yes this is a big issue rigidity and vibration damning have traditionally been done by increasing the mass and by using cast iron that has a natural tendency to absorb and dissipate vibration.
You can build very rigid light wait structures but most of these have harmonics like a tuning fork. These vibrations would not be good for your surface finish and on the other hand if you built every thing out of cast iron with a mass of several thousand kg your speed and acceleration are limited. The trick is to build a rigid machine with light moving parts that absorbs vibration. Every thing is a compromise trying to find a compromise that will accomplish what you need is where the fun is. linden On 15-11-08 10:10 AM, Kyle Kerr wrote: > Re: homing. Delta printers usually home at the top of their travel. When I > asked my CNC instructor about a delta mill he said the lack of rigidity > would kill accuracy. > > On Sat, Nov 7, 2015, 7:38 PM linden <[email protected]> wrote: > >> Thanks for taking the time to look at the pictures and read through my >> chicken scratch notes. Now I know that Linuxcnc should be able to >> control something like this and we have a potential solution for >> homing. I will keep going. I really need a real Internet connection or >> some card board to start pushing shapes around simulating to see how >> they interact with each other. One dimension is not to bad but trying to >> picture things in my head in 3 dimensions and 6 dof freedom is a bit >> much for my little brain sometimes. lol >> >> I am having second thoughts about using the lower set of trucks and >> additional links associated with them. The original idea and thought be >> hide this approach was it would give me a larger translational working >> area relative to overall tool hight and may help with rigidity. looking >> at the videos suggested earlier about rigidity and with the abb picker I >> am having second thought. I have also seen a Russian you tube model that >> has given me some ideas. once I have modled and thought a bit more I >> would like to pick your collective brains again. >> >> all the best, >> >> Linden >> >> On 15-11-07 06:30 PM, Philipp Burch wrote: >>> Hi Linden! >>> >>> On 07.11.2015 00:55, linden wrote: >>>> Hello All, >>>> >>>> I have progressed a little with the design and modeling of the machine I >>>> asked for help simulating. It has evolved a little over the last few >>>> weeks. have a look at the pdf attached for pictures Another month and I >>>> will be back to the world of fast Internet and I should be able to try >>>> some of the previous suggestions for simulation. >>> Looks nice :) >>> Since you have 6 actuators, your design will definitely be >>> overconstrained if you only want 5 degrees of freedom. But I have the >>> feeling that you could actually get 6 DOF by having the nominal angle in >>> C about 60° (or even 90°) rotated in respect to the picture you >>> attached. Since you can control the distance between all the joints on >>> the center plate to their slides, this will allow you to rotate the tool >>> plate as well. >>> Even if you don't need rotation around C, I'd suggest to implement this >>> angular offset, as it will most probably make the system more robust >>> against torsional forces. Similar to how hexapods are done. >>> >>>> The big question is: >>>> >>>> Could linuxCNC something control this? or do I have 2 many variables >>>> with no fixed base position? >>> When you get the kinematics right (which I'd expect to be about midways >>> between the complexity of a delta kins and a hexapod), this should be no >>> problem. >>> >>>> A few other questions and thoughts I would like to pick your collective >>>> brains on. >>>> >>>> How would you home something like this? >>> As with a hexapod, forward kins are complicated and may have many >>> (practically) invalid joint positions. I'd suggest to have the following >>> homing switches: >>> >>> 1. One for every pole at the top end, which gets activated when the >>> upper slide reaches the top limit. >>> >>> 2. One between every two slides on a pole which is closed when the >>> slides are closer than the nominal position (shown in your PDF) and open >>> otherwise. This must NOT be limit switches, as movement in both >>> directions over the activation point must be possible. >>> >>> Additionally, it will help to have an approximation of the distance >>> between every two slides on a pole. This could be done rather easily by >>> using a linear potentiometer. >>> >>> You could then implement the homing about as follows, using coordinated >>> movements in the XYZABC space (not on a joint basis): >>> >>> 1. When the system gets turned on, all you know is that it must be in >>> some mechanically feasible position. So start by a linear move towards >>> Z+, meaning that all slides run synchronously upwards. Stop as soon as >>> one upper slide reaches the top homing switch. >>> >>> 2. Use the feedback from the pots as an initial guess of the slide >>> distances for the kinematics. Using those, perform a move in the XY >>> plane perpendicularly away from the joint which has it's top slide >>> homed, i.e. moving approximately towards the center position. Stop as >>> soon as the second upper slide hits the top homing switch. Since this >>> movement will also adjust the distance of the slides on a pole, you may >>> also detect one or the other homing switch edge of those. Use this >>> information to correct the joint position guesses. >>> >>> 3. Do the same as in step 2, but this time with a movement perpendicular >>> to the connecting line of the two homed poles. So you should eventually >>> also reach the top homing switch of the third pole, meaning that all >>> upper slides know their positions and the tool is roughly centered. >>> >>> 4. Move all slides down by a few millimeters to get some "working range" >>> for the last step (i.e. coordinated move towards Z-). >>> >>> 5. Perform a movement to X0Y0A0B0C0, i.e. to the approximate nominal >>> position with an aligned tool base. This is possible using the >>> information form the pots. >>> >>> 6. Perform moves from C-30 to C30 (or less, depending on the accuracy of >>> your potentiometer feedbacks), so that all homing switches of the lower >>> slides see at least one edge. As soon as this happens, all joints are >> homed. >>> All this will require that the tool can rotate around a C axis (which in >>> my terminology is perpendicular to the machine table, NOT aligned to the >>> tool axis). This means that it will be required to have this angular >>> offset of the tool base plate as described above. >>> >>>> I stile haven't simulated the movement digitally yet or built a complete >>>> cardboard and drinking straw model, but i think i have finally settled >>>> on approximate geometry any questions or comments are gladly welcome. >>>> Has any one seen any thing like this before I am sure I am not the >>>> first? Is there any grate big flaw with this logic that I have missed? >>> At least from my feeling, I think that this should work with the >>> mentioned adaptation. I'm not very familiar with complex machine >>> kinematics, however. >>> >>> I find it a cool construction :) No idea if someone else already built >> that. >>> Good luck! >>> >>> Regards, >>> Philipp >>> >>> >>> >>> >> ------------------------------------------------------------------------------ >>> >>> _______________________________________________ >>> Emc-users mailing list >>> [email protected] >>> https://lists.sourceforge.net/lists/listinfo/emc-users >> >> ------------------------------------------------------------------------------ >> _______________________________________________ >> Emc-users mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/emc-users >> > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
