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  1. #1
    Carries on from here

    http://www.mycncuk.com/forums/diy-cn...al-please.html


    Right here's version 3 with the Z axis moved up into position

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    Quote Originally Posted by silyavski View Post
    I assume your gantry will be 160mm high, that means for the 600mm travel you will use something like 1m of 160x12mm plate which weights 15kg. For example 1m 160x60x4mm steel profile weights 9 kg and would be stronger than your combined plate+aluminum profile.
    I said I might add the plate. Are you talking about steel box section ? if you are the problem is it doesn't enclose the ball screw like the C shape.
    Last edited by EddyCurrent; 13-09-2013 at 10:34 PM.

  2. #2
    Bearings till look too close together on Z axis.?

    I'd also re-think having the profile for gantry bearing plates it's just not flat enough or strong enough. Profiled linear bearings won't tolerate much error so these areas are important, the profile will bend and impart twist on the bearings making them bind. To give an idea of how little just the thickness of piece of paper can be the difference between binding and smooth running much more and easily locked up.!!
    Last edited by JAZZCNC; 13-09-2013 at 10:46 PM.

  3. #3
    Quote Originally Posted by EddyCurrent View Post
    I said I might add the plate. Are you talking about steel box section ? if you are the problem is it doesn't enclose the ball screw like the C shape.
    Yes, i get it. I was suggesting a steel box section profile and 2x 30x30 or similar aluminum profiles fixed on one face so you can imitate your first design. Best would be your design made from steel box.

    Looking at the design, what i see is that you try to make flat the z but at the same time the whole Z assembly seriously overhangs on the linear bearings. So making it same but from steel, you could fix the rails further in front up to the near end of the steel box, which you could not do with aluminum.

    I have redrawn million times this and until now i couldn't find a real way to diminish overhang without placing the ball screw behind the gantry. For a better understanding is better to draw the spindle mount brackets, the spindle itself and the bit. On the 3d warehouse there is 2.2kw spindle with mounts. So you will have better picture what you gain and at what price and compare actual bit position.

    -using flat wide rails 15 size on the Z will gain you 15mm versus 20 size square rails
    Last edited by Boyan Silyavski; 13-09-2013 at 11:19 PM.

  4. #4
    Quote Originally Posted by JAZZCNC View Post
    Bearings till look too close together on Z axis.?
    Bearings now 150mm spacing, drawings show Z axis at extremes of travel.

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    Quote Originally Posted by JAZZCNC View Post
    I'd also re-think having the profile for gantry bearing plates it's just not flat enough or strong enough. Profiled linear bearings won't tolerate much error so these areas are important, the profile will bend and impart twist on the bearings making them bind. To give an idea of how little just the thickness of piece of paper can be the difference between binding and smooth running much more and easily locked up.!!
    Gantry rails are not on a profile it is steel box section to be bolted onto a base so it can be shimmed level. I've also read all about the epoxy leveling idea.

    Listening to what silyavski is saying I've done an alternative for the Y rails, one pic shows the original position, the other shows the rail moved forward but is mounted over the inside vertical edge of the middle 80x40 extrusion instead of down the centre of it. I'm thinking the original is strongest.

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    Questions
    1. I mentioned earlier that due to the standard ball nut I need 4mm shims under 15mm bearing carriages on Z axis so should I fit 20mm bearings instead without shims?
    2. Steel or aluminium for gantry ?
    3. Anything else to amend ?

    One I get the gantry finalised I can try to estimated the centre of gravity and get the X axis bearings better placed.
    Last edited by EddyCurrent; 14-09-2013 at 11:45 AM.

  5. #5
    Quote Originally Posted by silyavski View Post
    Yes, i get it. I was suggesting a steel box section profile and 2x 30x30 or similar aluminum profiles fixed on one face so you can imitate your first design. Best would be your design made from steel box.

    Looking at the design, what i see is that you try to make flat the z but at the same time the whole Z assembly seriously overhangs on the linear bearings. So making it same but from steel, you could fix the rails further in front up to the near end of the steel box, which you could not do with aluminum.

    I have redrawn million times this and until now i couldn't find a real way to diminish overhang without placing the ball screw behind the gantry. For a better understanding is better to draw the spindle mount brackets, the spindle itself and the bit. On the 3d warehouse there is 2.2kw spindle with mounts. So you will have better picture what you gain and at what price and compare actual bit position.

    -using flat wide rails 15 size on the Z will gain you 15mm versus 20 size square rails
    I'll make a drawing to see what this is like, also estimate the weights. I've had a good play with the spreadsheet but it can't do the aluminium extrusion however there is another calculator for that.

