Fixed z axis to gantry design

Hello everyone,
This is my first post here but i have been a long time reader of this forum and the likes of Jazzcnc and Jason and many, many others have inspired me to take the plunge and build a simple cnc design. I have limited hand tools plus a dropsaw for this build so simplicity is a major factor. I don't have a whole lot of experience working with steel but I can weld.

I will be milling aluminium so the machine needs to be rigid. i will be building it out of 75mm square box steel 5mm thick.

After perusing many designs i think (not carved in stone) I will be basing my design on the following machine.
Attachment 16314
http://www.youtube.com/watch?v=4Bj6er322Dg
With the following alterations:
rail size will be 900x600by300
Gantry will not be fixed but will move on 20mm Sbr linear rail as will all other axes. The cutting area will be too small if its fixed and i realise this will impact on rigidity. I know profile rail is preferred but its just out of my budget.
Will be using pulleys with HTD5 15mm belts on all axes.
Nema23 425oz.in motors on each axis driving 1605 ballscrew thread.
Reasons i chose this design:
Simple to build
Rigid z axis that i know I can build accurately.
Seems like a very strong design.

So any major design faults with this design?
Am I on the right track?
What would you change about this design to improve upon it?
Lastly, thanks so much for any help given:eagerness:.
Regards,
Lachlan

The design in the picture is a nice design and would work well as it uses a fixed gantry and moving bed, but if you want to have a fixed bed and moving gantry then I think you will run into problems if you make it like that.

The gantry is quite tall, which it needs to be to allow enough Z movement. When this becomes a moving axis you now have a long cantilever from the bearings at the bottom which is difficult to brace without having widely spaced X axis bearings and significant loss of travel. Getting the stiffness back could be a problem as the geometry is now against you.

Personally for a machine used mostly for aluminium then I would either go with the fixed gantry and beef up the gantry support a bit more than the one in the picture, or go with a conventional moving gantry with Y axis and then Z axis. I think it will end up stiffer and more effective than your in-between design.

Nema 23 would be fine, and 1605 ballscrew works OK (that's what I have), but many choose 1610 and then drive it with a 2:1 reduction pulley or 1:1 pulley depending on what they are cutting.

I agree with Rob completely increase the size and stay with this design or switch design completely.

Personally I'd stick with this design and increase size because it's a very strong design and ideal for cutting alumnium.
If you do need moving gantry and switch design then stay away from high gantry sides and go with frame that has high sides with rails on top and Gantry sat directly on bearings. It's the only design to use of your wanting to cut aluminium.

Also I'd re-think those SBR rails if your cutting Aluminium has you WILL REGRET it down the line.!! It's alot of work and money to spoil with cheap rails that WILL show there weakness in the finish and accurecy you achive.!

Thanks for the feedback.
Both comments make a lot of sense so I will have a think about it and come back with a better design. I will change to 1610 ballscrew. Still just learning Solidworks so it might be a while. If going with the fixed gantry design how best to beef up the gantry? Triangulate, gusset or just larger box section? Also does a design like this really need 2 ballscrews on the z axis? Would it be ok to have one in the middle?

The fixed gantry would benefit from stiffening in the fore/aft direction. Start at the top of the gantry in each corner and run a brace down to the front or rear corner of the machine. If you look at this machine from the side you would see the gantry sticking up from the base with little support.

If you go with only 1 ballscrew on Z then there is the risk of racking when cutting off centre. Imagine doing a plunge cut at one side of the table, the Z axis will try to rotate, possibly bind the bearings, but certainly loose some accuracy. To combat this you have to space the bearings apart (vertically) but then you start to limit Z travel, or have a much taller gantry and this then needs to be braced still further.

Small ballscrews at this length are not that expensive on e-bay, and you can use a single motor as per the design in the picture to save costs on two motors & stepper drivers.

That makes sense, I will use two ballscrews on the z axis and brace the sides of the gantry front to back.

Loved the movie, someone has actually sat down and thought about it rather than following the herd.
The gantry connects to the bed. Presumably linear blocks are screwed to the gantry, linear rails are screwed to the bed.
If you want to add dead weight to cancel out vibration, by all means bolt the gantry to the building.
Maybe I an missing something, but I don't see any need for bracing because the bed does not reference anything apart from the gantry.
Cutting steel badly indicates that it might cut aluminium alloy reasonably well :cocksure:

Hi Robin,

What I meant by bracing was if you view the machine from the side the 'bed' is effectively a capital "L" shape, with the stiffness of the vertical part dictated by the stiffness at the joint to the horizontal part. Better to join the top of the L to the front of the L to form a complete triangle.

But the horizontal part should be a mere decorative trim. It shouldn't connect to anything :untroubled:

Edit: It does have a purpose, it stops you putting your tea cup where it will get knocked over :hysterical:

Quote:

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Originally Posted by Robin Hewitt

But the horizontal part should be a mere decorative trim. It shouldn't connect to anything :untroubled:

Edit: It does have a purpose, it stops you putting your tea cup where it will get knocked over :hysterical:

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How do you work that out then.? It's the foundation of the whole machine and what the Fixed Gantry connects too. Rob is correct in that the Fixed Gantry would benifit from bracing to form a triangle rather than just being a tall pillow.!

