Build Log: 3-4 Axis Mill/Router

[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.

[IanS1]

After loads of thought I've decided to stick with a more traditional router design for my first machine. The frame work is almost built so now my attention is more focused on the gantry design. To start with, this machine will be used for cutting aluminium, plastics, acrylics, MDF and plywood. If the machine is successful I will eventually build a dedicated machine for cutting the acrylics, MDF, plywood etc and keep this one for aluminium cutting only, hence the strong heavy frame. Overall the cutting area will be approx 700 x 1400mm and I'm hoping to get around 100mm for the Z axis travel. The bed will probably be a 30mm thick aluminium plate.

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The frame work is built mostly from 80x80x5mm, the overall size of the frame is approx 1160mm wide, 2000mm long and 650mm to the bed height. All of the diagonal bracing is 40x40x3mm.

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The Y axis section is 100 x 50 x 8mm, I still have to add the bracing for either end of the Y axis. The extended Y axis is for future plans of a possible ATC at the rear, a rotary axis and a clamping system for machining the ends of ali plates at the front.

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I'm really pleased with the way it has turned out so far. Using a spirit level across the bed and across the X/Y axis indicates it's pretty flat but with the adjustment I've built in and if needed the use of epoxy I should be able to get it clocked in quite nicely I would think.
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Anyway, on to the gantry. The design I've gone for is fairly straight forward and hopefully strong enough. The gantry is mostly drawn in 25mm plate with the Z axis plates done in the 30mm. All plates attatching to the bearing blocks are drawn in 30mm. The overall gantry width is 1250mm and height is 270mm.
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The bearing blocks on the Y axis are spaced at 370mm end to end and the X axis are spaced at 230mm end to end. The linear rails on the Z are spaced at 150mm and the bearing blocks have a vertical spacing of 310mm.
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Would really appreciate some input on whether this gantry design will be up to the job.

[IanS1]

Any thoughts on the gantry would be very much appreciated.

Ball screws: Would a 16 be OK to use on the Y axis or would I be better off using a 20? It's going to be about 2000mm long.