CNC Motors

[JerryB]

I have friend (former student) who is taking vocational training in CNC programing and is helping me rebuild my Wells-Index mill. He has already completed 2 courses and is taking a third.

In our conversations he mentioned that perhaps I should just set up this machine as a very basic CNC rather than outfiting it with a DRO. This makes sense because "Home Machinist" magazine has started a series on the contruction of a home built CNC control unit.

I have all the computer stuff a person could want so there is a savings of about $1K right off the top of the $1,700 estimate to construct this unit. The article has the unit attached to a BP that is already setup for CNC.

Now the Question(S)....

What type of motors would I need to purchase to make this assembley work? Are they called servos or other names? I understand they give feed back to the computer in order to tell the number of revolutions of the motor and hence the amount of travel. If I need to run them through a gear box in order to gain enough torque, will the software/controller board take this into consideration? Is there a process that must be undertaken to syncronize the lead screw(s)pitch and/or gear reduction unit ratio to the computer?

AS you can see I ain't well versed on CNC. My only experience was about 30 years ago and we used a ticker tape machine to program the unit. I was in charge of making the adapters to contect the motors to the various machines so we could use one CNC to drive different machines at different times. We had to think in reverse order to program. That is we had to think "negative" to move right and "positive" to move left. After several hours of frustrations I gave up.

I have to place either a hand wheel or a power feed mechanism on the right hand side of the table, (mine lost the power feed decades ago) so why not a CNC motor?

Ok guys...tell me everthing I ever wanted to know about the mechanics and electrics of CNC.
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[AndrewMawson]

The more sophisticated machines use servo motors, that seek to a position using feedback from (usually) an indexing slotted disc on their shaft. Simpler systems use stepper motors, and count where they expect to be rather than use feedback.

Servos tend to be faster and smoother, but there are big strides being taken in stepper drive technology so don't rule them out by anymeans.

Look at

http://www.ahha.com/index.htm
http://www.geckodrive.com/
http://www.linuxcnc.org/

[Vampyr]

In general I'd say servos are better, *BUT* they also tend to be more complex systems and more expensive. A servo generally needs some form of servo amplifier system (usually PWM) I may have to put up some schematics some time... The best way to get servos is find someone dumping old equipment and acquiring them. I have a Pile of Koll-morgan Amplifiers and PMI servodisks that I acquired this way (pulled from out dated PnP machines). I have not been able to find a price on them new or even used for the servos, but the amps are about $350 used and those servos are $511 to rebuild.... Do you want to take a stab at new prices? However these are also very high end hardware with peak torques in excess of 1000 in-oz and 0 to 3000 rpm acceleration in 60 degrees.
Encoders, there is a cheapskate way to do those... There are two of them in every $10.00 mouse on the market.
Perhaps the best position feedback is optical glass slides.
Check this site http://www.lasermotion.com/Servo.html . I have never delt with them so I don't know about service quality, but I saw a lot of goodies for reasonable prices.

Well I have a few other things to take care of, I'll try to get back to you tomorrow

[JerryB]

So it appears that the most economical way to go would be with stepper motors.
Do these require anything different in regards to the computer or other input/output devices?
I see you mentioned amplifiers. I take it to mean these units take the weak computer generated signal and then enhance and increase it in voltage and amperage necassary to turn a motor.
If I go with steppers, will I need other hardware that the magazine article doesn't mention?

[JimGlass]

Hi Jerry;

You can do it. Below is a CNC mill I designed and built from scratch in my gargage. It cost me $350 for the steppers, driver board (Xylotec) and power supply. I'm having a ball with this thing. Took it to work for show and tell and 2 engineers had me run jobs it. Now they want to build one.

The software is TurboCNC and the license cost $20 and runs in DOS. I program from my laptop throught the printer port to the Xylotec board. The 3 steppers wire directly into that board and your CNC. The machine was built strictly from information obtained on the internet and a majority of the info came from this message board.
Gota run,
Jim

[JerryB]

I saw that unit and it is certainly impressive.
I am trying to outfit my old Wells-index with CNC.
I found these on ebay and they are within 1.5 hours drive of me.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... gory=25293

I am bidding on them because they worked on a Bridgeport so they must surely work on my machine.
Have I messed up? Will the power supply and controller board be a problem or will they be fairly easy to find?
Thanks for the help so far guys. It IS getting interesting.....

If you see anything on ebay or know of something I will need PLEASE don't hesitate to let me know.
Bridgeport stepper motors

[Vampyr]

If you win, great, but I'd look for a bit smaller... A drive circuit would be cheap enough to modify for the power consumption, but those puppies are going to require a fairly high output supply (read fairly expensive) If you can find a decent transformer, it shouldn't be much of an issue.... Look for surplus equipment, especially ex-military DC supplies. You may want to ask the seller about what ratings the old power supply had or what the controller was since he said it was a retrofit job....

[Doug_C]

Hi Jerry,

Those stepper motors were from the old BP Boss machines. Marty is using the same motors in his machine as it came with them. See our other discussion on here within the first thread of this forum. He is using the Gecko drivers.

Motor size will be dictated by torque required to move the axis. Mechanical advantage can be gained by timing belt pulley ratios. As you probably already know, that relates to work done over time. The more torque at the motor, the less time to get a mass moving from point A to point B. This is where reducing friction is advantageous. Ballscrews have done wonders for reduced motor and drive system size.

Stepper motors are the cheapest way to go, but can give the biggest inaccuracies with no feedback to the controller. Even though the controller thinks it stepped them 2000 steps, it does not know when some of those may be missed due to high slip loads. This is considered an open loop system. Not many in industry still use this application for potential high loading situations without feedback, but workable if you are willing to live with the limitations.

Closed loop systems are much more reliable in terms of accuracy and less hunt in interpolated moves. Either rotary encoder or linear encoder feedback will suffice. This is primarily for position tracking. Some closed loop systems also use a tachometer for feedback to gain axis velocity information. Basically it is a DC motor turned into a little generator that produces a very linear voltage based on RPM. With these all tied direct coupled to the servo shaft, this keeps the servo in control and out of a run-away condition. On some of the newer systems the encoder pulses are used in the same way a tach feedback is. Of course the CPU clock frequency must be capable of tracking pulses at that rate to make the conversion.

Once you have the system assembled, a software interface will need to be capable of taking in a text file with G and M codes that instruct the main controller board how to control the servo amps as beasts of burden. All the questions you asked about gearing ratios and resolution are taken care of by software parameters. Sure the hardware plays a role, but that is where most values to place in the parameters come from. These can be anywhere from max speed to travel limits to motor direction. Once set they are behind the scene.

After all that you are free to worry about the actual part programming. At times, that can be enough trouble in itself.
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DC