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Troll CNC: EP 3 - Design Considerations pt. 2


finally I've got time to write some thing. Still going on design considerations about construction. So the time came to decide materials and type of construction. For a 3 axis cnc your workpiece needs to move in 3 axes perpendicular to each other marked as X, Y and Z. I'll take the convention and name them following: when I'm facing the machine from the from the X axis is the one moving from left to right, the Y axis from back to front and Z axis up and down. There are 3 major construction types: mobile gate, mobile table and stationary tool (not sure if these are proper names, but i find them quite descriptive.

Mobile gate construction we have when workpiece is laying on the table which is still. Over the table moves the 'gate' back and forth which is the Y axis. On this gate is mounted linear motion assembly which enabled for movement in X axis and on this axis the tool is mounted. The tool mount is mobile in Z axis. This solution provides the best ratio of work area to machine dimensions. It is most widely used when large work area is needed.

Mobile table is the topology when the table with the worpiece moves in Y axis, what would be gate is stationary with X axis on it and Z axis is mounted on top. This type of construction provides most rigidity and stiffness, while giving lower work area to machine dimensions ratio.

Stationary tool type of construction is characteristic because the tool is mounted on a stable post or mount being able to move only up and down. The table is mobile in both X and Y axis. There are some large machines built like this, particularily most of traditional manual milling machines are. In DIY cnc machines such type of construction is mainly used when small dimensions are needed and there is no need for exceptionally large work area.

I've chosen to make my machine as the second type (that is mobile table) because when constructed with diy methods it can offer most rigidity. With mobile gate there are strong bending forces acting on the gate linear assembly, which means that they need to be more expensive to achieve rigidity comparable to the mobile gate type.

Next thing that needs to be chosen is construction material. There are 4 main materials that can be used for making a cnc mill: wood, aluminum, steel and concrete.

Wood is obviously the softest and cheapest one. And by wood i mean everything from ordinary boards, through MDF, OSB boards up to plywood and high-pressure compressed composites (like Lignofol). The obvious advantage is ralatively low costm which means that in case of some screw-up you don;t loose that much money, and believe me there WILL be srew-ups. The wood and it's derivatives are also easiest to work with (i.e. you can get virtually any tool for working with wood in a construction market, which is not the case for other materials). Another nice things is that you can get wood-like materials in every construction market (in Europe usually Castorama, Leroy-Merlin, Praktiker, OBI) and almost always they offer cutting boards to size (although limited to rectangular shape). Major disadvantage of wood is that it is problematic to precisely cut and drill because of some random features occuring naturally in a tree. It is also most prone to degradation because of humidity (holding a woodem construction in a basement with humid air is a bad idea). It is also not as rigid as other materials (but you can get thicker pieces which improves situation).

With aluminum the construction is usually better than that made of wood (unless poorly designed). There are two types of aluminum construction: made of extrusions or solid plates. Thing with extrusions is that you can buy them pretty easily, as there are many firms selling it (Wrocław, where I live is a rather big city with ~700k citizens and there are at least 5 aluminum wholesellers). Downside is that usually you need to buy whole trade unit which is usually 4 or 6 meters. This means that to keep costs reasonable you need to use one or 2 types of extrusion in whole construction. Another downside for me is that I don't have equipment or skills to weld aluminum (you can only do this with TIG, GTAW or whatever you call it, and this is widely recognized as the method of welding hardest to master). This meaqns that only method of joining pieces is to drill holes and connect them with rivers or screws (aither with nuts or cutting a thread in the hole). Such approach needs high degree of precision, which is unavailable for me. Second solution employs solid aluminum plates. There are companies which use high precision bandsawsm, lasers and waterjets to cut virtually any size and shape. so you can make the walls of 20mm thick aluminum plates which gives really stiff and rigid construction. The downside is taht you need to have holes threaded and you prorably cannot do that at home with precision high enough.
Large downside of aluminum is also the cost. Aluminum is sold per weight and standard cost is around $9-10/kg. My calculation was that I'd need at least 30 if not 40 kg of aluminum no matter if I uise plates or extrusions. downside of using solid plates is taht cutting custom pieces always costs extra.

Another common material is steel. Advantages of steel are that you can get it at total bargain from junkyard. You can easily get it in virtually any size and shape ranging from tiny pieces to extrusions of the size or train rail. Another advantage is that steel is easy to weld using any method available (MMA, MIG, TIG, you name it). Steel offers better mechanical parameters that virtually any wooden or aluminum construction. Unfortunately is has few huge downsides. For the one steel is very hard to work with, needs epxpensive tools. If you need to cut a 30mm diameter hole in a 10mm thick steel you have a problem and needed tools you prorably won't be able to get in a construction market. Steel is also very heavy meaning that a machine witk work area of A3 sheet size woukld prorably weigh upward of 150kg which makes it completly not portable.


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