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KRE Mill V3 - Concept

Discussion in 'CNC Mills/Routers' started by Kevon Ritter, Sep 13, 2018.

  1. Kevon Ritter

    Kevon Ritter Master
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    I don't like to post concepts or ideas as there is almost a 100% chance of them not going anywhere, but this is one that I want to get moving very early into the next year.

    KRE Mill V3 1.png

    Disclaimers:
    • This is not based on Openbuilds components.
    • I am in no way talking bad about any other machine, just stating that everything has it's place.

    The project started as a true mill with an XY moving table. However I couldn't quite get the work area I wanted, without having to go crazy on the frame. I wanted at least 8" (200mm) in the Y. I switched to a moving Y table, and a moving XZ spindle. Because that opened up so much space, I went ahead and just gave it a 12" x 12" (300x300mm) build plate.

    The revision of the XY table mill turned into this. It seemed like it would work just fine, but everything just felt wrong. The Z was admittedly always sketchy. After that was revised, it really highlighted the rest. (Old Z has the extrusion.) The upright support also didn't inspire confidence. The next issue was the X axis. It was based on two 4040 sections separated, then mounted to the uprights. That's only creating a greater leverage. Plus the 4040 may not be enough for what I want. I did add a back plate with an extrusion mounted to it, which should have reduced any fore/aft flex, but that brings us to the final issue. It was butt ugly. I believe in function over form, but you should still be able to extract form from function.

    KRE Heavy Mill V2 1.png KRE Heavy Mill V2 2.png KRE Heavy Mill V2 3.png

    That brings us to the V3. It is built from four 4080 extrusions at the base instead of two 4080, with some 2040 support. The uprights also share the same side face as the base extrusion which allows for an easy integration of a support plate instead of just the corner brackets.

    KRE Mill V3 5.png KRE Heavy Mill V2 4.png

    The X axis is now built from two separated 4080 extrusions, but with a slightly reduced gap. Due to the fact that the uprights are no longer attached to both X extrusions, the possibility for swaying is now a thing. To counter that, I simply used 80mm wide rear mounting plates. There are also short sections of 4060 inside to allow for ball screw mounting hardware, and to a provide a little more "squareness stability." Adding a web would be incredibly easy, but may not be a single bit necessary.

    KRE Mill V3 6.png

    The entire build is based on 20 series extrusion, "HGR20" assemblies, 16mm ballscrews, and a mix of 1/4" and 3/8" aluminum plate. 20 series uses M5 hardware and had a manageable slot gap. Lead screws can be used, but finding decent metric anti-backlash nuts that don't cost a fortune is a chore in itself. That would also affect the serviceability. The work surface, ballscrew mounts, and both z plates are 3/8". Although the Sphinx can machine the aluminum for this, I'd rather build this with acrylic first for two reasons: to find weak points and to buffer the upfront cost. Even with acrylic, I'm sure it would still be above the level of the v wheel system. A good test would be for it to machine its own plates while in its acrylic form.


    Goals
    • Efficiently mill aluminum
    • Use all of what the router/spindle has to offer.
      • Although not by a long margin, the Bosche Colt router was actually able to over power the wheels. Then a chain reaction would start. I'll be placing an order for a 1.5kW spindle in the next two weeks. That will go on my Sphinx, but is intended for this mill.
      • This isn't a machine fault though.
    • Easier maintenance and disassembly
      • C beam with inner wheels almost requires a complete breakdown.
      • Every axis on this mill can slide off after decoupling the nut.
      • Every axis can also be independently separated from the other axis as well as the frame without compromising alignment or integrity.
    • Tighter tolerances
      • Less flex to keep the end mill straight is a big deal in the finishing and time departments.
    • To have fun and learn more :p
      • Although it's for side business/hobby purposes, we're all still builders here.
    • Flood safe and easy cleaning
      • This was not an issue before, but I'd use 18-8 SS hardware.
      • I'm going to cover as many slots as possible. What cant be cover by plates would be covered by a printed part.

    I'm still working on the details, but I was just curious to hear what others make suggest.

