Beginner Moving Gantry Build? Read This!

[Tenson]

Rail Types


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.




Machine Frame


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.





Tips


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