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Workbee 750mm Aluminium Help

Discussion in 'CNC Mills/Routers' started by Benjamin Hoey, Feb 16, 2019.

  1. Benjamin Hoey

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    G'Day from Australia!

    I bought myself a Workbee 750mm x 750mm screw version.

    About the set up:

    1.5kw air cooled spindle with vfd - https://www.sharpsoftware.com.au/product-page/1-5kw-air-cooled-spindle-vfd
    Fusion 360 for cam/cad
    OpenBuild Control program using grbl, arduino, tb6600 stepper drivers.
    Steppers are 2.45Nm - https://www.sharpsoftware.com.au/product-page/nema23-stepper-motor-2-45n-m
    Cool mist system with oil/coolant mix as recommended by my local machine shop (120 psi air pressure to remove chips).

    I did my research and I expected that the Workbee would mill aluminium with enough preparation (watching ooznest YouTube videos have inspired me). I am only wanting to build prototype parts. This is not for commercial use. I plan on getting a working prototype and then sending the cad files to commercial cnc for production.

    But I have had serious trouble doing simple things with aluminium.I have had no trouble with MDF or plywood. And I was very happy until this point.

    I was able to get a part cut from aluminium using the absolute lowest settings - 100mm/sec feed rate and 50mm/sec plunge rate. I was making dust (which is bad I read) and the finish was not good, but it was passable for prototype - BUT the main problem here, was the time it took to produce the part - for one of the parts, fusion estimates 6 hours.

    I slowly sped things up but things got worse.

    I dont know what the problem is, but below are some questions/problems to help figure this out. Any help/advice for any of the questions would be appreciated.

    BITS?:
    I started with cheap 6mm 2 flute hss flat end mills with the intent on buying decent carbide when I get everything going. I have one 3 flute 6mm carbide bit which I tried after the hss failed. This did not seem to make any difference.

    First observation:

    All the bits seem to slide around the aluminium.
    Watching as the z axis comes down, I noticed that the stepper seemed to have enough torque, but the shaft adaptor was flexing so much that there was not much pressure on the aluminium.

    I have made sure that the eccentric spacers are adjusted properly. There is almost no play in the z axis in any direction, except when I pushed up the z axis with my hand, and saw flex from the shaft adaptor.

    First question: Am I using the correct shaft adaptor (came with kit) - is the flex I am getting normal?

    6.35mm x 8mm Flexible Coupling - Maker Store


    IMG_2616.JPG IMG_2617.JPG


    Second observation:

    The backlash nut block on the z axis is supposed to be adjusted? I have not adjusted this at all because of the location of the nut (in between the z axis plates) and also because I dont believe I have any backlash.

    Second question: Does this adjustment also affect the torque of z axis coming down?




    Here are some of the setting I have used (none have created 'chips'):

    Tried this first:
    10000 rpm (also tried slower and faster)
    100mm/min cutting feed rate
    100mm/min lead in feed rate
    100mm/min lead out feed rate
    50mm/minramp feed rate
    50mm/minplunge feed rate
    0.1mm pass depth and 0.3mm and 0.5mm

    Following fswizard calculator online version.
    8000 rpm
    833mm/min cutting feed rate
    100mm/min lead in feed rate
    100mm/min lead out feed rate
    50mm/minramp feed rate
    50mm/minplunge feed rate
    0.1mm pass depth and 0.3mm and 0.5mm


    Following Ooznest Youtube Video:
    16000 rpm
    730mm/min cutting feed rate
    100mm/min lead in feed rate
    100mm/min lead out feed rate
    20mm/min ramp feed rate
    316mm/min plunge feed rate
    0.5mm pass depth


    IMG_2604.JPG IMG_2620.JPG

    Thanks very much,
    Benjamin.
     

    Attached Files:

  2. GrayUK

    GrayUK Openbuilds Team Elder
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    I don't know if you've seen this video, but check it out.

    Estlcam is a great little program.
     
  3. Benjamin Hoey

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    I hadn't seen it before. I dont have the cash to buy it. But I will get the trial and give it go tomorrow morning.

    This doesn't feel like a solution to my problem though. All the successful aluminium cutting videos I have seen using the Workbee do not use this method.

    Thanks.
     