    Kev
     

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    #1 Kevon Ritter, Sep 13, 2018
    Last edited: Sep 13, 2018
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  2. Andreas Bockert

    Andreas Bockert Veteran
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    Looks nice! What pitch are you planning for the screws? I’ve been looking at 16mm screws but I’m a bit concerned with the 5mm pitch type.

    Do you need the height? To me, that seems like the one obvious possibility for improvement. Intuitively, it would seem that the vertical 2040(?)s that support X/Z us the weak spot.

    I hope you’ll post build details when you build it!
     
  3. Kevon Ritter

    Kevon Ritter Master
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    Regarding the screws, I'm still on the fence. 1604, 1605, and 1610 all share the same nut dimensions. 1616 is larger by a tap, but I don't care about this variant. I was thinking about going with 1605 all around I haven't done the math yet on rpm/torque/feed yet, but the Z will have the 1605 for sure. I don't see myself wanting more than 1200mm/min during operations, and 2000mm/min free travel. Maybe it's just resonance, but 2000mm/min begins to sound scary on the 500mm 8*8 lead screws.

    Regarding the height, I made it that high to account for different vises. The space between the bottom of the X assembly and the build plate is a safe 110mm. That's accounting for up to a 1/2" plate. I've been looking at various vises and just came across these little guys. They shouldn't be too hard to replicate, and consume very little height. With that said, I can definitely drop the gantry. I just need to be able to work with up to two inch high material (plus vise height).

    All of the following pics are with a 1/2" (12.7mm) plate instead of the 3/8" (9.525mm) plate seen above. The end mill is just a round representation of a 1.25"-2.5" fly cutter set to 1.5". There is enough side clearance to allow entry from the left and right to surface the plate. I would keep the minimum 1.25" though.

    220mm - current
    KRE Mill V3 7.png

    220mm view without plates
    KRE Mill V3 8.png

    220mm with larger side plate - I did not change bolt locations for the pics, but you get the idea.
    KRE Mill V3 9.png

    195mm with larger side plate - Notice the change in spindle mounting height. You can also take a look at the distance of the stepper from the top of the picture frame. This is also showing min and max Z.
    KRE Mill V3 10.png KRE Mill V3 11.png

    After the 25mm drop, It still has a safe clearance of 85mm. The ironic part is that I make the "do you really need that height" argument all of the time. I do agree that the upright is more than likely the weak point. It's 4080, but then again, so is everything else. I can fill is with something. I should be able to use these brackets instead of the L's for the inside. I dropped the gantry another 5mm in these two pics.

    KRE Mill V3 12.png KRE Mill V3 13.png

    I'll definitely be posting whatever I can. I'm more than likely going to order the spindle in the next week or two. The itch needs to be scratched. I would like to get a water cooled ER16 220V 1.5kW spindle. The issue is that I don't have 220V. Tapping 220V isn't exactly rocket science, but it's not my space, and I don't want to pay an electrician to do something I know I can do. I'll probably just grab a 2kW transformer for the time being. Whatever happens, I do have 220V where I live and can play around with it to confirm it's working. If I don't order the spindle, I'll be ordering the extrusion instead.

    Speaking of spindles, another concern was the mount. I absolutely despise the generic cast mounts.
    • Weight is good for dampening, but I don't see THAT weight being beneficial. I'd rather just fill the frame with sand.
    • I wouldn't be able to precisely add mounting holes where I want. I don't have access to a press.
    • That are ugly.
    • They could help with heat, but heat isn't an issue unless something is catastrophically wrong.
    I'm kind of against the printed route, but is there actually an issue with this? Would a printed mount fatigue? My design is 120mm tall with a 15mm thick border (except the back center). The mounting holes are outside for squaring purposes. I also gave it five holes on each side with a 20mm spacing. The idea is to maybe use some for accessories such as coolant brackets or lights or a roast beef sandwich maker. The clamps are spaced 30mm apart. Everything is M5. I would definitely use washers for the mounting bolts.

    KRE Mill V3 14.png


    Further info:
    • 300mm ball screw and guides on Z
    • 500mm ball screw and guides on X and Y
    • all M5 hardware, which was another reason for keeping the 20 series
    • 267oz-in NEMA 23 steppers in the CAD model
     
  4. Andreas Bockert

    Andreas Bockert Veteran
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    First a caveat, I'm a computer guy, not a mechanics guy so take anything I have with a grain/pinch/mound of salt.

