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  • hsm cnc turning center

    I've just watched a couple videos, one showing a cnc machine making a crankshaft, and the other making a huge gear. I'm still amazed at the capabilities of these machines- not surprised, but amazed. I've been thinking about what mechanisms go into these machines to give them the accuracy, freedom from play, and what must be a long life between maintenance.

    I've seen the odd home shop guy build using ball bearing slides, etc- just wondering how far anyone has gone with their own build of a lathe, mill, or multi-axis machine?

    Part of my question relates to either the use of sensors to 'see' things, measure things, or simply stepper motor and hope the various slides and rotating devices follow the steps exactly without getting out of sync. Some of these elaborate cnc centers must have a full 'self-examination' capability- perhaps actually the ability to 'see' with deadly precision to know exactly where each cutting edge is in space, and the same for the working material. How much of this is actually blind?

    I'm not intending to try and revive the thread about composite machine bases, but it would seem that at least some of those interested in that might be wanting to build a multi-axis and highly capable cnc machine for themselves. I suppose you would have to want to for the sake of the build itself, as much so or more so than the usefulness it would offer later. You could farm out a lot of cnc work and it would still probably cost you less than building even a modest, fully functional cnc machining center. I'm one of those who would build the machine as much for the fun of it as getting the use from it, but even doing a cnc conversion to an existing machine would be a daunting task for me. My brain would turn to mush-

    Well, just wondering if there's an example of someone who's gone fairly deep into this-
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    I have done a few retrofits. A manual bridgeport with steppers, a servo bridgeport boss retrofit, a 7x lathe with linear rails etc, a gang tool american way lathe, a 14x40 lathe. I have since moved and am now in the process of a RF45 mill retrofit.

    As for accuracy of a cnc, the key is quality parts. Most of the home builds you see are done with rolled ball screws, cheap ballscrew support bearings, and other low cost parts. Real factory build cnc machines use high precision parts and as a result they get much better accuracies. It is often overlooked how important those ballscrew support bearings are!

    After doing it wrong a few times I learned. I now prefer used high quality ground ballscrews from ebay, the cost is not far from cheap rolled screws but the performance is light years different. Often, these used screws come with the precision angular contact support bearings that would cost several hundred dollars if bought separately.

    Accuracy of position starts with the motor. Steppers are far less accurate in this regard to servos with encoders. Typical encoders are several thousand steps which normally translates into well below sub tenth thousandth accuracy. From there, its down to how good the machine ways and other iron is, such as rigidity.

    Bottom line, you can't use cheap import low tolerance parts and expect to split tenths with the finished machine. It takes the good parts to get good results.

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    • #3
      I don't have much experience with conversions or home-built CNC; so far, I've only worked on converting my little Sherline mill, which is utterly simple mechanically. Building the control has proven to be quite the challenge, though. That said, I may be able to offer some insight into the world of high-speed, high-precision CNC machines, as that's been my job for the past few years. Most of this is from personal experience, though I have generalized it somewhat.

      First, the machine does not truly know the position of the tool tip - that much is completely 'blind'. You can put the wrong tool in the spindle and run a known good program on a known good setup, and the machine will butcher the part or crash. The medium- to high-end machines have the ability - or at least the option - to make the machine stop in the event of an improper, but not harmful (to the machine), cut by spindle torque monitoring and such. The machine has absolutely no idea where the part material is. You have to set the work offsets correctly to make sure that the material falls inside the programmed tool path. That's why training and clear procedures are so important - the machine will go to amazing lengths to prevent itself from being hurt, but it's helpless in the face of blithering idiocy, or perhaps a simple moment of inattention or carelessness. Yeah, I've been there!

