View Full Version : CNC control systems 101
10-16-2011, 02:58 PM
Can someone please explain what makes one control better than another? I have been reading everything I can find on the subject but I am a bit confused. I read Jerry (Macona) making a reference to some machines having a real control as compared to Mach III. This confuses me I had a machine dealer over telling me that the machines he sells use the Prototrak control and that it is the easiest control to use and that I would probably not use my cad system to draw parts if I had it. Is this the big difference that there are some wizards that allow you to make common bolt circles, shapes, etc and different operations and making a cad drawing and creating toolpaths in cam is not needed?
For my use I am not a job shop building many different parts I build parts for my small business and I draw them all in cad and do the toolpaths so does that make the expensive control not needed for my use. My use is in making the same parts over and over and ocasionally making a jig or repair part.
The machine dealer had a used machine for sale and when I asked about thread milling he said the older version of the Prototrak control did not support thread milling so I asked if it could be upgraded and he said yes $11K. That surprises me hell out of me the Tormach with Mach III is less than that and I have seen guys on youtube thread milling on a Tormach. I am just left wondering why are these controls so expensive and just what do they do that is so much better to justify the $$$?
10-16-2011, 04:09 PM
A few basic differences are that Mach was developed for hobbyists to get into CNC for a price way lower than a professional system for example, this is done by the PC doing the servo/stepper move algorithm and passing it via the parallel port to 'intelligent' drives which take a step direction signal and position the motors.
The PC has no idea whether the motion took place, i.e. no feedback to the controller.
With a professional system such as Mitsubishi, Fanuc, Fagor etc, the PID (positioning) loop is passed back via encoders to the controller processing the (G code).
This is know as a closed loop system. There are specialized Motion cards that fit in a PC slot which will emulate the closed loop system, such as Galil Sytems
Or an outboard port type such as Dynomotion, which use the PC as a HMI and does the closed loop processing and CNC servo control outboard of the PC.
For accurate threading or synchronized tapping you need a closed loop system that can gear one axis off another. Galil and Dynomotion can do this as well as the commercial systems.
10-16-2011, 04:48 PM
Max has it pretty much down.
I would really be surprised if the prototrak can't do thread milling. That would mean it can't do helical interpolation which is really odd. What it might not have is a canned cycle for thread milling, if this is so cam can put out the necessary code to do it. What the prototrak probably does not have is rigid tapping. That requires synchronization between the spindle and the Z axis. Mach cannot do that without some creative coding.
A lot of companies like Haas have all these extra features all ready built in. A code turns it on after you pay them your next of kin.
A real control, which is not that expensive, is much more stable, more compact, and provides better movement. A lot of controls have S-curve acceleration profiles that make a smoother running machine. They also tend to have less glitches that can ruin your day.
A friend of mine recently paid about $3k for a full milltronics mill control, I believe a Cent 7. Nice control, fiber optic link to the breakout. There are a couple chinese vendors on ebay that have been selling FANUC controls at about that price point. You can even get them with step/dir outputs.
10-16-2011, 08:14 PM
Thanks this helps. The machine dealer was talking about a canned cycle because I asked him if you had the G-Code would it do thread milling and he said yes. The machine that he has used several of them are the Trak DPM SX3P the one in the picture in the link I posted was a new Trak DPM SX5P.
10-16-2011, 09:48 PM
Still sounds like a good machine. Still has a 40 taper spindle which a ton is available for. You don't have to rely on the NMTB flavor. You can use CAT or BT with a different drawbar (Slightly longer) And the power drawbar they are putting on those is the same one I have on my machine. Blows the kurts out of the water.
10-17-2011, 12:41 PM
The used Prototrak machines have an AG3 control and they do not have 4th axis control. I need a 4th axis for some of my products I want to make.
I have a lead on an older Fadal 15 that comes with a 4th axis for around the same price but it would cause me to build a small shed for that machine. I may just go with the Tormach it will make life simpler for now and if I find I need something more I will have to trade up or add another machine then. I am not going to do anything for another 60 days unless something perfect comes along so I am still looking for the right machine.