On Saturday 06 October 2018 08:46:35 Les Newell wrote: > > No, I plan to support 50 mm bars every 600 mm more or less. I'm > > attaching some pictures of the design I'm working on > > If you really want to go with this I would second Chris' suggestion of > using HDPE or UHMW. You are still gonna wear those shafts out pretty > quick unless you use chromed shafts. Hydraulic rod is hard chromed and > accurate diameter. I don't know if you can get 50mm hydraulic rod in > those lengths but if you can it would still be pretty costly. The > bushes will be service items so make plenty of spares! > > > I also thought about > > reducing the 3000 max RPM with the worm and gear to 100 RPM on the > > shaft > > Don't use a worm or planetary gearbox. They have backlash (especially > worm boxes) and don't like repeated drive reversal. Harmonic drives > work well but don't have enough advantages to offset the cost in this > application. > You are looking for an overall 10:1 ratio so that can be done by two > belt reductions. Say 3.2:1 from the motor to the shaft then another > 3.2:1 from the shaft to the pinions. Cheap, simple and reliable. > > > I uploaded the pictures because the list doesn't allow me to attach > > them. Here's the link: https://imgur.com/a/7kLUWsq > > I'd like to see some triangulation in that frame. Try to make sure > the screw/rack/whatever is well below the top of the table. You want > to be able to easily slide sheets on and off the table without fear of > damaging anything. > > In my opinion screws are out unless you use high pitch screws, say at > least 25mm pitch, 30+ mm diameter. Even with rotating nuts there are > limits to how fast you can spin the nut before the balls start jamming > in their guides. > I have used endless belts with a fixed motor, as Roland suggested, on > a couple of machines. One was a 2.5m x 1.5m table machine and I had > problems with the long belts flapping around, especially when changing > direction. This was a plasma cutter so there wasn't much load. The > other machine was a 3.5m long feeder which only needed to push > accurately in one direction before retracting. I didn't have any > problems with the belt flapping but stretch under load was a > significant issue. On the plus side, that machine has been running > every day for 10+ years with no noticeable wear in the drive system. > Using a belt that is fixed at both ends and looped over the pulley is > more than twice as rigid as an endless belt and it won't suffer from > the flapping issues. For a machine your size I'd use two 30mm wide > belts on X with lots of tension. I really like the servo belt idea and > am thinking of using it on a machine I am planning on building. > Keeping dirt out could be an issue. > I have serviced a lot of commercial CNC routers and generally they > either use high pitch screws or rack. > > Todd suggested a minimum of 5KW for the spindle. It does depend a lot > on what you are doing. You can do a lot with a 1/2" cutter which will > work fine on a 3KW spindle. If you want to go bigger you will need > more power. Don't forget about extraction. You'll be generating a LOT > of dust. Making a good extraction hood isn't nearly as easy as you > would first think. If you need multiple tools for each job you might > want to allow for a tool changer. If you can't have a tool changer at > least have a tool setter. Tool setters are easy enough to make and > save a lot of time. Just make sure it is well protected so you can't > damage it when loading/unloading sheets. > > > If indeed its that critical, one would need a dynamic distance > > detection method of some sort riding the work pretty close to the > > tool... > > Whoa Gene, before you get too esoteric the solution is actually really > cheap and simple. Make the top of the table out of something that is > easy to machine, say MDF or plastic. After building and leveling the > machine skim the whole table using the router. The distance from Z to > the table will be pretty darn accurate over the whole area. As long as > the machine is built reasonably accurately the table will be pretty > close to flat. Remember this is a wood router, probably cutting large > sheets. If the machine has a slight bow the sheets will bend enough to > sit flat on the table, especially if it is a vacuum table. > > This brings up another point. How do you intend to hold the work down? > The simplest solution is to have a MDF bed and screw your work down > but that can get old fast if you are working with big sheets, > especially if you are cutting lots of different parts. You only need > one miss placed screw to wreck a cutter and probably the job as well. > Vacuum works well. There are two