Moving Table vs Moving Gantry; Dremel vs Router


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Thread: Moving Table vs Moving Gantry; Dremel vs Router

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    Gold Member chuckknigh's Avatar
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    Default Moving Table vs Moving Gantry; Dremel vs Router

    For a homebuilt unit, which is a more robust design? I can see benefits to both approaches...

    My understanding of them is based, primarily, on photos of other people's machines, and of Cranky's 7th Sojourn and Routezilla designs.

    Both look well engineered, and both would be excellent designs to emulate. (OK...copy) But which, for someone starting with a "blank sheet of paper" is the better design to emulate?

    I *like* Routezilla better -- something about it appeals to me. But, my concern is the potential for the gantry to rack while the machine is under load.

    Is this a real concern, or is it more of a theoretical thing? I'd be using twin rails -- one on each side with fully supported homemade linear bearings -- and a screw drive mounted in the center.

    THANKS!

    -- Chuck Knight

    P.S. For the cutter head, would it be better to go with a Dremel/RotoZip, or a small router?

    I already have both a Dremel and a trim router, so it's not a matter of using "what I already have."

    And then there's this router:
    http://www.harborfreight.com/cpi/cta...emnumber=44914

    I've seen it on sale for $20 from time to time...a truly exceptional price. Something cheap that I could dedicate to this project, next time it goes on sale.

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    Registered anoel's Avatar
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    Default hmmm

    I built the 7th Sojourn and liked it well. The table was a bit small for my specific needs. With a little help from John I just finished up the mechanicals for a Routezilla sized machine. It's more of a hybrid of the Routezilla and Pheonix as I migrated the X and Y axis from the 7th to a moving gantry style machine. So far so good. I've not cut anything with it yet because it's going to need some faster speeds to cope with the large size of it. It's got a 28"x16" cutting area. I'm moving to 4v steppers and a 24v power supply in the next few days, which should prove to be much more powerful than what I've been using. I hope to get slightly better than 25" per minute with it. I cut all of the plastic parts and the gantry uprights with the 7th Sojourn and alignment was extremely good on this one. I can turn the lead screws very easily with my fingers and move any of the axis with less effort than I could on my all hand cut 7th. I'm pretty stoked. I can't wait to get my power supply. Where's that damn UPS man?

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    Registered HomeCNC's Avatar
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    I never liked the moving table because the screw under the table is exposed at times, and it requires a larger floor/table space.

    Thanks

    Jeff Davis (HomeCNC)
    http://www.homecnc.info


    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Gold Member chuckknigh's Avatar
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    Why does it need higher speeds? Is it just a matter of practicality vs patience, or is there an actual reason to have the higher speeds?

    To start out with, I'm thinking of getting "whatever" steppers I can find, adapting them through pulleys/gears to the torque I actually need, and then upgrading as better parts come available.

    By driving it from a smallish shaft to a larger pulley, I should be able to increase the effective torque and resolution of "whatever" I can find. And, it doesn't seem to be too hard. It might make the machine painfully slow, but it should work...and suffice till better parts can be had.

    I'm going to use that braided air hose coupling trick that was mentioned on the board -- good tips on here, and on the RC forum. I've already mocked up one of my screw drives, and it works just fine. Support bearing on each end, air hose coupler design, etc. Works great, even if it is only 6" long at this point. Like I said -- it' a mockup.

    I have a computer flea market event coming up next weekend -- it's obvious what types of things I'll be hunting for. There's usually at least 1 guy down there with steppers and otical components...and another with broken LASER printers and Xerox machines. Both should prove to be good sources for steppers.

    -- Chuck Knight



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    Default just because....

    Is'nt "Just because" the excuse our parents alwas gave us?

    The need for speed is the fact that I need to route soft and hard wood. Too slow Feed rates cause the wood to get burn marks. 6 inches per minute on a 28 inch long axis would be dreadful, think about it. It'd take over 4 minutes to go from one end to the other. 12 inches per minute is almost as bad. I'll accept full length travel in a minute though, anything less would be like watching paint dry.



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    Gold Member chuckknigh's Avatar
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    :-)

    Anyway, seriously, I always wondered what the big deal was about speed. While there is a practicality aspect (1/2" per minute would be painful!), almost any speed is still workable.

    In my case it's the precision, and not the speed that I'm interested in.

    Having said that, though, I'll be overjoyed if I find some 450 oz-in steppers at that sale this weekend. :-)

    Any particular devices I should look for, for scavenging big steppers? Big printers, old IBM typewriters, LASER printers, mainframe tape drives, copier machines. Anything else that would be a good choice? Last month I turned down an old IBM mainframe for $5. I'd have taken it for eBay, had I had any way to arrange for shipping. I'm telling you, *ANYTHING* technological can (and does) show up at this sale.

