Build Log: 3-4 Axis Mill/Router

[Jonathan]

If you were making a fixed bed (moving gantry) machine, then I would suggest the following:

Since you only need the Z-travel to machine plates 'stood on end', you can leave the bed at the best position for cutting what you will cut most often and just clamp the plates on the end of the bed to machine them. If the bed ends just inside the X-axis travel, then you can move the gantry (just) off the end of the bed for machining the plates.

Here's an example in case that's not clear:


If you put a vice of some sort on the end of the bed then it would be quite efficient.

However, since the bed is moving you wont gain so much from doing this, maybe 100mm less Z-travel required. That's still a big gain for virtually no change in the design. You might even be able to get the full 220mm length to fit by allowing the plate to pass between the bed supports. Clearly that severely limits the X travel in that configuration, but you don't need much.

You really need some diagonals on the back of the frame. Don't leave it open like that as it will make the stiffness parallel to Y quite poor. Getting the gantry sides cast would be nice, for the right price, however if this option is unrealistic there's no need to use 40mm thick plate. A solid cross section is a waste of material when subjected to bending forces, so in this case a box section would be better. For example you could use four 20mm plates to create box-shaped gantry sides and the difference in strength between that and solid material would be small. If you can give the dimensions of the gantry sides then I can say roughly how big the difference would be. The reason for this effect is the material towards the center in a solid cross section is subjected to very little stress, so contributes very little to the overall stiffness.

[IanS1]

Thanks for your input folks.

JAZZ: I think there's a video somewhere on here showing a much smaller machine with a similar design. Can't remember who posted it now though. I had considered it but thought it would be way to difficult/costly to scale up to the size I'd need. Will have another think about it.

JONATHAN: A moving gantry design would be my preference as I originally wanted to build the extra axis for turning at the end of the bed, not a huge problem with a fixed gantry as I could mount the turning axis on to the table as and when needed. Baring in mind this machine will only be used to cut ally and sheet steels would a moving gantry design work as well as a fixed gantry? I was under the impression a fixed gantry would be generally better.

[Jonathan]

Bearing in mind this machine will only be used to cut ally and sheet steels would a moving gantry design work as well as a fixed gantry? I was under the impression a fixed gantry would be generally better.

You could design a moving gantry machine as strong as the current design without too much difficulty.

[Ross77]

Since you only need the Z-travel to machine plates 'stood on end', you can leave the bed at the best position for cutting what you will cut most often and just clamp the plates on the end of the bed to machine them. If the bed ends just inside the X-axis travel, then you can move the gantry (just) off the end of the bed for machining the plates.

If you are doing a lot of repetitive work of the same size then you could also make a slot in the table and clamp it down low.

I have seen vertical moving gantry's on routers before, in fact I mentioned it when I first started and got talked out of it. As John said it is just transferring a massive load to the gantry sides and the distance vs bearing spacing would be huge. Not impossible just difficult and defiantly not a cheap or quick fix.



Just bolting a fixed gantry to a moving gantry design is also not the most effecient way to do things. I would redesign as new and if you went for the fixed gantry then you can get rid of the high sides at each end as they are not doing anything. It would be stronger if you went for an 'A' frame arrangement (two a frames opposite each other. Main gantry at the top and transmitting load to the ground through the legs, the bed can sit at the horizontal section of the 'A' and extend as far as required to support the table. As the gantry is fixed the cutting load is only directly below the cutting head therefore all the force is contained and distributed within the A frame. table only needs to support its own weight


The moving table also needs beefing up, as it is there will probably be deflection in the middle. Most designs I have seen have the rails at third points. The table itself will then need strengthening to overcome deflection in the middle and the cantilevered edges. To some degree having a fixed gantry just inverts the flexing problems associated with the z axis and to the table as this is now compromised.

A fixed gantry will get you closer to cutting steel as you can really beef up the design, the same weight and strength on a moving gantry has to be accelerated and decelerated which means big motors and drives.


Edit
The moving table is all wrong. It should be the other way up. bearings fixed to the frame and rails mounted on the table. that way the work is always fully supported under the cutting tool and the bed support can be drastically reduced. And will fit within the 'A' frame I mentioned, look a VMC designs. You basically looking at a 2 sided mill arrangement. Fixed gantry machines are a completely different design to moving.

[Ross77]

Ok a bit more thought and I think your answer is little bit of everyone's idea.

That is a fixed gantry design that a has a short Z axis mounted on on it say 50 70mm for machining operations but then use Jazz's suggestion of adjusting the height of the whole beam using a clamp/unclamping system to change to height depending on the work piece. The A frame idea could easily have this added as long as there is still a brace bar at the top.

This is not exactly revolutionary as this is the same principle that mills have been using for years!

[IanS1]

Thanks for your input Ross.

The A frame type design is interesting I'll look into it. The mechanically adjustable height on the gantry is worth some thought too but immediate concerns are, being a pain to set the gantry parallel to the bed every time.

The table definitely needs more work with regards to support/beefing up. I am very intrigued though about the idea of fixing the bearings to the bed and the rails to the table, I will look into it.

[Ross77]

Thanks for your input Ross.

The A frame type design is interesting I'll look into it. The mechanically adjustable height on the gantry is worth some thought too but immediate concerns are, being a pain to set the gantry parallel to the bed every time.

The table definitely needs more work with regards to support/beefing up. I am very intrigued though about the idea of fixing the bearings to the bed and the rails to the table, I will look into it.

Yeah adjustment would be the tricky bit. You can either predrill holes at say 50mm ccs and its just a case of un bolt, move, rebolt (could still give alignment issues) or go for linear rails and lead screw to guide and adjust, then reclamp to to make it solid.

This is the way I was going to do it on mine if went down that route, except id add encoders to the lead screws and a dual display to show the position, i figure ballscrews are ott as the weight of the gantry will keep the load on the leading edge of the screw so no backlash will be present. Add hydraulic clamps job done.