  4. Alex Chambers

    Alex Chambers Master
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    Are you saying that the flexible coupler is compressing when you press up on the Z axis, or is it moving sideways. In either case there must be a problem with the mounting of your leadscrew. Check the bearings are seated properly in the recess in the end plates and that the lock collar is pressed firmly against the bearing and its grub screw is tight. Trochoidal milling might help once you have eliminated any hardware problems, but fusion 360 has an equivalent - I'll post more detail later when my computer is running.
    Alex.
     
    Rick 2.0 likes this.
  5. Alex Chambers

    Alex Chambers Master
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    Fusion 360's version of trochoidal milling (in 3D) is called SPIRAL. Couldn't find anything similar in 2D, but there are various ramp options as well as the facility to do a roughing cut (eg adaptive clearance) first then a finishing cut for the final mil or 1/2 mill.
    Alex
    PS: on a new machine it's unlikely you will have backlash on your leadscrew. The plastic anti-backlash nut is a tight fit to start with. You can adjust it with an Allen key, but it will need very little adjustment and only when the plastic nut starts to wear. I haven't found a way to tighten the locknut yet - I think I will have to make a right angled spanner to get at it.
     
    #5 Alex Chambers, Feb 16, 2019
    Last edited: Feb 16, 2019
    Colin Mccourt likes this.
  6. Benjamin Hoey

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    Thanks Alex for confirming the backlash nut. I wasn't sure.

    I rebuilt the lead screw assembly and only found a problem with the flexible adaptor, it seemed worn out (more flexible than a new one). I replaced it with a spare one I have. I have also purchased a 'rigid' adaptor, it should be here on Wednesday. Maybe that will help?

    6.35mm x 8mm Rigid Coupling - Maker Store



    After installing the new adaptor everything was solid again, no flex anywhere on the z axis.

    For a simple test I went to mill a 30mm x 30mm x 3mm square to see what would happen.

    The first square was terrible, a lot of chatter in the machine.

    1.jpg

    Settings I used:
    6mm bit
    40% step over
    0.5mm depth of cut
    500mm/min feed rate
    50mm/min plunge rate
    8000 rpm


    The second square, I changed back to the hss bit and it was better, but the bit broke.

    2.jpg

    Settings I used:
    6mm bit
    40% step over
    0.5mm depth of cut
    500mm/min feed rate
    50mm/min plunge rate
    12000 rpm

    I also note that my eccentric spacers were loose after each operation. I am going to assume that they are getting loose during the operation, which is obviously bad for accuracy. Any ideas how to fix this?

    Thanks.
     
  7. Christian James

    Christian James Journeyman
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    I cant really advise you as I have yet to cut aluminium (I will be soon though). But, personally, I would use bits made especially for aluminium. I would also not use 6mm cutters but rather 1/8th or 4mm. Also, do you know what alloy your aluminium is?
     
  8. Benjamin Hoey

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    Yes, it is 5083 plate. I also have 6060 T5 bar. The 5083 is what I need for my application.

    Here is a video of a machine like mine cutting aluminium with a 6.35mm bit, so I know it is possible. I used their settings, the only difference I can see is that they used a carbide bit – cutting starts at 43 seconds in.

    Ooznest Video:


    Thanks.
     
  9. Alex Chambers

    Alex Chambers Master
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    Your problems with the flexible coupler are a bit of a red herring - it is being damaged by problems elsewhere rather than being the cause of your problems. I put a modified version of carbide's speeds and feeds chart in the helpful tools thread. Your settings aren't far off but carbide suggest a speed of 17500 rpm for 6061 aluminium and a 1/4" bit. Your speed does seem a bit low for a single flute bit. I would suggest the next attempt should limit the DOC to 0.2mm.
    Your problems are caused by vibration and lack of rigidity, which is shaking your Z axis loose.
    Which eccentrics are working loose? Z axis, X axis or both? Is everything tightened up, including spindle mounting to Z axis? Use new nylock nuts if they seem to be going on to the screws easily (the screw thread can form itself in the nylon and they don't grip as well.) Is there any lack of rigidity anywhere else in the machine that could be allowing vibration to build up?
    Apart from suggesting places to look someone who is not standing in front of the machine can't help much.
    Good luck, Alex.
     

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