1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. PLEASE: Read the FORUM RULES BEFORE registering!

    Dismiss Notice

Slant-Bed CNC Lathe Build

Discussion in 'CNC IN THE HOME SHOP' started by barnbwt, May 4, 2017.

  1. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    I have no idea if this is the right place for the build thread, but my home is above my shop, and this CNC project will be in it :)

    I'll say up front this is a big, big project. Especially for something that's only the size of an oven. This first post will be a sort of rolling summary (so long as I remain able to edit it) of milestones passed & yet to be completed. I will try to mention all the questions I've had to find answers to in order to progress, so that someone embarking on a similar adventure might take a short cut. If it looks like I'm missing anything, or especially if anything needs clarification or is outright wrong, by all means speak up! My starting point is as a total ignoramus who only has a couple years' experience with a small manual lathe, and a degree in aerospace engineering. Having seen what a *well made* machine can do --even a small one-- and the near-limitless capabilities of even simple automation compared to my own abilities, I think it's worth reaching for the stars a bit.

    I will have a lot of questions I'll ask here, every step of the way to be sure ;). I'm not posting on CNCZone since that place is lousy with abandoned machine-build threads, and it's depressing. It seems like plenty of ambitious projects happen here, and will way less pretense. Thanks in advance for tagging along, and of course for any help or advice.

    So away we go...

    Overall Concept:
    -Benchtop size, trying to keep it inside a 3ft cube
    -Workpiece size 2" x 7.5" (with boring capacity to that depth)
    -CNC lathe profiling capability, including threading/rigid tapping
    -Light milling capability with a live tool (engraving, slots, helices --real word)
    -4000rpm spindle (well over 1000sfm for a 1.25" 'average size' profile)
    -Capable of good results on all commonly machined metals (steels, aluminum, brasses, Delrin)
    -5C Collet/Closer spindle, with D1-4 mount for chucks
    -Design for future; tool changer, tailstock, flood coolant, full closed loop axes, chip auger
    [​IMG]
    That last one (growth capability) is why the machine isn't about 1/3 shorter.

    Design Constraints: All Good Designs Have Restrictions
    -Residential 110VDC/60Hz 1-PH @ 15A, grounded GFCI outlet
    -3ft cube external envelope (again)
    -500lb weight limit (this is flexible within reason)
    -"Easily" moved; it won't be on casters, but I plan for it to have lifting points for a hoist
    -Bench top; heavy wooden workbench, but no 'proper' machine platform to stiffen it (also flexible)

    Design Criteria: Brass Tacks by Category
    Complete Parts List (with Purchase Price):
    -Used Baldor BSM80C-375AF servo motor (135$ with extra 10:1 gearing) & pulley
    -Used Baldor FMH2A09TR-EN43 (267$) & cables
    -Nema 23, bipolar, 425 oz-in, 4.2A steppers (tentative) & mount
    -Smooth Stepper ESS motion controller (tentative)
    -Used Cherokee International QT6A1D 24VDC @ 5A, 5VDC @ 20A Power Supply (24.99$)
    -80VDC @ 8A stepper driver power supply (tentative)
    -DM856 Leadshine 80V stepper drivers, qty (2) (tentative)
    -5VDC cooling fans & heat sink, qty(2) (tentative, for stepper drivers)
    -24VDC ducted cooling fan (tentative, for servo driver)
    -16mm OD, 4mm pitch SFU1604 ballscrew & nut, 300mm X axis (30.58$)
    -16mm OD, 5mm pitch SFU1605 ballscrew & nut, 500mm Z axis (76.89$)
    --BK/BF12 ballscrew bearing mounts qty(2) (0$ part of Z axis order)
    --Stainless steel spring coupler 1/4"-10mm, qty(2) (0$ part of Z axis order)
    -28mm steel ballnut mounts for 1604/1605 ballnuts qty(2) (20.52$)
    -25mm x 400mm linear guideways qty(2) & HSR20CA carraiges qty(4) , X axis (238$)
    --25mm x 600mm linear guideways qty(2) & HSR20CA carraiges qty(4) , X axis (0$ part of X axis order)

