I had considered making it so that I can bolt the Z axis to the Y axis at different heights which, if I understand you correctly, would have the same effect as your idea. I don't like this idea so much since it means I've got to zero the axis to the part multiple times. From my experience with the smaller blades this is definitely not something I want to be doing.
Earlier in this thread I asked about drilling holes in the standard round 20mm rail to enable me to add supports. 'routercnc' said that this could stress the rails causing them to be deformed. Is this view shared by others? Not to say routercnc is wrong, I just want to be sure that this option isn't viable before forgetting it. If I used the adjustable bearings then any slight imperfection shouldn't be too big an issue?
Great project you have there, interesting stuff.
There's no easy way to commit to an own design, and you'll no doubt end up tweaking whatever you make anyway. Making these CNC machines is a mix of previous experience, guesswork, copying parts of existing designs, understanding forces, making calculations, and fabrication skill. You've obviously thought about the problem, the risks, and the compromises, so it's probably time to give it a go.
My concern on drilling the hardened rails is that I don't know if they are stress relieved as part of the hardening process. If not then drilling a hole may allow the stresses contained inside to distort the rail. The bearings won't help you to 'make it straight again' between the span. But I'm really interested in a second opinion on this as well. I'm going to post this question seperately to put it to a wider audience.
If I do go down this route which bearings are a better bet...the adjustable or fixed ones?
I could always support the rails underneath without actually fixing them (hmm loctite!?). That would at least stop them deflecting in one direction.
I've emailed the seller to list 600mm - that should be cheap enough :)
The postage is the same as the product..........£120 would get you a nice linear rail system from gary at zapp or if you are patient then wait for S/H linear rails not unsupported round rail crap for that price....
Last edited by Ross77; 22-08-2010 at 08:03 PM.
I bought my 300mm Z axis profile rails, and my 700mm Y axis profile rails from Korea. FA system:
And very pleased with them. Wouldn't contemplate buying these things new because of the price. Remember to allow for import tax, handling etc. and a 2-3week wait if you go down this route. I got lucky on the 700mm rails (import wise), but the 300mm rails cost around £20 for import etc.
If you go with open bearings on an unsupported rail (or supported) remember that under certain load conditions they will 'open up' to some degree. Thanks to Ross for pointing out that obvious fact to me some time ago! There's alot to think about, or blissfully ignore!
I used unsupported rail on my build but it is 30mm in diameter, i wanted to go for the truck type and profile but cost was a factor. If i could i would have gone for this option. At some point i will change it to accommodate it but not now. My advise is if you can stretch go for the right thing in the first place. It will save in the long run. I have another use for my parts at a later date so that is a saving for me and my design allows for that change as an upgrade. IT could cost you double by skimping because if it does not work out then those parts are wasted?If the nagging gets really bad......Get a bigger shed:naughty:
I sent a message to the seller and got this:
2 SBR16-600mm supported rails +4 blocks:
Air shipping cost is included.
2 SBR20-600mm supported rails +4 blocks
Air shippping cost is included.
I've just looked at the 'HGR20' profile rail from Zapp. It comes to £129 for 2x600mm rails and only two blocks. I simply cannot afford to spend that much on rails!
If I decided to use the supported round rail for the Y axis (or just round rail supported with the holes etc) is 16mm going to be sufficient or should I go for 20mm?
Also, how to I work out the optimal spacing for the rails on the Y axis? Is the torque from the Y screw a significant factor here since that would suggest I should put the rails close together?
Thanks again for all the advice - there certainly is a lot to think about!!
Last edited by Jonathan; 22-08-2010 at 10:24 AM.
Yes I'm in the same situation with regards to cost. The profile rails are roughly twice as much as the 20mm supported rails.
Surely the supported rails mounted on a 15mm are not going to bend much at all? I'm well within the 860N load rating of a single bearing.
The Z-axis assembly will weigh roughly 13kg.
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