CNC conversion thoughts.

[bpimm]

I have a Grizzly G1007 that was converted to CNC when I bought it. Since i bought it i have worked it over quite a bit to get it tighter. I have come to the conclusion that it is not big enough to do what I want to do so enter the Jet JVM 942F knee mill, Now I want to convert the Jet to CNC. One of the things I want after using the Grizzly is it needs to be CNC and manual not just CNC so I need to come up with a way of quick switch over or be able to use it in manual mode just turning the motors with the hand cranks.

Some questions:
I know steppers don't turn freely when not powered, do servos turn freely when not powered?

How do you figure out how big of motor (stepper or servo) you will need for a given mill?

On a servo motor can the encoder be used for the DRO when in manual mode?

I read about a Knee mill conversion where the Z axis was using the knee instead of the quill for more Z axis travel, It's an interesting idea to me, just wondering what your thoughts are on this idea. I already have one part to build that has the need for over 4" of Z axis travel...

Has anyone made or seen an enclosure for a knee mill to keep the flood coolant contained?

I'm thinking I want to use servos instead of steppers and will probably add a motor to the knee for power feed at a minimum. I'm open to any ideas on the conversion.

Thanks
Brian

 

[DMS]

I have been converting a used import knee mill that I purchased. It's been a long process, but I actually had it up and working fairly quickly. Since then, I have been "improving it". Currently I am working on a ballscrew conversion.

My setup is pretty similar to what you are thinking about, I have DC-servos on all axes driving the screws indirectly using "L" style toothed belts at a ratio of 2:1 on the X and Y, and 1.4:1 on the Z. For the Z axis, I did converted the quill, something that I often regret due to issues with quill length. I haven't yet run into an issue where this has prevented me from proceeding, but it can be annoying. The quill conversion was pretty simple, the knee conversion is a bit more involved and is something I may tackle at a later date.

As far as being able to turn servos when un-powered, yeah, that works just fine. You can turn steppers too, but they are not as smooth. With 1.8 degree steppers, you probably would not have an issue either. The main benefit of servo's over steppers is that you have a closed loop so that you will know if things get out of whack, instead of the mill just humming along and slicing off metal you meant to keep. Steppers, properly used, should not have this issue, but when it does happen, it can leave you scratching your head.

I do wonder why you feel the need for hand wheels? I often do things "manually" with the control powered just by using the keyboard. If you do go with hand wheels, then I would recommend making the handles spring loaded so that they collapse, that or using a dog-clutch so that you can disengage. Otherwise you will have some very nice wrist breakers built in

As far as using the rotary encoders as a DRO, if you use the servos to feed, you can get the same effect, though they won't be as accurate as glass scales mounted on the slides. If you run in manual mode, I don't think you will get the same effect, at least not how I have things set up. For me, the encoders are not attached to the control, but rather to the motor drivers. With a different setup it might work, but at that point you already have to have the software running... so again, why use the hand wheels.

 

[bloomingtonmike]

I recently installed an Ajax Centroid controller over the last 3 weeks. My machine is a 1987 Millport 1630 Rhino Knee Mill. It uses 29 in lb servos and 4000ppr encoders. These encoders connect to the Ajax allinonedc controller. Using the Centroid handheld milling pendant I can more easily control this machine than if I was using hand-wheels due to my inexperience. Not only that I have consistent motion through the cut with the controller. You may find you do not need handwheels as much as you thought when you get done and have a pendant. If I want precise movement I just use the MDI screen and enter the gcode manually to move the table and spindle.

I get 10 thousandths level of displayed feedback accuracy with my encoders - not sure 100th thousandths or more is needed at my skill level. I am a newbie though to the mill.


The pendant on my DIY Joes castcnc 4X4 CNC router is a XBox wireless controller.

 

[Rbeckett]

How's about some pics of these projects fellas? I would love to see some different ideas and approaches to solving the add on type of CNC gear. I have been tinkering with the idea of adding power feeds to my mini mill from HF. My hands shake so bad that I cannot get a smooth finish without relying on powerfeed to smooth over my jittering. Thanks!!!

Bob

 

[bloomingtonmike]

A walk around video work OK for you?

[video=youtube;qFE-9ClgFx8]http://www.youtube.com/watch?v=qFE-9ClgFx8[/video]


Video of coolant under software/pendant control:

[video=youtube;XfYi_r7AQoY]http://www.youtube.com/watch?v=XfYi_r7AQoY[/video]


Pics of what it originally looked like and progress pics are here:

http://s39.photobucket.com/user/BloomingtonMike/library/CNC Millport

- - - Updated - - -

How's about some pics of these projects fellas? I would love to see some different ideas and approaches to solving the add on type of CNC gear. I have been tinkering with the idea of adding power feeds to my mini mill from HF. My hands shake so bad that I cannot get a smooth finish without relying on powerfeed to smooth over my jittering. Thanks!!!

Bob

Also a mini mill is probably more of a nema23 stepper sized setup. If you want pics/video of my router controller using a Gecko G540 I have that too. That would be a perfect controller for that machine and includes 4 axis if you ever add a cheap rotary device. Max about 3.5amps for the Geckos though so pick a controller before stepper motors so you size everything correctly.

You might be able to do what you want with an arduino and a stepper maybe.

 

[DMS]

How's about some pics of these projects fellas? I would love to see some different ideas and approaches to solving the add on type of CNC gear. I have been tinkering with the idea of adding power feeds to my mini mill from HF. My hands shake so bad that I cannot get a smooth finish without relying on powerfeed to smooth over my jittering. Thanks!!!

Bob

If you have not done so, take a look at BillGrubby's power feed conversion for his 9x20 cross slide. I believe he used a small worm gear motor.[thread]16439[/thread]

 

[CNC Dude]

I know steppers don't turn freely when not powered, do servos turn freely when not powered?

How do you figure out how big of motor (stepper or servo) you will need for a given mill?

On a servo motor can the encoder be used for the DRO when in manual mode?

Hi Brian,

To answer some of your questions:

1. Steppers don't run freely WHEN powered. If you unpower them, they should just rotate almost like any other motor. Due to the way they are constructed, though, they tend to cog a little bit. I imagine the mechanics will pose way much more resistance than the motors, though. With that being said, a Servo motor will give you way even more resistance than a stepper WHEN powered, because a Servo is programmed to maintain a position, which means they will resist changes in position.

2. To size the motor you will need to know which torque you want. That question is tougher to answer as it will depend on what you want to cut, at what rates, etc. Most CNC machines process material at power rates measured in HP. I don't know much about this, but hobby mills will do something smaller than 2 HP, whereas industrial machines will do anything larger than this. Although this power is mostly referred to the spindle power, you also have to factor in feed rates, which is what your axis motors will be in charge of. If you want to go slow, small motors will do. If you want to go fast, then you will need larger motors. Stepper motors are rated in torque as oz-in or similar. The higher the torque they offer (usually the larger they are as well) the quicker you will be able to feed. There is plenty of complexity on this topic, and I think a book could be writen on it!

3. On a servo system you could use the Shaft Encoder as a DRO, but this is not a very accurate way of measuring your position, unless you account for backlash. As mentioned before, an axis glass scale is the best feedback mechanism as it is absolute. Motor shaft encoders, however, only tell you where on the angular position the shaft is located. If there is no backlash, then it is a no brainer to translate this into linear motion/position. I think MACH3 (and I imagine other CNC control SW suites) has a configuration panel for backlash. How this changes with wear, however, is a question I do not know how to asnwer.

 

[DMS]

I finally got around to posting my progress on the ballscrew conversion here [thread]18812[/thread]