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Thread: The "Ultimate Tabletop Machine" for Woodworkers

  1. #16
    Thanks for the excellent discussion Gary, very informative! I'm wondering if you didn't leave out another important parameter for your 250/250/125 test - depth of cut and I suppose type of material? Seems like these would affect deflection.

    You are so right about the marketing stuff, but of course that is what marketing people do, accentuate the positive and deflect attention from the negative. And in a relatively new field with so many neophytes jumping, we are very vulnerable. Without real knowledge or experience, people can only search online for information, on manufacturer sites and forums, and everyone becomes an expert, and the misinformation spreads virally.

    If we go to the extremes it become easier to look at. Everyone would agree that a router built of mdf, v-way guides, leadscrew, laminate trimmer, running mach3 is exponentially less of a machine than a massive cast iron floor router with precision linear rails, ball screws, high speed spindle and commercial controls. The problem is the vast middle ground, where almost all of us have to dwell.

    Clearly we all have to make some serious compromises, and that's hard to do without good, accurate information, such as yours above. What do we really need in a machine? Can we even reasonably articulate that? CNC offers the promise of degrees of accuracy that most of us are not even able to measure, much less see, not to mention incredible speed. And it's hard not to want it all.

    PS on edit - I have to plead guilty on all counts. As an elderly, semi-impoverished, semi-retired woodworker who misspent his life in a quest for excellence instead of financial stability, I just don't have the funds (yet) to purchase what I want (if I knew what it is). I don't feel I have the time or electrical/computer skills to build a machine, as much fun as that would be. So I have to rely on the used market as I did for my heavy machinery, but there is hardly anything there yet in the size and quality I want. Maybe it's time to liquidate the wood stash...
    Last edited by richard newman; 01-22-2018 at 2:54 PM.

  2. #17
    I guess the question then becomes, what categories of machines make sense from a financial and capability sense? I've certainly found my Shapeoko and Nomad to be quite useful, and accurate and efficient enough for my (hobbyist) needs (ob. discl. I work for the company), but will grant that the material removal rates for both are quite limited, and not something one could choose for high speed production if one can get the project budget to cover a faster machine.

    Sonny Jeon, the current lead developer for the Grbl firmware which has helped a lot to make CNCs accessible to hobbyists is very fond of an old poem, "The Wonderful One-Hoss Shay", noting that every aspect of the machine is a tradeoff. I'd like to think the Shapeoko 3 has struck the best possible balance at the low end for the budget-conscious hobbyist, but would be interested to know of any alternatives or additional information --- I try to help maintain a list of CNCs suited for hobbyists on the /r/hobbycnc subreddit wiki page.

  3. #18
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    One more thought. I think there is room in the market for a small machine specifically designed to cut aluminum and other soft metals. Of course that would require stronger steppers and a more rigid frame. It would also require high torque at low RPMs. It would also need a way to dispense lubricant/coolant that doesn't make a huge mess. I am not currently interested in such a machine but might very well be in the future. I would expect to pay maybe $10,000 for a small machine.

  4. #19
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    Very well said Gary - the part about stepper motors I find particularly relevant and something that a lot of people don't really realise. I didnt really think too much about microstepping until I had to program a stepper driver for a client at work (because they were too stupid to let us just buy one..) The whole getting pushed into a fullstep by cutting force, or the fact that a microstep is just an approximate hover of the motor between two full steps is lost many people, A 1/16th microstep is 1/16th the torque, so very easy to lose a lot of tiny little steps over a 2-3hr carving (especially if you drive your machines like me haha).

    This is why I push for closed loop these days, preferably a servo as those are just a dc (be in brushed or brushless) motor with an encoder strapped on the back and have no real concept of steps (other than cogging and all the other fun magnetic stuff... but thats another discussion!). A 4000 count quadrature encoder is pretty cheap these days, and even a poorly tuned motor will hold within 5 encoder positions pretty reliably - an average or well tuned driver will hold to 1-2 pulses very reliably - during accel/decel you might get 10-15 pulses out for 10-20ms, under constant load you should stay within a couple of encoder pulses of the target pretty reliably - and you'll never lose a step unless you crash the machine (and if you do crash the machine it wont rip itself to bits like a stepper driven machine will, as encoder error will climb so quickly and trip the driver into error).

    It's actually really interesting to hook an encoder up to a loaded stepper motor's shaft and watch the error - its actually pretty surprising to see, i wish i had access to some of the graphs we generated.

    If i had to boil my list down to just the bare minimum of 5 items, based on 12 years of building and running hobby CNC routers:
    * Ball screws! Even terrible quality ones are so vastly, vastly superior to anything else. Really really cheap ballscrews are cheaper than leadscrews + nut, AND better!
    * Linear rails
    * VFD spindle
    * Encoded motors
    * Rigidity.