    -1.5" box tubing, 1/4" and 3/16" thicknesses (tentative)
    -3/8" or 1/2" steel plate (tentative)
    -1" steel plate for carriage & slide table surfaces (tentative)
    -1" square cold roll steel bar for spacers (tentative)
    -2" square aluminum bar for stepper motor mounts (tentative)
    -Spindle assembly (ball bearings, spindle shaft, spindle casing, pulley)

    Machining Envelope:

    -Intended part size is 2"OD x 7.5" stickout (min cutter travel 1")
    -Spindle bore pass through at least 1" diameter
    -No tail stock at this time
    -Ability to move to tool changer dock along X-axis; total travel must be 6"
    -Boring depth capacity of 7.5", means Z-travel of carriage must be 15" total
    Machining Capability (Forces/Power):
    -Capable of machining steels, aluminums, plastics, brasses, and irons
    -Carbide cutter speeds (>1000sfm, for a ~1.25" OD part is 3000rpm, so spindle = 4000rpm)
    -Shallower/faster cuts acceptable so long as surface finish is not compromised (if possible)
    Optimizing Rigidity:
    -Beefy, welded truss frame, with several hundred pounds minimum weight
    -Optional attachment of mass/dampening material later (iron plate or epoxy granite)
    -Heavy gauge plate steel motor & spindle mounting plates at headstock
    -Thinner gauge sheet closeout bolted to exterior
    -Direct-driven stepper motors
    -25mm timing belt drives spindle
    -16mm ballscrews, 25mm linear guideways both have far higher load capacity than needed
    Optimizing Accuracy:
    -Use of linear guideways to constrain axis motion with very little off-axis backlash
    -Use of ballscrew drive for axes to reduce backlash (also friction)
    -Use of servo motor on spindle to precisely track its position & speed for threading & milling
    --May ultimately add a secondary encoder to the spindle to even more precisely monitor position
    Minimizing Size:
    -Work envelope and machine travel kept to minimum practical size
    -Simple single-belt spindle drivetrain
    -Z-axis carriage and X-axis cross slide made from 1" steel plate with inset mounting surfaces
    -45deg bed slope is the best compromise between height & depth, and a short load path down

    Electrical Components:

    -Baldor BSM80C-375AF Servo motor is ~1kW, with max RPM of 4000rpm, 2500 pulse/rev encoder, generating 3.6N-m continuous force, and operating at rated 6.29A & 300VAC. The power figures triple at maximum, but for a lathe such violent power moves are generally unnecessary and indicate more serious problems (crash). That, and the 1kW resultant power draw make me confident this motor won't blow my breaker in operation (inrush, perhaps). The way these AC servos operate, is a fancy VFD called a servo driver feeds them a continuous current across the whole speed range, but increases voltage to drive it harder, while increasing frequency to keep time with its spinning (greatly simplified). This constant torque quality manifests as a nearly flat torque curve across the operating range of speeds. While the motor is not hugely different from any other induction unit apart from its low inertia, the real magic of servos stems from their precise control; an encoder tracks the exact angular position & feeds that data to the servo driver in real time, where high performance control systems adjust power delivery to quickly react to applied loads on the motor or commands from the controller.
    -Servo driver is a 9A unit set up for analog command signals, but digital on the inside so it is also capable of accepting step/direction inputs just like a stepper motor (which are then parsed by a computer & sent to the motor, then adjusted as the encoder reports the response). The driver actually replicates & outputs the encoder signal, so it can be sent back to the motion controller itself for a complete closed-loop of feedback for all components. The controlling input pulse is a low level signal, and the computer must be isolated from the enormous & variable motor current, so a separate 24VDC supply is necessary
    Component Block Diagram:
    I will update this with an actual diagram at some point
    Controls Block Diagram:
    -Laptop running Mach3 CAM software -> ethernet -> motion controller
    -Motion controller -> servo & stepper drivers -> servo & steppers
    -24VDC power supply->motion controller, servo drive, stepper drives
    Power Consumption:
    -15A 120VAC available, 16A total peak demand (dang, no tunes!)
    -9A continuous draw by servo motor drive (at max power)
    -2A/120VAC continuous draw by 5A-24VDC/10A-5VDC power supply
    --motion controller requires 1A 5VDC
    --stepper driver fans require .5A 5VDC
    --servo driver requires 3A 24VDC
    --servo controller cooling fan requires .25A 24VDC
    -5A/120VAC max draw, I think, from 8A 80VDC power supply
    --4.2A steppers (4.2*2*.67) comes to 5.7A 80VDC
    Mechanical Components:
    -25mm linear rails; these carriages are large enough to have zerk lube fittings, and also have external flanges that broader their mounting footprint. They also seem to be made to a slightly higher standard than budget smaller rail sizes.
    -16mm ballscrew; the 5mm pitch is plenty for my needs (I'd prefer tighter, but is very $$) and the screw itself is far, far, far, far stronger than any load I'll be applying to it via the stepper motors or cutter.
    -Ballnut mounts will be an off the shelf variety paired with my ballnuts
    -Stepper motors will be mounted directly to the ballscrews on machined standoffs, the shafts connected by helical couplers
    Spindle Construction:
    -The spindle is a 'cartridge' type that is mounted into the frame in alignment with the axis rails
    -The spindle will be a 5C collet bore with space for a tubular closer, and D1-4 faceplate for chucks
    -Two opposed & preloaded roller or angular contact bearings will support the business end, one set of simple rollers or deep groove bearings will support the side load of the pulley on the tail end
    -All bearings will be sealed to avoid the need for lube systems & gasketing
    -The casing will be a simple hollow cylinder bored for the bearing races, with a large square mounting flange welded at each end that will bolt to the frame for mounting
    Tooling:
    -At least initially, there will be an AXA quick change post mounted on the slide that holds the cutters
    -I plan to add a quick-change capability to the surface of the slide itself, of a wedge type similar to the Aloris system, that will be operate as part of the automatic tool change system

