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Thread: How frequently should I screw?

  1. #1
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    Default How frequently should I screw?

    Let us assume I am building some sort of CNC router sorta gizmo... and the base is 1" thick aluminum and the Y linear rails will be put on 7" high 1.25" thick aluminum side walls, with a 2" wide by 1" thick aluminum cap, and reinforced with buttresses made of 1" thick aluminum plate. Sort of like this:



    I plan to join the pieces with countersunk machine screws. How far apart should I place the screws to optimize strength while minimizing work? And what size screws would people recommend?

    Or more importantly - what are some good URLs so that I can learn the basics of how to design fastened joints? I can find a lot about riveting and bolting lap joints, but finding texts about this sort of assembly has escaped my google-fu.

    So why build it on a 1" thick aluminum base? Because this followed me home from an auction and my wife is going to let me keep it... although she says it's my machine base and I have to feed it and clean up after any messes. It's a nice size - 42" x 68".



    Please don't recommend welding - I totally suck at welding aluminum.

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    One would have to know the maximum horizontal load applied to the rails to be able to design the fastenings.

  3. #3
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    Well, you get the thread title of the month award.

    Oh, yeah, the ONLY good way to do what you want is a really good welding job on the pieces...........
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    Now that we have that out of the way, bolting or using screws is probably not a bad way to handle the thing, since you really do not want to have to lay the whole thing on the bed of your mill and face the top and bottom. Welding would be solid, but I would expect it to turn the assembly into a bit of a pretzel, at least with respect to the scale of the errors you can tolerate with that kind of base.

    That said, the flatness needed on the various surfaces to prevent it from being just as bad as the welded up part may be, well, "significant".....

    Can you change the design so as to allow the top piece to float to where it wants to be, and then use other methods, like scraping, etc, to achieve the final flatness needed? That would avoid having strains or misalignment built-in to the top rail,

    I am thinking of it being much as you show, but with the top rail NOT being the surface the "ways" go on, and having that piece be perhaps steel or a CI bar that is bolted to the SIDE of the upright. I see that as a piece which you get ground flat on top "outside of house" before you attach it. You could then arrange the "rail" with shims so that it is not strained, and then fill in with an epoxy in the gaps for solidity.

    That process would also allow your option of ways to get the tops of the "rails" aligned with each other. The actual linear rails would then be bolted to the Ci or steel sub-rails.

    My idea is to allow somewhat imprecise construction to be compensated for. I do not imagine that getting everything flat and perfect is going to be possible with either "screwing" or welding.
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    Quote Originally Posted by J Tiers View Post
    Well, you get the thread title of the month award.
    Thank you...

    Quote Originally Posted by J Tiers View Post
    I am thinking of it being much as you show, but with the top rail NOT being the surface the "ways" go on, and having that piece be perhaps steel or a CI bar that is bolted to the SIDE of the upright. I see that as a piece which you get ground flat on top "outside of house" before you attach it. You could then arrange the "rail" with shims so that it is not strained, and then fill in with an epoxy in the gaps for solidity.

    That process would also allow your option of ways to get the tops of the "rails" aligned with each other. The actual linear rails would then be bolted to the Ci or steel sub-rails.
    I'm a little worried about bimetalic construction of fixing aluminum and steel tightly together - although I already have it since the linear rail is steel and the bed is aluminum. I haven't looked into the differential expansion coefficients yet...

    I did buy a 4' by 8' table with a 1" steel top at the same auction... and I could theoretically make the entire tool out of steel and that would eliminate the bi-metallic problem. Oddly enough I can get (and have) lots and lots of thick aluminum stock for scrap price, but very little steel. I'd have to buy steel at market price. It would also mean I'd have to use much beefier motors, etc, and that means bigger drivers, etc. Plus it's a lot easier for aging me to swing big chunks of aluminum onto my mill table... and it's so easy to work "shiny wood".

    SO for all of those reasons I'm making the mostly aluminum version first (and maybe the only one I make)

    I figure there is going to be a lot of hand scraping, maybe some shimming, and hours spent with the indicators. As the lamented Mars-red would have said ... "The Joy of Precision".

  5. #5
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    Why not make it out of mdf but a few percent bigger..
    then take it to a foundry and get it cast in iron..

    Might not be what you want to hear but may be cheaper, stiffer in the long run..

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    I was a bit worried about opening this thread too....

    I think if I were doing something like this I'd use both fasteners and dowel pins. I like the idea of fasteners because they don't produce the same potential for distortion that welding does. But I'd also suggest that you don't really want flat edges. Rather the edges of the side rails and braces should be relieved a few thou in the middle so the inner face of the rail contacts the plate along a 1/2"(?) band on the inner side, then comes up .005 to .010. And that rise is reflected in the feet of the braces out to a lower contact pad of about 1" square at the outside.

    The screws along the rail from below would pass up through the base and into the rails such that they split the edge of that raised transition in the rail and up through the middle of the square pad. See where this is going? You get specific lines and points of contact between the rails with braces and the base plate which should produce a really stable positioning by having a wider spreading out of the "feet". The bolts would pull the rails down solidly to good wide spread points and lines of contact.

    I'd do the same along the top edge with a relief between a 3/8" band along the inside of the edge and some 1/2" wide pads on the braces. But now you would only need one bolt down through the middle.

