5'x3' Ideas and build.

[JAZZCNC]

The primary goal for this project is to be able to cut hard/soft woods and be accurate/repeatable in doing so.

Specs so far:

Cutting area maximum 5'x3'.

X Axis driven by 2x 20mm ball-screws with a lead of 5mm driven by a NEMA34 motor connected by cam-belt/pulleys.

Y Axis driven by a 16mm ball-screw with a lead of 5mm driven directly by a NEMA23 motor mounted to gantry side-plate.

Z Axis driven by a 12mm ball-screw with a lead of 2mm driven directly by a NEMA23 motor.

I see why your thinking 20mm screws to avoid whip so you can have 5ft cutting but in all honesty they are really going to compromise this machine just for wood use.?
Why not drop the cutting area to 4ft which is standard sheet width or better still make the machine 4x4 to accept half sheet.?
This 300mm less screw length puts you within the 16mm screws range, yes it's near the limit but still usable. This will then allow you to use 10mm pitch screws on both X & Y axis which are really what's required to give speeds needed for good wood machine.
Nothing wrong with joining screws(my preferred way) with belts or using 34 motor so long has realise you'll get slight less speed than you will from the single 23 driven Y axis. Thou that said thats the same if you drive X with 2 x 23motors due to having to keep a good safety margin top avoid stalling. Also the speed imbalance can be regained with a bit of gearing.
Whether you use 2 x motors slaved or larger single motor is debate argued many times before so won't go over again.!

The way you have the machine spec now you'll be lucky to get 5mtr min rapids and be in the 3-4mtr cutting limit. This means the motors will be working near flat out all the time while cutting wood and at times not fast enough. This opens up lots of potential for dropped or missed steps when working hard due to being near the motors corner speed all the time. 10mm pitch doubles the speeds which means you'll be cutting with far more torque available at the same 3-4mtr/min making the machine far more accurate and reliable. It also means that you can cut faster if needed which you haven't got an option with 5mm pitch.

Yes the 5mm pitch can be geared 1:2 to give same speeds has 10mm and I know it works because I use my machine like this. But at the time I had no realistic cheap alternatives so had to take that route but thats not the case these days and if building again I'd use 10mm pitch.

Some questions so far.

I'm using 15mm 6082 aluminium plate for this build. I'm hoping to use M8 35mm hex socket head cap screws to bolt this all together, which means the screws will bite 20mm of the thread I'll tap into the plate edges. Does this depth sound robust enough? Is M8 too big for the edge of 15mm plate?

To allow for a bit of adjustment I'll be making the holes in the face of the plates 10mm in diameter so there will be a bit of slop to play with before torquing everything up once it's all aligned. Does this sound okay?

Where there are slots cut for plate to pass through (the arm which passes through the side-plate, for example) - rather than cut this slot to be exactly 15mm wide, should I make it slightly wider (say 15.25mm) to ensure that the plate passes through okay? .25mm too much?

Obviously there won't be any internal right-angles because the end mill will have a radius. Where there's a square edge that fouls a radius, I'll manually chamfer off the corner to make it fit (see pic below).

M6 or M8 will be fine and 20mm thread is plenty.

With the gantry and slot then I wouldn't make it like that at all and your creating lots of work and expenses. just use flat plates bolted to each other far easier and cheaper than wide piece of Ali plate.

now on to the bed that the gantry sits on:

A general and straight-forward question here. Is this viable?

Is there anything here that represents an alarmingly bad design decision? Or could this actually work?

YES this bed design is a VERY BAD IDEA and the base board going under the rails will be a HUGE MISTAKE.!!
First it will absorb moisture and swell turning the machine into a shape shifter with temperature or humidity changes. Even if sealed it's not good.
Also if you mess up and cut thru the bed which is easily done or you spill drink or water anything that will cause damage then it's a major job to change.

DON'T DO IT:!!

If you can weld then weld the frame together and drop the spacers and just bolt base board to frame. Much quicker and easier to build has well.!!