    Frame Construction:
    Maintaining Alignment:

    Mechanical Assembly:
    Electrical Assembly:
    Function Testing & Adjustment:
     
    Last edited: May 9, 2017
  2. Boswell

    Boswell United States Hobby Machinist since 2010 H-M Supporter-Premium

    Likes Received:
    87
    Trophy Points:
    28
    City:
    Cedar Park
    State:
    Texas

    -Return to Top-

    Barnbwt, This looks like it is going to be an interesting build. Looking forward to following your build.
    what makes a Slant Bed lathe special? In other words, what can you do on a Slant bed lathe that you can't do on a flat bet? Just curious
     
  3. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    The slant bed is cool for a number of reasons, but only one really has to do with the slant itself; the spindle rotates 'backerds' what we manual guys are used to, so the chips fall down off the upside-down cutter on the back side of the spindle, and fall straight down, below the ways. Chip clearing at the cut is improved, and machine cleanliness in general is improved.

    The slant also makes for a slightly more compact machine volume, front to back, because of the incline and because the carriage is on the back side where the motor is already taking up space, and not sticking out the front where your hands can reach the handwheels on an apron ;)

    The other advantages, which have nothing to do with the slant layout, involve the use of linear guideways as opposed to V-ways. This is just more a modern high tech development, basically extending the rolling vs. sliding concept behind ballscrews to the surfaces that constrain the linear motion of the axes. The result is incredibly low friction (basically none compared to motor forces) and extremely low backlash/play in non-axis directions (which you only get from V-ways on larger/heavier machines with tuned up gibs). With the unpredictable friction out of the way, it's now far easier to control the machine, especially at really fast travel speeds, which is how the modern fancy tooling centers can run hundreds of ipm cutting moves while dynamically playing with ballscrew forces to compensate for chatter or other cutting forces and achieve a perfect finish.

    For the home builder (or commercial builder, I suppose) the advantage of these rails is that it's much easier & more straightforward to build a machine that is square & in alignment. All I need to do is provide two roughly-correct sturdy surfaces to bolt the rails two in my frame, then face those contact areas in a larger mill or grinding setup, and then scrape or shim the rails to final tram upon installation. No more pain in the butt V-ways that require lots of material removal in gigantic grinders followed by induction hardening, that also can't be replaced when they wear out much faster than the hardened ballbearing races.

