Possible gantry weight problems

[samualt]

OK, so I let my newbie impulses rule me and I went and bought some really big servo's. These babies do 1500 oz-in and weigh 9 lbs (6.5" long w/encoder and 4" in diameter).

Question:
Assuming the router is very rigid and can handle the weight (it won't collapse or rock), at what point does the weight become a problem because of inertia? I mean, you get a 100 lbs gantry moving and it might be hard to stop! LOL.
What is the heaviest you would make a gantry?


P.S. I was thinking of building Ninewgt's design. CNC Router

[High Seas]

I had the same questions and was not here to ask!

I couldn't find in my lone research any good data for when/why a moving gantry vice a moving table so went for a similar style.
I too await inputs from our forum experts on this question too.

It seems that the design you're looking at would handle the weights and motion ok, and have less opportunity to get out of square than a verticaly cantelevered gantry.

, Jim

[balsaman]

I don't think 100 lbs is unreasonable for that machine. I don't know if you have seen my machine but I am guessing the gantry weighs 60 lbs.

Eric

[ninewgt]

Sam,

Dont worry - If you are building my machine - the gantry will handle those motors without a problem......

I did a little test - real low tech.....

I actually sat on the gantry and rode it back and forth...... I weigh about 160.... and my son could push the gantry with a few fingers and he is a little guy (11 years old) thats how easy it moved.......

So - dont worry - the frame, and gantry will handle that and more

[HomeCNC]

Ninewgt, I think he was asking about stopping the mass. How easy was it for your son to STOP your movement?? How many inches did it take to stop you?

[Verboten]

My knowledge of Physics isn't what it should be. But, assuming that ninewgt isn't gaining any more energy than the initial push, and is actually losing energy due to friction, then wouldn't it actually take less energy to stop ninewgt over the same period of time than it did to initially start him moving?

- but let's not test this by putting someone infront of the runaway gantry

[ninewgt]

Jeff

I didnt tell you that part - he couldnt stop me and me and the gantry went crashing to the floor.... haha-kiddin.... He didnt push me that hard......and I stopped myself... But the added weight will be of no problem - especially with 1500's I have 470's on the machine now and have run it quite fast, with pretty fast ramp and decell rates - all without loosing steps / position. The X axis is driven with a rack and pinion, and it is geared down via a timing belt pulley, and driveshaft that the pinions are mounted on..... I did a lot of research on torque, and inertia to figure out the thing because I was worried about the 470's not being sufficiant - but they do great under the drive ratio i set the machine on........

[epatsellis]

well, as a data point, my 8'x12' router has a gantry that weighs about 1200 lbs. I drive it with a 34 frame 550 oz. in. motor reduced 10:1 to a 30 tooth 24 d.p. gear.
steps per inch is about 5100 (gecko 201 driving it..) it accelerates and decelerates just fine. However I am replacing the motor with a 1 hp. ac servo that I just acquired to increase the acceleration.

erie

[avsfan733]

epat, is yours a homebuilt, or a machine constructed by a company? I am looking at making my gantry very heavy because I have grand plans of machining metal on it, and am preceding with design and drawing ideas for such. what is your Z-travel?

The bearings I am going to use are .002 cOf which would mean about 6 pounds of force on each of the four bearings...but thats only downward, I am worried about the momentum as Sam is...is there any sort of listed figures as to power off internal resistance to movement that you know of?

[epatsellis]

It's both, Originally it was built by ESAB, as a plasma cutter, Cybermation bought them and converted them to router operation with a Precise 40Krpm spindle, and a PDP-8 based control. When I bought it, the control was dead, so I stripped it down totally, re bearinged, and used (in order) EMC, CNC Pro, and finally Mach 1 and 2. I've been busily fabricating new motor mounts and I will finally get some serious accelleration out of this thing. I'm shooting for at least 1 g, preferrably more as I do a lot of carving on this machine.

z travel is limited by gantry height at the moment to 10", I have a stage for my z axis that has 16" of travel, but due to bearing design on the y and x axis, limited to the 10".

the interesting thing about the way that ESAB designed this thing is that all the bearings are standard, off the shelf bearings, due to the weight of the gantry there are only bearings on 3 sides, riding on a 1" thomson rail, on one end and a single cam follower riding on a shaft on the other end, reasonably rigid, even when cutting .090 aluminum.

erie