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Simple and accurate home "switch"

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John Haine01/04/2018 10:41:39
2500 forum posts
132 photos

After quite a lot of further experimentation I have settled on this arrangement of an "X" home switch on my Super 7 conversion.

pic_5_cmp.jpg

Basically it is an isolated copper pad, consisting of a rectangle of PCB glued on top of 1/4" Tufnol. Soldered to the PCB is a 4mm receptacle, actually one of the terminals from a piece of "chocblock", to take a 4mm plug that connects to the digitising input of my BoB. The whole thing is attached to the face of the apron with (blush) superglue. A bracket on the stepper mounting carries a 50mm stainless bolt with its end formed to a smooth dome to contact the pad. The bolt is a running fit through the hole in the bracket and is spring loaded, so it has a bit of "give" if the slide over-travels. Note that the cable to the plug is screened, the screen being earthed only at the controller end to avoid an earth loop.

The Mach 3 homing and limits config is set up to home at 2 mm/minute. One simply jogs the slide in until the bolt is a mm of so away from the pad, makes sure the plug is connected, and clicks Set X Home - the slide moves slowly until the pad earths, then stops. My convention is to use this position as the X zero for machine coordinates, so having homed the slide I just click on "Zero World X".

This is part of an offset calibration system I'm working on, the idea being that having calibrated a set of tools in Dixon toolholders I'll be able to set the lathe to turn to a specified diameter with a calibrated tool without having to reference again to the material diameter.

As far as I can tell, since the material can be chucked in any position along the Z axis, there is no particular benefit in having a Z home switch.

Muzzer01/04/2018 11:52:53
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2904 forum posts
448 photos

Bare copper probably isn't the best choice, as it will soon tarnish and become intermittent. If your arrangement results in a brushing movement, it's possible that might clean the contact area with use but on the other hand that suggests it will wear rapidly.

There's a lot to be said for using a pukka microswitch, ideally a sealed one. You've proved the benefit of a home switch but the last thing you need is a dodgy contact some weeks or months down the line.

Murray

John Haine01/04/2018 12:11:56
2500 forum posts
132 photos

It will get cleaned.

John Rudd01/04/2018 12:42:47
1365 forum posts
58 photos

So, instead of the tarnished copper pad or the mechanical micro switch, how about a slotted opto isolator based electronic switch? ( I did think of a Hall based, but then the magnet would attract all manner of swarf...)

I conceed the idea isnt as simple, but perhaps more reliable?

Just a suggestion.......blush......

Hopper01/04/2018 12:55:40
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3598 forum posts
72 photos

Swarf?

Douglas Johnston01/04/2018 12:58:40
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573 forum posts
32 photos

The only problem with a slotted opto switch or other electronic device is a possible failure due to a power loss for the electronics. I would stick to a microswitch for reliability. The last thing you want is a crash landing!

Doug

not done it yet01/04/2018 13:32:24
2918 forum posts
11 photos

Sorry, but any ‘simple’ switch will fail - sooner or later. There are two alternatives, as I see it. 1) A back- up ‘emergency’, should the normal switch fail, or 2) the design must be of a ‘fail-safe’ nature. The former is the easier, I feel.

John Haine01/04/2018 14:46:11
2500 forum posts
132 photos

This is NOT a limit switch. It gets used once when the machine is switched on and needs homing - and is clean from its last brush down. It's no sweat to give it a quick clean before using it. Experience shows that not much swarf gets to its location anyway. The machine gets carefully watched whilst homing and if it does overrun there's a spring to take it up, then after a mm or so the ballnut contacts the endplate and the drive stalls - but (a) it's running slowly, (b) steppers don't object to stalling, (c) soft limits should stop it before then anyway.

I've gone into this microswitch thing before. A homing switch has to be very consistent, microswitches with guaranteed consistency are very expensive, and need careful mechanical protection from overtravel. This cost me effectively zilch, and is more consistent than the spec of a high quality limit switch.

Andrew Johnston01/04/2018 15:36:41
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4640 forum posts
522 photos
Posted by John Haine on 01/04/2018 14:46:11:

I've gone into this microswitch thing before. A homing switch has to be very consistent, microswitches with guaranteed consistency are very expensive, and need careful mechanical protection from overtravel. This cost me effectively zilch, and is more consistent than the spec of a high quality limit switch.

We need numbers!

The commercial (microswitch) limit/home switches on my CNC mill are repeatably better than 2 thou. I still zero on the reference for the work by other means though, just to be sure.

Andrew

Edited By Andrew Johnston on 01/04/2018 15:38:27

Robert Barker 101/04/2018 19:31:08
6 forum posts

hi

ii have a denford orac cnc lathe , for the spindle speed & threading I use a proximity on the end of the spindle , this is round switch about 3/4" dia & has 3 wires , for the axis homes i use a diferent proximity switch which is smaller only 5/8" x 3/8" , these have been on the for 3 years with no problems , some times my machine is working 8 hours a day & 5 days a week

robert

Mikelkie01/04/2018 21:06:30
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87 forum posts
9 photos

I think John's arrangement with a copper pad is ok, I put a similar type of contact on my lathe to operate a home made electric kickout for the power feed also with a copper contact. Worked well for 8 years now never cleaned it, but it's covered to keep out swarf and coolant.

