Manual and CNC

My son bought a Turn Pro knee mill bare bones, no power feed or DRO. It has almost no time on it, it still had the heavy grease on the ways, after checking on the cost of two servo power feeds for X and Y axis and a good quality DRO, I suggested converting it to a 2 axis CNC. Is it possible to install servos to the lead screws but leave the handles in tact for full manual operation and still use the CNC as a DRO?. Would it be better to use servo encoders for the position display or get a long linear scale like a DRO uses? He agreed to change out the acme lead screw for ball screws so it would be accurate. He is new to machining and still needs manual control so this CNC conversion would be for simple bolt patterns and milling out a pocket or a boss in a part. Does any one know of a dependable way to add a CNC Z axis control to the quill handle?
Thank You
The Farmer and the Farmers Son.

You better have a full machine shop at your disposal if you choose to convert a knee mill! The table is very heavy, and must be removed to replace the acme cross nut!
Thee lubrication pump must be converted to an automatic oiler, as the surface area on the ways is large for the fast rapids of servos! So they must be kept well oiled!
The old Hurco CNC knee mills never had handles to turn either!
I would never use a CNC mill as a manual one, the neutral motors are too much to turn by hand

Well, you CAN make a silk purse out of a sow's ear... Not always the best course.

Yes, I put handwheels on my camsoft knee mill at first. Note wheel, not handle, when running or it will whack you hard under power. Anyway, with no power applied (MO command), turn the wheel by hand easily and the position updates just like a DRO. I used it this way when learning before I added an MPG and physical jog buttons.

back to your quill question, With a good zero backlash right angle drive and ball screw a good machinist should be able to design and build the brackets to mount it all up.


In my view, BY FAR the best way to get a CNC mill is to buy one with a dead control, then add Camsoft (or Linux CNC for very low cost). For example, I bought a second Excello with a dead 6M control for $350. its my parts donor machine. My son just bought a tree mill with a dead Dynapath for $1500. This is the best iron for a knee mill I've ever seen - box ways, CAT40 spindle tool head and tons of cast iron.

Karl

On the CAM SOFT web site there is a video of a Knee mill conversion, watch the long version. It shows the man pushing a button on the screen and the mill converts back to manual mode with no servo lock or hold. The Control then works as a manual DRO. How did they do it?
Thank You
The Farmer

Piece of cake, just issue MOTOR OFF. The servos free wheel and the encoders work just fine.

I used to do this for edge finding instead of using the keyboard jog function.

Karl

I use my Acer Knee mill CNC in manual mode often. A button on my Camsoft screen does the MOTOR OFF command Karl refers to which frees up the motors. When turned off, the X and Y axis spin freely (no motor drag - is's a DC Servo system, not stepper motors). I can use the X/Y, knee, and quill manually to machine while Camsoft acts as a DRO. Then push the button again and I'm back to CNC. I forget if I have to home the machine again after doing this, but that's only because I tend to either machine 100% manual, or 100% CNC. It doesn't matter because that's really a matter of how your control is programmed. I don't know if I have to home _my_ machine again because I haven't tested if the programming (done by a previous owner) was done that way (preserved home after shutting off the motors). But I know it can be done.

For example, my machine is programed with a 2D mode. Push a button and it disables the Z axis. Then I use CNC commands to position X and Y by CNC, but I use the quill to drill or tap a hole manually sometimes. If your quill is mechanically configured to work right, you could use Camsoft as the Z axis DRO (mine doesn't do this, however - see below). There are many possibilities with this, but you'll have to program them because many of these scenarios do not come pre-programmed with the Camsoft package. Examples are provided, but you must modifiy them to fit your needs and tastes.

Remember, Camsoft is a retrofitter's tool, not a canned CNC machine out of the box (which is true of pretty much any CNC package). Programming will be needed to get the machine you want. Also, the mechanical way you hook up your motors affects how useful the machine is.

For example, it is a little bit bigger problem getting the Z axis to move freely. Most knee mill CNC conversions motorize the quill. And most of them I've seen do not allow the quill to move easily even if the motor is turned off. You have to physically disengage the motor from the quill. Elrod Machine has some videos that show an example of this. You can see their CNC conversion here: DIY Knee Mill Retrofit

My machine is a little different. My Z axis (my entire head) moves up and down on a slide (it's a MillSlide from Milltronics). So my quill (and the entire head) work like a manual mill all the time. But that's a rare bird. I don't think Milltronics sells it anymore. Mine is the only one I've seen. I'm sure the Millslide product was quite expensive. Probably why they don't sell it anymore.


A CNC conversion is not a trivial task. Do it because you want to have fun building your own CNC machine. Don't do it because you are hoping to save money on a CNC machine. Before you are done, you'll spend significantly more time (and maybe even more money) building the machine vs. buying a pre-made package, or a complete CNC machine. Said another way, decide which is more important to you. Making a CNC machine, or making chips on your CNC machine. Then do 1 or the other, but don't expect to do both at the same time.

Regards,
Mike

Is it possible to put a linear scale on the Z axis so when you disengage the ball screw on the quill you will still know where you are, as in 2D milling?
Thank You for all of your input
The Farmer

Yes, they are common and even relatively cheap if you just want to read right off the scale itself for manual quill work. Many of the cheap ones even have a digital output that can tie in with a CNC controller, but that might require some kind of converter depending on your CNC controller as the signaling for the cheap scales isn't the same as a more typical A/B signaling on a glass scale. I'm talking about stuff like this:
6" Digital DRO Quill Table Readout Scale for Bridgeport | eBay
or this:
Electronic 6" Digital Read Scale Ruler Caliper DRO Quill Vertical Bridgeport | eBay
I just randomly chose those from a quick search on eBay. Note that these types of quill scales work similar to a digital 6 inch caliper (same technology exactly).

For hooking into a CNC controller, however, it is more typical to use a glass scale on the quill as they are more accurate, and durable (over time - repetition of use), but much more expensive. This all depends a lot on budget, and the kind of CNC conversion kit you get. And I'm probably not the best to comment on such things anyway as I haven't messed with this much myself.

You or someone else asked about encoders vs. glass scales. Glass scales are better as they tie directly to the table and quill, but are expensive. Encoders normally are placed on the end of the motor and accurately show the motor position, but don't show backlash when you reverse direction. Even a ball screw has a very small amount of backlash. Any good CNC controller software should have backlash compensation which solves this problem. Camsoft does. Even Mach3 does. But you have to measure the backlash and enter the number in the software when using encoders, and check it every now and then to maintain accuracy. With glass scales, you don't need backlash compensation.

My knee mill has encoders, and though I wish I had scales, the encoders work fine. The machine is still very accurate - especially with backlash compensation turned on.

Regards,
Mike