  6. #6
    Quote Originally Posted by EddyCurrent View Post
    Gantry rails are not on a profile it is steel box section to be bolted onto a base so it can be shimmed level. I've also read all about the epoxy leveling idea.
    Not talking about rails I'm talking about the profile you show the gantry sitting on, or should say sat on top of the X axis bearings.

    Regards the rail position on the profile.? You are showing or presuming the profile is flat and smooth across it's surface but most i've used have some recess down the centre or between slots so you may find they interfere with plans.
    Also if your using 15mm rail then you'll find the slots are wide with chamfered edges, this means the rail sits awkward and can cock when fastened down.

    Q1: First are you sure you only need 4mm shims.? When I make Z axis even with 20mm profiled rails I need to machine approx 6-7mm channel down centre of both plates to give clearance for nut and bearings.?

    Q2: Either.!!. . . . Steel is cheaper and strong but it resonates more so ideally needs filling . Profile is OK but not has strong and often not flat has folks realise and can make rail setup/alignment harder not easier like folks think.

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  8. #7
    Spot on, I think there's enough info here and in the other current build logs to embark so I'm going to start ordering some stuff. Rather than add to this in dribs & drabs I'll update at strategic points in the build.
    Hopefully I won't have to ask many more questions.

  9. #8
    On the other hand, went back to basics for a rethink, used spreadsheet and this set up should be very strong with not too much metal bashing.

    Gantry width 1000mm, 80x80x3 steel box section
    Gantry sides 120x80x5 steel box section with top open end plated.
    All other plate 20mm aluminium
    Rails mounted on front to remove need to have extra brackets if they were placed top and bottom.
    150mm travel on Z axis.
    Front could have thin slotted plate for chip protection or the whole front could have bellows fitted to cover rails as well.

    Rail are spaced at 170mm centres and Z carriages are same spacing vertically
    Y rails, not shown, mount directly under plate at bottom of gantry sides.
    Hole for Y ballscrew to exit at one end (not shown) may require double plating around it for strength.

    Any thoughts on this ? especially best way to engineer the various butt joints at end of each box section.

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    Last edited by EddyCurrent; 15-09-2013 at 08:06 PM.

  10. #9
    Yep that's getting better.!. . . couple of things worth a mention.?

    The way you have ballscrews fixed into gantry ends using FF/FK blocks will cause you problems with alignment and getting in and out because it's effectively trapped.
    With this design you'll be better with BK/BF blocks either sat on box section or fasten thru sides of blocks into ends.

    How you going to fasten the 120x80 box to those plates which I presume are Aluminium.? You'll have to make sure those ends are cut or machined perfectly 90deg otherwise they'll be a pain and if you weld plates on ends and bolt into Aluminium the plate will need milling perfectly flat.

    Fastening the ends to gantry box section will also need careful end machining and attention, Esp if bolted together. If you weld it together you'll need to pay very careful attention to it twisting. . . . Personally I'd weld it together then Epoxy level the front so both rails are on the same plane. This way doesn't matter if it twists has the epoxy will remove any so all you need to ensure is that it's square which is easy done with careful measurement.

    I'd also increase those little up stand brackets, basicly make them triangles that go at least half way up the Box section height.

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  12. #10
    Quote Originally Posted by JAZZCNC View Post
    The way you have ballscrews fixed into gantry ends using FF/FK blocks will cause you problems with alignment and getting in and out because it's effectively trapped.
    With this design you'll be better with BK/BF blocks either sat on box section or fasten thru sides of blocks into ends.
    You're right, I was thinking the gantry sides might be bolted together thus allowing access to fit the ball screw.


    How you going to fasten the 120x80 box to those plates which I presume are Aluminium.? You'll have to make sure those ends are cut or machined perfectly 90deg otherwise they'll be a pain and if you weld plates on ends and bolt into Aluminium the plate will need milling perfectly flat.
    I can see the 90deg aspect will be a problem whatever method of construction is used. I was thinking of bolting a block to the aluminium that was a tight fit into the box section, then bolt the box section to the block. Welding a bottom plate as you say and milling might be a better idea.

    Fastening the ends to gantry box section will also need careful end machining and attention, Esp if bolted together. If you weld it together you'll need to pay very careful attention to it twisting. . . . Personally I'd weld it together then Epoxy level the front so both rails are on the same plane. This way doesn't matter if it twists has the epoxy will remove any so all you need to ensure is that it's square which is easy done with careful measurement.
    I was thinking welding would be best, I would make a jig to hold it.

    I'd also increase those little up stand brackets, basicly make them triangles that go at least half way up the Box section height.
    I had them like that at first, will change back.
    Last edited by EddyCurrent; 15-09-2013 at 09:48 PM.

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