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  1. #21
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    Default Re: CNC improvements

    My friends thanks a lot for your answers! My mommus cnc currently has an accuracy of + - 0.2mm. I want to use it for luthiery work and this accuracy is not very good! In advance i want i to cut a bigger surface so as to curve neck for guitars. At this momment i am thinking of a possible way to drive the gantry with only one motor but from the center (in some machines i have seen it ) i dont know buti have to ask is there any good modification to momus that will not need alot of extra work and also allow me to attach lead screws ?or ballscrews!~?



  2. #22
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    Default Re: CNC improvements

    Quote Originally Posted by pikolo View Post
    My friends thanks a lot for your answers! My mommus cnc currently has an accuracy of + - 0.2mm. I want to use it for luthiery work and this accuracy is not very good! In advance i want i to cut a bigger surface so as to curve neck for guitars. At this momment i am thinking of a possible way to drive the gantry with only one motor but from the center (in some machines i have seen it ) i dont know buti have to ask is there any good modification to momus that will not need alot of extra work and also allow me to attach lead screws ?or ballscrews!~?
    I would say if you want to carve necks, then there are some cutting forces to contend with where a screw can have an advantage. I think which ever way you go, it will need work. Luckily for you, YOU HAVE A CNC of which you can cut parts with! You have to design you parts within the accuracy of your machine but it's not out of the realm of possibility. And likely you're going to have to do the leg work.

    If you use ballscrews, my biggest concern would be, being the machine is enclosed, ballnut contamination with dust. You'd have to ask for matching sweeps, but they don't really work as well. They make bellows for the screws but they take away some travel. Maintenance, mainly preventive, is key.

    As far as tolerances. The instructions quote a tolerance of about .005", which is due partially to the mechanical resolution of the drive system, but also the components and materials. You have to ask yourself if investing in screws with more than double the suggested tolerance than the other parts in the machine are worth it? Likely you won't see much advantage unless you also upgrade to linear rails. Also remember the fixed and free end bearing blocks needed, some form of alignment and shock absorbing couplers for the motors with the requisite proper motor mounts... Your Momus will no longer be. You may want to investigate getting higher quality pulleys and belts with a more accurate tooth profile designed for more precise automation, like GT3...

    ACME screws can have a tolerance from .004"-.009"/ft. It may sound like a lot but in practice I found the tolerances are not that bad. Delrin anti-backlash leadnuts can be had rather inexpensively from vendors like DumpsterCNC, the quality is very good, and they last a long time. I've had the same nuts on my machine for over 6 years, with no appreciable signs of wear or lash. You can "tweak" the settings in Mach3 for more accurate results, but I find them fine as-is. Buy them from a reputable source; they have to be precision ACME screw. A 2-start, 4tpi (as in turns per in) would be a good choice and balance of speed and resolution (1/2"-8, 2 start or 1/2"-8, .250" lead is what it would be called out as from a supplier.) You'll have lower rapids, but better torque and resolution at cutting speeds.



  3. #23
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    Default Re: CNC improvements

    Hello friends! i have thought of an idea that maybe can solve my problem....The solution i am talking about is an overhed leadscrew...i jave modeled it a bit in a harry in sketch up....I dont know if it is a very good idea but alot easier that other \s i have thought of ...what do you think? Also the movement from left to right isnt finished yet buit it is obvious where i will place the motor

    Attached Thumbnails Attached Thumbnails CNC improvements-momus-1-jpg   CNC improvements-momus-2-jpg  


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    Default Re: CNC improvements

    Unfortunately, the screw above the gantry is not a good place for stiffness. Think about 2 forces: the cutting force near the table, and the driving force at that screw/nut. The forces are separated by a mile. That will introduce a large torque, which will rotate that overhead support and the whole gantry. The Momus design has the belt drive very high up already. Ideally, this drive method is placed at the same height as the cutter -- then no such torques are created.

    David Malicky


  5. #25
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    Default Re: CNC improvements

    I seeyou are definately right....I have made another model and i think it is better....now the gantry moves with one motor on the back of the momus like solsylva and with two lead screws with antibackshlash nuts driven with a timing belt>The motor is placed in he middle...On the y axis i have placed the motor in the way you see in the pics...what do you think of this design?

    Attached Thumbnails Attached Thumbnails CNC improvements-new-momus-modifications-jpg   CNC improvements-new-momus-modifications2-jpg   CNC improvements-new-momus-modifications3-jpg  


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    Default Re: CNC improvements

    Well, having the screw in an ideal position, and one in the best position possible given the current design can be two totally different things. So just design it within the constraints you have, unless you're also willing to redesign the entire gantry bridge.

    The redesign I think would work. would like to see how it turns out.



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    Default Re: CNC improvements

    I agree with Louie, I think it would work well. For the X motor, I'm assuming you would use 2 idler pulleys to achieve a good wrap angle around the motor pulley. Just fyi, most dual screw machines use a motor/drive for each screw -- stiffer and more precise, but more $.
    By 'ideal', I just meant to clarify the best case, not to redo the whole machine.

    David Malicky


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    Default Re: CNC improvements

    redisigning the gantry is something that i have thought about too...but it is very time consuming i believe....with this design i already have i will be able to cut pieces 60 cm long in the x axis but i will have to extend the cnc table (i think not a big deal to have it a little bit ....yes the pulleyw are going to be like you said....All the screws are going to have antibackshlash nuts the motors are 425 oz in steppers so i think that the one in the x axis can handle the bigger load...What do you think of the possition of the y axis motor ? will it be a problem?



