'Otter'. Build log.

[Blazing Black Beard]

Don't worry about the steel quality - I didn't! I just bought something from my local steel supplier, probably generic "construction" steel. Doubt if it's stress-relieved or anything else. It is described as "Grade 43a", whatever that means. Welds and cuts OK, and I knew from the outset that I would need to build in adjustment/levelling, etc. Being fussy about steel quality is OK if you can actually buy the quality you are after in small enough quantities (a lot of suppliers will charge more in carriage than the cost of a small quantity of steel - that's not criticism, just a reflection of commercial reality) and if your design is actually sensitive to the steel quality. My design, such as it is, could be beefed up significantly by using, say, 4mm wall thickness rather than 3mm, and for the static bed and frame components, that might not have been a bad idea. However, 3mm works fine for the gantry and does keep mass down.

Don't think you've said how big this is going to be, or what you want to cut on it. This drives some aspects of the design. For example, my machine has about 1.5x0.75m cutting area. For that, I have used 2005 ballscrews. If I were only cutting wood, then 2010 would have been better - faster rapids and cutting speeds. However, I end up doing quite a lot of small fiddly bits and the speed isn't too critical so 2005 works well for me. Similarly, I use NEMA23 3Nm motors; for faster speeds I could have gone to 4Nm for a bit more torque but I'm OK at the moment. However, stepper motors need volts to get performance out of them; don't be confused by spec sheets that say things like "4.2V". That's a red herring. Most of these size motors will work best from about a power supply giving about 68V via a suitable stepper driver. There are plenty of threads that look at different drivers and how to build power supplies.

I'm using a "standard" 2.2KW spindle and matching VFD. Don't know much about ATC spindles apart from the fact that a decent one will cost big bucks.

Neale again thanks for all the info. My gantry will be somewhat like yours I assume so my 3mm thick 100x50x1250 box section should do fine for the X axis (left to right on gantry and table ends).

I will be cutting small aluminium parts with high accuracy and superior finish, larger aluminium plate with accurate hole cutting but still relatively small compared to the size of the machine which will be approximately 1000mm x 1500mm cutting area footprint. I also aim to easily cut steel sheet and small accurate steel parts from time to time.

As for the ballscrews, I seem to keep reading that 20mm diameter x 5mm thread seems to be pretty common and you use them, so I think I may settle for these when I come to buy the rest of the components after I have finished the first phase of the 3D design.

I know virtually nothing about spindles or their VFDs. You mention 4Nm for the NEMA 23 motors. I'm assuming the holding force is better for heavier gantry arrangements that have more inertia, therefore need more force to ramp speeds without skipping position? Looking at motors and drivers is as mind boggling as spindles and the various (and sometimes exotic) materials that some people use in their builds.

I have been looking at a marble bed and gantry arrangement and I am having a hard time drawing myself away from that expensive venture (eek!). I don't think I will ever do it but it's hard to stop looking at these machines and what I am inclined to think about their accuracy. They must be extremely rigid and very heavy, not to mention unwieldy should one want to move the machine for any reason. I can also imagine that accidents on a marble bed/gantry machines can be costly.

Anyway, back to the real world where I think I will stick with a 100x50x1250 box section gantry. Would moving to 4mm-6mm thickness be of any benefit for my 1800mm sides? Is there going to be any benefit from using thicker box section on the top part of my bed frame? I'm guessing that the rigidity of the 4+mm box section is only of any benefit where there is an axis directly connected with linear rail and bearings and the thinner 3mm box section would be adequate to form the lower part of the frame and the supporting uprights and diagonals? Does that sound about right or am I missing something?