Maximum deflection to aim for?


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    Default Maximum deflection to aim for?

    So I'm designing my new CNC, my Openbuilds based build just isn't cutting it. I've designed my machine strongly influenced by the Xzero machines. I'm curious as to what I should be aiming for as far as deflection during cutting goes.

    The design has a 1" thick x axis plate, .625" gantry riser plates, and 1.0" plate for the z axis. All Mic-6 aluminum.

    For my simulation, I modeled the machine being fixed at the bottom of the feet and applied a load to the lower 1/3 of the z gantry of 100 lbsforce. That's about three times the amount of cutting force experienced when taking a .100 DOC with a .25" end mill at a .0037 chipload. The results show about a .002" deflection maximum on the gantry.






    Is this a good result? Can I expect to take a cut like what I programmed in the FSWizard?
    I would at the least like to be able to cut aluminum this well:



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    Default Re: Maximum deflection to aim for?

    How big is the machine?

    Do you need that much gantry clearance?

    If you plan on cutting mainly aluminum, I'd probably want it stiffer.

    I'd use thicker side plates, a larger gantry beam, and lower gantry height, if you can.

    Gerry

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    Quote Originally Posted by ger21 View Post
    How big is the machine?

    Do you need that much gantry clearance?

    If you plan on cutting mainly aluminum, I'd probably want it stiffer.

    I'd use thicker side plates, a larger gantry beam, and lower gantry height, if you can.
    Sorry the machine is 1000 x 750. The z axis is 300mm long. The z clearance is about 6 inches. I thought it may be a bit tall but wanted to leave room for mounting a vise on the bed. Maybe ill lower it another inch or so. Also the x axis profile rails are mounted on 1x1" 1018 ground steel bar which are botled to the 1" gantry beam. Would you still recommend something thicker? Thanks for the tips!



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    Default Re: Maximum deflection to aim for?

    The load situation in your FEA is not that realistic... The point of attack is obviously the bit tip which is usually close to the table which not only loads the gantry with a directional force but due to the leverage also with a torque.
    From my own experience (see the BigBamboo thread in the build log section). I found the FEA quite realistic as long as only the gantry/table was analyzed and very rigid (~10,000 lbs/inch) in reality as well as FEA. However, when the z-plate, spindle and linear bearings got involved, the FEA became impossible and the practical results went down to something 3000 lbs/in in y-direction which is still very usable for a router but not as rigid as I had hoped. Most of the flex comes from the round supported rail bearings so you may be better off with your profile bearings.

    But I suspect you are going to sacrifice most of the capabilities with the skinny gantry uprights. Some bracing or box profile should go a long way and this is something that you can optimize with the FEA,

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    Default Re: Maximum deflection to aim for?

    That video isn't demonstrating a huge capacity in my mind, so should be fairly easy to duplicate. I'm seeing a shallow depth of cut and step over in the video though obviously I could be wrong.

    In any event If you have an aluminum machine right now that doesn't cut it, why build another machine that will at best be marginal? I ask in all seriousness because building machine #3 a year or two down the road is hard on the budget.

    I'm not sure about all of the dimensions but the gantry as a whole just look to be a bit "thin". That includes the riser, the beam and the spread of the linear bearings that support the risers. I'd take a good look at the stickies as there is one that dives into gantry beam design fairly deeply. Even without engineering analysis I'd consider a beam with bigger cross section and new risers most likely also made from box sections.

    As for materials I'd suggest leaning towards steel for most of the structure of the machine. Mainly this is because steel is so cheap compared to heavy duty aluminum extrusions. Either way (aluminum or steel) You will be better off with parts of the machine welded up ( X axis frame / base) and post welding machined to give you reference surfaces for the linear rials and so forth. Yes this adds an expense to the project but If you are trying to improve on what you got it is a good use of your money. I'm not a big fan of T-slotted aluminum extrusions for a machine frame as I've had really mixed results with how well some of these T-Slotted aluminum extrusion builds hold up.