    The disadvantage, is that between the carriage and tool position, you really couldn't run one manually if you wanted to. At best, it'd be a fly-by-wire system with no feedback, where you couldn't see what you are doing very well. So it's really a dedicated/optimized CNC only kind of deal. I already have a manual Chinese lathe for those jobs, and I learned a lot about what I can do with it, what I can't do, what I'd like to do going forward, and what capabilities I'll need to do that. I want to be able to make Glock barrels from blanks & AR15 bolts from raw stock (not those exact parts necessarily, but those kinds of mostly-round widgets)

    TCB
     
    JimDawson likes this.
  4. Eddyde

    Eddyde Active User H-M Supporter-Premium

    Likes Received:
    366
    Trophy Points:
    63
    City:
    Brooklyn
    State:
    New York

    -Return to Top-

    Wow, an awesome, ambitious project! I look forward to following this one, Thanks!
     
  5. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    The current candidates for the stepper motors. I could actually use some advice, or at least confirmation of my assumptions here;
    -23HS33-4008D: 400 oz-in NEMA23 stepper, 5.66A bipolar parallel (2.83A serial), and 7.2mH/1.8mH inductance
    -KL23H286-20-88-ALY: same thing, but rated at 425 oz-in, 2.8A bipolar parallel (1.4A serial), and 6.8mH/27.2mH inductance

    The vastly different inductance figures for each in parallel/series configuration suggests they are wound quite differently inside, despite the identical external profile. From what I understand, the higher-current draw & much lower serial inductance figures of the first one suggest it's meant to run stronger at higher speeds than the second. Now, seeing as I have limited current/power to work with, and the moving carriage is only ~50lb (20lb for slide), and the axis travel is not very large, I am looking at the second KL stepper in series configuration. If I understand correctly, it may lack in power at speed (during rapids at hundreds of rpms), but would be just as powerful when resisting cutting forces & directing the cutter during turning moves, which require far lower speeds (tens of rpms). I think this may be one of those cases where the greater serves for the lesser.

    And the 24VDC power supply (ya'll be sure to let me know if I should run independent PS's for the stepper drivers & control boards);
    -used Siemens 20A 24VDC power supply.

    By my calcs I should never be running more than 13-14A 24VDC, and that's for the worst-case bipolar parallel stepper configuration. This power supply, in addition to being overbuilt for the task as well as German in origin, is also much more efficient in how it uses its source power than the claimed figures I've seen from the only alternative; the Meanwell clones that are visibly 'lesser' in every way. I figure the extra saved .5A is worth an extra 20$ or so for the used unit (even if I end up having to replace some ancient capacitors upon first powerup). I'm told the greater always serves for the greater when it comes to power supplies ;)

    Servo drive showed up today. It also looks like I'll need to add a cooling fan to the servo driver for sure, and probably also the stepper drives for good measure. I figure if I'm gonna bother adding a 12VDC step-down line for the fans (there aren't a lot of cheap options for 24VDC) I might as well work up something for the motors themselves while I'm at it. Maybe attach or integrate a heat sink & fan into the aluminum standoff the steppers mount to? Maybe make that giant 3/8"-1/2" end-plate the servo motor bolts to out of aluminum? The stationary items are easier to cool, just put a draw-fan nearby to suck the heat away.

    TCB
     
  6. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    I have decided to split the stepper drivers off into their own independent power supply for isolation reasons, since even steppers can dump current into the control devices when slowing down. This also gives me the ability to run higher voltages for the drivers, for better high speed performance & torque control.

    I have a proposal for my stepper driver setup for each axis I'd like to get some thoughts on;
    For the Z-axis, use a nice Gecko-type driver that has both a high microstep count (my carriage travel is .001/full step) with smoothing, and high speed morphing to single-step so the longer axis has full rapid speeds. Unless it breaks the bank, this drive would feature closed-loop encoder input capability for a later upgrade (not sure how important this is since the motion controller board will also have this feature for sure)

