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  1. #31
    Mmmm! not sure what you mean here?
    Ok better explain myself in case I'm wrong.

    Just to make it clear thought that its only because open bearings and supported rail are being used with a small bearing centre to spindle nose distance ratio. Ideally the bearing centres (on plan) should be increased but given Mike has already redesigned it a few times I didn't want to offer any more changes without justifying them with calcs.

    If you are making a box to encompass the supporting rail then you are actually increasing rigidity.
    Yes but its not so much the bearing mounting but the bearings themselves, open bearings can have quite a large radial clearance (10 to 50 micron), with the current set up and using a conservative 10 micron clearance (could be to 50), 100mm bearing spacing and 200mm spindle distance that results in 39 micron of free movement at the spindle. moving the ball screw the other side increases this to 51 micron.

    This is free movement just to take up play, if you add any deflection due to load then this will obviously increase further. for cutting Ali the open bearings can run close to the max load capacity and as rigidity is a function of imposed load and load capacity then deflection will be high.

    On the face of it 10 micron is a very small amount but remember this is only One element and doesn't include deflection in the part. if you lose 10 micron at every junction then it all adds up to a considerable amount.

    Any way that's my take on it!
    Attached Thumbnails Attached Thumbnails Ballscrew position.pdf  

  2. #32
    Ross i did say that the plate could be moved further back. In this design there is no reason why the ballnut could be more central and the spindle moved closer to centre. There is no need to have the nut pushed further away from centre.
    If the nagging gets really bad......Get a bigger shed:naughty:

  3. #33
    Quote Originally Posted by Ross77 View Post
    Ok better explain myself in case I'm wrong.
    I thought your first explanation was good, but the latest one is even clearer. I agree with the 39-51um figure and as you say it's likely to be more. Any compromise like this will reduce the stiffness of each element. You can analyse the stiffness of the overall machine as springs in parallel, so if you reduce one a very small amount it's not likely to make a tangible difference, however if one is reduced substantially such that the stiffness is lower than other elements, then that will dominate the system, causing the overall stiffness to be low. This means compromising one part can waste money you've spent on other quality (stiffer) parts. An extreme example would be using profile rails on two axes and unsupported rails on the other. In that instance changing the profile rails to supported rails isn't likely to make a difference, since the weak point would still be the unsupported rails. Similarly, 'making a box to encompass the support rails' wont make much difference, since that part of the assembly is already at least an order of magnitude stiffer than other parts.

    You can get the ballnut in a more optimal position with the current design - it doesn't have to go behind to do that. Similarly the current design could easily have a 'box to encompass the supporting rail' added.

    Quote Originally Posted by Swarfing View Post
    Switching would save a lot of hassle having to sort extras like seals and way covers etc.
    Just stick a 15mm shaft seals, like these, either side of the ballnut and the swarf is wiped off easily. They're working well for me and not showing signs of wear, although admittedly I've not been using them for long.
    Last edited by Jonathan; 28-04-2013 at 06:47 PM. Reason: spelling
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  4. #34
    Ross i did say that the plate could be moved further back. In this design there is no reason why the ballnut could be more central and the spindle moved closer to centre. There is no need to have the nut pushed further away from centre.
    Paul I think we must be looking at different drawings

    If you look at the Z axis drawing in post #18 there is barely enough room to flip the plate over. if you then centralise the ball screw you will have to move the Z axis assembly further away, making the situation much worse.

    As I said there are other more pressing issues with the design but as the parts are bought and he wants that size working envelope then there's not much that can be done other than maximise the other components. Another reason for not moving the ball screw is that despite the large plate on the back the y axis could still twist if Mike decides to cut larger pieces of Ali. with the ball screw in the front it is a simple case of widening the y beam to increase its torsional rigidity.

    Anyway I think we will just have to agree to disagree on this one. We all have different ideas and its up to mike to decide as ultimately he knows what is needed from the machine.

  5. #35
    Right, I have made one small change to the Y axis to tie it all together. So no more changed till it is built. I ordered some 6082 aluminium plate and have started to construct the Z axis.
    I can only do a little at a time before the pain is to severe and I have to stop, It will be a slow build but I will get there. Anyway one drawing of the change and two pictures to keep you going.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  6. #36
    looks like some good progress Mike. that back plate will help stiffen it all up.

    glad all the comments didn't put you off. keep it up

  7. #37
    Ok here is another part done I had to cut 3mm off the aluminium plate, as they came to large. Now the only way I could cut the 3mm off was with a hand held router and a M6 end mill with the plate held in a wooden jig. That was hard going. Anyway with stops to rest I did start to assemble the bed.

    Click image for larger version. 

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  8. #38
    Looking really good. Any estimations on how much you think this project will cost?

    Just wondering.... Where did you source the aluminium extrusion from? Thanks

  9. #39
    Bit late now but a chop saw would have been a good investment for you if you have problems using a hacksaw.
    With the right blade you could have cut all the aluminium both plate & profile yourself which would have made things a bit easier, plus it would also be handy for cutting aluminium to size before machining it once the router is up & running.

  10. #40
    Quote Originally Posted by robump View Post
    Looking really good. Any estimations on how much you think this project will cost?

    Just wondering.... Where did you source the aluminium extrusion from? Thanks
    To get it up and running I hope it will not be more than 1500. I tried to work out what It would cost then added 22%, I cost the bolts at about 40 but it has cost 59 in bolts so you need to add about 22% to what you think it will cost I have done this with everything that I have made and it's stood me well. It will always end up at the higher price.
    Last edited by Bush Flyer; 30-05-2013 at 09:49 AM.

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