    With my machine that is rigidity challenged when it come to aluminum I have to take very shallow cuts. Shallow cuts means you need even more speed to achieve proper chip thickness. Admittedly I have done very little aluminum cutting but I have done test cuts in the 2-3000 mm/min range that were confidence inspiring. (Btw, my jog rate is 8000mm/min ). This is with 8mm OB screws.

    So, I've been pondering a little bit about lead screws. My machine is not as rigid as I would like and I need to compensate with speed. It thus makes sense to have medium torque at higher speeds rather then high torque at low speeds. Since steppers drop in torque as their RPMs hence it would make sense to use a screw with high lead to reduce the needed RPMs. Please, if I've got my head on backwards I would love to hear why (from anyone).

    I do like the look of the longer brackets.

    Regarding mounts, I have a 65mm copy of the OB style router mount. It's nice and solid but it moves the router away from Z and increase the leverage for cutting forces. In the beginning I used 2 of the heavy duty hose clamps + 90deg angle irons from OB (See Yet another 1x1 Sphinx). To be honest, aside from looks I think that solution was superior and I might go back to it or manufacture a nicer version of it.
     
  5. Kevon Ritter

    Kevon Ritter Master
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    What do you consider shallow? I was cutting 900mm/min at 2mm DOC with 2mm stepover with a 1/4" diamond o-flute under flood, and 900mm/min at 9mm DOC with a 0.5mm stepover on a 1/4" variable flute ZRN 3 flute under flood as well. 8000mm/min... I'm honestly scared to even try that. I think I capped mine at 2500mm/min. I will definitely try bumping that up slowly. I admit that it's a mental block more than anything. I haven't messed with ball screws yet, but they are much lower in the friction department.

    I completely agree with you on the torque-to-speed statement.

    Keep in mind that the OB mounts are made for routers with larger tops. The back is only 5mm thick, pretty much there just to keep it together. The thing for me is pressure. I'd rather have very little pressure over a larger area than a high pressure over a smaller area.
     

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  6. Andreas Bockert

    Andreas Bockert Veteran
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    So far I've pushed the machine to do

    1/4" 2 flute, 6mm DOC, 0.5mm stepover ("optimal load" in F360) at 2800 mm/min.
    1/8" 2 flute, 6mm DOC, 0.5mm stepover ("optimal load" in F360) at 2600 mm/min.

    It seems to me that I can run int pretty fast as long as I keep the stepover under control.

    The OB mount has 19mm towards the back so I like the 5mm design you have. It also makes sense to make it tall. If you prefer a machined look you could create 2,3 or 4 clamps out of 3/8 or 1/2" aluminum.You could also check out the ones that @Chris Laidlaw manufactures (Designed to Order Router/Spindle Mount for Almost All Routers/Spindles and CNCs | eBay)

    At least with my Makita there wasn't any interferance between the router and the back plate when I used the hose clamps.
     
  7. Kevon Ritter

    Kevon Ritter Master
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    Someone else running more than 250mm/min at 0.2mm DOC! Did we just become forum buddies? Your post made me go back and check "ideal" feeds and speeds. One critical factor missing from both of our numbers is RPM. Plugging everything back in, the 36ipm (900mm/min - switching back to imperial since that's the more used system for chip loading) works out to be 18k rpm for each tooth at 1200 SFM with a 0.002 chip load. The 3 flute should be at 108ipm (2700mm/min), but I was running it as a finisher. The Bosch did begin to bog so I had to set it to 22k rpm.

    I ultimately went with the 2mm DOC and 2mm optimal load for balance the tool use through that specific operation, but I do want to move to full depth from the get go. The goal here is to get that depth (possibly greater than 12mm) and a decent step over/optimal load.

    Since I should be able to better control RPM, I'll be properly recreating my F360 tool library.

    I was actually going to message Chris to see if he could make the Z axis rail spacers. I can do it on mine, but it's just one of those things I'd rather leave to a big machine.
     
  8. Andreas Bockert

    Andreas Bockert Veteran
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    2 mm optimal load? Whoa, now I see why you need a more rigid machine.