    For the X-axis, use a much cheaper Chinese two-phase driver with minimum 16 division microstepping, but no fancy smoothing or high speed morphing functions, or even idle-current cutoff. My thinking is the X-axis is always going to be nearly stationary for any lathe or milling operation, so the fancy motion control features won't be that useful, and the lack of 'sleep mode' will keep the stationary stepper fully energized to resist X-axis cutter forces at full holding torque (since the X won't be sent pulses at all during a simple profile move, for example). I also found out they make cheaper rolled 4mm ballscrews, so I may go with the slower speed for this shorter axis vs. a more precise motor/control setup (though I'm torn since it is also the higher-stressed axis, and a 4mm screw is 1/4 weaker, 9.2kN vs 13kN, but still over 2000lbs capacity)

    TCB
     
  7. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Another question that has occurred to me;
    Besides the convenience of using ready-made ballscrews with their journal profiles already completed, is there any advantage to placing the preloaded/fixed end of the ballscrew at the opposite end from the motor-coupling? It seems to me, that by placing this end at the lower end of the table, the loaded length of the screw is greatly shortened, resulting in a more rigid system (albeit only slightly compared to other factors). The more common approach with the motor right at the fixed end seems tailored for CNC mill/lathe conversions, where the motor uses the space previously occupied by the handwheels, where there is also the structure to solidly mount the fixed-end screw support. The slant bed is kind of backwards, in this regard.

    TCB
     
  8. cmantunes

    cmantunes United States Active Member Active Member

    Likes Received:
    15
    Trophy Points:
    8
    City:
    Herndon
    State:
    Virginia

    -Return to Top-

    If your budget permits, consider replacing the steppers with integrated servo motors, such as ClearPath. Not only will you get 6400 steps/rev if desired (w/ enhanced version) but the quietness, the smoothness of movement and lack of vibration will considerably enhance the build.
     
  9. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Of course a full servo-controlled closed-loop system on every axis would be nice, but that is far out of the budget for this project (hoping to keep it under $1500 for starters). For now, I mainly want to make sure I am not boxing myself out of such an upgrade at some future date. If I have problems with threading or engraving, I'd consider it. A friend is running steppers & getting good results with engraving on his mill, but they are geared down 1:3 or so; it is possible the direct drive's reduced resolution & holding torque could be a limiting factor, in which case a precise/sensitive servo is a good solution. I do think that simply going with a tighter ballscrew would get me most of what I'm looking for, for the time being, for not much money, though.

    I spent today re-tweeking the frame a bit more; now the slide is 1.5" wider (basically 8" square), but I was able to tuck the ballscrew supports between the linear guide carriages, reducing the length of the carriage about 1.5" up where the stepper mounts, and bringing the lower edge of the carriage in by an inch. Also added a horizontal stiffener bar to the upper end of the carriage (a section of 1.5" box tube) since the trench I had to mill for the X ball nut clearance left it U-shaped in a structural sense. End result, the ballscrews are a bit shorter and now hidden, the linear rails didn't change much at all.

    Resizing 1.png

    This means the depth of the machine can be reduced about 1.5", and now that I've also updated the models of the servo and stepper motors to be more accurate, I have a much better route to stiffening the headstock area to support the spindle. Two X-braces three inches away from each other & filled by two thick shear plates should be crazy stout.

    I'm pretty happy with the structural layout at this point, with only one possible, but unlikely modification; I think there is just enough room to put flip the stepper motors around and place them beneath the ball screws. Instead of a fat block mounted to the ballscrew supports that mounts the motor, there would be a thick plate that hangs below for the motor face to bolt onto. The idea of fully hidden motors is very appealing, and this would allow for some degree of gearing to a different ratio (namely on the X-axis). However, I also believe it wouldn't be quite as rigid, both due to the belts and the cantilevered motor mount. I'll have to think on it some more, and decide if it's enough to matter to the ballscrew.

    TCB
     
  10. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Next round of stuff bought, pretty much everything but the bulk steel & electronics;
    2X 400mm linear rails (X)
    2X 600mm linear rails (Z)
    300mm 1604 ballscrew & nut (unfinished) (X)
    500mm 1605 ballscrew & nut (Z)
    24VDC & 5VDC power supply
    2X BK/BF12 ballscrew mounts
    2X stainless steel 1/4"-10mm couplers

    The cost this time was 408$. Provided I don't go crazy with the motion controller, I'm on pace to squeak by under 1500$ in materials. The first post will be updated with the particulars of these latest items.

