New to CNC

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Chris Hollands
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New to CNC

Post by Chris Hollands »

Being new to this area and not knowing a lot about it ,I thought I would start to DIY CNC mill. I bought a 9 by 36 mill in as new condition (10 years old)about 1 year ago knowing this will be a long term project ,I also bought 4 DC servo motors a little latter ,800 oz in ,I have very little info on the motors ,they are brand new and I got them for $90 each which I thought was a good price.
I have installed ball screws to start with and will buy the drive belts and pulleys next.
Questions -
What ratio do people advise ,I was looking at 2:1 / 3:1 is this ok.

Does anyone have any info on the motors - Electro Craft corp - Model E670 - part no - 0670-06-061 - Sn - H02132. - No -600276-101F
there is also a reference to Reliance Electric.

Any help would be great.
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Rick
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Re: New to CNC

Post by Rick »

I would go 1:1 with those size motors.

Check this link out

http://www.cncroutersource.com/linear-f ... lator.html

I entered .200 pitch and 90% effic. and got about 1400lbs thrust, more than enough for that mill.

I will see what I can find on the motors, when I have a bit more time
Rick

“We make a living by what we get, but we make a life by what we give." Sir Winston Leonard Spencer Churchill (1874-1965)
"Peace is that brief glorious moment in history when everybody stands around reloading". Unknown
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redneckalbertan
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Re: New to CNC

Post by redneckalbertan »

I don't know much about CNC machines, but to my way of thinking I would figure out what the maximum speed you want the table to travel at and using the motors maximum RPM figure out from there what your gear ratio should be. It is better, in my mind, to have the motor spinning faster as opposed to slower.
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Chris Hollands
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Re: New to CNC

Post by Chris Hollands »

The higher the ratio the better the encoder reading (the count),Im not sure if that is the way to state it but I think people understand what Im trying to say ,I just cant think of the right term at the moment.
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Rick
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Re: New to CNC

Post by Rick »

Chris Hollands wrote:The higher the ratio the better the encoder reading (the count),Im not sure if that is the way to state it but I think people understand what Im trying to say ,I just cant think of the right term at the moment.


True, if the encoders are mounted to the motor and not the screw. What count encoders do you have ? I am not sure how much resolution you are trying to achieve.
There are a few thing to consider when designing your system. And as with most mechanical systems there has to be compromise. There are a few items that intertwine with each other and you have to decide which one or ones need highest priority. A few are rapid speed, resolution and thrust. Then the dynamics of the system come into play. As far as resolution goes there is a point where too much can be a problem and too much can be also just useless. Take into account what kind of machine and its inherent accuracy you are dealing with. What type of ball screws did you install (ground with zero backlash?) The old Hurco CNC's came both with .001 or .0001 resolution. the .0001 was better but In my opinion on a knee mill anything smaller than .0001 resolution in the control is useless. The old rule of thumb in QC is your measuring instument needs to have 10 times the resolution of what you are trying to measure. ie to measure .001 you instrument needs .0001, now this is more for statisical process controll but it still makes some sense on machine resolution. I am sure that there is more clearance in a knee mills gibs than .0001 so trying to hold that kind of tolerance (on a regular basis) is not practical. Not saying you are trying to do that just making a point. I guess what I am trying to say with all of this rambeling is dont go overboard with things. You have plenty of power with the motors you have. If you dont have encoders picked out then you have a bit more flexabilty on how you can set the drive system up. A inline direct set up is much easier and simpler to achieve with out backlash.

Also you need to make sure how fast of an encoder pulse input your controller can handle, your documentation should state a hertz figure. calculate it using your max motor rpm (convert it to rev per sec) and multiply by the encoder counts per revolution
Rick

“We make a living by what we get, but we make a life by what we give." Sir Winston Leonard Spencer Churchill (1874-1965)
"Peace is that brief glorious moment in history when everybody stands around reloading". Unknown
Murphy's Law: " If it can go wrong it will"
O-Tool's Corollary: "Murphy was entirely too optimistic"
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Chris Hollands
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Re: New to CNC

Post by Chris Hollands »

Thanks Rick

To go a step further ,I was looking at using the Centroid control system and they recommend 2000 resolution encoders ,that is what they sell as part of there package.

The encoders will be attached to the servo motor.

Do people have other options /encoders at a reasonable price and quality or sellers they would recommend .

Attached is the spec sheet for centroids encoder , see what you think.
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Rick
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Re: New to CNC

Post by Rick »

Chris

What pitch are your ball screws?

I am not really up on who has what as far as encoders go, sorry.
Rick

“We make a living by what we get, but we make a life by what we give." Sir Winston Leonard Spencer Churchill (1874-1965)
"Peace is that brief glorious moment in history when everybody stands around reloading". Unknown
Murphy's Law: " If it can go wrong it will"
O-Tool's Corollary: "Murphy was entirely too optimistic"
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Bill Shields
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Re: New to CNC

Post by Bill Shields »

I would put the encoders on the motors and do a 2:1 or 4:1 ratio. perm-mag motor like to spin as opposed to grunt from stall at low speeds. if you had a/c servos would be a different story.

Any reasonable incremental encoder of 2048 or 4096 pulses / rev will do you just fine.

Be sure to get one with a once / rev pulse necessary for the 'home' position.

Suggest you decide which amplifier / control boards you are going to use and see what pulse / revolution they support, and purchase encoders accordingly.
Too many things going on to bother listing them.
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Chris Hollands
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Re: New to CNC

Post by Chris Hollands »

Thanks Bill & Rick

Rick the ball screws are rolled type and 0.200 pitch
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