On Thu, Jan 1, 2015, at 04:40 PM, andy pugh wrote: > On 1 January 2015 at 02:16, poormansairforce H > <poormansairfo...@gmail.com> wrote: > > > I have been trying to join the forum but can't get past the registration > > page, says there are errors:( > > You will get better answers here anyway. > > > I have a question about setting up a bipod hot wire cutter. Simply, in CAD > > I have figured out that on my dream machine setup, at a 4' 6" cutting > > length when the wire is angled 22.5 degrees on the vertical axis the wire > > moves 3mm from its intended position due to the attach points swinging in > > to compensate for the shortening of its length. > > I don't quite get what you are saying. Is this an effect in addition > to the simple calculations in the kinematics module? > > Can you expand the "its" into the actual components being discussed? > Maybe you are looking at the effect of the pulley diameter not being > infinitesimal? > > --
I think he's saying he has two "Hektor" type cable bipods, each controlling one end of a hot-wire to cut foam. Normal Hektor kinematics assumes that the controlled point moves in a plane. In this case, there are two parallel planes, separated by the length of the cutting wire - but only when the cutting wire is perpendicular to each plane. When the cutting wire is angled, you have two possibilities: 1) the controlled points remain in the original planes, but the distance between them (and thus the length of the cutting wire) changes. This case lets you use the standard hektor kinematics, but has difficult mechanical issues with constraining the controlled points to stay in the planes, and managing the variable length cutting wire. 2) one or both controlled points are pulled out of the original planes so that the cutting wire length doesn't change. Pulling a controlled point out of its original plane invalidates the hektor kinematics. The error is small when the controlled point is far from the pulleys, but gets larger close to the pulleys. (And, when you get sufficiently close to the pulleys with a sufficient angle, the whole thing fails.) I'm pretty sure he is asking about case 2. This will require custom kinematics. In addition, case 2 is under-constrained. When the cutting wire is angled, it will pull inward on both controlled points. How much does each controlled point move inward? The corrections needed depend on how the inward movement is divided between the two bipods. Also, I have a simple mechanical question: How is the cutting wire kept tight? Cable bipods don't have any ability to resist tension perpendicular to their plane. If I was building such a beast, I'd be tempted to use cable tripods on each end. Turned sideways, so that the tension on the cutting wire keeps all six tripod wires tight. I'd add a spring in-line with the cutting wire to absorb any minor errors in the kinematics. It would still require custom kins, but the system is fully constrained. First compute the X,Y,Z position of one controlled point, then use U and V angles plus the length of the cutting wire to compute X,Y,Z for the other controlled point. Then compute the lengths of the six tripod wires by solving for the distance between each controlled point and the winch points. -- John Kasunich jmkasun...@fastmail.fm ------------------------------------------------------------------------------ Dive into the World of Parallel Programming! The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
