Y axis: one motor + pulleys/belts vs one motor and undertable beam


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Thread: Y axis: one motor + pulleys/belts vs one motor and undertable beam

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    Default Y axis: one motor + pulleys/belts vs one motor and undertable beam

    Hi,

    was planning to build something like Joe2006 which has the gantry sides connected underneath the table and the central motor/leardscrew attached to it. However I ran into some post discussing bed frames and full supported design vs partially Supported Y- axis & fully supported X-axis and it put a bug in my ear. The bottom line in the post was that a fully supported design (with all the axis solidly on the table) is much better as far as avoiding flexing. The obvious solution would be two motors/controllers/leadscrews, however that's out of budget. I then found out that one of Solsylva's designs was solvingthis problem by having pulleys and belts on the back controlling 2 leadscrews on the sides. An example can be seen in this post:

    https://www.cnczone.com/forums/diy-c...tml#post777324


    This is still a little more expensive, but maybe worth doing. I've looked at several dozens build threads and never seen anything like it, a joe2006 with one motor + double leadscrew, and I'm wondering if the reason is because it's a dumb idea/no real gain.

    thoughts?

    thanks

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    Default Re: Y axis: one motor + pulleys/belts vs one motor and undertable beam

    Welcome aboard Spikes!

    You pribably think that your question is simple resulting in a simple answer, unfortunately we might see weeks of discussion instead.

    The first thing to realize is that there is no perfect machine design, they all have trade offs. The one big advantage under the table linear systems have is protection of the leadscrew and bearings. To some extent anyways as does gets everywhere.

    Im not familiar with the designs and being on a cell phone at the moment wont be looking them up. In any event the number one consideration in my mind is that suppirted rails are used, if not the design is not stiff enough to consider. So if we are past that the next question is how far apart are the bearing blocks? Twist is a functionplay in the bearings and the stiffness of the rail and bearing block assembly. The fartger apart you can separate the bearings the less of a problem twisting becomes. Of course this creates an issue where you have a lot of dead space on the table where the gantry cant reach.

    Now some people obsess over that dead space, having worked a bit with turret mills im not bothered. In the end you figure ot the work space you need and size the machine accordingly. Im pretty sure one could go thriugh the mechanical engineering to figure out what an acceptable bearing spread would be for the size of machine you are interested in. Before even doing that though you need to nail down work area size and what you can accept for over all machine dimensions. In any event it isnt impossible to get a working machine, that gives acceptable results, with a leadscrew running down the center. It depends upon many factors but the ratio if table lenghth to width can have a big impact on viability.

    In any event lets say that you have a large table that would benefit from dual leadscrews. Driving them from a single belt can certainly work but again it is the details that matter. How far apart are the leadscrews, the belts type, belt support solutions, even belt width are factores. So yes it can work and actually work well if carefully engineered. The gotcha is that the belt will need to be tensioned tight and thatdoes require stiffness in that area of the machine.

    In any event if you look at alternative designs for large table machines you almost universally see dual leadscrews in part because the bearings are on top or the side. The problem with many designs in my opinion is that the X bearings are too close together supporting really narrow columns holding up the Y axis. So you still have an area for twisting to happen though it might be more of a rocking. It comes back to guys wanting the machine to be as short as possible. As long as they understand they are trading off stiffness for machine length there isnt sn issue. However if their goal is be best possible accuracy and solid alignment it doesnt make a lot of sense to me.

    Now there us a huge number of qualifications here the rail type (round or profile, its size the bearing block preload, the machines construction materials and etc all come into play. If you want a machine with under the table linear motion components id say go for it understanding the trade offs. If you consider all the issues and you dont want to address them consider another design. In the long run dual lead screws and the required steppers isnt as damaging to the budget as some alternatives. For instance for a single motir drive dual lead screws drive yiu will need to buy suitable pulleys, belts and iflers at a minmum. One of those pulleys will need to allow for phase adjustment or the leadscrew bearing block will need to be adjustable for position ( for final gantry square up). Also this axis "MAY" require a bigger stepper if only one is driving.

    I expect you will get a lit of opinion expressed in this thread but in the end a viable build is all about engineering and build quality.



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    Default Re: Y axis: one motor + pulleys/belts vs one motor and undertable beam

    Hey Wizard,

    thank you once again for all the time you put into quality feedback. No problems about weeks of discussion, I'm in no rush and appreciate all the input I can get.

    Agreed on the tradeoffs, I'm totally onboard with that and why I'm not taking some of the stock designs for granted as they may give better results, but there may be tradeoffs I can take to save money while still getting results that are good enough for my needs. Per other thread, this is the design I'm leaning toward, size 2'x4' work surface:

    https://www.cnccookbook.com/wp-conte...c-router-5.jpg

    So rails are supported and I don't have a problem with dead space so I'll look into spacing those bearings as suggested , thank you. Do you think 4-5 inches are a reasonable amount?

    As to the consideration of two leadscrew respective to table size, do you think a 2'x4' would sustantially benefit from it or it's small enough that one central leadscrew would be plenty?

    I didn't understand what you meant here: The problem with many designs in my opinion is that the X bearings are too close together supporting really narrow columns holding up the Y axis . Also I've seen people referring to the bridge as the Y or the X, so I'm not sure which one you meant there.