    In any event what you want to accomplish is up to you. If you can better define what you want to do with the machine we can possibly offer better advice. The vast majority of builds attempted here in my mind are under engineered for what they user wants to accomplish. Of course there are lots of reasons for that, but I come back to the idea that this is your second build where the first machine just doesn't cut the mustard. This is why it is very important to nail down you expectations. In the end deflection of the machine is just one component that makes or breaks a machine. You also need a machine that has the axis square, straight and perpendicular as the case may be. All machines, spindles, deflect to some degree or another and obviously less is better here but If the machine has one axis out of square to another by 0.020" over 12, then the little bit of deflection doesn't mean much.



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    Quote Originally Posted by wizard View Post
    That video isn't demonstrating a huge capacity in my mind, so should be fairly easy to duplicate. I'm seeing a shallow depth of cut and step over in the video though obviously I could be wrong.

    In any event If you have an aluminum machine right now that doesn't cut it, why build another machine that will at best be marginal? I ask in all seriousness because building machine #3 a year or two down the road is hard on the budget.

    I'm not sure about all of the dimensions but the gantry as a whole just look to be a bit "thin". That includes the riser, the beam and the spread of the linear bearings that support the risers. I'd take a good look at the stickies as there is one that dives into gantry beam design fairly deeply. Even without engineering analysis I'd consider a beam with bigger cross section and new risers most likely also made from box sections.

    As for materials I'd suggest leaning towards steel for most of the structure of the machine. Mainly this is because steel is so cheap compared to heavy duty aluminum extrusions. Either way (aluminum or steel) You will be better off with parts of the machine welded up ( X axis frame / base) and post welding machined to give you reference surfaces for the linear rials and so forth. Yes this adds an expense to the project but If you are trying to improve on what you got it is a good use of your money. I'm not a big fan of T-slotted aluminum extrusions for a machine frame as I've had really mixed results with how well some of these T-Slotted aluminum extrusion builds hold up.

    In any event what you want to accomplish is up to you. If you can better define what you want to do with the machine we can possibly offer better advice. The vast majority of builds attempted here in my mind are under engineered for what they user wants to accomplish. Of course there are lots of reasons for that, but I come back to the idea that this is your second build where the first machine just doesn't cut the mustard. This is why it is very important to nail down you expectations. In the end deflection of the machine is just one component that makes or breaks a machine. You also need a machine that has the axis square, straight and perpendicular as the case may be. All machines, spindles, deflect to some degree or another and obviously less is better here but If the machine has one axis out of square to another by 0.020" over 12, then the little bit of deflection doesn't mean much.
    Thanks for the critique. I would love to use steel but have no experience or means of fabricating. Im in an apartment garage so I cant be grinding and welding for hours unfortunately.
    . I may use steel tubing for the base instead with epoxy to level the rails but im not sure. Im a bit contrained in my build requirements in that the build has to be small enough to go in a sound proof enclosure i built for my first machine.

    The gantry is a bit "thin", I agree. I designed the riser so that I could use some aluminum plate I have laying around, maybe ill bite the bullet and get something thicker or add some bracing to it. I also did some deflection analysis on a 80x160 aluminum extrusion with pretty much identical results to the 1" thick aluminum plate. The plate gave me more y travel so I went that route. I find it hard to believe that the plate is more rigid than the extrusion though.

    I dont have any real goals for the machine, just anything better than my current machine. The Openbuilds system is really weak and the wheels have way too much play in them. Cutting aluminum takes .01 passes and still makes parts out of tolerance because the x axis is so floppy. My main objective is to build a machine that will hopefully bring me reasonable accuracy (within a few thou?) and wont take forever to make a part. I dont need extreme precision or to be able to plow through parts. Im just a hobbyist who wants a better machine without breaking the bank.

    As far as alignment goes, I think ill be okay. Im leaving the hole clearances wide enough to provide adjustment and will be making some screw adjusters to help tram the machine in once its finished. Ill eventually get a second ballscrew for the y axis to help keep the gantry in alignment. I will make do with one ballscrew for now.



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    Default Re: Maximum deflection to aim for?

