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Cnc That Keeps Manual Wheels, Too?

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Chris H

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As I understand it, since a stepper motor only moves a set distance when told by the computer and otherwise doesn't know where it is, there is no way manual handwheels could be used with steppers, without re-homing the machine. Since servo motors use an encoder, they should theoretically know where they are and readjust if moved by handwheels.

First, is that even right?

Second, if so, what are the disadvantages to using a handwheel with a servo set up on a cnc machine? Would it be better with a belt drive system? Is it even possible with a direct drive set up?

I'm asking because I can see where a "quick need" could come up that would make running through a cnc program more trouble than it would be worth, and the handwheels would be an advantage.

Or, is this where just investing in a control pendant and running the machine from it would be the best course of action?
 

JimDawson

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I am a real fan of a fully convertible machine for prototyping and general shop use. You are correct that sometimes trying to run a simple part under CNC control is just not worth it. My machine has hand wheels as well as a disconnectable Z-axis. I can go from 3 axis CNC to full manual in about 10 seconds. I can also run 2 axis CNC with a manual Z, works great for doing hole patterns. Just yesterday I needed to mill a flat on a shaft, my mill was tied up but I have Alloy's mill in my shop working on a retrofit. His mill does not have hand wheels so I had to use the pendant to do the job. It was a bit of a PITA.

You are also correct that a stepper system has no idea where it is. This is called an open loop system. If you were to move the table manually, it would be lost. It is possible to use encoders with steppers and make it a closed loop system. There are also some hybrid stepper systems that have on-board encoders and electronics that make them pretty much like a servo. I have no experience with those so I really don't know what the capabilities are.

With the proper stepper drives, encoders, and controller it is possible to run a stepper just like a servo with an analog command voltage. The controller doesn't even know it's running a stepper. This is the way my mill Z axis is set up and all 3 axis on my CNC router.

The best method of using encoders is to put the encoder on the load (table) using a linear device like a glass scale or magnetic reader. This eliminates any lead screw or backlash error.

There are no disadvantages that I know of to using hand wheels on a servo system. A belt drive would be the preferred method. It is possible to attach the hand wheel to the end of the motor shaft but that would make it a long reach.
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RJSakowski

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My Tormach is a CNC only machine in there is no way to manually drive it. However, the Mach 3 flavor that it uses has a manual jog/shuttle feature that allows me to jog in fixed increments or to move a chosen axis in one of seven speeds from a fraction of an inch per minute to about 60 in./min. I use the latter to do "manual machining" as my typing is not trustworthy enough to use conversational control.

While it does not have the tactile feedback that you get by manually running the crank, it is fairly similar to having a power feed feature on a conventional mill. When youo get close to your feed endpoint, it incremental jog will take you there precisely.
Bob
 

Tony Wells

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Most CNC machines have a manual wheel that is a pulse generator so you can move the axes "manually". But of course, they also have encoders to track their movements and therefore keep up with position. If you chose to use a PG, and were content to manually rehome when you were ready to hit the green button, I'm sure it could be switched in and out as needed.
 

Baithog

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Years ago it was recommended to me to install DRO's before I converted my original mill. I didn't because I was being cheap, but now that I am using the mill more, I'm gonna do it. The DRO always knows where the machine is in space. It you need to do something manual, then restart a program, the DRO makes it quite easy. Just remember to write down the current coordinates before shutting off the motor power and twiddling the hand wheels. To restart the program, set the machine back to the DRO coordinates saved and restart the program. The controller and the program have no idea what was going on while stopped and blissfully continue on. Forgetting to set the machine back or failing to restore power to the motors could get exciting. A pendant is another possibility. They are not overly expensive and can control all possible machine motions. They have 2 disadvantages. The first is that they do not provide the feel and flexibility that true manual control does. The other is that the pendant is working through the machine controller. Not being totally separate can cause execution problems when doing mixed manual and CNC. I will be rebuilding my mill after the current project. Mixed mode operation is definitely on the list.
 

chevydyl

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On my machine I have servos, using Mach3, there is no motor encoder feedback, even tho the encoders will tell the drives where they are, it doesn't tell Mach where they are, when I use manual I just disable the drives with a switch, and it still counts in ultraware, but it means nothing to Mach for position. So, if your running mach, it's still open loop, even tho the drives and motors are closed loop. A control like prototrak uses both glass scales and motor encoders. Although older prototrak mills have a roller read head with no actual scale
 

Surprman

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I'm building a CNC mill and chose stepper motors with dual shafts so I could install handwheels if I want. Installation of a DRO would allow manual operation as other noted above.

Rick
 

chevydyl

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Something else you guys should be aware of is turning certain motors by hand can create back emf I believe it's called and fry drives unless there is something in place to handle it. I think stepper motors may be on that list, can't remember for sure you'll have to find out, brushed DC servos are on the list, my brushless AC servos don't create the dangerous back Voltage when turned by hand, or rather by hand crank through the belt drive
 
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