Muzzer01/04/2018 21:18:55
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2904 forum posts
448 photos
Posted by John Haine on 01/04/2018 14:46:11:

A homing switch has to be very consistent, microswitches with guaranteed consistency are very expensive, and need careful mechanical protection from overtravel. This cost me effectively zilch, and is more consistent than the spec of a high quality limit switch.

Funny that a crude, exposed copper pad is more consistent than a high quality switch. You should patent it and then let the suppliers know where they have been going wrong!

What numbers do you get when you home say 10 times with a decent 1um DTI? We should be told!

Murray

Tim Taylor 230/12/2018 18:23:11
66 forum posts
8 photos

I use sealed micro-switches on my converted G0704 mill. They are used to home to min-X, min-Y, max-Z in machine coordinates. Since you would rarely, if ever, mount the part to be machined at physical home on the table, accuracy of a few thou is plenty, as you have to zero to the part before machining anyway....

For John's application, his solution is simple and reliable. My only real concern would be having the BOB input directly exposed to the environment - I would probably run it through an opto-isolator......

Tim

blowlamp30/12/2018 20:00:31
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1185 forum posts
82 photos

I've never installed any limit or homing switches, but if & when I need to, I think I'd consider using a set of vehicle contact breakers or an optical kind of setup.



Martin.

Tim Taylor 230/12/2018 21:23:39
66 forum posts
8 photos

I have seen some inductive proximity sensors that will repeat very accurately, but they are also kind of expensive.

Tim

Robert Atkinson 230/12/2018 21:39:54
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237 forum posts
17 photos

Neither slotted optical or inductive sensors are much use for a index or refernce sensor without extreme care in the target and electronics, Both the raw signal generated by the sensor and the trigger level of the electronics vary with voltage, temperture etc. Ambient light is an issue for optos unless modulated signal are used.
A directly operated, goldplated contact with some overtravel capability is the best option. Contacts taken from a unused small signal relay or microswitch are a good choice. Normally closed (break) contacts are normally more reliable.
Some of the most accurate sensors used on CMMs have 3 sets of parallel round bar contacts spaced at 120 degrees. The contacts are wired in series and each pair of bars is shorted by a ball. The 3 balls are on a spider held down by a spring and with the input shaft at the center opposite the spring. Pressure on the input shaft in any direction will break the circuit.

Robert G8RPI.

John Haine30/12/2018 22:38:44
2500 forum posts
132 photos

Funny this thread has reactivated! There was a question that I hadn't responded to above about the repeatability of my arrangement. I's already reported that in the "what did you do..." thread on 15/02/18, I'm getting better than 10 microns and probably better than 5. Since originally posting I have improved the system by making the fixed contact surface beryllium copper which is a better material, resistant to corrosion; and eliminating the 4mm connector and permanently wiring to the home input.

Tim, the whole point of having an accurate X home switch on the lathe is that you don't have to reference to the material! Rather, all the tools are calibrated and offsets stored; then if you accurately home the machine and select the tool you can turn straight to the desired diameter. And all the inputs on my BoB go through opto isolators.

John McNamara31/12/2018 01:09:49
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1288 forum posts
113 photos

Hi John

I provided housings for standard size plunger type industrial micro switches for the Epoxy V mill I am building. these switched are very robust and can be purchased for under 20 dollars from a known manufacturer.or a lot less from unknown Asian makers. I used these switches on another CNC project a router they continue to work well in spite of the wood dust. repeatability is around 0.003"

The housing is tab and slot construction 2.5mm laser cut steel (not shown is the m3 screws that hold it together although the tabs do most of the work. they are very rigid.

micro switch.jpg

This article is worth a read

In particular the section on precision micro switches with Micron level repeatability.

**LINK**

I did a google search.

**LINK**

I quickly found these as an example, but did not do an extensive review of what is on offer.
For around 40 - 50 dollars pretty good accuracy 5 - 10 micron 120 dollars 0.5 micron

I will stick the the mechanical switches for my project however if retro fitting a lathe where a single home switch is needed I would be tempted.

**LINK**

Regards
John

PS Link to my project **LINK**

Nick Hulme31/12/2018 01:54:50
673 forum posts
37 photos

I recommend

1. Enclosed.

2. Silver Contacts.

3. Ditch the "Shade Tree" engineering, it's not funny, clever or useful.

Nick Hulme31/12/2018 01:58:07
673 forum posts
37 photos
Posted by John Haine on 01/04/2018 10:41:39:

As far as I can tell, since the material can be chucked in any position along the Z axis, there is no particular benefit in having a Z home switch.

A Z axis Zero will be a known distance from the work holding equipment (Chuck) and allows your CAM to know where to stop to avoid collision.

Simples.

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