    Quote Originally Posted by the_hamturdler View Post
    Thanks for the critique. I would love to use steel but have no experience or means of fabricating. Im in an apartment garage so I cant be grinding and welding for hours unfortunately.
    Well that can be a problem. The thing here is that even if you go aluminum extrusions you still have to cut and otherwise machine the parts or order all the parts precut. This ends up being a big expense if you are trying to minimize costs. If you are to pay somebody to cut aluminum to size you can just as easily have somebody cut steel to size. You may very well save some money with steel to afford to have a professional welder rebuild the frame for you. I don't know what your local situation is like but there are a variety of shops around here that can handle various part of a project like this.

    On the other hand people with limited shops can really benefit from off the shelf kits. It is probably the one areas where aluminum T-slotted extrusion kits make sense.
    . I may use steel tubing for the base instead with epoxy to level the rails but im not sure. Im a bit contrained in my build requirements in that the build has to be small enough to go in a sound proof enclosure i built for my first machine.
    You need to be careful with artificial limitations. A more robust machine in the same exclosure may mean you loose working area.
    The gantry is a bit "thin", I agree. I designed the riser so that I could use some aluminum plate I have laying around, maybe ill bite the bullet and get something thicker or add some bracing to it.
    If you have material laying around that is one of the best ways to control costs. Find a way to use the materials but at the same time enhance robustness of the machine. As far as the gantry beam goes that is perhaps the best place to sues steel if you don't want to use it everywhere. In many machines the gantry is the weak point followed closely by a weak Z axis. The obvious issue that it is unsupported with no way to really beef it up after the fact.
    I also did some deflection analysis on a 80x160 aluminum extrusion with pretty much identical results to the 1" thick aluminum plate. The plate gave me more y travel so I went that route. I find it hard to believe that the plate is more rigid than the extrusion though.
    Aluminum extrusions, well T-slotted extrusions, aren't the world best structural members in my mind, certainly not for machine tool building. This especially if you bolt the linear rails directly to the T-slots. Not only is it ought to align those linear rials the T-Slots themselves aren't as structurally sound as would be liked. Take a good look at the cross section of a common T-Slooted extrusion and I think you will see what I mean.

    At work we use a lot of aluminum extrusions but not for critical structural members.

    I dont have any real goals for the machine, just anything better than my current machine. The Openbuilds system is really weak and the wheels have way too much play in them. Cutting aluminum takes .01 passes and still makes parts out of tolerance because the x axis is so floppy. My main objective is to build a machine that will hopefully bring me reasonable accuracy (within a few thou?) and wont take forever to make a part. I dont need extreme precision or to be able to plow through parts. Im just a hobbyist who wants a better machine without breaking the bank.
    Yeah the Openbuilds is a bit of a joke for CNC routing.
    As far as alignment goes, I think ill be okay. Im leaving the hole clearances wide enough to provide adjustment and will be making some screw adjusters to help tram the machine in once its finished. Ill eventually get a second ballscrew for the y axis to help keep the gantry in alignment. I will make do with one ballscrew for now.




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    Default Re: Maximum deflection to aim for?

    Great advice, thank you. I do have a means of fabricating like a chop saw and drill press, just not steel really. I don't have a welder, bandsaw, and cant be grinding welds and making sparks all day without ticking off my landlord. I can make aluminum parts slowly on my machine, though, and I'm trying to design the parts so that tolerances aren't much issue.

    For the extrusions, I'll be getting them from Misumi which has a .5mm length tolerance from what I'm seeing. If they are off, I'll find a way to get them to correct length, either with the chop saw or maybe find someone with a mill in the area.

    I think you are right about using steel. I may have to go with aluminum for simplicity, but I thought I'd get some quotes before deciding.Right now I'm working on a gantry design that I can have laser cut, cnc bent and welded at a company that made a custom bumper for a friends FJ40. They did great work and were under 500 dollars for a bumper. I think that may be a good route for getting a steel gantry with minimal fab work.



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Maximum deflection to aim for?

Maximum deflection to aim for?