Gantry advice please

[Crunch]

If it helps while you wait for the time difference my gantry must be close to 100kg as it is double beam with lots of thick aluminium plates on Y and Z, double screws etc. I have 2 ballscrews driving it at 1605 size and pitch with no issues but would happily go to 1610 if I did another one to get the speeds up for wood. Then swap pulley out to 2:1 for aluminium if required. They are just over 1000 mm and no whip issues.

Each screw is driven by a 3Nm stepper of about 3mH inductance at around 70V. A very strong set up. I’m running legacy (old!) analogue stepper drivers but for a new build go digital.

If you are yet to order the screws and want to go belt drive then make sure you ask for enough machined length on the end to get the pulley on as the standard offerings can be a bit short. Check the drawings they provide against how you will use them to be sure.

Your other question about the 1200 mm ballscrew on Y - this is getting long for screws but I would say still doable. I’ve not built anything with screws that long but my thoughts would be stay with 1610 to avoid having too much inertia and provision for another ballscrew support at the floating end to add some pre- tension. Also I’d think about adding 2 ballnuts on the Y axis unit spaced as far apart as the Y/Z axis allows to hold the ballscrew in 2 places and give slightly less free length when the Y axis is off to one side.
If someone has direct experience of 1200 mm screws then let that be your decider as the above are just my thoughts based build quite a few slightly smaller machines.

As for the letterbox idea it is tempting but as has been mentioned you will loose a lot of stiffness. Mount the screw somewhere on the outside even if you have to space that axis away from the gantry a bit.

Looking great so far by the way �� (and welcome to the site)

Thank you for your welcome to the site.

Thank you for the information that's a great help.
I was mainly wanting to fit ball screw inside gantry to save space and keep it compact. Based on comments not a good idea as it will weaken it to much .
I even thought of welding a round tube inside the gantry for the ball screw to run in, still cut the front out and weld it all up so it braced the open parts together.
I don't know if that makes sence. Lol
Also dont know how much that would weaken it.
I will think on how to mount it on the out side.
Thanks for advice on ball screw length , I've been talking with Fred from bst ( I think that's what his company is called )
He has been very good. I've not ordered linear rails yet as I want to order ball screws at the same time but not sure what to order hence questions.
I'm not ordering any electrics until mechanicals are finished.
What do you think to closed loop steppers vs open steppers.
Regards Paul


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[AndyUK]

What do you think to closed loop steppers vs open steppers.

I think the majority of people on here use open loop steppers or servos - don't forget you can get open loop stepper drivers which can very reliably detect missed steps and feed back an alarm signal.

[Crunch]

I think the majority of people on here use open loop steppers or servos - don't forget you can get open loop stepper drivers which can very reliably detect missed steps and feed back an alarm signal.

Hi Andy
Thanks for information,
Ive been following your build

Regards Paul

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[AndyUK]

I been following your build

Thanks! :) I too started with the idea of a steel box section gantry, but then temporarily lost access to the Bridgeport and milling the faces became an outside job. Mild steel isn't near straight enough to mount rails to directly.

Once I did the deformation calcs and realised HD aluminium profile could perform just as well, and was an aweful lot easier to build with, it tipped the balance.