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

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

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