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  1. #71
    Quote Originally Posted by Tenson View Post
    I find that quite surprising. I rather thought with the very short length and least weight of any axis, the Z would be okay running unsupported. If it really is that bad I'll get some supported rails.

    Anyone want to buy 2x 30cm unsupported rails? ;)
    Ask Jazz about it, he came around to help me out with the machine and saw it. Its the cutting forces that will really show this up but even pulling back and forth on the z axis by hand will tell you how sturdy it is. The more rigid the design, the better the finish and accuracy you'll get.

    If you do it right you'll be cutting MDF with very tight tolerances and even aluminium won't be an issue. Just keep asking loads of questions on here and tell folks about every design decision your making as they'll be able to advise if its a good one or a bad one. Building a good machine doesn't mean its more expensive but building a bad one will cost you to fix it.

  2. #72
    Quote Originally Posted by Shinobiwan View Post
    Building a good machine doesn't mean its more expensive.
    Bloody seems like it!

  3. #73
    Quote Originally Posted by Tenson View Post
    Bloody seems like it!
    lol, ok well not that much more expensive. You'll have a really nice machine if you do it right and it'll last you years.

  4. #74
    Is there a big advantage in build quality terms using profile rails for the Z-Axis rather than supported round rail?

    I must admit I can't see an easy way to use supported rail so that the bearings oppose each other on the Z axis. I have seen Jazz used HiWin rails facing out, not opposing, so is this only okay with profile rail?

  5. #75
    Quote Originally Posted by Tenson View Post
    Is there a big advantage in build quality terms using profile rails for the Z-Axis rather than supported round rail?

    I must admit I can't see an easy way to use supported rail so that the bearings oppose each other on the Z axis. I have seen Jazz used HiWin rails facing out, not opposing, so is this only okay with profile rail?
    The advantage of profiled rails are much tighter bearing tolerences with a smooth action helping to reduce resonance and flex they also last much longer, esp in harsh conditions like a Z axis where it's constantly in the firing line and open to crap from the cutter. They don't really add much more strength than supported round rail if any.!

    You can use supported rails just the same as profiled like you have seen me do. Absolutely no difference other than they stick off the Z axis rear plate (which you would call running across X axis, me Y axis) slightly more due to height of supported rail.

  6. #76
    The main advantage with profile rails is that they are preloaded. This means there is negative clearance between the bearings and the rail, so there will not be any play. It also makes any deflection linear and crucially, with rails with 4 rows of balls which are the type you should get, the load ratings are equal in all directions. Conversely supported round rails have little if any preload and the load rating varies significantly depending on the direction the force is applied since the open side of the bearing clearly offers less support. Ideally we would mount supported rails so they 'oppose each other' on Z, however as you say it's quite difficult to accomplish rigidly without a somewhat radical gantry design. Hence most people mount them both the same way round. If I measure the deflection when a force is applied to my Z-axis parallel to X the reading varies quite a bit, due to the uneven load rating.
    However this is of course all relative - compared to unsupported rails the deflection is tiny. You wont regret using supported or profile rails on the Z-axis - ultimately depends on what you can afford.

  7. #77
    Thanks I rather thought it might be the case, though good to know open bearings on supported round rail are not a no-no. The HiWin rail is okay priced but the bearings seem a bit steep. How about other brands? RS Components sell one called IGUS DryLin T which for 300mm is quite affordable (GBP14 rail, GBP16 Carriage, I can't do the pound sign on this Korean computer) but doesn't offer longer rails. There are many other brands too.

  8. #78
    Hi Guys,

    I got some SBR rail from CNC-4-U with open bearings. The bearings look like they are normal ones that have been cut open. Is this normal? They also have little grub screws in the top that hold the bearing in and when tightened up they close the bearing up on the rail a little. The motion was a little jerky at first but after a quick sanding of the cut endge of the bearing thye are smooth now. They seem a bit home made. Should I be sending them back or are they okay?




  9. #79
    Quote Originally Posted by Tenson View Post
    They seem a bit home made. Should I be sending them back or are they okay?
    That's perfectly normal. Same as you'd get from linearmotionbearings2008 on eBay. I ordered some recently from china (not for myself I hasten to add!) and they came in less than a week. The're essentially the same except the grubscrew is now on the side of the bearings, not on the base.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  10. #80
    Thanks for that. I like CNC-4-U. They are UK based so I'm supporting local trade and the prices are not too far from the eBay ones. I got the 2.2KW VFD spindle and 4Nm steppers from them too.

    Jonathan, can you link me to some more pictures of your machine? I saw it in the video for the single flute cutters but I'd like to see more detail.
    Last edited by Tenson; 16-04-2012 at 11:31 PM.

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