  9. #29
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    Default Re: CNC improvements

    A single 425 oz-in for X sounds reasonable for this app. I don't know what voltage supply and drives you have; there are 2 versions of the 425; one is 3.8mH and the other 6.8mH.
    - The 6.8 has poor torque at higher speed, but good torque at lower speed and amps.
    - The 3.8 has more high speed torque, but requires more amps for the same low speed torque.
    If you have a lower voltage supply and plenty of amps, a better choice would be the 400 oz-in, 1.8mH stepper: US SHIP NEMA 23 Stepper Motor 2 83nm 4A 8 Wire 6 35mm Dual Shaft CNC Mill Lathe | eBay With enough amps, it will have much better torque at high RPM.

    The Y motor location seems fine to me -- looks like it is no further back than the rear carriage bearing, so you won't lose any travel.

    David Malicky


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    Default Re: CNC improvements

    After going through some pretty cheap steppers, I can say this is one area that leaves room for improvement with a little extra outlay in investment. Check out the steppers on the GeckoDrive website, made by Lin Engineering, and are high quality with better finish, wire and insulation and better bearings. Check eBay for Parker, Kollmorgen, Portescap, Vexta, Sanyo Denki, even Leadshine - all superior to some of the stuff out there now. Even brand new, Sanyo Denki and Vexta motors aren't that much more.



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    Default Re: CNC improvements

    So after a while i have thought of a different design for the momus...I want to buy cnc rails supported 1200 mm or maybe 1800 and also move tha gantry with one leadscrew ...i dont know if this design is better and that is something that maybe you can help me with..I want it to be better in accuracy and more ridgit ....please tell me your opinion on thisi ope you can understand what i want to do by looking at the pictures...tha plan is not yet completed but you can get an idea..Does this change woth the trouble making it >?

    Attached Thumbnails Attached Thumbnails CNC improvements-momus-1-jpg   CNC improvements-momus-cnc-kainourio-2-jpg  


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    Default Re: CNC improvements

    Quote Originally Posted by pikolo View Post
    So after a while i have thought of a different design for the momus...I want to buy cnc rails supported 1200 mm or maybe 1800 and also move tha gantry with one leadscrew ...i dont know if this design is better and that is something that maybe you can help me with..I want it to be better in accuracy and more ridgit ....please tell me your opinion on thisi ope you can understand what i want to do by looking at the pictures...tha plan is not yet completed but you can get an idea..Does this change woth the trouble making it >?
    At this point it is no longer a Momus design... The Momus gets its rigidity from the side walls also being torsion boxes, and you lose the benefit of that by going with the gantry legs as drawn. Especially if your intent is making guitar parts. Also those linear bearings are probably not as good at preventing the gantry from racking, or rotating... I think it's better to utilize the portal design of the Momus but then drive two leadscrews via timing belt and pulleys.



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    Default Re: CNC improvements

    xm....i see what you mean...my frame in the momus i have is made of iron tubes welded together (by the way)...The problem i have with the cold rold steel is that i cant find any really straight and in the dimensions i want///i want to make the cutting area around 60 cm and now it is 43 so i need 80 cm cold rolled steel.The only i found are 45 mm wide and 6 mm thick.I have tried to measure their straightness with a thread from side to side and they have a bend of 0.3-0.35 mm over the 80 cm distance...since then i am thinking all the time how to make them more straight but dont have access to other machinery.(any thoughts?) the other problem i have is that if i buy a lead screw i will have to end machine the ends so as to fit the bearings...also i will have to buy them from england maybe cause in Greece i couldnt find any accurate enough ...ball screw is still an option on the other hand they are preatty cheap from china...(also some say antibackshlash and some not...difference?matter so much?)



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    Default Re: CNC improvements

    So i have ordered ballscrews and now am thinking about the rails....shouls i just buy these CNC Set 20x 800mm 4x Linearwagen Linearführung Linear Guide Rail 3D Fräse | eBay and place them where the momus cnc had its rails or just find a way to make the cold rolled steel straight >? at this time they have a 0.35 mm bow in the 800 mm distance (tried it with a string tensioned and feeler gauges go under it )



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    Default Re: CNC improvements

    Quote Originally Posted by pikolo View Post
    ... My mommus cnc currently has an accuracy of + - 0.2mm. I want to use it for luthiery work and this accuracy is not very good! In advance i want i to cut a bigger surface so as to curve neck for guitars. ...
    Assuming your medium of choice will be wood, is it realistic to expect better accuracy than +/- 0.2mm in your parts? Changes in humidity can easily distort wood that much or more.

    No signature I would write will fit on only two lines.


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    Quote Originally Posted by the_tool_man View Post
    Assuming your medium of choice will be wood, is it realistic to expect better accuracy than +/- 0.2mm in your parts? Changes in humidity can easily distort wood that much or more.
    hi and thanks for your reply!i am thinking of making other tools with this cnc so i might mill alluminium even steel...so the rails will be a good choise?



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    Quote Originally Posted by pikolo View Post
    hi and thanks for your reply!i am thinking of making other tools with this cnc so i might mill alluminium even steel...so the rails will be a good choise?
    For aluminum maybe serviceable. For steel, maybe not



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    Default Re: CNC improvements

    So the ballscrews finally arrived and i am ready to assemble the machine...i was wondering if there is a n option to add a 4th stepper (so two steppers on the x axis) and not use a timing belt to drive both....Which is the best option? I have heard that moving the gantry with two motors is not that good and the dont co operate well ,But i have seen professional machines done that way....any thoughts?



  19. #39
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    Default Re: CNC improvements

    Yes, many machines work that way. In Mach3, there are functions for "slaving" one motor to another, and for squaring the axis if they get out of synch. You need separate drives for each motor, though.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: CNC improvements

    yes i will have to spend 130 euros more for a power supply stepper and driver but i will have more work if i choose the timing belt option and i dont know which way will be better in the end...i dont want to have to square the gantry or destroy parts :P



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