Start with a DRO

fester

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I have a HF mill (40939) I'd like to convert to CNC. The first thing to do will be to add a DRO.

Does anybody know of an appropriate DRO I can link to a PC?
 
Start with your planned controller, for instance Mach3 will display input from DRO scales but it will not act on the input.
You can get a DRO unit which uses a PC as the display but I don't think that is a step towards CNC either,
Regards,
Nick
 
The idea is to have a typical DRO display for manual mill use, then feed the display to a PC for CNC.
The PC would be the controller, feeding Raspberry PIs to operate stepper motors on the axes.
 
Typical DRO displays have no reason to be able to communicate with a PC and so don't, DRO controller units which use a PC as the display use bespoke software, you'd have to approach the software vendor for source code or a software module to give you access to the data.

Open loop steppers work perfectly with Mach3, Linux CNC and other controllers for CNC and Marlin etc. for 3D printing, is there a specific reason that you want to re-invent this particular wheel for this project, given that in a stepper system if you need feedback to correct position you haven't built it correctly in the first place?
 
Open loop by definition does not catch errors. I'm just in the habit of checking my work.

Open loop doesn't make "Errors" poor design, poor implementation and poor operation make "Errors", consider that as soon as you spot an error you've cut material you weren't supposed to, it's far better to build a good system if you plan to use steppers.

It's a common request from newbs on the CNC forum sites "how do I add feedback?" or "Close the loop" and it often proves impossible to explain that it isn't required and doesn't prevent errors, it just spots errors after they occur and so the best approach is to design and build a robust system which isn't sensitive to EM noise and doesn't generate lots of EM noise.
 
Almost any DRO scale system can be interfaced with a PC. You only need a piece of counter hardware between the reader and the PC and the software that will read the counter and display the result. I know of no DRO display that has a PC output.
 
I have a DRO-PROS unit with glass scales that I hooked to Mach3. I had to use a logic chip to drive the signals without taking much current. You can show the DRO coordinates or even the error between them. There are lots of ways to create quasi-feedback. You can use the macro pump to go through a reset routine 5 times a second or you can call out an M-code at points in the Gcode. The macro can keep adjusting the Mach3 DRO to match the encoders if the error is above a certain limit. The problem is you kind of lose track of zero. You can also get information from less expensive DROs with scales like calipers have. I have these scales on another mill but haven't tried connecting to Mach3. If your machine uses the 3-lug drives on the handles, I have all kinds of parts for driving these with stepper motors, including complete CNC packages. If interested contact me at pthomps2@gmail.com
 
I have a DRO-PROS unit with glass scales that I hooked to Mach3. I had to use a logic chip to drive the signals without taking much current. You can show the DRO coordinates or even the error between them. There are lots of ways to create quasi-feedback. You can use the macro pump to go through a reset routine 5 times a second or you can call out an M-code at points in the Gcode. The macro can keep adjusting the Mach3 DRO to match the encoders if the error is above a certain limit. The problem is you kind of lose track of zero. You can also get information from less expensive DROs with scales like calipers have. I have these scales on another mill but haven't tried connecting to Mach3. If your machine uses the 3-lug drives on the handles, I have all kinds of parts for driving these with stepper motors, including complete CNC packages. If interested contact me at pthomps2@gmail.com

Nice but it's just a bodge for a broken system.
 
Open loop by definition does not catch errors. I'm just in the habit of checking my work.
There are indeed systems that use dual feedback loops for motion control. here is a video on the Galil site that explains it. You have to sign up (for free) to view it, but its worth it as they also have other videos and downloads that are very informative.
http://www.galilmc.com/learn/online-videos/dual-loop-compensation-methods
I am on the same path, in the (slow) process of CNCing my mill with dual encoders, that will give absolute position. I also want full manual function. So far, I have the DRO installed, once I feed the linear encoders into the PC, it will give the XYZ positions on the monitor, the DRO readout will be reused on another machine. You will need a motion controller capable of dual encoder input. Galil makes some of the best but they are expensive.
 
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