options. The first is a pod system. > You have a number of vacuum pods like these <http://vacuumpods.com/>. > They use a high vacuum and hold well but are best suited to production > work on pre-cut blanks where you can set up and run hundreds or > thousands of parts. A matrix bed pulls a vacuum through the whole bed. > They rely on high flow and a lower vacuum. They are best for sheet > work, especially if you run lots of different parts. You need to make > sure unused areas of the bed are sealed off. > > For that size machine you are looking at a vac pump rated at 10kw at > the very minimum for a matrix bed, maybe 5-7kw for pods. My 8'x4' Rye > router with a matrix bed has 2x 4kw vane pumps and they are barely > enough. Another 8'x4' machine I use occasionally has 2x 3kw side > channel blowers and they are also only just enough. I have been > investigating using multiple vacuum cleaner motors (say 5 or 6 of > them). They are relatively efficient and should generate enough vacuum > for a matrix bed. They are much cheaper than proper vacuum pumps > though in this application they would need to be replaced at fairly > regular intervals. > > Les > Well, I did in the back of my mind figure it was a solved problem, even if the maintenance was higher, like for replacing the mdf bed when it got dinged up or even wet from spilled coffee. That last comes under the category known as shit happens. :( And each of us has their own problem solving tools behind the eyeballs.
Carving wood as you are also simplifies the problem with the reduced force needed to make the tooling move. But its been my experience that if mdf is used, and supported only at the edges, it takes several layers of 3/4" to be rigid enough over time to be useful as a level work surface. So the lifetime and replacement costs need to be added into your costs to produce the work even if its only 10% of the costs for resharpening or replacing the tooling as it dulls. I've got one table in the garage, one of those 2x4' 4 shelf assemblies that sells for about $30. The top table is 3 of the 4 supplied 1/2" mdf shelf panels, and in 5 years has still sagged at least a 1/2" low in the center, and the 4th shelf 9" above the floor, serving as a catchall, clamps, quarts of finish, pieces for jigs, etc, is probably 2" deep in the center. Probably shoulda had some 2x4's on edge glued to the bottom, but hindsight is 20-5. ;-) I'm not even sure it should be called mdf as the chip content is 5 or so times bigger than what you get if you go buy a sheet. Finer than OSB though. I won't even hid a piece of mdf inside a furniture piece because of its long term lack of stability. Ikea furniture does, but its generally tossed on the burn pile at about 2 yo, or put out as a yard sale item with a 50 cent price tag by then. That table has got to go, and a newer one made for use as an assembly table as I have everything but the lids already cut out for 3 more of those blanket chests so my boys will each get one before I miss morning roll call for good. I think there is some pix of it on my web page. Thanks Les for injecting reality. ;-) I keep our local radio station on the air, and a repair I did to his low power nighttime transmitter has the owner worried, because I'm the only one around that knows the toroid transformer I put in to replace a fried E-core transformer isn't a cobble job, but an actual improvement, doing the same job at an operating temp about 75F cooler than the E-core. And I know where there are 20 more of them, bought to make a bank of 1kw PA amps. A retired ex Navy, ex employee of mine that in his younger years, put on a show on his 40 acres out in the middle of nowhere WV, as a charity thing, getting such groups as The Greatfull Dead to come in and do several sets over the weekend, called River Rocks, since he has a river about 25 feet from his house. But that stopped after all the DeadHeads that follow them and bring in grass by the 20 kilo bag all got stopped and arrested for drugs as they were leaving. Theres no labyrinth of roads in and out of the place so there was no escape. So the local charity's are doing without the profits for what had been an annual event for about 20 years, making 20k-50k$ every year he did it. In my now 84 years, I've smoked maybe a dozen joints but one pull on the local hay was the end of that, 35+ years back. And I quit smoking totally 30 years ago so I'd be alive when the woman I was going to marry, needed me in our dotage. Nobody ever got hurt bad enough to make a trip to the ER. They all had fun, and raised money for local charities. So some hay got smoked, what the hell was wrong with that? Sigh... -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
