...
Ahhh, and the software is working flawlessly in my mine!
OK, now down to the nuts and bolts... I’ve got a few questions for you.
1. I believe there are 6 edges in your drawing. Will all 6 be able to elongate/shrink in length?
2. If so, then how are the legs mounted to the base?
3. How is the table mounted to the legs?
4. Could you put a horizon line in your drawings and use some color scheme to differentiate between moving and non-moving parts?
5. The original design you shared had the rotation about the z-axis as being an integral, however with this design, I’m not seeing it. Can you explain?
--bb99
There are 10 types of people in this world; those that understand binary and those that don't.
...
Last edited by jimbo; 01-08-2004 at 05:03 PM.
Thanks for the links! Lot's of stuff to go through.
--bb99
There are 10 types of people in this world; those that understand binary and those that don't.
:-) Exactly.Ahhh, and the software is working flawlessly in my mine!
Before I build anything, I've found that it's always a good idea to create a dynamic model in my mind. Once I *understand* how it's supposed to work, and how the parts all interact, I can draw it, and then build it. It's just the way I design things...
On a related note, I'm guessing you have a basic framework for the software, already...it might not be formalized, but it's something you're working out in your mind.
There are 3 ways to do it...OK, now down to the nuts and bolts... I’ve got a few questions for you.
1. I believe there are 6 edges in your drawing. Will all 6 be able to elongate/shrink in length?
First, keep the base triangle a fixed size. Make the upright legs of variable length, like one the hexabot platform a few messages back.
Second, keep the upright legs a fixed length, and vary the geometry of the base.
Third, make everything variable.
I think, for the purposes of my thought experiment, I'll focus on the first two. And, the problem of keeping the router orientation fixed is easier to solve on the variable base/fixed uprights version of the design.
I'm calling it the router end -- it is just the articulated "tip" of the machine. Conceivably, you could also mount a workpiece on it...and move it under a fixed spindle.
Have you seen the "hexaglide" design? It's an ingenious approach that uses fixed length struts, and a variable geometry "base" of sorts.
http://www.isw-uni-stuttgart.de/pers...e_primod1.html
I think it looks promising, even if it looks like a bit of a kludge. Very home-buildable, and a reasonable model for a 3-axis machine. I think the *6* drive screws are "in the way" in this particular implementation...3 screws won't be, though. And, it could be further modified by mounting the screws horizontally, such that the base of the tetrahedron would move in and out, rather than up and down.
It shows one of countless variations on the construction, though...and a particularly simple one, at that. The picture below is of this type of unit...they call it Modell, and it's based on the hexaglide designs.
These are some of those details that have to be ironed out, in physical models.2. If so, then how are the legs mounted to the base?
3. How is the table mounted to the legs?
My initial thoughts are twofold, depending on which design works better.
In design 1, the base would be fixed, and the upright legs would be of variable length. They would be fixed to the base with a ball joint, or possibly a u-joint...same difference, in practical terms.
In design 2, the base would be variable, and the uprights would be of fixed length. They, too, would be connected to the framework with ball joints.
In both models, I see the machine "suspended" over a fixed workpiece, and holding a router.
Once I start doing my own drawings, I'd be delighted to do that. The diagrams, above, are just "clip art" off the web. I think the irregular tetrahedra were from the Math Forum.4. Could you put a horizon line in your drawings and use some color scheme to differentiate between moving and non-moving parts?
Sure, I posted a rambling message last night, about a different idea I'd had...a simplification of the hexapod concept. By basing a machine on a tetrahedron, rather than an octahedron, we could create a 3 DOF machine with a space frame construction (which means averaged errors, rather than cumulative errors like on a traditional machine)...and do it with only 3 motors, 3 controllers, and 3 actuators...half the complexity, and half the cost of a hexabot. A hexapod is a 5 or 6 DOF machine (depending on whether it's a fixed base or a rotobot) and its capabilities would likely never be used by the home hobbiest.5. The original design you shared had the rotation about the z-axis as being an integral, however with this design, I’m not seeing it. Can you explain?
It's just a different, but related idea that occurred to me, and now I'm trying to figure out the dynamics. A home-buildable, simplified, "tetrabot." At least, that's what I'm calling it, for the time being.
-- Chuck Knight
This is something closer to what I have envisioned...but this one uses 2 tetrahedra to allow the other degrees of freedom. Just imagine it, but with only 3 legs hanging down, and the router head held rigidly vertical.
In fact, this might be a good design on which to base it, as it could add other degrees of freedom by adding another tetrahedral module on top.
BTW, this is a commercial unit designed by Turin, and marketed by Motorola.
-- Chuck Knight
Chuck,
Just a thought. Do you think that the "tetrabot" will be as rigid as a hexapod? Also how about using the tetrabot with a verticle rotating work surface for doing 3d? I have been also looking at the different designs and believe the fixed leg length with variable base would be easier and more rigid for the home builder like myself. I'm also thinking it might require less powerful motors to run it. Also wouldn't any backlash in the control threads running the variable base have less effect on the spindle than if the threads were directly in the legs to vary leg length?
