Help with Y-axis design

Afternoon,

This will be my first CNC router build and I'm hoping to make it as rigid as possible (within my budget) that's capable of wood, plastic, foam and light aluminum work. Right now I'm trying to figure out the best design for my Y-axis. The first picture (if I uploaded them properly, the first pic is the one that doesn't have the high sides that the linear bearings mount to) seems to be the most common design I've seen. With that design there is a large Delta distance for the gantry sides. If I did it that way I would have to space the bearing blocks far apart to keep it as rigid as possible, this would decrease my workspace. However, if I design it the way I have in picture #2 wouldn't that decrease my Delta distance allowing me to decrease the spacing between my bearing blocks and increasing my usable workspace? With design #2 I realize the router will have a large, unsupported length when it has to extend all the way down to the table top, this will decrease the rigidity. To fix that I figured I could use a removable "box" of some sort that would sit on the table so the router would not have to extend very far when working on smaller projects. These CAD drawing were just to get my point across about the Y-axis they are definitely not a finished design.

Any pros and cons on these two different Y-axis designs would be appreciated!

Thanks

Hi Curt - With the linear rails mounted on a bench "side" you have to be careful as you have no way to get them parallel unless you machine the rail lands. The workspace vs footprint issue is always a tug of war. All you can do is either define your workspace requirement and end up with the footprint you get or define the footprint and get the work area you get. Then adjust. It will take at least 10 or 50 design cycles if you have not designed a CNC before to get to where you want to be. With linear cars they need to be at least two lengths apart prefer 3x.

There's lots of commentary on cutting aluminium on this site. I suggest you look at a high rail design (your 2nd image is going that way) vs the column design ie don't have columns, set your rails at the height so the gantry goes straight across, no columns.. This does not affect your footprint or your work area but does improve the stiffness of the machine dramatically. If its a benchtop machine and you have the space look at a fixed gantry design, this maximises the stiffness of the machine but has a larger footprint vs a moving gantry design. If this is a construction aluminium section based design I suggest you buy one off the shelf. It will be cheaper than developing it yourself then you can do better when you design/build your second machine. Even make your own parts etc If you are going to invest a considerable amount of time designing your own CNC do not use construction aluminium sections. They are too much of a compromise vs your effort and the result will be close to someones kit. Plus you need to put effort into the tool holder area before you commit to the base area. If you start at the base you will run out of room for the Z stuff. You have to look at the whole picture often.

If you want a small router to do work buy one, if you want to D&B do that but don't use construction extrusions. Having been thru the exercise of designing 20 machines plus and committing to build 3 of these designs they all have cost more than expected. Then looking at commercial kits they are about the same price as I build a one off for. But my machines have always been better then the same cost kit. That's mainly because I use good quality linear rails vs extrusions and wheels. You will find these things out eventually, sourcing parts at the right price will be the main issue with a build like this. So let the people who have done this before do it for you if you just want a machine. I have recently been buying rails/cars from a company and they ceased stocking a certain chinese line so the price doubled for their korean brand. Really threw out the budget!! Then I bought integrated stepper leadscrews from a company for a production model and they told me replacement antibacklash nuts were available. I built three machines and decided to stock spares so ordered nuts... Sorry we don't stock these!? Try getting 2 start Tr8x4 nuts somewhere!! So welcome to the CNC club!!

Write down your machine objectives and budget and specs and review often. You will drift... Ideally Do not build or buy anything until you have a complete design resolved on paper and costed so you can make the buy/build decision clearly. If your a Maker and that's what you enjoy throw these guidelines out and keep Making...Cheers Peter

The "B" difference between the two would probably only be 2-3", which is negligible.

Raised sides make it more rigid, at the expense of more difficult loading and unloading, and a potentially smaller workspace along the gantry, unless you make the machine wider.

Thanks for the advice peteeng and ger21!

peteeng, so you're suggesting that mounting the bearing rails on top of the raised side frame would be easier to get them parallel? I've been reading lots of bad things about using extruded aluminum. I was hoping some manufacturers were better than others. Using the extrusions just seemed like an easier place to start with my first build. For me finding 15-20 mm aluminum plate, in small amounts, is proving to be difficult. Maybe I'll have to rethink using the 80/20 extrusions. I understand what you mean about starting my design from the tool tip and working backwards though.

Hi Curt - Yes put them on top unless you have a way to achieve parallel on the sides, mount them on top. The main reason to put it on the side is so a column plate can be bolted directly to it. But if you use a high rail design you will want the rail on top.

Extrusions are easy and convenient but are relatively expensive and heavy. T nut bolting to flange is not as stiff as bolting into continuous metal. Consider the word "easy" carefully. If its easy to do it means there are several compromises associated with it. Do things because they are the best or correct thing to do not the easiest to do. Extrusions limit your design to a certain ending or topology. May as well buy a kit as this is where you will end up if you start with extrusions. Take the ShapeOKO its an extrusion design with small belts and many bolts. People use them and upgrade them in various ways but it reaches a limit. Any machine is limited by its design DNA. Sorry to repeat some themes. I've attached an article I did on extrusions. This is from a structural viewpoint only but there are other ways to look at it.

There's nothing wrong in working with thin steel or aluminum. You just need to know how to get the stiffness. Thick aluminium plate is used because it's easy. Tubes are the way to increase stiffness and rigidity yet be lighter. All pathways will have challenges around what you can and can't do at present. The aim with Making something is to learn something new, so do something different. So give us a list of your resources and skills and this will help in the discussion. Peter