Z-axis design - help sizing materials please!

[Jonathan]

Dear Jonathan,
The best price to you:

2 SBR20-600mm supported rails +4 blocks with
2 SBR20-900mm supported rails +4 blocks
238 usd
air shipping cost is included.
If you accept this best price to you,please let me know your paypal email. I will send the paypal invoice to you.
Thanks!
Chai

Hmm very tempting - the best price too :lol:

[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?

[Swarfing]

Well i have just been out in the workshop to finish off the 'Z' of my small desktop machine and screwed it all up because i was interrupted. I cast and machined all the plate i needed, cut it all drilled and milled only to realise all the planes are wrong because i needed to stop and re-adjust my mill head. A point in question relevant here. You can spend a lot of time and effort to make as much as you can but not always effective. I will give up and now buy the appropriate parts. The cost of the time (which is the most precious cost) is what hurts most. The profile will cost more in the end but will make the job easier and quicker to get done. Better to wait a few weeks more and get it right than be in a hurry and see the cost mount up. Just think the time you have in the middle can be used else where?

[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 :)

[phill05]

. The cost of the time (which is the most precious cost) is what hurts most.

Look at the bigger picture, look at what you have learnt this time, next time you should not do it wrong.

On another note I built a z axis a few weeks ago, after reading through all the different information on here and other forums I decided on 20mm x 340mm profiled rails fixed to back plate and carriages moving, just got it all powered up and I don't see any deflection/movement whatsoever, I am getting a perfect cut finish and this is on alley.
The rails and carriages were bought off e-bay as used and have no problems with them.

Phill

[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]

On another note I built a z axis a few weeks ago, after reading through all the different information on here and other forums I decided on 20mm x 340mm profiled rails fixed to back plate and carriages moving, just got it all powered up and I don't see any deflection/movement whatsoever, I am getting a perfect cut finish and this is on alley.
The rails and carriages were bought off e-bay as used and have no problems with them.
Phill

Ok you've convinced me that this is the way to go - thanks! I'll buy them asap. Have you got a picture of your Z axis as I'd be interested to see?

2e0poz:
I had a similar experience. I started out by trying to make the Z axis with plywood and the bearings on aluminium angle method. Oops. It was horribly bendy...still as phil05 says, I've learnt not to try that again and I've not really lost much. One good point is the backlash was <0.005mm!

[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!