    -- Chuck Knight



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    Anoel,
    Your machine looks very good! You did a nice job..

    Did you use 1/4-20 threaded rod or something different?
    You might be able to get a little more speed out of 1/2-13 threaded rod.

    Last edited by WOODKNACK; 05-01-2003 at 01:16 AM.
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    Default 5/16-18

    I used 5/16" - 18Tpi threaded rod. It was the easiset to deal with without finding someone to turn the ends down. I did need to grind one end down to 1/4" to fit the motor coupling, but that was easy because I just needed to grind it down just till the threads were gone. Ginding down a 1/2" rod to a centered 5/16"/22mm to fit the thrust bearing would have been a bit more risky and if done wrong would cause binding and vibration. If everything works well I might jump in again and try 1/2" and have someone turn it on a lathe.



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    I was thinking about turning some 1/2 rods down. Lucky for me I have acces to a lathe. What are you using for a board? John's board? I can't wait to hook my new xylotex board to a machine. I have to get the lead out of my a_ _ and get working on it. With spring here there is much to do around the house.. Sucky...

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    Yeah, I'm using John's board. (or a 3 axis clone of it at least.)



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    Well keep up the good work and keep us posted. Would like to see something you cut on that machine. Again nice looking machine....

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    Anoel,
    What size is the foot print of your machine? How much room does it take up?

    HEY CHUCK,
    I looked at that router at HF. Looks like an easy one to adapt to a machine with that square base on it. I wonder if you can get different collets for it. Like say a 1/8 collet so you could use dremmel tool bits.. You could buy a route speed control at say Wood Workers warehouse to control the speed of the router!!

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    That HF router is not good as a CNC Spindle. The Spindle runout is pretty bad and the collet design is such that you'll not find a 1/8" collet for it. I bought the Homier equivalent, same router just a different color. It'd be fine to use for a hand held router but not for the CNC Machine. Step up to a name brand laminate trimmer that you know will give you good service and that you know you'll be able to get a 1/8" collet. I've been very happy with my Dremel Advantage and it's not very expensive and it's got a continuously variable speed contol built in.

    Nathan


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    I have heard that HF router takes an odd ball collet. Also lots of endplay in the collet.

    Gantries are smaller for the same cutting area.

    Eric

    I wish it wouldn't crash.


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    Member coherent's Avatar
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    my 4 cents worth...

    Speed is an issue with larger machines. If you're getting 5-6 ipm with a homebuilt system with small steppers on a 12" axis, you may not mind. Put the same drive system on a 48" axis, and you're talking 8 min, just to move from one end of the table to the other in a straight line. Multiply that multifold for a design or lettering and it'll take forever to cut. You may say you wouldn't mind, but I guarantee you (ask anyone with a machince) that you will after a little while.
    Moving gantry vs moving table has many arguments and both designs will suit a home build machine. The only real argument (other than the machines footprint if that's an issue) it that with a moving gantry, a piece of material can be in theory any length, and you could for example cut a sign 10 foot long sign by simply sliding it through the table and cutting in steps. A heavy piece doesn't have to be limited by the capability of a moving table either. Also, material clamping or a vacuum hold down system can be a simpler process with a stationary table, although simple t-nuts work well with either.
    A previous post regarding speed in realtion to your cutter is an important one. If you can't control your cutting speed to some degree, than you need to be able to control your spindle speed to avoid burning or melting the material depending on what you are cutting.
    Pulley, gears and all that stuff are great in theory, but keep in mind that even small steppers do a very good job of turning plain old 1/4 20 tpi threaded rod. And the torque you will get is normally more than sufficient to drive a small machine. A threaded rod is an inclined plane and the simplist way to get good torque to your table or gantry. There will be a tradeoff adding more mechanicals (gears, pullys etc) in a couple of ways... first, the more parts, the more play. And it can add up considerably if it's not precision fit/parts. Second, design such components carefully or you may find the initial added drag does not result in an overall torque increase.
    Lastly, go with a small trim router or dremel, especially if you have a speed controller for it. The cheap routers at harbor freight seem to have more play side to side at the spindle bearings than your dremel or a good trim router. (I know, I bought one and tried it). Why build a tight machine that could cut to within 1/32 or less, then slap on a cheap router that after a week of use, has 1/8 inch of slop? Best bet is to make a differrent mount for each of your tools, and use what best suits the job.
    Sorry for the verbosity
    -Marc



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    If you try to gear your steppers to get more torque, remember that the faster they spin the less torque tey have. And it drops off pretty fast, as they have the most torque at very low rpm's. So you may not actually gain any torque this way.

