7 ms·

Neat kinematics with a horribly flexible frame that denies any serious usage. That said - it's designed to a low budget that increases the potential audience, a

Neat kinematics with a horribly flexible frame that denies any serious usage. That said - it's designed to a low budget that increases the potential audience, and that is something that can only be welcomed. I'm particularly interested in the software as that will be an enabler for future variations that have mechanical rigidity. Nice website!

mabbo 5y ago
Are there similar systems- CNC with multiple heads- that don't have this problem? I played with a wood router CNC in high school and loved it. I've also played around with 3D printers a bit. The idea of having (and building?) a reasonable system that can do both strong appeals to me.
- joshu 5y ago
  no. anything that is strong enough to, say, CNC will also be very slow for 3d printing or laser. different processes demand different designs.
  - jjoonathan 5y ago
    Depends on the price class, of course. If you spare no expense, you can employ lots of iron and lots of power to throw around the iron. https://youtu.be/RcansEmVknA?t=150s https://youtu.be/RcansEmVknA?t=150s
    - sbierwagen 5y ago
      For context, that DMU50 mill is about $275,000 new.
- enragedcacti 5y ago
  The closest that can do both is probably MPCNC (https://docs.v1engineering.com/mpcnc/intro/ https://docs.v1engineering.com/mpcnc/intro/). it is built with that modularity in mind but as other commenters have pointed out building something for rigidity makes it slower and heavier. a cheap 3d printer like an Ender 3 in addition to the MPCNC makes more sense, especially since you can save on the MPCNC by 3D printing the components yourself. The main benefit to swapping a 3d printer head onto the MPCNC would be the pretty massive XY size you can build the MPCNC out to. If you need to print really large, flat objects it is probably cheaper than a lot of alternatives. But at that point it probably would make more sense just to mill it out of wood anyway.
ivanbakel 5y ago
>with a horribly flexible frame that denies any serious usage What's the risk from this? That CNC work will be affected by resistance from materials, causing the frame to bend? What kind of scales would that effect be noticeable on?
- _jal 5y ago
  For any real cutting, you need rigidity or nothing will be accurate. You will not be doing any real cutting on this machine.
  - ivanbakel 5y ago
    But to what extent will it be inaccurate? It seems unlikely to me that anything used for machining hard materials can be 100% rock-steady. Presumably, industrial machine are simply inaccurate to very small, tolerable degrees.
    - Arcanum-XIII 5y ago
      I have a small mill, it still weight around 1250kg… to be accurate at 0,01. And for that I would need to lock the unused axis and respect the power of the machine. This is not CNC, where the forces are often higher because they can work faster, with curve and some other things I’ll have a hard time doing.
    - abakker 5y ago
      Accuracy and Precision would be problematic with this design, but, lack of rigidity is more insidious. Most of these high speed spindles will use carbide tooling, which is highly susceptible to breakdown under vibration. A flexible horrible frame like this is going to vibrate a lot even in materials like delrin or HDPE. Also, with a vertical spindle and CNC control, there is no reason to have a vertical rotary axis. The reduced motion doesn't buy any capability upgrade. The 4th axis that is helpful would be angular. (you could put a horizontal spindle on this, I guess, but this machine is not up to the task.
    - deleted 5y ago
      [deleted]
londons_explore 5y ago
I would like to see software compensation for frame flexibility. Either open loop (ie. use mechanical models to predict frame flexing and compensate for them), or closed loop (have sensors to detect realtime head position). The whole idea of having a frame which is 100% rigid seems like a simplifying assumption that makes everything more expensive, with knock on effects too (eg. making stuff stronger makes them heavier, which in turn means a bigger motor is needed, which uses more power, so a bigger power supply is needed, etc)
- mankyd 5y ago
  I would think it would have to be closed loop. Material density is not always uniform (especially with something like wood) and that has one of the biggest effects on how your tool behaves. Even then, compensating for sudden release in tension, where the tool tries to spring back, seems perhaps insurmountable.
- jjoonathan 5y ago
  I expect it would be easy to compensate position (closed loop is already table stakes in the professional tier) and extremely difficult to compensate chatter. What would it take to drive the motors of a 5 axis CNC to responses well into the kHz? You would need that, to compensate chatter. You would also still need to handle the same peak force, whether or not you do it with negligible deflection.
  - prova_modena 5y ago
    There are some industrial machines that compensate for chatter by slightly varying the spindle speed when chatter is detected. However machines so equipped are fundamentally very rigid and usually experience chatter occurs when pushing end mill performance to its limits. I suspect the adaptive spindle speed strategy would be much less useful on a machine like the Polar Bear.
    - jjoonathan 5y ago
      Right, resonance dodging and deflection compensation aren't going to cut it here, not by a long shot. I still think a sufficiently good 5 axis control loop could damp acoustics: for one, the (fractional) millisecond timescale of the acoustic oscillations are geological by compute standards, and while I'd expect limitations from sensing, actuation, and power electronics, I don't see anything fundamentally prohibitive. I'm pretty sure this is an engineering problem, not a physics problem, and that eventually someone is going to crack this nut. It won't be the Polar Bear.
canadian_tired 5y ago
Exactly. I can't speak for laser, extruders, knives or such, but for subtractive work (i.e. milling / drilling operations with some form of rotary tool) this design would be an exercise in frustration. Typically, you would have encoders on each axis to tell the machine where things are happening. These encoders are excellent, but any flex/slop in the chassis or moving parts is not caught by the encoder... so the software will not know that the cutter is in the wrong place. Thus, crappy parts. Any reasonably useful CNC design takes rigidity and backlash prevention seriously. Take a look at PocketCNC for a good (and yes, expensive) design.
- Turing_Machine 5y ago
  You could probably use it for ECM or EDM, but yeah, not stiff enough for traditional hogging cuts as a lathe or mill.
- galangalalgol 5y ago
  could you use strain gauges to measure the deflection and fix it in software?
  - harperlee 5y ago
    Great question. Human arms are also very bad in rigidity, repeatability, etc. but we manage to do great work by adjusting. Of course we try to move and rest on things to get out of the error-prone postures (so in effect moving from this to a gantry type of configuration), and need to measure, create jigs and templates, etc. but there is something here to be covered by sw for sure.
    - morpheos137 5y ago
      Comparing human arms to CNC "arms" makes about as much sense to me as comparing a sparrow to an F16. Yeah both humans and multiaxis cnc machines can articulate their arm to apply a tool to a work piece and both a sparrow and an F16 can fly through the air.
ZeroGravitas 5y ago
Whats the definition of "serious" here? Is there a standard measure of tolerance or material compatability that it can be rated against to see if it would work for a specific need?
- icegreentea2 5y ago
  CNC (and mfg in general) will have different tolerance needs. And the flex on this machine will depend on the material and work speed. I'd probably argue that anything worse than 0.5mm (~20 thou) will start giving you headaches if your making parts that are supposed to interact with each other. That said, if you're making basically oneoffs and are willing to handfit, that tolerance is probably tolerable (ha!).
- SeasonalEnnui 5y ago
  Serious = harder (hence more useful) materials. As it stands I would say only materials that could be cut with a pair of scissors (card, foam) could be used on this safely without frustration.
- sbierwagen 5y ago
  It's not hard to make a mill that can cut shapes into a block of styrofoam. If I stretch I can maybe imagine some industrial uses for a styrofoam-only CNC mill. (Moldmaking?) But what would a regular consumer use it for? "Serious" just means the ability to cut more useful materials, like wood or steel.

mabbo 5y ago

Are there similar systems- CNC with multiple heads- that don't have this problem? I played with a wood router CNC in high school and loved it. I've also played around with 3D printers a bit. The idea of having (and building?) a reasonable system that can do both strong appeals to me.

joshu 5y ago
no. anything that is strong enough to, say, CNC will also be very slow for 3d printing or laser. different processes demand different designs.
- jjoonathan 5y ago
  Depends on the price class, of course. If you spare no expense, you can employ lots of iron and lots of power to throw around the iron. https://youtu.be/RcansEmVknA?t=150s https://youtu.be/RcansEmVknA?t=150s
  - sbierwagen 5y ago
    For context, that DMU50 mill is about $275,000 new.
enragedcacti 5y ago
The closest that can do both is probably MPCNC (https://docs.v1engineering.com/mpcnc/intro/ https://docs.v1engineering.com/mpcnc/intro/). it is built with that modularity in mind but as other commenters have pointed out building something for rigidity makes it slower and heavier. a cheap 3d printer like an Ender 3 in addition to the MPCNC makes more sense, especially since you can save on the MPCNC by 3D printing the components yourself. The main benefit to swapping a 3d printer head onto the MPCNC would be the pretty massive XY size you can build the MPCNC out to. If you need to print really large, flat objects it is probably cheaper than a lot of alternatives. But at that point it probably would make more sense just to mill it out of wood anyway.

ivanbakel 5y ago

>with a horribly flexible frame that denies any serious usage What's the risk from this? That CNC work will be affected by resistance from materials, causing the frame to bend? What kind of scales would that effect be noticeable on?

_jal 5y ago
For any real cutting, you need rigidity or nothing will be accurate. You will not be doing any real cutting on this machine.
- ivanbakel 5y ago
  But to what extent will it be inaccurate? It seems unlikely to me that anything used for machining hard materials can be 100% rock-steady. Presumably, industrial machine are simply inaccurate to very small, tolerable degrees.
  - Arcanum-XIII 5y ago
    I have a small mill, it still weight around 1250kg… to be accurate at 0,01. And for that I would need to lock the unused axis and respect the power of the machine. This is not CNC, where the forces are often higher because they can work faster, with curve and some other things I’ll have a hard time doing.
  - abakker 5y ago
    Accuracy and Precision would be problematic with this design, but, lack of rigidity is more insidious. Most of these high speed spindles will use carbide tooling, which is highly susceptible to breakdown under vibration. A flexible horrible frame like this is going to vibrate a lot even in materials like delrin or HDPE. Also, with a vertical spindle and CNC control, there is no reason to have a vertical rotary axis. The reduced motion doesn't buy any capability upgrade. The 4th axis that is helpful would be angular. (you could put a horizontal spindle on this, I guess, but this machine is not up to the task.
  - deleted 5y ago
    [deleted]

londons_explore 5y ago

I would like to see software compensation for frame flexibility. Either open loop (ie. use mechanical models to predict frame flexing and compensate for them), or closed loop (have sensors to detect realtime head position). The whole idea of having a frame which is 100% rigid seems like a simplifying assumption that makes everything more expensive, with knock on effects too (eg. making stuff stronger makes them heavier, which in turn means a bigger motor is needed, which uses more power, so a bigger power supply is needed, etc)

mankyd 5y ago
I would think it would have to be closed loop. Material density is not always uniform (especially with something like wood) and that has one of the biggest effects on how your tool behaves. Even then, compensating for sudden release in tension, where the tool tries to spring back, seems perhaps insurmountable.
jjoonathan 5y ago
I expect it would be easy to compensate position (closed loop is already table stakes in the professional tier) and extremely difficult to compensate chatter. What would it take to drive the motors of a 5 axis CNC to responses well into the kHz? You would need that, to compensate chatter. You would also still need to handle the same peak force, whether or not you do it with negligible deflection.
- prova_modena 5y ago
  There are some industrial machines that compensate for chatter by slightly varying the spindle speed when chatter is detected. However machines so equipped are fundamentally very rigid and usually experience chatter occurs when pushing end mill performance to its limits. I suspect the adaptive spindle speed strategy would be much less useful on a machine like the Polar Bear.
  - jjoonathan 5y ago
    Right, resonance dodging and deflection compensation aren't going to cut it here, not by a long shot. I still think a sufficiently good 5 axis control loop could damp acoustics: for one, the (fractional) millisecond timescale of the acoustic oscillations are geological by compute standards, and while I'd expect limitations from sensing, actuation, and power electronics, I don't see anything fundamentally prohibitive. I'm pretty sure this is an engineering problem, not a physics problem, and that eventually someone is going to crack this nut. It won't be the Polar Bear.

canadian_tired 5y ago

Exactly. I can't speak for laser, extruders, knives or such, but for subtractive work (i.e. milling / drilling operations with some form of rotary tool) this design would be an exercise in frustration. Typically, you would have encoders on each axis to tell the machine where things are happening. These encoders are excellent, but any flex/slop in the chassis or moving parts is not caught by the encoder... so the software will not know that the cutter is in the wrong place. Thus, crappy parts. Any reasonably useful CNC design takes rigidity and backlash prevention seriously. Take a look at PocketCNC for a good (and yes, expensive) design.

Turing_Machine 5y ago
You could probably use it for ECM or EDM, but yeah, not stiff enough for traditional hogging cuts as a lathe or mill.
galangalalgol 5y ago
could you use strain gauges to measure the deflection and fix it in software?
- harperlee 5y ago
  Great question. Human arms are also very bad in rigidity, repeatability, etc. but we manage to do great work by adjusting. Of course we try to move and rest on things to get out of the error-prone postures (so in effect moving from this to a gantry type of configuration), and need to measure, create jigs and templates, etc. but there is something here to be covered by sw for sure.
  - morpheos137 5y ago
    Comparing human arms to CNC "arms" makes about as much sense to me as comparing a sparrow to an F16. Yeah both humans and multiaxis cnc machines can articulate their arm to apply a tool to a work piece and both a sparrow and an F16 can fly through the air.

ZeroGravitas 5y ago

Whats the definition of "serious" here? Is there a standard measure of tolerance or material compatability that it can be rated against to see if it would work for a specific need?

icegreentea2 5y ago
CNC (and mfg in general) will have different tolerance needs. And the flex on this machine will depend on the material and work speed. I'd probably argue that anything worse than 0.5mm (~20 thou) will start giving you headaches if your making parts that are supposed to interact with each other. That said, if you're making basically oneoffs and are willing to handfit, that tolerance is probably tolerable (ha!).
SeasonalEnnui 5y ago
Serious = harder (hence more useful) materials. As it stands I would say only materials that could be cut with a pair of scissors (card, foam) could be used on this safely without frustration.
sbierwagen 5y ago
It's not hard to make a mill that can cut shapes into a block of styrofoam. If I stretch I can maybe imagine some industrial uses for a styrofoam-only CNC mill. (Moldmaking?) But what would a regular consumer use it for? "Serious" just means the ability to cut more useful materials, like wood or steel.