      In some situations, particularly for very large/complex/expensive parts, when changing setups frequently, or when the part material (forgings, castings) may not be consistent, it makes sense to include a touch probe at the beginning of an operation to establish known points on the workpiece. Those probes seem to be an option on just about every machine now. Some CAM software can program the probing routines, which makes things nice. You can then hand-edit the program to shift your offsets, add or remove certain cutting passes, alter the length of a particular tool path, call a subprogram, whatever... automatically, depending on the values indicated by the probe. Probes can also be used to check features after machining, to make sure that the part is within tolerance; that's commonly known as 'in-process inspection'. All that is still a ton of manual work, though, and it doesn't necessarily transfer from one part to the next.

      So if probes exist to check the workpiece, there should also be probes to check the cutting tools, right? There are, and they seem to come in two main flavors - touch pads and lasers. The touch pads are typically used just to set tool lengths on mills, though they can set both X and Z offsets for turning tools on lathes. I haven't see a laser setter on a lathe - though I have far less experience with lathes than I do with mills; perhaps some mill-turns or live tooling machines have them. The laser setter on a mill can determine tool length, diameter, and corner radius. Any of these sensors have to be set up themselves when the machine is installed by using a precision test bar, which is micron accurate or better. There's also something called a BTS - Broken Tool Sensor - that resides in the tool carousel or magazine and isn't terribly precise, but it can tell if the tool is within a few thousandths of where it should be and is a very fast check. The ones I know of are kind of like an arm that swings out and very gently hits the end of the tool. If it's not there, the machine throws an alarm.

      The machines do 'know' the position of each individual axis. Stepper motors are almost exclusively used for (in relative terms) very small machines, many but not all of which would fall into the "hobbyist" or "lightweight" category. Larger machines use servos for everything, including the spindle motor. Servos utilize a so-called 'closed loop' system, in which some form of feedback is provided to the motor driver to tell it whether the motor has moved by the amount that it is supposed to.

      Machines generally have some form of high-accuracy, high-precision linear scale - glass, inductive, and magnetic scales have all been used - for feedback on the linear axes. The computer will command the servo(s) to move a certain direction at a certain speed, then as the servo is in motion, the computer will keep track of where the axis position should be, and checks this against the reading provided by the scales many thousands of times each second. The computer will then command the servo to speed up or slow down as necessary to make it achieve the required position. Rotary axes are similar, except that they may either use either scales or rotary encoders. The scales/encoders and servos together are sufficiently precise that newer machines can move in 1 micron increments - accurately! Quite a feat when the ballscrews might be something like 40mm lead!

      The servos have to be 'tuned' so that the motors will move quickly and accurately to the desired position. If they are out of tune, the servo will either undershoot the correct position, or overshoot and 'hunt' - that is, it will move back and forth very rapidly as it tries to maintain the correct position, but always going past it. I have personally used a machine on which the servos were so out of tune that you could program a sharp 90° corner, and the machine would put a 1/8" radius on.

      The machine will usually have both a proximity sensor and a hard stop at the extreme ends of each axis. The computer has 'soft limits' set up for each axis - if the machine moves beyond that limit, the control will alarm out for overtravel and stop itself. All the machines I have used have to be manually moved back into the permissible range, but sometimes, especially on older machines, the machine will let you move the wrong way. If the axis moves far enough to trip the proximity sensor, the machine will go into an emergency stop mode and shut itself down. Failing that, the hard stops will at least ensure that mechanical damage is limited until the servos fault from overload...

      So, the machines can know the axis position, but that doesn't guarantee that it will cut accurately if the machine isn't mechanically aligned. A great deal of effort goes into the design and production of the machine frame so that it will properly support the ways or slides. The frame is designed for high stiffness, which raises the natural frequency of the structure and thus reduces vibration or 'wobble' from sudden movements, and is made sufficiently strong that it will deflect minimally under extremely high loads.