    To do this well clearly you'd want to make up the sides with braces first to slightly oversize. The braces being fixed to the sides with three screws and two dowel pins and then with things bolted and pinned so they are acting like one part then machine the edges so they end up as true and perpendicular as they can be.

    If desired the top rails could be rebated or given a bit of a slot upwards and the top of the rail treated similarly so the cap rail fits into a groove or down over a bit of a tenon. But either way the point of contact for the downward pressure should be on the inner strip along the rail and the pads at the top of the braces.

    I'm thinking that something like 3/8 screws used in pairs at the bottom of each brace and a single screw holding the cap down should be enough. The metal you're using is stout enough that it should be OK in between.......OR.... you could complement the screws with some manner of serious industrial adhesive that basically welds the metal rails to the base?

    As solid as the base plate should be with it being that thick I would suggest that a boxed diagonal setup from below to brace that table would be in keeping with the nature of how you're designing the rails. Like build up a lower set of braces that makes the underside look like one of those oversize cast iron surface plates from days of olden tymes. Then sit the whole shebang on a stand that bears up against the bracing at three points much like an oversize surface plate so you don't warp the table with changes in the floor and such. That would also stop the middle from flexing downward. Again such a lower bracing arrangement to be built up then machined "true" before fitting the top plate onto it with numerous fasteners. In that case to keep the stuff out of them I'd seal them in with epoxy before the top surface is skimmed so it's all one big happy and smooth surface.

    I know you have it on what seems like a nice strong and sturdy base. But that base is steel, right? So depending on how it's attached I'd be a touch worried about the different rates of thermal size change causing the top to be pressing outwards at warmer times and pulling inwards in colder conditions. Thus why I'm thinking it's own set of bracing in aluminium so it's all the same material and float that in a steel frame might be worth considering.

    And I assume you will be drilling and threading an array of hold down holes in the table? Through holes I assume so the swarf can be simply blown through?
    Last edited by BCRider; 01-25-2019 at 12:40 AM.

  7. #7
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    Default Violence to engineering

    Quote Originally Posted by Illinoyance View Post
    One would have to know the maximum horizontal load applied to the rails to be able to design the fastenings.
    To be accurate, yes.

    But can one "back of the envelope" this? Obviously this is heading towards a wood/aluminum/maybe-steel-but-probably-not CNC router. When cutting metal I can limit myself to certain DOC, cutter radii, etc. So, please don't scream too badly in pain when I commit violence to engineering...

    Lets say I have a 2 HP spindle. That's roughly 1000 foot-pounds/second. Assume something like 75% efficiency in getting from motor to tool edge, so thats 750 foot-pounds/second. That's very roughly 1000 N-m. Much of that force is going to be used in cutting the metal, accelerating the swarth, etc... but lets assume that 20% of that 1000 N-m will be applied to the X axis of the sliding blocks on the rail ... i.e, 200 N-m.

    Firing up the old Fusion 360 - assuming perfect attachment of the structural joints, constraints at the four corners of the table, and a static load of 200 N-m along the entire sliding bearing block - this is what the results of a static stress displacement study looks like:



    The displacement in this wildly optimisitic situation is .00009".

    But I know the joints between the bodies in the assembly are not going to be perfect. They will be little pegs of steel screwed in every so many inches between them. A study where the ends of the wall are constrained to the table but nothing in the middle - nothing on the wall or on the buttresses - shows a displacement of 0.0029"

    So that got me to thinking - the more screws the better... probably. But how many is enough? Is there a place where it's too many?

  8. #8
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    Quote Originally Posted by 754 View Post
    Why not make it out of mdf but a few percent bigger..
    then take it to a foundry and get it cast in iron..

    Might not be what you want to hear but may be cheaper, stiffer in the long run..
    Right. Definately the way to go if I could do it, and if I had a way of machining the rough casting. But two things argue against this approach.

    1) I can't machine such a large rough casting
    2) I can get a very large amount of nice cutoffs of surprisingly useful aluminum for scrap metal prices. Pieces such as 1"x 12" by 48", etc..

    So, the aluminum and assembly road is calling to me ...

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    Quote Originally Posted by Dan_the_Chemist View Post
    Thank you...



    I'm a little worried about bimetalic construction of fixing aluminum and steel tightly together - although I already have it since the linear rail is steel and the bed is aluminum. I haven't looked into the differential expansion coefficients yet...

    .....
    Sttel is, IIRC, about half of aluminum in thermal wxpansion..... BUT... you can anchor the steel solidly at one end, and make the other mount points just loose enough to let them expand without coming up against the bolts. Mounting on alignment pins with the bolts doing the clamping might have the best of many possibilities. Making that system adjustable might be even better.

    Some of that differential espansion would tend to run counter to the epoxy "grouting" though.... it seems as if one material all through is the best. At 23 um /mK, vs 12 um/mK, the differential is 110 um in 10C.... about 1/10 mm.... enough to make an effect if rigidlt bolted, etc, but not too much to allow a 1m part to expand and slide (it is 4 thou or so)

    When you KNOW you will have some lack of alignment precision, and/or expansion issues it's a good idea to build in ways to sidestep the problems that could come up.
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  10. #10
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    Self leveling epoxy is often used in situations like this.

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