22-08-2010 #41Dear Jonathan,
The best price to you:
2 SBR20-600mm supported rails +4 blocks with
2 SBR20-900mm supported rails +4 blocks
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.
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 :)
Last edited by ecat; 22-08-2010 at 12:55 PM.
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.
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?
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?If the nagging gets really bad......Get a bigger shed:naughty:
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.
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!
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