    Overall tho I get your point in your final comment, the saving may not be worth after all, I'll try to find something suitable and compare the numbers.

    thanks again



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    Quote Originally Posted by Spikes View Post
    Hey Wizard,

    thank you once again for all the time you put into quality feedback. No problems about weeks of discussion, I'm in no rush and appreciate all the input I can get.
    Im fitting you in between various steps in a drywall project at home. Well that and a pizza. Im working on getting my shop area organizedinsulated and such. So if the response seem odd or disjointed you now know why.
    [Quote]
    Agreed on the tradeoffs, I'm totally onboard with that and why I'm not taking some of the stock designs for granted as they may give better results, but there may be tradeoffs I can take to save money while still getting results that are good enough for my needs. Per other thread, this is the design I'm leaning toward, size 2'x4' work surface:

    https://www.cnccookbook.com/wp-conte...c-router-5.jpg
    [Quote]
    Cool! I think ive seen this one before. It is an interesting approach and i suspect relies upon using a precision template to route out each rail supporting member. Each would have to be very precisely made to keep the rails parallel in all dimensions.

    The big problem i see here is that the uprights supporting the gantry should be very stiff. From this picture it looks like rotating forces from the gantry would bow the supports outward.

    Considering all the trouble doing this linear rail solution will be im really thinking round rails would be just as economical. This especially if you have to buy tools to build the table and supports in the first place.
    .
    So rails are supported and I don't have a problem with dead space so I'll look into spacing those bearings as suggested , thank you. Do you think 4-5 inches are a reasonable amount?
    The rails are not fully supported but the design is much better than many ive seen.

    As for spread of the bearings that depends upon Z axis clearance. In the case of that picture i think they are a little close together. Roughly the should be as far apart as the Z is high. If not that id set the lower limit spread to 75% on a light duty design like this.
    .
    As to the consideration of two leadscrew respective to table size, do you think a 2'x4' would sustantially benefit from it or it's small enough that one central leadscrew would be plenty?
    With that design as is i dont see a lot of inherent siffnees. Dual leadscrews might help a bit but honestly id spend my money on other parts of the machine first.
    I didn't understand what you meant here: The problem with many designs in my opinion is that the X bearings are too close together supporting really narrow columns holding up the Y axis .

    What i mean there is that a lot of alternative designs come up short when it comes to gantry support even if they have dual leadscrews. On the X axis the bearings are too close together and they are supporting flimsey gantry support uprights.
    Also I've seen people referring to the bridge as the Y or the X, so I'm not sure which one you meant there.
    That is a problem on this forum. Personally i start from the top and work down in reverse to designate the axis. So Z = vertical that moves the spindle up and down, Y = the gantry and X = the table axis. This seems to set some people off but standards do make for easy communications.

    Overall tho I get your point in your final comment, the saving may not be worth after all, I'll try to find something suitable and compare the numbers.

    thanks again
    Dont forget all the tooling you may need to buy! I dont know what you have access to but tooling adds up fast. For example you will need at least one pattern following bit, a router and templates to build most of this machine. Solutions for the bearing brackets probably need a drill press at a minimum. Lets not forget a table saw. If you have access to some of this stuff already then the cost to you is lower.

    I liken this to metal working i do. The cost of the steel is only part of the equation, welding wire costs money, the right angle grinders cost money dill bits, band saw blades, paint and whatever impacts the cost of something.

    So you take the same basic design and redo it with round rails on the table. In the end does it have a major impact on costs? I really doubt it to be honest.



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    Default Re: Y axis: one motor + pulleys/belts vs one motor and undertable beam

    Hey Wizard,

    I don't know if right now I envy more the neat workspace or the pizza... I'm both hungry and left to deal with a dirty community workshop... oh well.

    About the stiffness of the gantry's sides, that's where I was thinking of maybe adding some thinnish (1/16, 1/8?) sheet of metal, but point taken.

    Regarding the rails and its supports, I think I can make a profile and borrow a router with a follow bit to keep things accurate, but I get the point about complexity and tools costs being a factor.

    With that design as is i dont see a lot of inherent siffnees.

    Care to elaborate on this? the table is a torsion box, the gantry's beam is another torsion box and I can reinforce the sides as discussed above. how is that not inherently stiff? And where would you spend your money then?

    Dual leadscrews might help a bit but honestly id spend my money on other parts of the machine first.
    where would you spend your money then?

    In the end I think I can keep the costs lower like this over round rails on the table (which would then also require its own bearings, otherwise what's the point?) by borrowing tools and stuff. Unless we're saying that it would be better if I could mount the pipe on a metal base with bolts , a sort of round rail look like, and still use bearings. Like this:



    Full details here: Krazatchu Design Systems

    would that be a better design? There's still some complexity on getting all the holes right, etc. This is also actually how solsylva does it, and then uses skate bearings with that.

    as to standards, oh well, that's a problem everywhere, but especially where noobs like myself jump in and use the wrong conventions which are picked up by the next noob joining - it's a tough one and I'm grateful for everybody's patience.



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Y axis: one motor + pulleys/belts vs one motor and undertable beam

Y axis: one motor + pulleys/belts vs one motor and undertable beam