Hovering over "Buy"... care to sanity check my thinking?!

[brumster]

Not much replies to questions, which is fine we're all busy people, so I wonder whether this now needs moving to a new build thread? Would that make more sense? I'm not saying it's definitely going to happen yet but maybe that will get it better visibility....?

I made a start on the design anyway, made some assumptions....



80x40x4 steel box welded frame, X on the platform and a fixed gantry (Y and Z), single central C5 ballscrew, external size 600x500. BGR15 linear rail (why not), NEMA23 motor, big design question at the moment is direct drive or pulleys/belts? I know some of you seem to be fans of pulleys/belts; any reason to go that route? I don't need it for the gearing-down/accuracy as by my measure, even direct will give me the resolution I need (and microstepping, even better). Are there other good reasons to belt-drive?

Appreciate it needs bracing...

I need to get some ballscrew dimensions to understand how the motor/linkage needs to be spaced... when you spec a ballscrew are the "unscrewed" parts of the ends always a fixed dimension or will it vary from one manufacturer to another? I took a quick look at one datasheet I found but there were no dimensions on there (from Zapp Automation). CNC4YOU have some specs up for C7 ballscrews but would my C5's be any different?

[JAZZCNC]

I know some of you seem to be fans of pulleys/belts; any reason to go that route? I don't need it for the gearing-down/accuracy as by my measure, even direct will give me the resolution I need (and microstepping, even better). Are there other good reasons to belt-drive?

The main advantage of belts other than gearing is to reduce resonance and smoothen motor performance. This is why most use it.
Resonance cripples the performance of steppers so anything to help reduce it is welcome. Using Digital drives makes huge difference to stepper performance and smoothness.
Also regards Micro stepping don't think of it to increase resolution because while it does up to a point it's main purpose is to smooth motor performance. Noticed you also mentioned running Steppers at 200 steps (Full step) this would make them run awful so expect to use Micro stepping.

but would my C5's be any different?

They are exactly the same regards end dimensions. But C5 screws require higher spec end fixings hence why they sell C5 end bearings other wise there's no point using C5 screws.

[brumster]

The main advantage of belts other than gearing is to reduce resonance and smoothen motor performance. This is why most use it.

Right, that's what I needed to know! Ok, I'll go with belts, albeit on a 1:1....! Thanks.

Resonance cripples the performance of steppers so anything to help reduce it is welcome. Using Digital drives makes huge difference to stepper performance and smoothness.

I will be going digital, yes, there seems only a marginal increase in cost of the drivers and like I said, this is an investment/reuse later so no need to buy cheap first off. There's no point; if I just want to do cheap then I might as well buy a 6040 :)

Also regards Micro stepping don't think of it to increase resolution because while it does up to a point it's main purpose is to smooth motor performance. Noticed you also mentioned running Steppers at 200 steps (Full step) this would make them run awful so expect to use Micro stepping.

Yep, my point was that I didn't *need* microstepping for any resolution aspect; naturally I'd use it.

They are exactly the same regards end dimensions. But C5 screws require higher spec end fixings hence why they sell C5 end bearings other wise there's no point using C5 screws.

Yes, I'd noticed on the website, that's fine. Wasn't going to attempt to mix ;) but to know the end dimensions are the same is just what I needed to know. I'll model off C7 drawings/dimensions, but just spec everything at C5.

Perfect, thank you!

[magicniner]

I made a start on the design anyway, made some assumptions....

Fasten the 4 fixed bearings to the lower and the rails on the underside of the component above, your support is then always optimally positioned for the point being machined.

- Nick

[brumster]

Interesting; can you elaborate (or link to somewhere that explains) the advantages in laymans terms? I don't think I've ever seen it done that way... any construction challenges (alignment?)...? I guess it's only the same but inverted....

As an aside, I reworked the motor mount to be underslung for a pulley arrangement... using a generic eBay-sourced motor mount bracket

[magicniner]

You want a design which keeps the alignment/support as close as possible to the cutting area, not one where the alignment/support moves away from the work area at either end of travel.
You can make the rails twice as long as the required travel and use multiple linear bearings to ensure support close to the cutting area.
Regards,
Nick

[brumster]

Right, I've made a decision! The design of a DIY machine can be parked because I'm going with this :-

http://www.mycncuk.com/threads/9412-Sieg-X3-and-Stand

...and I'll convert it over time. A home build came out at £1600 or so but my confidence that I'd build anything straight and true is low, probably more luck than anything. This X3 will let me do my aluminium parts without worry. Not sure how it'll be with accuracy on the PCBs but, pfft, we'll see. Nothing ventured, nothing gained - worst case I'll get an MF70 just for PCBs and convert that, using the X3 to make all the bracketry :)

If I had the space I've be doing a Bridgeport conversion but I don't have the space. If I was just doing large-area MDF and wood then likewise I'd be going with a home-made gantry system, but this approach just feels 'right' to me...

When I get round to starting it I'll post on here, might be a month or two yet, but I'll be honest at least :)

Thankyou all for your input; it has helped me make an informed decision and I've learnt a lot in the process, so that's been very useful and has helped me understand what I would be taking on.