Regards twin screws on Z axis then 100% needed and it would fail badly without them. Personally if your only cutting aluminium then I would stay with 5mm pitch has you won't require the extra speed 10mm gives but you will benifit form the extra torque and resolution 5mm gives. Even geared 2:1 so doubling the Torque/resolution 5mm pitch will easily give higher feeds than are required for aluminium.

If your thinking about cutting woods/plastics then go with 10mm pitch.

Quote:

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Originally Posted by JAZZCNC

How do you work that out then.? It's the foundation of the whole machine and what the Fixed Gantry connects too.

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At first glance you might think that, but the foundation of the machine is actually the rectangular gantry.
Everything refers to the gantry, that is why it is such a rigid design. The overhangs are minimalized.
Okay, you need something to support it so it doesn't fall over, but a strut? Does that not detract from the beautiful simplicity of the design? That tower in Pisa would get a lot more benefit from a strut than this, but it doesn't mean you should fit one. I mean, how gauche :thumbdown:

Quote:

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Originally Posted by Robin Hewitt

At first glance you might think that, but the foundation of the machine is actually the rectangular gantry.

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No it's not. Watch this video and you'll see the whole structure.


The Gantry rectangle is only has strong has it's connection to the base hence the massive plates. It would greatly benifit from more bracing higher up with connection to the lower frame. Yes Maybe not looking has simplistic but what we building here.? machines to do a job good or poser machines.!!

https://www.youtube.com/watch?v=qKdWBgGly3A

I didn't see that movie. It doesn't need a strut, it needs the rails fixing to the bottom of the bed and the blocks fitting to the gantry. Dump the overhangs, kill the vibrations. Simples :distracted:

Quote:

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Originally Posted by Robin Hewitt

I didn't see that movie. It doesn't need a strut, it needs the rails fixing to the bottom of the bed and the blocks fitting to the gantry. Dump the overhangs, kill the vibrations. Simples :distracted:

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Sorry but you'll have to point these overhangs out because I'm struggling to see any.!! Or any that will make a difference anyway.?

I agree the Bearings on the base and rails on the bed would be a better option but it's not the weak link in the machine. The High gantry frame without support is far more a weak link and source of vibration IMO.

Hi Robin, Lachlan,

Maybe this helps explain it. If you remember the days of 'free-body-diagrams' - the bed is part of the system. See picture which shows all the forces acting.

You can hopefully see how the extra brace can help. This is where myself and Jazz are coming from.

I think Wal created something like this (although smaller) and grappled with different layouts to make sure the 'tower' was stiff enough.

Can it work without the brace - of course. Is it much better with it ? - yes, significantly, and for little effort.
Attachment 16328

Thanks

Why does this chap spend a long time adjusting the rails to be perfectly parallel and then use a lump of pine as the base for cutting? Seems a shame, since pine obviously isn't flat and stable!

Quote:

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Originally Posted by Tenson

Why does this chap spend a long time adjusting the rails to be perfectly parallel and then use a lump of pine as the base for cutting? Seems a shame, since pine obviously isn't flat and stable!

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Ye did seem a lot of wasted time and to be honest I don't think he had a clue what he was doing judging on how he went about it.!!

HI EVERYONE,
Was surprised and happy to see so many responses. Thankyou! I think the triangular bracing of the gantry is a must for this build it makes sense as a gantry triangulated is so much stronger than a gantry just flapping in the breeze so to speak. I have some 50x50x4 box steel surplus, would that steel cut the mustard for this frame build?

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Originally Posted by routercnc

See picture which shows all the forces acting.

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But the diagram doesn't show the rails being screwed to the bed and the blocks being screwed to the gantry.

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

Quote:

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Originally Posted by Robin Hewitt

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

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Robin I agree the swapping the bearings and rails gives a good benefit because has you say the bearings are always in prime location providing Max support but the screw moving with the bed isn't needed and creates more Mass to move and complexity. Moving the Screw, Bearings/mounts, Motor etc. means the Negatives out weigh the advantages in this case.

Not being funny or picky here but please stop calling it the gantry it's the base Frame and I'm easily confuddllledd. .:stupid:

Quote:

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Originally Posted by Robin Hewitt

But the diagram doesn't show the rails being screwed to the bed and the blocks being screwed to the gantry.

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

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Hi Robin,
Yes, I totally get what you are saying. Putting the bearings on the bed and the rails on the moving part makes good sense but it is maximising stiffness in the vertical Z direction. It is similar to the debate around conventional Z axis where it is often better to put the rail on the moving part for the same reason.

However, the bracing on the tower is primarily to address stiffness in the X direction. I've re-drawn the sketch to show the moving rails and FIXED bearings but the forcing (for X direction) is still the same and suggests bracing of the tower is beneficial.

X cutting:
Attachment 16333

There is also some Z direction stiffness from the bracing due to moment created by the offset of the tool from the tower in X which is independent of the rail / bearing arrangement.

Z cutting/ plunging:
Attachment 16334

Quote:

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Originally Posted by JAZZCNC

please stop calling it the gantry it's the base Frame and I'm easily confuddllledd. .:stupid:

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With both methods the rails have a length equal to the table travel plus the overall width of the bearing block assembly.

In this topsy turvey arrangement I am advocating the block assembly width is tiny in comparison, so the rails are short.

If you could bolt the "base Frame" to a workbench this could look ultra neat.