Accurate Strong Gantry

[Ross77]

To get the best rigidity you need to keep the spindle inline with the support bearings so no overhang. You could rotate the rails so they are mounted end on and in the middle of the beam.

The vertical moving gantry beam looks a bit slender with regards to the end bearing spacing. I would guess its 200mm deep and 1000mm wide? Thats is not a good ratio for racking and I would make the most of the gantry height and increase the bearings to maximum so the moving beam is like 'H' shape.

That would naturally reduce the racking and take the pressure off the screws and rams.

Just how I would do it anyway :0)

[vargai]

To get the best rigidity you need to keep the spindle inline with the support bearings so no overhang.

Hope I understand well. I made a top view sketch (left figure) -I think the spindle can get much closer to the posts.
The higher H gantry beam is agreed.
Ideally it should be balanced like a VMC in the right figure- manufacturer apply 500-600 mm and more overhang without problem.
Theoretically a reversed moving balance can be done but it increases the mass on Y axis motor
There are tasks left.

[Ross77]

The spindle on a vmc has to be like that so the table can move. But the column is fixed and only the z axis moves. Don't get your designs mixed up, you have the opportunity to build some thing specific to your requirements so best look to optimise it.

If you want to get technical then you don't have to put the cutter directly in line with the bearing but you need to balance the y axis and put its centre of gravity in line with the rails as that will produce the smallest moment loading in the bearings and hence deflection.

I wasn't meaning to use a H beam but rather trying to explain the position of the beam relative to the vertical bearings, ie keep the beam 200mm deep but increase the bearing spacing To 400-500mm. Might need to draw something up.

[vargai]

Might need to draw something up.

Tried to catch the principle based on sketches but this is more complex and one detail can change the others.
Here are some slightly more detailed plans -the C/G is near the C/L of the horizontal ball screw so hope this setup works.

That would naturally reduce the racking and take the pressure off the screws and rams.

I have increased the bearing spacing-350 mm now.I do not think it is worse than a moving gantry since it has double Z screw. Forces on screws mainly come from the vertical feed force -assuming it about 200 N(??) while I would like to reach 1000 N machining force in the horizontal plane.
If something might be considered to apply one more shorter Z rail and +1 each carriage on both side-I do not know if it worth
Drilling only means center drilling in this machine and max 6-8 mm holes. The machining that requires Z force will be arranged in the middle zone of the table where the load is almost symmetrical on screws

Regarding to the profile I started to change them to bend opened ones from 6-8 mm thickness. All of them havv to be designed according to their function

By the way when I am taking about balancing I am thinking of the Z motor too. I want it to get relieved form beam mass. It cannot be balanced perfectly since the spindle is moving but I want to remove the dead load expect the spindle at least.

This vertical in-between railing makes me worry a little.
The implementation will not be too easy. At the moment I am planing to put together the Z frame from three bolted parts

[Ross77]

Looks good. I would probably extend the gantry uprights down to the floor so the more area would be available for welding and increase rigidity. the vertical rails could then drop down and increase the bearing spacing a bit more.

I think golden ratio is 2:1 so if the y axis is 800mm wide then you need 400mm bearing spacing. as you pointed out one change can affect another and you will need to look at flex in the mounting plates and possible bracing,


Do you think the gantry uprights need bracing as well? looks quite tall

It may have already been mentioned but how to plan to keep all the surfaces perfectly aligned whilst welding and given the surface tolerance on steel c section?

[vargai]

Looks good. I would probably extend the gantry uprights down to the floor so the more area would be available for welding and increase rigidity. the vertical rails could then drop down and increase the bearing spacing a bit more.

I think golden ratio is 2:1 so if the y axis is 800 mm wide then you need 400 mm bearing spacing. as you pointed out one change can affect another and you will need to look at flex in the mounting plates and possible bracing,


Do you think the gantry uprights need bracing as well? looks quite tall

It may have already been mentioned but how to plan to keep all the surfaces perfectly aligned whilst welding and given the surface tolerance on steel c section?

Extending the gantry uprights down seems good idea -this node should be rethink with the bracing and reinforcement plates in profiles.

My aim is to design parts that can be done by both DIY and machining method.
machine bad is Ok -it has surfaces to be leveled on top -preferably positioned on the same level-that is way stand the gantry leg here at the moment

Y beam is the same -can be done on both way

Z beams will be separated and bolted so it is similar to Y -I do not want to weld a U form and do something with frame -it makes my life hard
rather to make strong flange nodes and the careful assembly and adjustment will give the accuracy (especially the two parallel surfaces)
Nodes will be oriented and fixed with parallel pins.
That is only may plan for now -God knows nothing about it.

[JAZZCNC]

That design will be a night mare to build and setup which isn't required and will have no gain over other simpler designs. C channel is bad choice due to it being bendy/twisty in relation to boxsection.
I don't think you have really thought about the complexitys and realitys of building this design.?

The C section inner surfaces won't be flat so will need milling for the rails. Then you have the problem of making sure the uprights are perfectly parallel in 2 planes and perfectly perpendicular in the other.!! . . 99.9999999% Nailed on that you won't be able to weld this up and keep parallel/perpendicular in all planes.
Any surfaces that where milled flat and true won't be flat and true after welding so this will be a waste of time and you have no way mill when welded up.
There is a no provision for adjustment and the design is such that there is no room for any error to be taken out.

Boxsection will work far better and be much easier to setup or deal with alignment errors after welding.
The slight difference from ideal bearing loading which Ross suggested won't make any differance to the machine in real world performance or longevity but it will to your mental health and sanity when setting up.

Also the Uprights need far more bracing as any deflection here will show at the cutter and finish quality. Personally for best strength and bearing loading I would have double uprights each side of gantry with 1 rail on each upright and the uprights would be braced in 2 directions.

I feel If you go with this design you'll regret it come setting up time.!