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  1. #51
    ill do a sketch if i find time.

    Why do you want such a high ratio anyway?
    i want to cut holes and bearing recesses in a 50mm acetal cylinder and also engrave fine markings and im trying to avoid the complications of adding a disk brake... it feels like i might get the holding torqe i need at 50:1 (i should really do the maths)
    i may well use timing belts and pullies, im not fully commited to one thing or another at the moment

    with all the ice i managed to burn out my windscreen wiper motor so iv even pulled the old one apart to look at the worm gear as a possibility... thats 55:1 ratio, i think id struggle tameing the backlash on that though

  2. #52
    Quote Originally Posted by Jonathan View Post
    Can you think of anything better to use that's not too expensive? Maybe plywood, but I guess that's not much better. Vacuum table isn't much good for metals. Bear in mind it needs to be about 800x1900mm ... so clearly I can't afford a nice piece of tooling plate that size!

    I'm not sure about the epoxy idea either, wouldn't it just crack/crush?
    Jonathan any type of wood table is going to fail to sufficiently clamp metal to it

    Whatever you are machining/clamping to the table has to be weaker than or equal strength to the table itself. So the minimum would be ali but it would be cheaper to use steel.

    A steel plate with 50x15 bars with 70x15 bars mounted on top to form the tee slots your require, and steel would be my preferred material.

    Steel wont warp when wet/different humidity you would have to set your X & Y Axis to run parallel to the plate, now you might think this is a disadvantage not being able to machine the bed to suit the X & Y

    But you can only get a flat surface this way if the material is thin as a very thick piece will show any misalignment.

    Phil

  3. #53
    Whatever you are machining/clamping to the table has to be weaker than or equal strength to the table itself
    iv heard this said a few times... im not sure i agree with that phil, surley the table only has to "equal and opposit" the cutting force and carry the weight of the job.... you could use blu-tac as long as it did just this :)

  4. #54
    Quote Originally Posted by M250cnc View Post
    Jonathan any type of wood table is going to fail to sufficiently clamp metal to it...
    A steel plate with 50x15 bars with 70x15 bars mounted on top to form the tee slots your require, and steel would be my preferred material.

    Steel wont warp when wet/different humidity you would have to set your X & Y Axis to run parallel to the plate, now you might think this is a disadvantage not being able to machine the bed to suit the X & Y
    Surely it all depends on what size cutter / feedrate I'm using - if the cutting forces are not to high etc. I was thinking more using the T slots for clamping wood, and using them to clamp my large milling vice (6" jaw, opens up to about the same) to the table for when I want to machine metals. That's still limiting me to only relatively small bits of metal though. If I want to machine a big sheet of aluminium then it's not going to be more than 8mm thick (cost again), so a 6mm cutter would suffice. What's the cutting force on a 6mm end-mill on aluminium at a decent feedrate? Surely not so much that lots of clamps on MDF won't hold it?

    I agree putting steel strips on the bed is a good plan, but again very expensive - unless you know of a better supplier than me! I could skim the existing 18mm MDF bed with the router before putting the steel on top. That should get it reasonably parallel. I guess I could afford to make a smallish steel bed.

    (Sorry I probably could have made this post a bit less rambling, I'm just thinking aloud.)

  5. #55
    Quote Originally Posted by blackburn mark View Post
    avoid the complications of adding a disk brake... it feels like i might get the holding torqe i need at 50:1 (i should really do the maths)
    Could stick with a worm drive and compensate for the backlash on that, either in software or use a split wormwheel sprung to eliminate backlash. If you've got 3nm holding torque then clearly that gets 150Nm at that ratio. So say maximum radius of 75mm, that gets you up to 2kN cutting force ... plenty.

    Quote Originally Posted by blackburn mark View Post
    iv heard this said a few times... im not sure i agree with that phil, surley the table only has to "equal and opposit" the cutting force and carry the weight of the job.... you could use blu-tac as long as it did just this :)
    Yes, double sided tape is great for holding some things, especially PCBs. Effectively I would have 54mm of MDF, with some slots in it...plenty to carry the weight of the job I should think. Could try and calculate the deflection. I'm worried more about if I could do the bolts up tight enough without crushing the MDF. If I put a thin sheet of steel under the top MDF layer that should solve that.

    Dad's still making legs for router - wooden frames are always his department. Going to go and help now...

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  7. #56
    Sorry, I had not read the whole thread, I assumed that with a bed of the size you have, you would be machining wood etc. Phil, there are plenty of machines out there using an MDF surface, the MechMate springs to mind and that is one serious machine!! Others use the threaded inserts in the lower board and epoxy to secure them with a drilled replaceable spoil board above. However I guess to machine metals you will be using some form of coolant in which case MDF is a total NO NO.
    Tim G-C

    I disapprove of what you say, but I will defend to the death your right to say it.

    (attrib. Voltaire but written by Evelyn Beatrice Hall "The Friends of Voltaire" 1906)

  8. #57
    The point is, this type of clamping just will not work for a machine vice 4th Axis etc.

    The Tee Slot nut will just pull straight out through the MDF or you only clamp with finger pressure:rofl:

    Phil
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  9. #58
    Double Post

  10. #59
    Quote Originally Posted by M250cnc View Post
    The Tee Slot nut will just pull straight out through the MDF or you only clamp with finger pressure:rofl:
    That's why I suggested in my last post putting a thin sheet of steel under the top MDF layer, so that T-nut is pressing on the steel, not MDF.

    I like the idea of threaded inserts, but they're not so versatile as T-slots.

  11. #60
    Quote Originally Posted by Jonathan View Post
    That's why I suggested in my last post putting a thin sheet of steel under the top MDF layer, so that T-nut is pressing on the steel, not MDF.

    I like the idea of threaded inserts, but they're not so versatile as T-slots.
    Jonathan, you have a lot to learn.:whistling:

    Phil

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