Beginner Moving Gantry Build? Read This!

[Tenson]

Hello All,

What follows is an attempt at a ‘sticky’ guide for beginners wanting to design a moving gantry CNC router / mill. I don’t consider myself an expert but I have been reading this forum for a few years and built my own mill.

I’m posting this as a draft. I hope that you can all add more details and good info before we make the thread sticky.

Let’s get started.

Gantry Geometry:
Regardless of the material you choose to build the gantry beams from, the geometry is an important factor in the strength of the construction. This first example (what I will call the Equals Sign ‘=’ gantry) is I think the most common I have seen among beginner gantry designs, but it is not a good solution.



The two main beams of the gantry, be they rectangle as pictured or square, are spaced apart from each other vertically and bolted end-on to the gantry side plates. The advantage of this layout is space saving of placing the ball-screw down the middle. However, these two beams are basically no stronger than a single beam because they are not braced to each other. Any upwards force on the Z-axis assembly is likely to be borne by the lower beam while downwards force will pull on the top rail.

I’ve seen people place a strong panel across the back of the gantry but this will add very little in strength to the beams.

Let’s look now at two common approaches that appear to work better.

The ‘Box Beam’ gantry. This is simple – rather than two beams we use one large beam, again bolted end-on to the gantry sides. It’s very strong and simple to build.



One theoretical downside is that the linear rails are not spaced so far apart as the previous design meaning you lose a little strength there, but using a decently large section box beam, say 120mm, it doesn’t present any bottleneck to performance. The second problem is that the ball-screw can’t be placed between the rails, so you need to put it on top, below or even behind (that last option is not my favourite idea).

Next we have what I will call the ‘L’ shape gantry. This uses two rectangle beams much like the first design but placed one vertically and one horizontally to get strength in both axis. This has the potential to give you more distance between the linear rails than the Box Gantry.



The downsides? Well, you need to bolt the two beams together well and how do you gets bolts all the way down? (Someone will probably answer this after I post!). Secondly the ball-screw tends to be placed in the gap at the back of the beams, meaning it is furthest from the cutter head and linear rails of the three designs.

Despite the mentioned downsides, the Box Gantry and the L-Gantry have both been used by many builders here to good effect.

Did I miss anything important? Let me know.

Z-Assembly

Taking any of the above designs we need to think about how we will attach the Z-Axis to the linear rails that move along the gantry. A very common method is to bolt thick ali plate (~20mm) at right-angles. This can work well, but of course the joints are an area of weakness. I often see a plate across the back of the Z Assembly in an attempt to make it stronger.



Unless you need the rear plate to attach a ball-screw or other device then don’t bother – it adds no strength because the Z assembly can still flex in a trapezoid shape. In the above image it is surplus weight.

A good alternative to strengthen the Z assembly is to bolt on end-plates which are cut from a single piece to unify the construction. You still want to bolt the right angle joints as well.




The last option for the Z assembly I will suggest is directly coupling the main Z plate to the linear rail bearings. We can do this if we place the linear rails on the front of the gantry beams rather than the top and bottom. It could even give room to squeeze the ball-screw in there too, getting it between the linear rails. This will produce a much stiffer construction but we now have the Z assembly hanging forwards to make space for the rails. If cutting area is tight it might not appeal.



This image shows the ball-screw mounted at the top of the Z assembly plate which is also a good option. However, note that one rail is attached to the vertical gantry beam and one rail to the horizontal beam. As we need both rails as parallel and in-line as possible it would probably be best to have one larger gantry beam for the vertical section and bolt both rails to that.

Material

There are basically three options for the gantry beam (and also machine frame) material.

Aluminium Profile – Easy to use because you can bolt straight into the end of it and also use ‘T-nuts’ to join things along the length. Go for heavy gauge type. Profile extrusion like this is very straight and most has perfectly flat faces which makes accurate machine building easy. To bolt profile rail to the face of the profile extrusion you will most likely want to use flat ali plate to help bridge the T-nut slow and support the rail a little.



Then you have Aluminium Box Section. Basically a box tube. Is hollow and comes in large cross-section with perfectly flat faces. Bolting anything to this is easy-peasy and the large cross section makes it very strong. You could possibly make a slice in the front to allow the ball screw to run down the centre and attach to the Z assembly. However I’m not certain what effect this would have on strength.



Steel Box section. The strongest and cheapest, but also least easy to use option and the heaviest. Steel box is not extruded, it is rolled from sheet which means it has rounded corners and the face is not 100% flat. Do not expect to bolt profile rail directly to this and get a smooth running action. It needs to be levelled in some manner. One popular option is to use self-levelling epoxy on the face for the rails. Another option is to bolt on a thick plate of cast aluminium. If your goal is to build a wood router then go for aluminium and save yourself trouble.