Z-axis design - help sizing materials please!

[ecat]

Cost is the killer and it all comes down to where are you willing to make the compromise. From what I've picked up so far you chose your target tolerance and design so the inherent flex etc of the machine is better than 1/10 of you target. So, for 0.1mm tolerance you design for better than 0.01mm deflection, or the other way around, if you suspect an assembly will flex by 0.1mm then you should expect no better than 1mm tolerance.

Unsupported rail will flex but the bearings should be consistent in all directions (?). Is the flex significant to you?

Supported rail suffers from much less flexing but what about the opening in the base of the bearing? How much extra movement does this allow?

Profiled rail + blocks cost a lot more than the equivalent round rail set up but minimises the issues with both unsupported rail and open blocks and does so in a smaller footprint. Are the benefits worth while and the cost acceptable for your design?

Second hand parts? Just how second hand are and why were they removed? What would the cost be of replacing a single worn part? The true cost of second hand = price + testing + reassembly + replacement_part_costs - how_much_you_enjoy_the_process

It wouldn't be so bad if there where a cheap and easy upgrade path but that's not the way the critical parts are designed. Going from the £70 round rail set up to a £200 profiled set up involves changing all the widths and lengths and offsets and leaves you with a pile of used parts to sell or the idea that a second machine could be built someday.

Your compromise, your budget, your choice :)

[Jonathan]

...0.1mm then you should expect no better than 1mm tolerance.

Maybe I've got to the stage where I should just try it and see and just accept that I may have to compromise on cutting depths to reduce the cutting forces.

Supported rail suffers from much less flexing but what about the opening in the base of the bearing? How much extra movement does this allow?

That's a good point. Looking at the drawing for the bearing blocks if the force is in the 'worst' direction with the supported rails then it's only supported on 2 of the little ball bearings compared to 4 on the unsupported ones.

With this in mind maybe I should mount the supported rails 'back to back' on the Z-axis aswell as the Y.

Second hand: true, you never know.

Your compromise, your budget, your choice :)

Indeed

I think I'll go for the 600 and 900 supported rails for £155. That's already much more than I was intending to spend and a much better design.
Has anyone here bought from that seller / can confirm that they're good?

[routercnc]

I think I'll go for the 600 and 900 supported rails for £155. That's already much more than I was intending to spend and a much better design.
Has anyone here bought from that seller / can confirm that they're good?

I haven't bought supported rails from Chai, but I have bought a water cooled spindle and some ballscrew support blocks (BK12) from him and can vouch for his good communication and good service. One of the square nuts in the support block set had a poor thread, but a quick email to him had one on it's way over from China. Remember to allow for import tax, duty, handling etc. in case Royal Mail search your parcel, but even with these the price is still tempting.

[Jonathan]

I haven't bought supported rails from Chai, but I have bought a water cooled spindle and some ballscrew support blocks (BK12) from him and can vouch for his good communication and good service. One of the square nuts in the support block set had a poor thread, but a quick email to him had one on it's way over from China. Remember to allow for import tax, duty, handling etc. in case Royal Mail search your parcel, but even with these the price is still tempting.

That sounds good. I did a search on his feedback for 'quality' and found lots of praise and only the odd bad one.
Do you think tax is likely? Did you get charged tax?
He's now listed the rails for me :)

[routercnc]

Did you get charged tax?

Listing my various imports:
Spindle no, bearings no, one set of profile rails no, one set of profile rails yes.

So possibly not, but allow for it.

[Jonathan]

Listing my various imports:
Spindle no, bearings no, one set of profile rails no, one set of profile rails yes.

So possibly not, but allow for it.

How much were the rails that you got charged for? Customs only change if it's above £18 if I recall correctly, so it depends to what extent the seller fakes the value on the form since alas, no rail is that cheap!