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.
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.
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.
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.
Last edited by Tenson; 21-09-2015 at 07:50 PM.
There are basically three type – Round Rail, Supported Round Rail and Profile Rail.
Round rail with closed bearings has pretty tight tolerance (little bearing play) but since the rail itself is not supported it will flex. Even thick unsupported round rail has pretty limited strength for a decent size router. If you are on a budget then I’d consider a thick gauge it for the Z axis and possibly the X axis if you will exclusively cut soft materials, but certainly not the long Y axis. Just don’t.
Supported Round Rail alleviates the problem of flex in the rail by supporting it along the length and allowing you to bolt it all the way along too. However it is really let down by the bearings. The open bearings are just normal closed bearings cut open. They have a lot of play in them by comparison to the closed bearings or profile rail. I’d really discourage any use of this type of rail.
Finally you can opt for Profile Rail. This is what all professional machines use and for good reason. It costs a bit though. The rail is profiled, so that although the bearings are open they can’t pull away. The bearings are usually pre-loaded with force too, so they stay really tight with minimal play. Most DIY machines will want to use the minimal pre-load option. The rail needs a very flat face to bolt onto, since it is rather flexible itself compared to the supported round rail. Hiwin are one of the most famous brands but there are many others too.
I’m not going to give examples of how to build the machine frame as the details will depend on your choice of gantry design and ball screws etc.. However you again have the choice of steel box section or aluminium profile. The frame is a really important aspect of the machine, don’t underestimate it. Flex in the frame is as bad as flex in any other part. For this reason I would personally recommend a steel frame. You could use a combination too, such as steel uprights for strength (not less than 60mm section) and aluminium profile horizontal beams for the ease with which you can bolt into the ends.
Steel box section can be bolted together or welded. I know nothing about welding so I hope someone will step up here and add some guide info for MIG / TIG / Gas methods etc... Welding is of course stiffer than bolts but both can work well.
Use diagonal brace members on the frame!! It’s no use building a substantial frame and leaving it all as right-angle joins. A box will flex to a trapezoid shape when force is applied.
If we add a single diagonal member where we see ‘open squares’ it will stop this happening. As the rectangle / square wants to ‘lean over’ it will either tension or compress the diagonal member and stay good and true.
Also remember to do this not only as you view the machine frame from the sides and front / back, but also as you look down on the machine. Near the top, the machine table will likely brace the shape keeping it true, but at the bottom you might want to add a diagonal member.
Think about the size of what you are building and where. Can you get it through the door once It is build?
Can you access components such as ball-nut mounts to adjust (and oil) them without disassembling the machine?
Put adjustable feet on the frame to level it.
Consider dust extraction at the point of machine design, it’s not an afterthought.
If you need to tap lots of holes get yourself a ‘point tap’ and stick it in your cordless drill. Use a squirt of oil. The point tap is designed for automatic machine tapping having a lead-in that helps to guide the tap into the hole. It will go 100% faster than your standard hand-held taps (that break off in the hole). If you need to tap ‘blind holes’ that don’t go all the way though the material get a ‘spiral flute point tap’. The spiral flute pulls swarf up out the hole rather than pushing it down.
Get an external motion controller. The parallel port on your PC or laptop can be used to output motion pulse info but the conversion of the code to timing pulses for the motors is done in software and is not very reliable. A hardware motion controller will take that burden away from the software and parallel port giving you more reliable performance.
Vacuum tables. If you want to hold small metal pieces a ‘high vacuum’ pump is good, such as those used for sucking out air conditioner fluid. On the other hand if you want to hold down large sheets of wood you need a ‘high volume’ pump to keep up with air leaks where cuts are made and also because material like MDF is amazingly porous. I’ve been using a domestic vacuum cleaner to hold ¼ sheets for a couple of years with good success but the pros use serious turbines with multiple horse power.
That’s all I can think of adding right now. Simon
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