      Incidentally, I have seen both box ways and linear slides on high-end machines, box ways will handle shock loads much better and are generally quite a bit more rigid. That said, linear slides handle very high side loads quite well, and Makino's T1 has a so-called 'hybrid way' construction on one axis, in which it uses both box ways AND linear slides. It's an unusual design, but it's meant for heavy-duty milling of hard metals, specifically titanium. The high-end machines also frequently feature liquid chilling of the machine frame, ballscrews, and spindle core to reduce temperature-related inaccuracies. The spindle, ballscrews, and ways all have automatic pressurized oiling systems to make sure they never run dry. Some of the spindle bearing oiling systems get pretty exotic, oil fog systems and the like - sometimes even the spindle taper has oil forced through it, to keep out contaminants.

      The machines are leveled after installation, then checked with laser interferometry equipment and adjusted until the machine is truly straight and square. At this stage, some machine controls can have a 'map' created so that any small errors in the 3D volumetric position are compensated for. Sometimes a machine axis may be driven by multiple ballscrews, and if so, the servos are synchronized to each other, which is mostly a one-time 'dumb' setting and is aimed at equalizing the load. Backlash is adjusted - this is usually a very small number, but usually not zero, even on a brand new machine with preloaded ballscrews. Again, we're talking microns, and it probably has something to do with compensating for tiny inaccuracies in the servo drive. The home position and, if different, the tool change position of the machine are set, then the pivot points of any rotary axes are set, as well as any error in the relative positions of those axes. If a ball-bar test setup is present, it's generally used afterwards to verify the adjustments.

      After all that, it's awe-inspiring to watch a final test as a .002mm/div (2 micron) indicator is placed on the surface of a precision ball, and the machine dances around, always keeping the indicator needle poised exactly on the surface of the ball...

      TL;DR - The high-end machines aren't necessarily that 'smart' when it comes to cutting a part - it's just that they are designed to microns, manufactured to microns, aligned to microns, machine parameters are set to microns, and then have robust error reduction features or compensation algorithms in the control so that the tool is where it's programmed to be, when it's supposed to be there. Automatic pressurized oiling and well-designed seals and way covers keep out contaminants for minimal maintenance and long life. That's why a relatively small high-end 5-axis machine can cost almost $1 million. It isn't that hard to crash any of them - trust me, a bad program, incorrect material, or a wrong offset will ruin your day. It's a great feeling, though, to do a bunch of work, then turn a complex new part over to production and just have them roll off the machine.

      -Brian

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      • #4
        Great post Brian! Now I want to go out to my shop & throw bin my cobbled together CNC lathe & mill & then beg, borrow or steal a "real" machine.

        (Naah, havin' too much fun with 'em.)
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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        • #5
          Very interesting and informative post Brian.

          I have a question. While I have never programmed a metalworking machine, my day job involves programming the control algorithms of a machine with many multi-axis gantries and pick&place mechanisms. We use stepper motors for all motions and encoders are on all of the critical axes.

          In your experience, are the stepper-driven machines also using scales or do they just have "home" sensors? If using scales, do they use the steppers as servos (i.e., correcting the motion) or are the scales just used to indicate pass/fail of a move? Since we use encoders with steppers, this is a conversation that we have many times and it would be useful to get more knowledge of how it's done in a different industry.

          Lyndon

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          • #6
            I don't have any direct experience with closed-loop stepper systems and therefore couldn't really comment, though I know that they do exist; in fact, when I was going through tech school back in '04, we had what I think was a Taig mill conversion with encoders on the steppers, but I never did anything on it. Given that many different types of controls exist for these machines, I suppose that it could be either a pass/fail or real-time correction, depending on who made the control. I personally have used only large servo-driven machines, and very small open-loop stepper machines with home/limit switches - closed-loop stepper rigs, in terms of absolute numbers, would be a (very?) small minority in the machine tool world. I gather that most of them have encoders, not scales, though I suppose that either could be used (I think) given that they had a quadrature output - that's not really my area of expertise. That's part of the reason I'm trying to convert this little Sherline, so that I can learn more about the inner workings of the controls, plus gain some experience with electronics in general.