    TCB
     
    JimDawson likes this.
  11. rowbare

    rowbare Canada Active User H-M Supporter-Premium

    Likes Received:
    12
    Trophy Points:
    8
    City:
    Ottawa
    State:
    Ontario

    -Return to Top-

    You have a great start here but I think there is some room for improvement. Note that I am not writing from practical build experience but from having followed dozens of build threads and absorbing comments from people who know more about this than I ever will. So take what I say with a grain of thought but please understand that I hope my comments will help shorten your path to success.

    I think you need to bring up the stepper voltage considerably. Performance at 24 volts will be poor. Anyone who, like me, bought a Taig mill with an Allegro based controller running high inductance steppers should be able to attest to that. According to Gecko Drives stepper voltage should be 32 * sqrt(inductance) for best performance. Thus the first motor you listed wants about 43 volts and the second one about 83 volts (in parallel, series is much higher). I don't know what drives you are planning to use but the first motor draws a fair chunk of current so something like the Gecko G540 would be out of the question. On microstepping: its main purpose is to smooth out the motor at low speeds. It isn't the greatest way to increase resolution since the distance between microsteps isn't always even. There is a point of diminishing returns with microstepping. That is around 10 microsteps according to Gecko Drives. See: https://www.geckodrive.com/gecko/images/cms_files/Step Motor Basics Guide.pdf

    As for the relationship between the motor, support block and screw, one of the roles of the support block is to isolate the motor from the violence of the screw. Support blocks have robust preloaded angular contact bearings while motors have comparatively wimpy deep groove bearings. With the mounting arrangement you are proposing, the motor will be subjected to all kinds of stress as the screw expands as it gets warm or if it starts to whip. In any case for short screws like you will here, you don't need to support both ends providing the screws are a sufficient diameter. The 16mm screws you are specifying are more than adequate. Look for the Critical Speed calculator on this page: http://www.nookindustries.com/EngineeringTool/Index to figure out what will work. It is interesting to note in their fixity examples, none has the motor opposite a bearing block.

    I don't know where Mach 4 is at but Mach 3 has well documented issues with threading and dealing with index pulses (it can only deal with 1 per rev so it doesn't track very well). You might want to consider Linux CNC. It is more work to get going but is more robust than Mach 3.

    If you have access to FEA software, run an analysis of your design. I think you will find that very little of the material above the spindle/bed plane contributes to rigidity. Take that part of your weight budget and put it where it will do more good and build up the enclosure. For instance, extra mass around the spindle and the spindle/bed interface is rarely wasted.

    Have you see this build? It is similar to yours: http://www.cnczone.com/forums/vertical-mill-lathe-project-log/281738-cnc-cad.html

    bob
     
    jbolt and JimDawson like this.
  12. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Absolutely right on the stepper voltage; I'm learning a lot and thought the drivers had a step up switch in them vs. just being a high-tech Morse code switch that passes current/voltage straight through when pulsing. I think my servo drive is similar (really hard to tell what all it does, what with the multiphase conversion, voltage/current regulation, and pwm; I just know it's recommended for this motor). I'm looking at some 60-80vdc supplies for servos, and possibly the 220 transformer I was hoping to avoid for the servo motor.

    This really begs the question; why aren't there voltage step-up stepper drives? Seems like it'd be nice to have the same adaptability you have for your output (current & voltage selection for different motors) on the input side, so you only need one big nice/regulated 24vdc supply unit, for instance.

    I know what you mean about the screw mounts always being fixed at the motor end. I suppose thermal growth is a very valid point, especially in big machines where they could get toasty. My X is only 10" long, though, plus there's a spring on the end of it at the motor. But by this same logic, there's no advantage to mounting the thrust bearing at one end or the other. I suspect the wider footprint of the fixed mount may be a bigger factor for rigidity in supporting a cantilevered motor than anything.