You lose me on the parrallel mechanism to keep the spindle straight. Can you please clarify.
I have downloaded the EMC Manuel and am reading thru it. It does appear to have a part about 6 axis control.
I am a little slow to pick some of this up sometimes, but then all of a sudden it will dawn on me so please hang in there with my questions.
Ron
There are only a few basic platonic solids...the tetrahedron, with 4 faces; the hexahedron (cube) with 6 faces; the octahedron with 8 faces; the dodecahedron with 12 faces; and the icosahedron with 15 faces.
The tetrahedron and the octahedron are the two lowest order forms, which exhibit inherent stability.
Now, in English, that means that all their faces are triangulated...triangles make things stiff. A cube is a platonic solid...but none of the sides are triangulated, so it's not inherently stable.
The tetrabot would be at least as rigid as the hexabot, and possibly more rigid, if built to the same tolerances. The higher the number of "edges," the less severe the angle between the faces...less severe angles can result in play in the system.
This is an issue with geodesic domes, which when they get bigger and more complicated (more edges) allows them to sometimes "dimple" inwards. There's just not enough difference between adjacent faces, to keep them apart. The tetrahedron hs the most severe angle between the faces (dihedral angle) and is the strongest and most rigid of the potential structures.
As for the parallelogram linkage, it's a standard engineering solution. Ever seen a "Luxo" lamp? It uses 2 parallelograms to make sure the lamp "head" remains in the same position, regardless of how the arm is moved. Same idea...I want to keep the router in the same position (vertical) regardless of how the arm is moved.
I'll draw up some pictures, etc, and post them, when I get some time.
-- Chuck Knight
Guys I am going to state my ignorance righ here right now, but for the life of me I can't see a real world need for something like the hexapod when it looks to me like anything you can machine with a pod cal already be machined with a 5 axis router that has plenty of software already available. Can someone please explain to me in layman's terms what the "real" advantage of a pod is over an good 5 or 6 axis std machine. Now on the other hand if this quest is because " I want to do it" I fully understand and wholeheartedly cheer you on. I just need a little help understanding the why!
Mike
No greater love can a man have than this, that he give his life for a friend.
Here's. what they say at http://www.hexapods.net/hexapod.htm
Cost might be an advantage in the right application. I heard the number $80,000 being quoted for a hexapod to sit under a brideport type mill. Compare this to a comparable HAAS 4 or 5 axis mill and it may look attractive in the right circumstance.
I did some playing around trying to see how a person would make a G code translator and noticed something interesting. A 1" move in X, relates to much smaller moves in a hexapod. All 6 axis must move, but each one in a smaller amount than the 1" X move.
It seems to me that this would equate to a faster overall run time.
The hexapod is a very rigid platform as Chuck has mentioned throughout his postings. It might not matter when cutting wood, but in the brain surgery application shown on the hexapod website, it might be vital.
I would say that for 99% of existing applications, a hexpod is overkill, but its very cool and if more developers work on the design, overall costs will drop, applications be discovered, and it may become a mainstream mechanism for CNC work.
Chuck,
Wouldn't a hex with 3 movable axis (arms) in a triangle make just regular 3 axis motion (x, y, z)? If so then it's way more complex than a conventional 3 axis machine but offers no advantage. The 6 legged one creates the 4th and 5th axis and is the reason hex's exist in the first place....no?
Eric
I wish it wouldn't crash.
but it's so elegant!
Well, of course it offers some advantages.
To start with, it's inherently stiff. We go to such pains to ensure rigidity in the gantry or moving table, on traditional machines...then we have to increase the weight, which increases the cost of the bearings and linear slides, which in turn requires us to increase the size of the machine...there has to be a better way? This might be it. After all, there are NO linear slides or bearings, on this type of machine...just ball joints.
A major advantage is that, with a parallel system, error is averaged out, rather than added together like in a tradtional machine. What that means, in practical terms, is that hardware store allthread could deliver the same accuracy level as a good quality ACME, and ACME could deliver accuracy rivalling ballscrews.
A tetrahedron based machine should be of similar cost and complexity to a "traditional gantry machine," since it uses the same number of actuators...hexabots use double the number. It might even be cheaper, since there is less "structure" involved, for larger volumes, and the parts can be of lower quality. Being "forgiving" is a major advantage for a homebuilt machine.
A possible advantage is a much greater working volume, without a tremendous increase in weight and cost, or a loss of rigidity. See above. Take a look at the Z-axis heights of the hexapod and othe rmachines that I linked to...it's HUGE!
It would also provide an "entry level" model into parallel kinematics...possibly even upgradable by adding a second tetrahedral module on top, like in one of my more recent pictures. That machine could have provided 5DOF, with ease.
And, of course, it's just so darned cool! :-) Hey, the geek factor is always a major player, in my world!
-- Chuck Knight
Last edited by chuckknigh; 01-07-2004 at 06:58 PM.
Well, it is cool.