            -Brian

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            • #7
              I started this journey about 12 years ago, with this old round coloum mill and some Gecko drives, it did the job then fell into disuse, I lent it to a friend but recently wanted to get back into it. I bought a Kflop motion controller, and have spent a few days getting it in a box ready to hook up with the old mill.
              Accuracy wont be awesome, still has acme screws, and hundreds of places for errors to accumulate but will do the job , if I had any belief in all this I would be converting my Induma mill, maybe next year.

              The beauty of the Kflop is it handles acceleration etc and can work with steppers in open or closed loop, servos and has buckets full of IO and d/a for anything you can dream of, there is a plugin for Mach 3 , it can be hooked to a relativly slow pc and run at high stepping rates as it doesnt need to attend to the windows and other pc functions that go on in a pc

              Should be up and running in a few days, if it works out to what I want Ill look at my other mill, as it gets around many of the shortcomings of this RF20, its all a bit of an adventure





              My neighbours diary says I have boundary issues

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              • #8
                Ive run a few professional cncs and have been studying the various designs quite a bit lately. Ive actually got a small VMC design on the backburner now, been doodling as time permits in Pro/E a few years now on it. As others have more eloquently said, mechanically there isnt much to them other than ultra precise fit of the parts. Theyre actually getting to be somewhat like many "Harleys" - an assembly of others' parts and rather expensive considering the end product.
                "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

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                • #9
                  Great replies- thanks especially to Brian for that lengthy but informative post. This is the kind of stuff I was looking for.
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                  Comment


                  • #10
                    Originally posted by lwalker View Post
                    In your experience, are the stepper-driven machines also using scales or do they just have "home" sensors? If using scales, do they use the steppers as servos (i.e., correcting the motion) or are the scales just used to indicate pass/fail of a move? Since we use encoders with steppers, this is a conversation that we have many times and it would be useful to get more knowledge of how it's done in a different industry.
                    I'm not aware of any contemporary, industrial metalworking machine tool that uses steppers. I believe some of the early Bridgeport Boss machines did, but that's all I know off the top of my head.

                    I have a Tormach mill, which uses steppers and home/limit switches only so is true open-loop. The Shopbot routers are available IIRC with either steppers or servos, and the steppers are the same as my mill. Stepper-driven DIY machines are also the same.

                    IF you used any kind of feedback with steppers, it would essentially have to be pass-fail, or close to it. If you have a properly-designed stepper system, and send 200 steps clockwise to motor A, it should turn 200 steps in the CW direction. If it doesn't, you either have a crash, obstruction, or electronic fault somewhere in the system. Unlike servos, steppers are always supplying the maximum torque possible for a given rotational speed. While I suppose you could do something fancy like slow the whole machine down to improve torque, the reality is that properly-matched motors should have enough torque at normal operating speeds to snap the tool, so if you're running out of torque, you have a different problem.

                    Also FYI, while the fancier machining centers like OP asked about do have all the gizmos that Brian mentioned, there are a great number of low-end VMCs like your base-model Haas that don't have linear scales (many offer them as an upgrade), only encoders on the servos.

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                    • #11
                      Here is a CNC lathe I built in 2009, it uses servo motors with encoder feedback to the controller.
                      This machine is still my main production lathe in my shop till today.

                      http://bbs.homeshopmachinist.net/thr...ward-cnc-lathe

                      Werner

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                      • #12
                        I did not know the Tormach was open-loop. I no longer have as much respect for them, now..... Who would want a "we hope it's right, but we have no clue" CNC?

                        Feedback seems a MUST.....
                        CNC machines only go through the motions

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                        • #13
                          Yes, tormach is open. And really slow. I mean slow.

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                          • #14
                            Oh and FWIW, linear scales on a CNC are almost always a build to order option. When the machine is built it is screw mapped like mentioned above so the necessity of needing scales is minimal.

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