    The upper frame is intentionally overbuilt, but is 3/16" vs 1/4" thick tube. I won't go thinner since I'm not a great MIG artist. I plan to mount a tool carousel in there, and at this point I may be putting some/all of the electronics in a cabinet/hutch up there, possibly on a thick aluminum slab for heat conduction (servo drive requires a fan *and* a cold plate). The lower frame will likely get heavier though; I am contemplating another tube along Z across the rail supports to better secure that critical plane against warp caused by the rest of the structure. I only want to mill the rail seats planar once ;)

    TCB
     
  13. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    I'm sure my beer budget will dictate some changes, but at this point I'm thinking;
    -Gecko G214 high-res stepper drive for X and Z axes (plenty of growth room for stronger steppers & closed loop in the future)
    -80VDC unregulated linear power supply, 4A capacity qty(2)
    -23HS33-4008D: 400 oz-in NEMA23 stepper (lower current usage for peak holding torque; speed is not so important)

    Is the higher voltage itself an advantage? I guess perhaps better second-order motor response due to electrons being pushed harder? Neither velocity nor acceleration are as important for turning jobs, at least compared to static holding torque and smoothness (which is why servos would be ideal, if only they were affordable)
     
  14. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    "Have you see this build? It is similar to yours: http://www.cnczone.com/forums/vertical-mill-lathe-project-log/281738-cnc-cad.html"
    That build & dumpsterCNC were my inspiration (that poor guy lost everything to the tornado near Emory, Texas just a few weeks ago as I was first diving into this, otherwise I'd probably be pestering him with these questions, lol). My hope is to scale that design down slightly, but not having that big box structure beneath the bed supports & above the bench top, and to make more efficient use of a small envelope with a stationary tool carousel (the tool holder will spin to index, and the carriage/slide will 'dock' with it to load & unload tools). The servo spindle is more of an added capability than an improvement over the original design, meant to make this thing way, way, waaaay more useful than my manual lathe is.

    TCB
     
  15. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    A few more odds & ends, in anticipation of the servo motor's arrival & testing;
    -High voltage connections, 8-pin & qty(2) 2-pin Molex connector terminal blocks; $5.78, $10.16
    -Low voltage connections, 14-pin mini-Molex connector terminal block,
    -Encoder feedback connector, 15-pin D-sub; $18.87
    -Encoder feedback connector, Fancy-pants mil spec 15-pin round bayonet connector; $46
    -Computer interface, RS485 cable; $6
    -Servo power cable; $52.98

    The power cable is Keb brand, but the 8-pin round connector & insert arrangement appears identical to the Baldor. I will probably end up buying a short length of shielded cable with 8 twisted pairs for the encoder line; while their prices are insane, Baldor was nice enough to clearly explain what is needed as far as the cable requirements in their installation manual. Once I have the RS485 cable in hand I should be able to put 24VDC into the servo drive to turn it on, and talk to a computer via the (apparently crummy & poorly supported) Baldor Mint Workbench software, which is where I can play with step/dir gearing settings and so on.

    One lame thing about this servo driver is the input pulse speed of only 1 MHz; with the 2500 line encoder on the motor, that's like 400rpm tops at 1:1. Cost of doing business in 1998. Through this software, I believe I can increase the encoder pulse/drive pulse ratio by a factor of ten (250 steps/turn) or greater and reach the top speeds desired. I need to learn some more to find out if this is a setting that can be modified dynamically via M-code during a program; if that is the case, step/dir would definitely let me have my slow & precise cake and eat the high speed stuff, too.

    Should I wish to run the motor in velocity (or even torque) mode via the analog signal, I may have some problems. This drive responds to -10VDC to +10VDC signals, but all the motion controllers out there seem to be 0-+10VDC outputs, with relays that reverse the output poles for the opposite rotation. Chapter 5.2 (no page numbers, Baldor?) of the manual linked below shows that the inputs can be set up as differential or single-ended. I think that even if I reversed the polarity of the motion controller board output, I would have problems with the single-ended configuration, since the +10V lead would be hooked straight to a grounded terminal. What I don't know is if putting +10V on that same ungrounded terminal for the differential configuration would actually work, or if its circuitry is still looking for a negative-to-ground voltage, albeit an unreferenced one. There's an "internal reference" resistor on the circuit diagram in this area that could still be going to ground for all I can tell.

    http://www.baldor.com/Shared/manuals/1919-803.pdf

    I know enough about circuits to know some simple MOSFETery can pass-thru a 0-10VDC signal, or turn it into a -10-0VDC equivalent depending on a binary direction signal voltage & power supply, but I didn't see such a ready-made device amongst the other spindle-control doohickeys at the usual vendors. If anyone has any suggestions I'm all ears, since I *suspect* interacting with driver settings via Mint Workbench as little as possible vs. external solutions is a good approach.