    Gerry

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    Gold Member chuckknigh's Avatar
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    Thank you all for so much help! I can't tell you how much it's appreciated.

    I think I'm going to hold off on the final design (is there such a thing?) of my router, till I find out what parts I have to work with. Like I said earlier, there's a surplus computer equipment flea market coming up on Saturday -- I'll see if anything promising shows up out there. (I hope, I hope, I hope)

    If nothing else, I'll definitely pick up some old printers, or something similar, and rob them of their steppers.

    Once I have some materials, I'll "finalize" my design as much as I can...maybe even draw it up and submit it for review.

    As to my opinion of speed...faster is obviously better. I'd never argue that... My point was only that my intentions for this machine are to provide precision I cannot manage by hand. Slow is OK, so long as it doesn't end up burning the edges of the workpiece. (Thanks for the warning!) Like I also said above, it won't hurt my feelings if someone shows up at the flea market with 450oz-in steppers for $1 a piece. :-) I'm only half joking...

    As to complexity, I definitely don't want to add complexity. However, if I'm using an O-ring to drive the allthread rod from the stepper, there will almost definitely be a difference is the 2 diameters of the shafts. That's the "pulley" system I was referring to...a smaller shaft driving a larger shaft WILL result in a higher torque and lower speed. The precise increase will have to be calculated, once I get the parts. I hope it's a 1:1 coupling...but I know that's probably not going to happen. I'm assuming that I'm not going to use a direct coupling to the driven shaft...but it's only an assumption.

    Moving Gantry vs Moving Table. The racking I was concerned about doesn't seem to be a significant factor, based on some of the responses here. I'm delighted to hear this. Consequently, I think I'll focus my efforts on a moving gantry design, to start with.

    Harbor Freight tools. I was not aware the bearings in the HF router were of such remarkably poor quality. I think I'll just stick with my trim router or Dremel tool...my thought was to have yet another tool (can you ever have enough?) that could be dedicated to this project. The price was DEFINITELY right...I'll see if they have something reconditioned by DeWalt, instead. My DW scroll saw and planer are top notch.

    Thank you all, again. I really do appreciate it.

    -- Chuck Knight

    P.S. now...noone had any another suggestions as to what machinery to look for...big old impact printers, older LASER printers, copy machines, old IBM electric typewriters. Anything else have good sized stepper motors in it? This flea market is 100 miles from my house (in Dallas...I live in a small, country town) and I'll be going ONCE this month. My next chance would be the following month, so I want to get it right.

    P.P.S. ANOEL, your machine looks fantastic. I'm envious that you've gotten to this stage already...but in my defense I've not been on this forum for very long. :-) I hope you and Cranky don't mind if I "borrow" a few ideas from it.

    Thank you for the inspiration.



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    Hi, look for old HP LaserJet l, ll, or lll printers. They have 100 oz., 5 volt steppers in them.

    You don't want 450 oz steppers. They use too much current and the driver board will cost you a fortune. Look for 50-100 oz 5-6 volt 1-1.5 amp steppers. They run great on unipolar driver boards and surplus 12 volt PC powersupplies. With 1/4" to 3/8" or even 1/2" threaded rod you will get 8 to 18 inches a minute.

    O rings wont work as a belt drive. You need toothed (timing) belts or direct drive. I would go direct. You already get a lot of mechanical advantage in your threaded rod/leadscrew.

    Gantries are more complex than moving table designs. If you want cheap and simple and accurate and learning, keep it easy. Build a movable table. easier to calibrate too.

    K.I.S.S.

    Eric

    I wish it wouldn't crash.


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    Chuck, what kind of driver are you going to drive your steppers with? That has a lot to do with what motors you can use.

    Gerry

    Gerry

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    Chuck,

    I'd look for an old line printer ... that prints on fanfold continuous forms. They are dinosaurs now and will have not only a stepper motor but the gearing and belt to hook up to your threaded rod. Also, there will be a power supply for the stepper. Depending how cheap you can bargain down, you may be able to cannibalize the thing for good use.

    > micro-switches for your limit stops. Most have a hinged arm so that the frame of the moving table will push against it and press in the switch.
    >Then there's those little multi-conductor wire connectors to put the wiring harness together and make it easier to replace an item if need be.
    >Flexible wiring harness "loom" sleeves to protect the wiring.

    I can think of all sorts of "what-not's" to fill a shopping basket for usable parts.

    Happy shopping ....



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Moving Table vs Moving Gantry;  Dremel vs Router

Moving Table vs Moving Gantry;  Dremel vs Router

Moving Table vs Moving Gantry;  Dremel vs Router