantry weight

[reefy86]

Hi ive been calculating the weight of my gantry and its 132kg without the spindle and motor weight and most of that weight comes from the 200x100x10mm steel tube weighing 87kg alone. what is a good thickness for the main gantry beam? is 10mm too thick

cheers

Ash

[Boyan Silyavski]

What you say is my dream gantry come true :yahoo:, people should take a note here. That's the perfect beam for simple gantry . Some good 2 x 400w 220Vac servo motors should move it flawlessly / rotating ball nuts both sides/ . Mine weighs exactly the same and motors are geared 1:1.5 / 20t:30t pulleys/ and it flies around at 20000 mm per min with 3000s2 acceleration , which is hell of a scary thing, so i have limited it now to 10000mm/min which is also quite scary until you accustom to it.


You could read also my 2nd build log #32 (http://www.mycncuk.com/threads/6619-Quite-an-Unusual-one?p=52445#post52445) and next ones where people explained to me what snappy is, cause at the time i was just imagining and not very sure what exactly i need


here is when i was testing it at 10000 mm/min and 3000 acceleration so you get a better idea what we are talking about. It could move as double as faster, though at the moment i see no point :


https://www.youtube.com/watch?v=ohU3zJLcR1E

[reefy86]

thanks boyan its an interesting thread so far :) i was originally going for nema 23's 3nm on a 2:1 ratio. i cant remember where i read it from but are servos alot more complicated to use then steppers? my actual goal is to get as much resolution as i can while keeping the machine nice and quick, i had in mind with the nema 23's running 2020 screws on a 2:1 rotating nut so i get some of the speed from 20mm pitch and resolution from a 10mm pitch. my dream goal was to have a resolution of a 5mm pitch but still have a really quick machine but ive learned thats not possible on steppers and im not quite sure yet if its possible on less expensive servos.

cheers

Ash

[reefy86]

edit to my previous post, it may well be possible with steppers but im just a newbie :)

Ash

[Boyan Silyavski]

The steppers that could change the servos on that weight will not be that cheaper than servos, drives must be AC high voltage, etc. I have not used, but the cheap servos from BST will do the job fine .

If you gear them like me on 2510 screw / 2020 or 2010 is too small for that job and price wise is same from china/ at 20t puley and inside rotating nut 30t puley, speed and resolution will be more than enough.


2510 screw makes 10mm per rev, so 3000x10x1.5=20 000mm/min max speed , so at 2500ppr / the KRS servo motors from BST/ will give you 2500ppr/10mm=250x1.5=375ppr resolution which is 1mm/375=0.0026666666666667 resolution on the axis and bigger on Z as there screw would be 1605.

So not so bad after all:yahoo:, 99% of people working on cnc will envy you to have such a machine at home


PS thats exactly the resolution of my motors
ps2/ 2048, not 2500 , only my z is 2500 ppr, so thats why at my video you see other number than 375 steps per rev/

[Robin Hewitt]

Just to make sure my poor old brain is still working...
If you put a 1m radius pulley on a 3Nm motor
One rev gives you 3N over a distance of 2http://www.mycncuk.com/vlatex/pics/3_4f08e3dba63dc6d40b22952c7a9dac6d.png meters
One rev on a 10mm screw only moves you 0.01m
so the force increases in proportion (3 x 2http://www.mycncuk.com/vlatex/pics/3_4f08e3dba63dc6d40b22952c7a9dac6d.png ) / 0.01 = 1885N force
1N will accelerate a 1kg mass at 1 m/s/s
So 1885N will accelerate a 132kg mass at 1885/132 = 14.2m/s/s
Sounds okay, 1G is 9.8m/s/s and 1G with a safety margin is good acceleration.
Also we know that 132kgf will accelerate a 132kg mass at 1G because that is what happens when you drop it.
So you get about 132kgf to drive the tool dead slow with the same margin.

[JAZZCNC]

What you smoking Robin.? . . . 1m radius pulley.

Also mach3 works in mm/s/s and nobody runs machine any where near 1G (9800mm/s/s).

[Jonathan]

The mass of the gantry alone is of little concern. As has been highlighted, the mechanical advantage you get from ballscrew is significant, so the motor doesn't directly 'see' the mass of the gantry. You have to consider the equivalent inertia of the gantry, i.e. the mass the motor 'sees'. This combines the gantry mass with the inertia of any pulleys you use and the motor itself. Choosing large pulleys can have the same effect as adding 10's of kg to the gantry mass.

Ideally, you pick the motors to match the gantry. If the motors required aren't affordable, I'd sooner use undersize motors and have the machine acceleration be a bit less, than have the gantry less strong.


Just to make sure my poor old brain is still working...

Better than most ;)

[Robin Hewitt]

What you smoking Robin.? . . . 1m radius pulley.

I gave up smoking 7/3/13 :distracted:
It's a 3Nm motor, the motor is specified in meters.
However, I am confusing my table types. Large accelerations are only necessary for plasma tables and even then 1G would be excessive :ambivalence: