Building a CNC Router

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markbresler
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Building a CNC Router

Post by markbresler »

Hi,
I have been watching a lot of YouTube videos about building CNC machines as I would like to drill holes in 1/8" wood sheets precisely for LED strips to shine through. Many years ago I used the LM629 motion control chip from National Semiconductor to have controlled motion using a brushed DC motor and optical rotary encoders. From the YouTube videos I have seen, everyone seems to be using stepper motors, and have not seen rotary encoders used.
Can someone explain why?
Thanks,
Mark
dyarker
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Re: Building a CNC Router

Post by dyarker »

The rotary encoders aren't needed with stepper motors. A microprocessor counts the number of steps sent to the motor to know how far it has gone.

A stepper motor has several pairs of electro-magnets. Power is sent to the next pair, the motor rotates only to line-up rotor with that pair and stops. (A step.) To move more, power is sent to next pair. To change direction the previous pair is used.

Keeping power on pair acts like a brake. The load being "pushed" with not move the load.

Cheers,
Dale Y
markbresler
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Re: Building a CNC Router

Post by markbresler »

Yes, I realize encoders are not needed with stepper motors if you don't ask them to go so fast or torque so they lose steps, I guess my question is are there efficiency, simplicity, or cost benefits of using steppers vs DC motors and encoders?
Thanks
dyarker
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Re: Building a CNC Router

Post by dyarker »

Not having the encoder and associated code.
Regular motor has no braking.
Dale Y
markbresler
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Re: Building a CNC Router

Post by markbresler »

ok thanks
patmcf18
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Re: Building a CNC Router

Post by patmcf18 »

Regular brushed DC motors have a wound armature which has a lot of inertia compared to stepper motor & servo motor rotors. Also, some ac induction motor drives have quasi-servo response & can hold the rotor at zero speed while applying full holding torque. Of course, you'll pay more for servos or vector drives with ac induction motors. Steppers are generally a poor-man's approach to motion control. However, today's stepper systems have proven to be pretty reliable, but you'll pay more for high-performance stepper systems. I've seen stepper systems used with encoders and they are very accurate assuming the micro stepping of a good stepper drive/ controller has the resolution you need.
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dacflyer
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Re: Building a CNC Router

Post by dacflyer »

DC motors can be ok if you have a gear box and high gear ratio... all depends on your use.
I have torn down huge plotter printers and they had DC motors and encoders, and high geared. and the encoder tape. I couldn't even see the lines on it unless i used magnifying lens.. big gear ratios are easier to stop the load accurately ..
steppers, depends on the accuracy you need, gear box could be handy too,,I can't remember if steppers had hold between steps or not.
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haklesup
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Re: Building a CNC Router

Post by haklesup »

Seems like adapting an off the shelf 3D printer kit can accomplish this with different file type input and less component selection. There are also off the shelf CNC kits with controller and steppers but not necessarly the gantry frame components. Drilling is wood isn't necessarily demanding for torque and a high-speed Flex shaft or modified DC drill could be macgyvered in if you were being creative. you could set the speed manually or modify the control switch.

Kits for entry level 3Axis cutting I can see going for less than $200. Reinventing the wheel will cost more than that I would think.

If the only task were drilling holes in a linear array, a positioning jig would do the job much simpler.
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