Limit switches

[robtown]

I recently did a long running 3D carve over the span of several sessions (days).

I shut everything down on the second day, and didn’t get back to it for a week.

When I started it all up and continued, it was painfully obvious that my machine hadn’t zeroed in exactly the same position as before.

This has me thinking about my limit switches and how accurate they are.

I’m using whatever proximity switches came with my shopbot some 18 years ago.

I wonder if I’d get better accuracy by switching to a mechanical limit switch? Or I’ve seen recommendations here over the years for a barrel type proximity switch that may be more accurate (repeatable).

I’d love to get some opinions on this.

Attaching an image of the carve. This one was for fun, doing a shakedown run of my machine after having it in storage for three years.

[Richards]

I had good success with 1/2" barrel proximity sensors and also the "slab side" proximity sensors that came with my PRT-Alpha. To get bet results, I flattened the head of the adjustable target bolt head on a lathe so that the sensor would not have to deal with a arching surface. Also, I adjusted the homing program to quickly move to the sensor, back off two inches and then slowly approach the sensor. Using a cheap Harbor Freight dial indicator, the homing was repeatable to less than 0.002". That was good enough for me on that 5' X 10' machine that was in a garage with no temperature or humidity control.

[suntravel]

I am using the cheap chinese SN04-N Centroid recommends

https://www.centroidcnc.com/centroid_di ... _gear.html

Costs nothing but gets homing within 0,01mm

Uwe

[Gary Campbell]

The old PRT type switches are notoriously inaccurate. Old technology used not quite in a way they were designed for. I like to use 8mm barrel proximity sensors with mounting brackets and bolts with modded heads similar to what Mike R does above. Make sure that you put one on all 3 axes and that you keep the target within 0.040" of the sensor

[Richards]

Gary's post reminded me of the frustration that I had using limit switches on my PRT-Alpha when I first set it up. Just as he said, the limit sensors were inaccurate. They were trying to run sensors rated at 10-30 volts from a 5-volt supply. I installed a +12 volt supply and they worked as expected after I flattened the bolt heads that were used as sensor targets. Also, as Gary pointed out, the distance between the sensor and the target is critical. I adjust sensors to just barely clear the target so that the signal is as strong as possible. I've even epoxied a rare-earth magnet to the target to intensify the signal.

Some people prefer mechanical switches. They work fine if the switch is good quality. Stay away from cheap switches with long, springy arms. If the switch uses a roller ball, verify that the roller is snug and not wobbly. Any play in the roller translates to error. A switch with 0.003" of play in the roller will have at least 0.003" of non-repeatability. 0.003" is about the thickness of a piece of copy paper. That's not much. On a CNC router cutting wood, that might be perfectly acceptable, but on a tight CNC mill or lathe, that might be 3X too much error.

[Muzzer]

These proximity switches work with non ferrous targets too but they are less sensitive than with a steel or iron target, so a brass target results in a shorter detection distance and smaller error. A small difference but it all helps.

[tblough]

My personal preference are Euchner mechanical limit switches. Trip dogs are 100% accrurate, switches don't phantom trip, and no magnets to collect swarf.

[ShawnM]

The standard, and favorite of many DIY builders, NPN NC proximity switches (square or threaded cylinder) work very well when powered with 24v. Don't use 5v. They use ferrous metal flags, not magnets to collect crap. Keep the flags close for very repeatable homing. I am a router guy but machine aluminum aircraft parts with great precision.

Many of the industrial machines I have retrofit in the past, Biesse, Holz-Her, have Balluff limit switches on them. They are industrial, mechanical switches but cost upwards of $300 each.

[suntravel]

These Euchners are very good, I have them on mechanical lathes switching 10 times a minute 24/6 and they last several years, longer than most Balluff NPN on the CNC machines...

But for hobby use the cheap SN04-N are working perfect also, even when triggerd from a piece aluminum.

Uwe

[Nigelo]

I also have Euchner mechanical switches on my Wabeco mill as OEM fitment and find them highly repeatable. Swissi's ProbeApp has a Z axis repeatability validator (homing test) which proves the point