I am not so sure it's more rigid. It also has a MUCH larger footprint for the amount of cutting area it has.
Don't let me discourage you. It's interesting to be sure!
Eric
I wish it wouldn't crash.
Anyone got any ideas for how to turn 2 screws with 1 motor? I know I can do with gears, belts, etc...but these two screws would be changing angles relative to one another...possibly distances from one another.
I could do it with a "flex shaft" type coupling, but that wouldn't be a particularly good solution...lots of backlash, I would imagine.
BTW, I think I've solved two problems tonight...
First is that I figured out how to hold the router, rigidly vertical, while not compromising the rigidity of the tetrahedron.
Second, I figured out how to provide for upgradability to 5DOF...the 3DOF variation is just a "special case" of the 5DOF machine. It should make a good "Kleinbauer-approach" entry level design.
Of course, in the process of solving these two problems, I've created about 10 more. Engineering can be fun!
-- Chuck Knight
Chuckknigh,
Two shafts one motor. How about using splined shaft into a socket for the change in distance with a universal joint to the splined shaft to take care of the angle change. Use belts or gears to run both shafts from motor. If you could engineer it right maybe the splined shaft could run through a bearing it could slide in and out of while rotating and the splined shaft would be the gear for the shaft running against the motor gear. Similiar to the way the drive shaft and velocity joint goes into the transmission on front wheel drive cars. Don't know if this makes sense to you if not let me know and I will try to draw a crude diagram. Just a thought Ron
That's a very good idea...and one I may have to borrow. If I understand it right, make a flexible joint that will telescope in and out, but which is held "together" by use of a spline. Makes sense.
Thank you. I have about a dozen possible ways to do it...all have tradeoffs, of course, including this one.
The ideal would be if I could turn the drive nuts, rather than the drive screws...that way they'd stay in basically the same place, but change only in angle. That, I could accomplish with any number of flexible joints...next time I'm in the hardware store, I'll have to play in the connectors aisle.
As to basic overall design, I think I'm going to start with the arrangement used by Turin...stacked tetrahedra. That should provide both ease of construction, and easy upgade to 5DOF at a later date. It's a good starting point.
As to the limited working volume that Eric mentioned, I think the limitation comes, not from the size of the machine, but the limited range of travel of the big linear actuators. I have an idea of how to address that limitation, on the cheap.
-- Chuck Knight
Wouldn't you have to have one Rod LH threaded and the other RH threaded to make the gear thing work with one motor and two Screws??? Not a problem with belts but gears counter rotate.
Nathan
Sounds like you could use something like a turnbuckle. RH and LH threads (have to work out the ball-joint ends), but you'd spin the body of the turnbuckle and get the in and out motion. The ball joints though - that might build in backlash.
Example: As you spin the body of a turnbuckle, the ends rotate too until they hit their stops and then continue in (or out) as the main body is turned. You could probably "capture" the movement like the heim joints on a suspension system though.
Just thinking this out as I'm typing here - not intentionlly mudding the effort. Seems like a rapid prototype could be built up to prove concept using timber and some turnbuckles - and no motors to check your movement theories for tetra, quadra, et all versions.
Jim
A good idea, but no. These two screw drives would not be in line, like a turnbuckle.Sounds like you could use something like a turnbuckle. RH and LH threads (have to work out the ball-joint ends), but you'd spin the body of the turnbuckle and get the in and out motion.
Here, take a look at this basic geometry -- two stacked tetrahedra. This image shows a 5DOF machine (used for LASER inteferometry...that tells you how accurate it is), and if you think about it, moving the upper and lower corresponding struts in unison, would keep the tool vertical. 3DOF is just a "special case" within this 5DOF machine.
Imagine if the top and bottom struts were just threaded rods, joined at the center with a small triangular plate...making 2 complete tetrahedra, sharing a "base." Now, just let the linear actuators be nothing more than threaded rods, and the nuts could be mounted at the side supports. We'd have to build in a little bit of tilt and swivel at the 3 corners, but that should give a tremendous amount of linear motion for each strut, at minimal cost.
Down side is simple...the "unused" portion of the threaded rod will "stick out" the side of the machine. But, it should allow *all* of the interior volume of the machine to be accessed, if it works the way I think it's going to work.
Read some of my recent messages -- stream of consciousness is the polite term -- rambling nonsense is more accurate.Just thinking this out as I'm typing here - not intentionlly mudding the effort.
Which is why I'm going to the hardware store, tonight, to get some small allthread. :-) Thinking about it is a very good idea -- playing with the geometry is a better idea! This isn't exact intuitive, to me.Seems like a rapid prototype could be built up to prove concept using timber and some turnbuckles - and no motors to check your movement theories for tetra, quadra, et all versions.
-- Chuck Knight
chuckknigh,
Just a thought, have you considered using rack and pinion instead of allthread and nuts? That way the pinion on the two same side shafts could be driven together off of one motor using some type of belt or gear drive with the splined unversal joint idea. I can see it in my head but man it's tough to get the idea on the computer screen in words. Ron