Reference positions in DIY CNC

[davidp]

I hope this is the correct sub-forum for this question.

I have studified a lot of posts and links for DIY CNC machines and there is one issue which doen't seem to get covered. How do you set a reference position rather than a limit switch?

To me a reference or home position is the base for axis position counting and is reached via dedicated buttons or programming G28 XxYyZz. It sets the basis for G54 to G59 or similar work shift systems. How is this done with DIY CNC?

Can a reference mark be added to a stepper motor? Also not all encoders have reference marks either.

With machines that can have indicators in the spindle or can use edge finders this is not a problem. Even lathes usually run G92 or G50 position setting. But some machines like grinders can't 'remove' their 'tool' in a repeatable fashion.

I personally don't think a limit switch would be repeatable enough to act as a home position, certainly as I see their use.

Is this an issue which must be put to one side for cost reasons?

Regards

Davidp
Bristol, UK

[Geof]

do a search on 'home switch' and you will get 316 hits. On the first two pages you will find about 8 mentions of limit switches and home switches.

[davidp]

Geof,
Thanks for the advice. And yes, I have been through the threads. Picked up a lot.
In short the home (reference) position strategy is contained in the control and the better systems (Mach, Turbo etc) have various strategies to cover a variety of needs. Most engineers here implement micro switches as a position sensor fro a home switch.
One idea I found from an earlier post by yourself was to consider adding an optical switch and disc to a stepper to replicate a reference mark. This would sidestep most of the issues of switch repeatability caused by switch bounce.
It's taken a few hours to go through the threads but its rained most of the day - I have learnt a lot.
Regards
David

[Geof]

Originally Posted by davidp ....One idea I found from an earlier post by yourself was to consider adding an optical switch and disc to a stepper to replicate a reference mark. This would sidestep most of the issues of switch repeatability caused by switch bounce.David

Did you find the one where I made a semi serious reference to a magnetic reed switch.

http://www.cnczone.com/forums/showth...ht=reed+switch

I realised afterwards that a reed switch could be a very good option to replace the optical switch and disc. Electrically it is much simpler and it is sealed so you don't have to worry about dust or liquids. Also reed switches are very reproducible in their actuation point relative to the magnet.

I proved this to myself years ago when I made a timer using a wall clock, two magnets and a bunch of reed switches. I glued little bar magnets on the minute and hour hands, glued one reed switch at the 12 so it would be actuated by the minute hand magnet and other reed switches wired in parallel but in series with the minute hand switch at the required hour locations on the face of the clock. When both the hour magnet and minute magnet operated reed switches the timer closed a latching relay that started a sound and light sequence above an aquarium display. One of the curatorial assistants at the aquarium timed the start of the first display on successive days with his fancy wrist watch and found it occurred with a second of the same time every day. He was blown away.

[davidp]

Geof,
Thanks for the reply. I think that it's the quality of the support circuits that makes the sensors useful.
I was speaking to one of our elctro-machanical engineers at work today and he suggested this item. A photo-reflective sensor - Kodenshi SG2BC. http://www.kodenshi.co.jp/english/se...upter_ref.html
It's less than a UK pound (about $1). He uses them as limit switches. He suggested a reflective strip on the slideway that ends when the limit is reached in either direction. This way it could always fail safe. A quick google turned up some interfaces, too.
Thanks again for the good advice.
David
Nr Bristol,UK