Looking for DRO system with an output pulse on zero

[Clive FosterParticipant @clivefoster55965]

I'm a confirmed fan of bed stops but setting up the usual form of multi-position one on a lathe is a faff and milling machines are pretty much limited to one setting at a time. It would be nice if they were as quick, easy and routine to set as an decent micrometer bed stop.

On the Bridgeport mill I frequently use the zero position memories as a sort of bed stop subsitute by preloading several with appropriate stop positions for the axes and working up to the zero position. Either all manual or, usually, letting the power feed do most of the work and finishing off by hand. Fundamentally the same as the technique I use on the lathe when running up to a fixed stop.

Seems to me that if the DRO unit gave an output pulse when it hit zero I could wire that up to control the power feeds giving me as many stop positions as I could reasonably desire. Same thing with the lathe.

Unfortunately I've been unable to find a commercial DRO box with the requisite outputs.

Does anyone know of one?

Or is there an alternative way of getting to the same place. I understand that LinuxCNC and maybe Mach3 can display DRO type outputs. Presumably an "end of travel" function can be made to give an output when zero is reached. Not that I want to tangle with a CNC program but whatever works.

Thanks

Clive

[Clive FosterParticipant @clivefoster55965]

[Michael GilliganParticipant @michaelgilligan61133]

That seems such an inherently sensible idea that I thought I should have a look:

This looks promising … with its

• Near-zero audible warning

: **LINK** : https://www.touchdro.com/resources/dro-manual/features.html

MichaelG.

Edited By Michael Gilligan on 30/04/2021 22:23:18

[Clive FosterParticipant @clivefoster55965]

Interesting find Michael

Not sure that I'd be entirely happy with remote wireless communication in the workshop with motors, VFD and other interference sources about the place. Especially as it needs to be real time communication. Objectively Bluetooth is fast enough at the speeds I'd need though and the range will be very short so plenty of signal.

But there is a certain emotional re-assurance about physical wires.

Touch screen sounds vulnerable in a workshop but I guess a screen protector will solve that. easy to change when it gets too dirty.

As you say the "near zero audible output" implies there are no software issues in giving an output at a specific DRO position. Just a matter of coding and providing some form of output to turn off the feed.

Maybe I'd drop him an E-Mail and see if he thinks it a feature worth adding.

Clive

[DC31kParticipant @dc31k]

As you say, Mach 3 and LinuuCNC have the capacity to do what you want. Both ot them will have an internal variable in the stlye of CURRENT_X_AXIS_VALUE. You would have to write a macro that runs continually and compares that value to your required one and then dives off into an interrupt routine that rings the bell when the values become sufficiently close (not necessarily 'equal' ).

To do this in principle in either program is not difficult, but it is the time and effort spent learning how to do it that will weary you.

Someone has posted here an Arduino sketch that does a DRO summing (quill and knee). It would be possible to use that as a building block for what you want. You can intercept the signals from the scale, let the relevant part of that program decode them and them perform the routine described above for Mach. You would need to give it both the current value and the desired value so it can do its work.

The main thrust of this is to suggest you do some trials on the human-machine interaction aspect of your idea before going much further. Stopping something moving on the sound of a buzzer (triggered by your pulse) is not repeatable. What you do at the moment is to look at the DRO display, see how close it is to zero and how fast it is changing and adjust your handle turning to suit.

Rig up a dial indicator and a microswitch-triggered buzzer on the machine. Set the bzzer to sound when the indicator is at zero. Now cover the indicator and drive into the microswitch. See how close to zero you get. Repeat nine more times.

An analogue dial with a moving hand (or its on-screen equivalent) or even watching the numbers of a digital display count down will help you a lot more than a single tone.

Think of it in reverse: starting an athletics race or the Christmas tree at a drag race. The best athlete is the one who goes on the 'B' of the bang, not the 'G'. The winning car is the one that takes off as the lights start to dim, not when they are fully extinguished.

If you want to take the quadrophenia route, then the parking sensors on a modern vehicle might be a good model. The programming is not trivial as it has to take into account not only distance but speed and acceleration. It would be interesting to experiment to see if you can stop your car the same distance from the brick wall every time, just using the onset of the continuous tone to guide you.

Edited By DC31k on 01/05/2021 11:39:44

[duncan webster 1Participant @duncanwebster1]

I've had a breadboard setup using an Arduino which reads the DRO scale allows you to set zero by moving the table and pressing a button, the when the table reaches zero from either direction it switched an output on the Arduino. A sort of idiot teach and repeat. This could be used to switch the table drive motor. I got it working and made provision for it in the hardware that drives the X axis stepper motor on my mill, then, as is so often the case with me I got distracted. If anyone wants the code/circuitry let me know, but it won't be just download and go, you'd need to adapt it to your exact needs

[Clive FosterParticipant @clivefoster55965]

DC31k, Duncan

The issue with DIY is how to avoid re-inventing the wheel by building a zero-finder running in parallel with the actual DRO box. By the time you have done that you might as well code a full on DRO and be done with it.