    I ran 1/4" at about 15000 and the 1/8" at 25000. I think the Makita has electric speed control to keep constant RPMs under load.

    Also, when taking light cuts you need to consider chip thinning. So while 1800mm/min is optimal when doing diameter/2 cuts this will increase to 3400 when doing 0.5mm cuts in order get the desired chip load. 1/8" bits will give a lot reasonable numbers.

    I'm amazed at the punishment a cheapo 1/8" can take. It makes me wonder what you could REALLY do with a 1/4" if you had the machine for it. It remains to be seen if this will work when doing longer cuts or if I'll start breaking bits.

    (Good visualization of chip thinning is at about 5:20 )
     
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  9. Kevon Ritter

    Kevon Ritter Master
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    Chip thinning is something I've thought about, but I've never actually put it together with optimal load/stepover/RDOC. In order to compensate, you must advance the cutting edge further along than normal. That 2mm doesn't sound absurd anymore, but it hurt my DOC for sure.

    This is a pretty good formula for adjustment. How To Combat Chip Thinning - In The Loupe - Machinist Blog
    [​IMG]
    Following that, dropping from 50% to 25% optimal load increases the desired chip load by 4.

    Ultimately, the greatest indicator is the chip itself. If you are throwing good size chips, there shouldn't be any heat issues, nor will thinning be an issue. The only thing that wouldn't be optimized is tool life.

    All of this isn't worth much when trying to use wet noodles to force end mills through materials. :ROFL: I'll let it sit for a couple of days (actually hours) before going for the extrusion. Maybe the little router can survive with new machining methods.
     
  10. Andreas Bockert

    Andreas Bockert Veteran
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    For smaller end-mills the weaker machines should begin to catch up to bigger ones since the weakest point will be the end-mill. So with a 1/8 end-mill the Sphinx should start to close the gap between it and a proper CNC. Consumer grade routers higher in RPMs which (in theory) should be good to achieve proper surface speed for small endmills.

    The obvious problem with using high speed toolpaths (especially if they're twisty-turny like trochoidals) is that it will be hard to reach that max speed. Even though the drivers and steppers seemed to handle pretty high acceleration it just didn't feel got to jerk the poor machine to a start.
     
    #10 Andreas Bockert, Sep 16, 2018 at 10:37 AM
    Last edited: Sep 16, 2018 at 10:44 AM
  11. Kevon Ritter

    Kevon Ritter Master
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    After replicating the Mod-Vise, I released that the build plate didn't have a dense enough pattern. So here's the updated table and vise.

    KRE Mill V3 16.png

    I've been struggling with limit switches. I want the machine to home the rear right (+x, +y). To that, the bed has to slide forward. I don't want any protruding from the front. These are the larger lever limit switches. It's either this or a proximity switch.

    KRE Mill V3 17.png KRE Mill V3 19.png

    Then there is cable management. I would like to run the cable chain to the -y side of the z stepper. That should save a bit of machine height. The only potential issue is how close the exit may be to the top of the spindle. It would have to fit three water lines, the spindle power cord, and the z limit wire. I left more than enough accommodation for whatever.

    KRE Mill V3 20.png


    I also did try higher travel rates. I didn't realize it, but my machine is hitting a resonance around 1500mm/min, but quiets back down at 2000mm/min. It did 3000mm/min just fine, but either slipped or stalled at 5000mm/min. I didn't really sit down to diagnose. Speaking of resonance, I plan to fill the extrusion on this new machine with sand. No epoxy granite, just sand.
     

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  12. Andreas Bockert

    Andreas Bockert Veteran
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    Grbl has a max pulse rate that can limit the max speed. I’m guessing you are using 1/16 microsteps? If you drop to 1/8 you might be able to go faster.
     
  13. Kevon Ritter

    Kevon Ritter Master
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    I didn't even think about that. I'm pretty sure it's set to 1/16. I'll give it a shot.
     
  14. Kevon Ritter

    Kevon Ritter Master
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    The extrusion is on order! I also started recreated all of the plates in Fusion.
    A big issue I was having was nesting the plates. It took some time, but I finally got a decent layout.
     

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