Thanks to all who responded, That's more responses as I was hoping to get. Kirk Wallace wrote: > On Sun, 2008-07-20 at 14:35 -0700, Rafael Skodlar wrote: > >> Hi guys, >> >> I'm following this mailing list for a while but still have a few >> questions about designing (ideal :-) CNC machine for use with woodwork >> mainly. >> > > I assume that you are planning a gantry/router type of machine? I hope > to build one someday, so I haven't had any experience with this. I can > only offer my opinion. > > Opinions from experienced people are well taken. >> Here are the preliminary specs: >> >> tool motion: 120 x 90 x 20 cm (X-Y-Z) >> frame material: T-slotted aluminum >> > > Those numbers are not carved in stone of course. They depend somewhat on other (possible) components like guide rods, lead screw or whatever I end up using.
> 1.2 + meters of aluminum channel may not be the stiffest material. My > T-slot material is stronger than I expected. Steel is good for the basic frame but it's harder to work with. At least for somebody without welding or or other heavy machinery. Aluminum is the metal of my choice. > plan was to use steel and weld the joints. It may be that surplus steel > would be cheaper (though less convenient) than extruded aluminum > channel. Personally, I really enjoy welding. You may want to consider > taking a welding class, so you can complete your project and pick up an > enjoyable skill. > > You have to be good welder to do it without warping etc. I tried welding few times using my brother's welder when Reagan was a president. Well, training is not a bad idea but my boss has different plans for my training since my job has nothing to do with wood or metal work. It's strictly a hobby at this point. The easiest for me would be to trade welding with somebody for "computer work" but I don't know anybody in South Bay to do so. >> motors: 3 stepper >> drivers: 3 geckorive (1 opt.) >> > > I have a bias against steppers, especially for traversing over a meter > quickly. > > Hmm, not encouraging.However, steppers semm to be very popular even in commercial products. Price is significantly lower than DC motors as far as I can tell. >> controller: Mesa 5120 (?) >> power supply: linear 35-55V 600-800W (?) >> X-Y motion: moving arm (X-Y axis) (with ball bearings and steel rod) >> > > The ball and rod slides I have seen are usually mounted on stand-offs > which doesn't seem rigid to me. I like the rectangular rails with ball > carriages using a horizontal and vertical set. > > Steel rod mounted on the side of T-slot would be very rigid combination IMO. It would be relatively easy to mount without too much work. I was thinking of using ball bearing for the gantry move. BBs are cheap and last very long without maintenance. >> Z axis: >> optional: 4th axis for rotating parts >> table: fixed >> Software: EMC of course >> > > I think EMC is a good choice ;) > > >> Questions: what size of the stepper motors should I pickup for X-Y? I >> was thinking in a range of 500 - 800Nm for X-Y and 200 Nm for Z but am >> not sure about it. Brand names are also a question. Are >> http://www.anaheimautomation.com/hightorquesteppermotors.aspx good >> enough? Is brake mandatory for Z-axis? >> > > I hate to admit it, but I tend to look at similar machines and then > guess at motor size. I figure if I get it wrong, there is eBay. > > :-) Same here. School is too far behind me to calculate exact forces and power needed to do the work. Simplistic approach of calculating mass of moving parts, acceleration, and travel distance should bring me in the ballpark. >> What kind of a moving mechanism would be the most cost effective for >> each axis? Lead screw, >> > > Too much friction for my taste. > > And not very fast either. I was afraid of a backlash and other maintenance issues also. >> pinion gear and rack, >> > > I have seen industrial routers with rack and pinions. I would consider > this. > > Plasmacam uses this I believe. One possible trouble is dust since it needs to be well lubricated to prevent wear out. >> belt, >> > > I would think belts would flap at any length close to being considered > long. > > True but I've seen some large printers with belts. Of course, printers don't need to carry a router. >> or something else? >> > > How about hydraulic cylinders? (Half joking here, but ... ) > > or steam :-) Here in CA we will soon face power shortage, a solution from our governator. >> I haven't seen anybody using steel cable to move the arms around in CNC >> machine for example. >> > > I have wondered about steel cable. Because of the weave, I would think > cables would be stretchy. Maybe you could preload the stretch out. A > steel tape might be better, but you might still have a flapping problem. > > I believe that weaved cables would be a good choice. I don't think they stretch much if any. Maybe it would be possible to compensate that with spring loading somehow. Since I haven't seen anybody using them they might not be that good after all. Local hardware store caries such cables of different thickness. There seem to be two kinds of cables. One is bare and the other is covered with protective plastic of some kind. >> What comes to mind here is an old HP plotter model 7221 which uses a >> rather complicated but efficient and precise mechanism with steel cable, >> pulleys, and two motors in fixed position to move the pen. While the >> cable will wear out faster than a lead screw for example, it's way less >> expensive and easier to setup with no backlash IMO. Steel cable doesn't >> collect much dust either making it suitable for the job IMO. >> > > Give it a try, and let us know. > > Need to look at the plotter again to see how it's done. I might even post few pics from it to give you an idea. >> I tried to find a source for plastic cable track to protect and guide >> the cable but Google did not respond kindly. What kind of flexible >> cables are used for connecting power/data to stepper motors and encoders? >> > > The higher the number of small strands in a wire, will be more durable > under flexing. I suspect that if you have a large arc or loop between > the ends, most any common cable will work fine. Just keep an eye on it, > and at worst, you might have to replace it in a few years. Oh, and maybe > stiffen or support any areas of localized flexing. > > Good to know. >> All suggestions are welcome but note that there is a budget limit :-) >> which should be significantly or reasonably lower than a commercial >> product of the same size and capabilities. >> >> Thanks, >> > > Keep and eye on eBay and Craig's List for surplus items or something > I'm not too confident on online offerers from strangers. I just don't like to dangle my credit card out there before I can touch the thing, but I might need to do it at some point. > that can be "repurposed", or even better find a machine in your area you > can convert. Sometimes it's fun to try to make other people's trash into > something useful in unintended ways. Watch for machines that are similar > to what you want, that are known to work well. Remember, plagiarism is > the sincerest form of flattery. > > Points well taken. Thanks again to all who responded and opened my eyes in many ways. Will need few days to digest all the responses about PSUs, motor drivers, mechanisms, etc. -- Rafael ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