I do wonder just how difficult it would be to intercept the display driver for the usual inexpensive import DRO box and interrogate that to find all zeros so the power feed can be stopped.

I'm somewhat surprised that Mach and LinuxCNC don't have an easily accessible alarm at "number" function for each axis that could easily be adopted for this purpose. It is, after all, basically the same as setting soft travel limits. Using Match or LinuxCNC as a DRO is a bit overkill though.

Learning to code just for this is more than I'm prepared to do although I do have experience with similar things when building computer controlled test set-ups when I was working. Mostly LabView based though. Also overkill.

Thanks.

Clive

[DC31kParticipant @dc31k]

Posted by Clive Foster on 01/05/2021 13:13:31:

I do wonder just how difficult it would be to intercept the display driver for the usual inexpensive import DRO box and interrogate that to find all zeros so the power feed can be stopped.

But even if you did that, it will not mimic a fixed mechanical stop or a manually-controlled stop on a DRO zero. And it is that part of it, I do not think you yet appreciate.

Just switching off the power feed will stop at an indeterminate and unrepeatable position. Just think what might influence it: cutting load, slide friction, speed it is going when you switch it off.

Under the hood in Mach, etc. there is a lot of maths going on before the soft limit switch or zero count is reached. It looks ahead and takes action because it knows how far away it is and what it is doing on the approach to it. The stop comes as no surprise.

In a similar way, it is why you would always rehome the machine after you hit a hard limit (or the big red button). For that situation, the stop signal comes as a complete surprise to the machine. It does all it can to stop as fast as possible but the actual position in which is eventually comes to a halt is indeterminate.

[Clive FosterParticipant @clivefoster55965]

Posted by DC31k on 01/05/2021 15:06:16:

But even if you did that, it will not mimic a fixed mechanical stop or a manually-controlled stop on a DRO zero. And it is that part of it, I do not think you yet appreciate.

Just switching off the power feed will stop at an indeterminate and unrepeatable position. Just think what might influence it: cutting load, slide friction, speed it is going when you switch it off.

True where a CNC machine is concerned but in a practical world when using as a bed stop equivalent over-run will be pretty consistent for any given set of conditions. Basically you set the stop position up dynamically by running a test and offsetting the zero so the stop is actually at the correct physical position. Its irrelevant if the DRO actually reads – 10 thou or whatever when things come to a stop so long as its sufficiently close to the same each time.

Whats important is the scatter or variation in the final stop position. A machine tool is a high friction device so there won't be much variation for any particular set-up. Allegedly the Bridgeport mechanical switch system, an inherently rather sloppy installation, is repeatable to within a thou or three when in good condition. Easy enough to finish the cut by hand if its really a problem. My normal practice with a lathe bed stop anyway.

Obviously its better to have a controlled slow down as the end point can simply be set without testing without worrying about feed speed, machine drag and cutting loads et al. But simple power switch can work well enough. I've built systems with micron level repeatability doing it the simple way and nanometre repeatability with controlled slowdown.

The obvious alternative for a mill is to finagle in a screw driven semi-hard stop carrying a probe to control speed ramp down running on its own slide. Stepper motor drive with step counting digital position read-out will be fine. In principle easy enough to do on a Bridgeport style machine with plenty of space around the table but the operational ergonomics are a little questionable. There is an envelope somewhere in my maybe file with a sketch Bridgeport installation on the back!

Won't work with a lathe of course because the drive comes via the gearbox from the spindle.

For a lathe electronic control of a single tooth dog clutch unit is the complete answer to precise stop positioning. Equally effective on milling machine installations too but its all extra engineering.

Clive

[SillyOldDufferModerator @sillyoldduffer]

Just a few thoughts about what an Arduino solution might look like:

The idea is an Arduino taps into the data stream being sent by the sensor head to the DRO, and implements the stop as a separate unit by cutting power to the machine, shown above being done by a relay, but other methods available.

I believe all DROs communicate with 4 wires, positive & negative volts, plus data and clock signals, often with a USB plug. The wiring and voltages depend on the DRO, and it varies: might need logic level conversion.

The Arduino reads the data. Zero is set by pressing a button which records heads current position. The relay is triggered when the head reaches that point again, or gets close to it. Pressing reset restarts the machine.

The controls need more thought!!! At least one other switch is needed to activate and deactivate the stop and it would be sensible to show if the stop is active or not with a LED.

Other hard part is decoding the data stream; this is encoded in various ways – binary, BCD, 2s-Complement, quadrature etc. Depends on the make and model of the DRO, so research needed.

Dave

[duncan webster 1Participant @duncanwebster1]

That's pretty much what I had, I used a pair of decoder chips twixt the JB and the Arduino to read the RS432(?). I think I could have done it in software, but for less computer savvy people like me the hardware solution was easier

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