    Controllers make a big difference too, as do the type of motor driver you use. An 8 bit arduino based controller can't do motion interpretation anywhere near as good as say a 32bit ARM based SmoothieBoard, which in turn can't beat something running on an FPGA (ie: ESS/MESA/Industrial controllers)... but again thats another discussion. Likewise, on my first routers upgrading from the well regarded Gecko G540 to leadshine digital drives was a MASSIVE improvement in heat, noise reduction and motion smoothness, and single chip drives can't compare to a full digital driver, especially the cheap allegro ones.

    If you were to offer a cleaned up and tuned up version of chinese CNC router frames, I'd bet you could do a good bit of business assuming you chose the right frames... but at that point why not just get the frames machined from your own drawings?

  5. #20
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    Quote Originally Posted by Art Mann View Post
    One more thought. I think there is room in the market for a small machine specifically designed to cut aluminum and other soft metals. Of course that would require stronger steppers and a more rigid frame. It would also require high torque at low RPMs. It would also need a way to dispense lubricant/coolant that doesn't make a huge mess. I am not currently interested in such a machine but might very well be in the future. I would expect to pay maybe $10,000 for a small machine.
    I think such a machine is kinda what we've been talking about rigidity wise - the air assist I have on my router was actually a fogless mister I added and had machined at work (only to find the mixer blocks are about $2 on aliexpress haha - the machinist did do a beautiful job though). The problem I had with running aluminium in my router was I had a good 15-20ft long jet of burning hot aluminium shavings blasting out of my router... my shop looked like a glitter factory vomited into it. Running a 5mm/3flute cutter at 18,000rpm 3mm deep in aluminium was so hot that the, at this point almost flood lubricant, was getting vapourised on contact, so my fogless mister was slightly ineffective in that the shop was full of vapourised cutting fluid haha. There's a reason they put enclosures on VMCs I guess!


    This is pre-servo upgrade, and still on a 1.5kw spindle (was trying to save money at the time, ended up blowing the VFD pulling too many amps too often, so upgraded to a 2.2kw when i got the new vfd):

    The swarf on the back wall is a good 1/8" thick in places.


    Poor picture quality because my phone was covered in cutting fluid.

    That's surfaced with a 5mm endmill (too lazy to do tool changes!), and felt perfectly smooth to the touch with a fingernail.

    When we moved out of the house over a year later there was still aluminium swarf friggen everywhere despite many shop cleanups, rearranges and such! There's still swarf on the ceiling.....

  6. #21
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    Here is an article on micro-stepping I found eons ago. https://hackaday.com/2016/08/29/how-...epping-really/ The electronics is the part of the machine I like working on the least and I know the least about it.

    Thanks for the information reply Gary. Having good honest info is useful.

  7. #22
    One can do pretty well with machining dry, though I'll admit I'm fascinated by the idea of air cooling (found a fascinating academic paper on it a while back), and the Nomad 883 Pro does well enough (albeit slowly) to have machined some fairly large parts, and has been used to machine titanium.

  8. #23
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    For those that don't want to read the full article that Brad linked to, here is the pertinent data:

    Microstep chart.JPG

    So as you can see, most steppers set at the most common (1/8) setting only have around 20% of their published torque. Be aware that that is when stopped. (Holding Torque) Assume 30% less than that when moving at normal cutting speeds, even less when rapid moves are executed.

    Remember that those "honkin beast" 370 ozin nema23 motors on 3:1 belt drives with a 25 tooth pinion gear will develop no more than 15 pounds of force when moving. Less when hot. If on linear rails which have 3-5 lbs of frictional force, is it any wonder why I recommend against NEMA23 for commercial applications? Any wonder why machines fail an hour or two into a cut file? Any wonder why a rotary axis wont hold against off center cutting operations? Any wonder why rotary axes fail most often when doing a rotary raster that everyone loves so much?

    I don't.

    And remember that the 370 ozin motors I refer to are about the largest used on small machines, most are substantially less.
    Last edited by Gary Campbell; 01-22-2018 at 8:13 PM.
    Gary Campbell
    CNC Technology & Training

  9. #24
    The way it was explained to me on another forum was:

    A stepper motor develops the most torque when forced one full step away from the commanded position. If you make it deviate more than that, it will lose steps (always in multiples of four full steps). Regardless of microstepping, the motor will give the same torque when forced one full step away from wherever you want it to be. If you force it half of a full step, it will apply about 71% of the maximum torque toward where you told it to go. If you force it only a quarter of a step, it will apply 38% of full torque. And so on: if it's already there, in the commanded position, it will generate no torque at all. It follows that a stepper motor moving against the load is always a little bit off from the theoretical position; how much depends on load, but it's up to one full step at maximum load.

    Now, if you want to increase precision, you may try to increase microstepping. This improves the resolution (you can command the position with a much finer increment), but it does not improve how accurately the stepper follows the commanded position. It's still the same one full step away from it at the same maximum load. If you want the accuracy to increase in the same proportion as the resolution -- and this is the unstated constraint -- you limit yourself to a smaller deviation than one full step. The only way to do that is to decrease the load: to 71% if you want to double the accuracy, to 38% if you want to quadruple it, and so on.