    TCB
     
  16. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    When I'm not torturing my mind with electrical/programming stuff, I'm working out the spindle for the next parts order. Originally I'd planned to use a pair of opposed angular contact bearings at the business end, but I'm leaning more toward a single-race dual-row angular contact setup for a couple reasons;
    -takes up less space & is simpler
    -is more available in a shielded configuration
    -50mm seems to be a very common size for HVAC uses (so much cheaper)
    -plenty strong for any cutting loads the ballscrews can resist without damage themselves
    -No PITA preload adjustment necessary once assembled

    That last one is where I'm not so sure. Not needing to squeeze the two halves of the support is nice, but I also worry that the loss of this ability is a bad idea for a lathe spindle. It seems to work fine for A/C compressors, but who knows whether that has anything to do with the loads I'm contemplating? Would the standard light pre-load bearing be rigid enough to prevent obnoxious acoustic patterns from fouling all parts turned at speed? I suppose another solution would be to use two of these two-row bearings, and lightly tension them against each other similar to how we normally do the single-row jobs (possibly one at each end of the spindle)

    https://www.vxb.com/3210-2RS-Bearing-Angular-Contact-Sealed-50x90x30-2-p/kit16655.htm

    TCB
     
  17. jbolt

    jbolt United States Active User H-M Supporter-Premium

    Likes Received:
    791
    Trophy Points:
    113
    City:
    Mountain View
    State:
    California

    -Return to Top-

  18. JimDawson

    JimDawson Global Moderator Staff Member Director

    Likes Received:
    3,714
    Trophy Points:
    113
    City:
    Sandy
    State:
    Oregon

    -Return to Top-

    Depends on the motion controller. When you have a motion controller that will output a +/- 10 V signal, it is normally setup as single ended output, with the command signal going to the drive command input and then the controller ground going to the drive signal ground. I have never done it, but setting up as a differential input might be the way to go in your case.

    I would not think a relay switching the command signal polarity would be very fast system, but maybe it works. I guess for the spindle, it wouldn't really matter.
     
  19. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Well, it is for velocity/torque control & not position; I can't imagine needing to rapidly vary the speed instruction at speeds beyond the step/dir regime. I also think the negative values are for reversing the spindle (not simply slowing it) which would be an even odder command for an already-turning machine.
     
    JimDawson likes this.
  20. barnbwt

    barnbwt United States Active Member Active Member

    Likes Received:
    26
    Trophy Points:
    18
    City:
    Greenville
    State:
    Texas

    -Return to Top-

    Thanks for the link, jbolt, lotta material to go over there
     
  21. JimDawson

    JimDawson Global Moderator Staff Member Director

    Likes Received:
    3,714
    Trophy Points:
    113
    City:
    Sandy
    State:
    Oregon

    -Return to Top-

    While you are correct it is for velocity/torque control, but it positions quite well also in velocity mode. Just requires a closed loop and a motion controller designed for that system. Up until the new digital ModBus systems were recently developed, it was the defacto standard for most industrial CNC systems. It is still shipping on new machines today. With the proper stepper drive it is also possible to drive a stepper in velocity mode with an analog signal.
     
  22. cmantunes

    cmantunes United States Active Member Active Member

    Likes Received:
    15
    Trophy Points:
    8
    City:
    Herndon
    State:
    Virginia

    -Return to Top-

    Have a look at ADCGAIN and ADCOFFSET on Mint Workbench help file. Together with a digital I/O pin, you may be able to adjust these on the fly. Depending on the state of the digital I/O, you would set ADCGAIN equal to 100 or -100, for example.
     

Share This Page