Build Log: 3-4 Axis Mill/Router

[IanS1]

Being fairly new myself I tend not to be to much help when it comes to machine design but why don't you look at buying a proper mill & possibly converting it to cnc if you want to cut mainly aluminium & other metals. Would seem like a far better investment for what you want a machine to do.

Good point but I feel table travel size could be an issue. Most of the ally I want to cut will be 500 x 500mm plus. Correct me if I'm wrong but I think most mills are quite narrow on table depth/travel. Being able to cut tool steel really isn't an issue, it was really just a nice to have feature. Thanks for your input though.

[martin54]

There are guys on here that run machine shops so probably better placed to answer this but at a guess I would think it would depend on intended use, budget & space. OK so I'm old school having done all my machining on manual machines & I have to admit to being a bit spoiled being in the RN because the workshops had just about everything.
Small mills seem to attract a lot of attention & sell well probably because a lot of people have a limited amount of space & are looking for a small hobby type machine, add to that the fact that they are normally single phase so plug straight in to most peoples domestic set up & can be shipped with relative ease & I suppose it's no wonder they sell.
Take a bigger industrial type machine that's going to cause more problems & expense to move, plus needs more space & either a 3 phase supply or someone who knows enough to run it off a single phase supply & the interest drops & you can pick up bargain machines if you look about.
If looking at a serious business machine then that's what I would be looking to do, not sure how easy conversion would be but would certainly investigate. If your looking at a hobby type machine that's more for your own use then a router might be a better choice. Speak to some of the guys on the forum who know what their talking about & they will tell you that routers aren't really suitable for machining any metals although you can use them for that sort of work.

[IanS1]

I've finally managed to get some time to make progress on my mill design so your thoughts/critique would be much appreciated please folks.

As I want to machine Ally and other sheet steels I've taken JAZZ' advice onboard and decided on a fixed gantry / moving table design. The overall length is 1500mm ish, overall width is 1200mm ish and the Z axis has 220mm movement. This should give me approx 750 x 750mm cutting area.

The 220mm Z axis movement is required for machining/drilling 200mm wide Ally plates stood on end. Most of the time I will be cutting Ally plates which will be no thicker than 10mm which gives me the dilemna of a huge Z axis movement or some sort of adjustable bed/table.

I have thought about having a fixed bed with some sort of stackable table which will allow for less Z Axis travel but this obviously opens up potential problems with stability of table because of the distance from the rails/bearings to the cutting surface.

How difficult would it be to implement an adjustable bed/table? Any thoughts on the stackable table?

The gantry ends are currently drawn with the intention of looking into getting them cast, could be way to expensive though. They're approx 450mm width x 400mm height and 80mm thick. If the cast turns out to be to expensive they'll be machined out of 40mm thick Ally and sandwiched together.

The gantry ends can also have some triangular supports added to the outsides for even more stability if needed. I also have a 20mm thick plate across the back joining both gantry ends and hopefully adding even more strength/stability if needed. Would bolting the gantry ends all the way through the 80 x 40 x 5 frame with QTY 8 x 16mm bolts per side be sufficient?

The gantry crossbars are drawn at 80 x 80 x 1100mm and braced from the back with a 25mm thick plate.I'm still not sure if this will be strong enough, any thoughts?

The Z Axis/Spindle support plate is drawn at 600 x 240 x 30mm thick. Will this be thick enough? Any thoughts about the actual spindle? Is the 2.2kw chinese spindle any good for cutting Ally or will I need something better?

The plate thickness around the gantry carriage is 30mm front, 20mm rear and 40mm top and bottom for the bearing plates. The motor mount/support plates at the top of the carriage are 25mm thick.

The frame is made up mostly of 80 x 40 x 5 box section accept for the four corner posts which are drawn at 80 x 80 x 5mm with 10mm thick base plates which will have an adjustable foot attached to each corner. I want to try and keep the front of the frame open to allow for easier loading of Ally plates.

The table still needs some work doing, perhaps a 40 x 40 box section frame underneath to give it some support. The table plate is 900 wide x 750 length x 30mm thick. Would that need extra support underneath?

Still loads to do but I'm hoping the basics for a strong machine are there.

[JAZZCNC]

Hi Ian,

Ok I can give you a solution that is perfect answer to all your issues but it's a bit off the Norm and cost's a bit more, will also mean a redesign. . . BUT. . . The plus,s are many and well worth the trouble if your cutting Ali and Odd sized material.?

Make it so the gantry lifts up and down not the Z axis.? Infact you don't have a Z axis the Gantry becomes it. The spindle is fixed on a plate that just moves across the Gantry.
There are only a couple of advantages to this design but they are BIG ones.?

Finish is improved massively because you only have the tool extension from spindle extending down from Gantry so stiffness and resonance is improved massively so tool chatter is far less, it's about good as it gets in tool rigidity and therefore finish.

There's virtually no restriction in material size that can be machined and still have Ultimate Tool strength has the tool Extension doesn't change only height of gantry. The Only limiting factor is how high you build the Columns that the gantry run up & down on. The flex and rigidity of tool will never change whether you machining 2mm Ali or Top of an Engine block has the distance form gantry to spindle never changes because spindle is fixed to plate running across it. Only gantry goes up an down doing the job of a normal Z axis giving the absolute minimum amount of flex, which can only come from the tool it's self.

This design will need ballscrew down each side to lift gantry up and down but could be driven with single motor connecting screws with belts across top. The Columns can be braced across the top so still very strong and rigid and provide perfect mounting for motor/belts etc. If heavy gantry then counter balance down side each column will neutralise the weight so won't need massive motors.

In your position with your needs then this would be my route without shadow of a doubt.!!

Hope this helps.

[John S]

The flex and rigidity of tool will never change whether you machining 2mm Ali or Top of an Engine block has the distance form gantry to spindle never changes because spindle is fixed to plate running across it. Only gantry goes up an down doing the job of a normal Z axis giving the absolute minimum amount of flex, which can only come from the tool it's self.

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So if the gantry is mounted on spindly rails both sides this will not be transmitted to the tool. ?

A Z axis usually has twin rails close together on one plate.
You are proposing two Z axis so 4 rails well spaced apart ?

[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!