    So the only way microstepping reduces torque is if you're talking about torque per microstep (meaning the torque developed when the motor deviates one microstep from the commanded position), which is a very specific measure. You still get the full torque if it deviates one full step, and that's still the maximum you can go before you lose steps. For most practical purposes, in a machine like this, it's the latter that counts -- maximum torque before you lose steps -- and that stays the same. The extra positioning resolution does not hurt anything, even if it does not translate into increased accuracy; it actually helps, because it reduces a specific type of resonance that can cause lost steps.

  10. #25
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    Nice bit of info William...
    For the most part I agree, but I would clarify that it assumes the start position was in a "full step detent" to make the first paragraph true. These magnetic torque detents are explained in the article that Brad linked to. I believe that once a motor looses a step it is much more likely to stop or rest in a full or half step. If it had started in an eighth step it may only move 1,3, 5 or 7 until it rested in a full or half step detent. In many or most cases the number of microsteps lost is on the hundreds. A typical 5mm pitch screw with 1/8 microstepping has 8,128.16 steps per inch, or 1/4 microsteps half that. In either case a mere 1/16" lost is over 500 or 250 respectively. Even a low resolution setup with 1500 steps per inch will loose near 100 at .060" lost position. We all know that sound.

    Here is a visual representation of the torque detents. You can why the motor looses such a great percentage of its torque with each increase in microstepping. You should also be able to see why once a stepper looses position, it is more likely to stop at the stronger detents.

    Step Detents.jpg

    What many don't realize is the silent loss noted in the last sentence of the first paragraph. Take the example of a small capacity machine cutting a circle in hard plastic or aluminum, maybe even hard wood. Low torque motors will produce a very uniform "scalloped" edge. This is often referred to as chatter, but if you look closely you will see that the scallops line up from top to bottom even tho a dozen or more passes have been made. The fact that they line up removes the possibility they were random, eliminating chatter. They are evidence of "detent jumping". Circles, and the changing torque requirements as the 2 axes change speeds and reverse direction are difficult to cut perfectly, and therefore used to spot the issue.

    IF you are a reader, Tormach has published a white paper pdf that explains this very well. Google "Tormach Design Analisys"
    Gary Campbell
    CNC Technology & Training

  11. #26
    On their engineering.html page?

    ED10223: Series 3 Design Analysis Whitepaper (949295 kb)
    Tormach Series 3 CNC Mills Design Analysis Whitepaper

    Thanks.

    Rather ironic that the paper starts off w/ Voltaire's classic statement, Le mieux est l'ennemi du bien --- "'The best (some render this as perfect) is the enemy of the good."
    Last edited by William Adams; 01-22-2018 at 11:44 PM.

  12. #27
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    I don't think high speed routers need a lot of micro stepping, its just a mind over matter thing. Or how accurate can I be? CNC Routers were designed to cut wood, not build the SpaceX or NASA rockets.
    Retired Guy- Central Iowa. , LightObject 40w CO2 Laser and Chiller, MakerGear M2 3D Printer. Fine Line Automation 4x4 CNC Router- Mach4 ESS

  13. #28
    AIUI, one advantage which micro-stepping affords is that it allows the motors to run more smoothly. That said, for difficult cuts, I will admit that I have ripped a page from 3D printing and figured out what depth per pass settings would line up at full step positions and used those numbers.

  14. #29
    Quote Originally Posted by Gary Campbell View Post
    Low torque motors will produce a very uniform "scalloped" edge. This is often referred to as chatter, but if you look closely you will see that the scallops line up from top to bottom even tho a dozen or more passes have been made. The fact that they line up removes the possibility they were random, eliminating chatter. They are evidence of "detent jumping". Circles, and the changing torque requirements as the 2 axes change speeds and reverse direction are difficult to cut perfectly, and therefore used to spot the issue.
    Interesting. There's an image of a pen-drawn circle or logo from the original Kickstarter version of the Shapeoko which I believe depicts what you've said.

    I'm also curious if what you are describing would explain why there was one incredibly slight disconnect along the outer curves of the locking register calipers I cut on my machine:

    WIN_20150914_192800.jpg

    Discussed it w/ a couple of folks, but nothing obvious jumped out as the cause --- since it could only be felt, not really seen wound up taking it off when finishing. Need to revisit those and make a pair in brass on my Nomad.

  15. #30
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    OK, now that we have either solved, or at least discussed all the problems that are the reasons I build my machines the way I do, can we get back to the original topic?

    Those that have seen previous posts or videos have seen most everything mentioned above. I am throwing in, all touch screen (no keyboard or mouse) operation, auto lube for linear bearings & ballscrews, "no touch" fixed tool measure, air operated magnetic dust foot.... the rest is on you guys........... think... forget what it would cost.
    Gary Campbell
    CNC Technology & Training

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