First build - Mid-scale gantry router design

[routercnc]

Welcome to the forum,

My machine has hiwin type rails on Z (15mm x 300mm) and Y (20mm x 700mm) but the X axis (900mm) is 16mm supported round rail. I can cut aluminium well and hold a good enough tolerance to press fit bearings into a bore, so it can be done with a mix like that. However, the hiwin (square / profile linear rail) are so much better and stiffer and the prices these days means that if you are starting from scratch you stand a better chance with hiwin all round.

As for 500x500 single ballscrew, my experiences are this is OK for wood, but aluminium is less than ideal as the gantry will rack a bit when you cut at the extremes. I upgraded from single to twin and it is much better. You might be OK cutting in the middle for very small parts but I personally wouldn't go there again.

Spindle to bed clearance is a personal design choice, and 150mm or so is popular, but if you go larger you will loose stiffness when cutting a full depth. Some designs use an adjustable bed, or use a purpose made 'solid packer block' to bring work up to the cutter. For me simple is best and 150mm or so is worth starting at in your design to see if it will cut everything you need.

You haven't mentioned gantries yet but an elevated X axis allows a superior gantry design(i.e. minimal / no gantry sides). Have a look at the recent build logs as most people go this way now - Jonathan, Silyavski, Eddie, Neil etc.

[Ellis]

Thanks all for your responses.

@Clive: I had heard about Chai, but found his prices don't seem to compare particularly greatly to other sellers. I realise of course that any of these vendors could be shipping sub-par kit. Is there a reason particularly why Chai is popular?

@Geoffrey: exactly that had crossed my mind as I can quite see a fixed gantry would potentially be much more rigid than a "live" one. I dismissed it because the footprint of the machine to work area ratio is so much greater with a moving gantry. However it's more important to me that I can hold tolerances reliably, so that style of machine is certainly a potential. Thanks for the thought.

@routercnc: great to hear you have experience with almost exactly my proposed setup! After finding the seller on aliexpress I linked above, who appears to offer every mechanical part for reasonable prices, I think I may push for all-Hiwin rails.

I had always pictured using dual ballscrews driven by a central stepper and belts. It feels right from a mechanical standpoint and it's good to know it's popular. I will be making the Z travel minimal for the reasons you stated. 150mm sounds about right to me.

Thank you for bringing up the raised X rails. That makes absolute sense. I won't even bother with the design I have currently which has the rails actually below the bed height. Cheers!

Thanks again all,
Ellis

[Clive S]

@Clive: I had heard about Chai, but found his prices don't seem to compare particularly greatly to other sellers. I realise of course that any of these vendors could be shipping sub-par kit. Is there a reason particularly why Chai is popular?

I think BST is now the preferred source and better quality. ..Clive

[JAZZCNC]

Without doubt if you want high tolerences then you need to go with a Fixed gantry. Better still use a design that doesn't use your typical Z axis approach.?
To get the best surface finish and hold tight tolerences it's critical that tool length is controlled and kept to minimum. Typical Z axis is always a compromise between material clearence and tool extension.
A far better way is to not have a Z axis as such but rather have the Axis which spindle runs across raise and lower. This ensures the tool extension is always the minimum it can be regardless of material height. The only extension is the tool stickout from the spindle. This results in much less chatter, less vibration and higher tolerences.
Check this out to see what I mean just make a better job of it and use a proper spindle.!!. . .


Make it very strong and Dense to minimise vibrations.

Wouldn't even think to use round linear rail if you want high tolerences as it will be the worst thing you could do and you will regret it for the slight difference in price.
Building a machine to hold tight tolerences is very very hard to do and will require substancial effort so don't spoil it for sake of few quid extra.