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Author Topic: Accuracy of Homing Devices? Pro's and Con's of Different Sensors  (Read 30029 times)

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Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #10 on: October 16, 2012, 03:11:46 AM »
For me homing has to be accurate, if I run a part one day and want to run the same the next day I want to know I can just place in the soft jaws of the vice and press start and away it goes.
 On a lathe it is especially important for the tool dia, I dont want to have to take a test cut each time I start the lathe, measure the dia and then set the tool offsets. I have a turret and I want to switch on, home and then when I call a tool and tell it to cut a diameter I know it will be there.
I use Index homing but I do it external to Mach as my servo drives support that homing feature. What happens is I send a signal to my drives from Mach, they start homing and look for the home switch input, when they see that input they then search for the index pulse from the encoder and then they stop and set that as home and send a signal to Mach which then sets the machine coordinates.
 There used to be a board that could do similar, it was made by CNC building blocks but sadly Ed no longer makes them. It worked in a similar manner except Mach actually did the homing moves, it was just that the building blocks bob delayed the signal to Mach. Hers how it worked. Mach started homing and saw the switch being activated and reversed the axis, the BOB however would not tell Mach the switch had again closed until it had seen the index pulse of the encode. It would then tell Mach the switch had closed and Mach set the machine coords.
 
As mentioned in my previous  post however if you use optical switches you can also get very accurate homing, that was proven by the tests I did, so if you dont have encoders and drives capable of homing then optos are a very cheap and accurate choice.
Hood

Offline RICH

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #11 on: October 16, 2012, 06:41:21 AM »
Guess I am a little off topic relative to the original question but if you'll excuse me.

We alluded to the relationship of homing to axis movement and tool postion. You need repeatable
and accurate axis movement and tool position. The turret can provide accurate tool position while other methods leave something to be desired. Each part of the system must be as good as the other.
Perfect homing using switches will be of no value if capable axis movement and tool position are not available.
Thus you'll be right back to manualy doing touch off in many instances.

Said a different way, assuming that homing is spot on, it's what's happening prior to and after homing that
makes or breaks the value of homing automation.

BTW, the turret realy comes into play when coupled with a Cam program providing code for a complex part on the lathe. One of these days i will make one..........

Till then,
RICH

Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #12 on: October 16, 2012, 07:01:14 AM »
Another situation where accurate homing is important.
I set my gang tooled lathe up for a large production run of one particular part using 4 tools.
The machine ran 500+ parts per day for months and was shut down every night.
It was imperative that the operator fired it up, homed it and went right on making parts.
One dimension (thread PD) was critical so it had to home accurately.

I experimented with several brands of micro-switches and found Omron (model ?) to be repeatably accurate to .0001" to .0002" which was close enough.
I much prefer Hood's servo homing and would have used that except this is a stepper based machine.
My next one will used the encoder and servos.
Russ
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #13 on: October 16, 2012, 08:56:34 AM »
The machine ran 500+ parts per day for months and was shut down every night.
It was imperative that the operator fired it up, homed it and went right on making parts.
Exactly.  I don't run near that many parts and the operator is always me but having the machine start from the same point is important to me as well.

My other machine, crude as it is, has old style stepper motors with very strong cogging (proper term?) without power.  I guess it's dumb luck but the machine has no motor movement whatsoever when powered down or turned on.  I did a run of 1100 parts over a period of 4 months and the only time I had to reset the zero point is when I screwed up & crashed it.  All I had to do was shut it down properly, close the lid to keep out pesky cats and I knew it'd be right where I left it the next day.

The new machine's more modern steppers turn easily when no power is applied and jump a bit when powered on, hence my desire to have an easy, precise method of zeroing it.  They are double shaft motors so I guess I could rig a mechanical lock on the unused end to lock down before turning the machine off for the day.:D

I was hoping you experts would say "Oh yeah, piece o' cake, put a sensor & slotted disc on the stepper, $2 Radio Shack switch on the carriage, write a simple macro in Mach and Robert's yer Father's brother.";)
Milton from Tennessee ya'll.
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #14 on: October 16, 2012, 09:26:02 AM »
Here's a thought ... a poor mans encoder / index homing.
Simple slotted disc and photo sw. (or cheap enc w/ index) on the back of a stepper. Totally enclosed, all sealed up.
Mach allows jogging while Home switch is active, so .....
Manually jog the axis to within 1 turn of finding the index pulse and then home normally.
It would have increased accuracy resolution due to the screw pitch as well as any belt reduction.
Bit of work involved, but it would be quite accurate.
Just 4 fun,
Russ
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #15 on: October 16, 2012, 10:04:00 AM »
Thank's Russ, sounds good!  Should be easy enough even for me.  I'll be experimenting with that down the road a bit.
Milton from Tennessee ya'll.

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #16 on: October 16, 2012, 10:22:03 AM »
Why not just make up some optos in a sealed box, you will have very accurate homing and wont have to mess about fitting pseudo encoders.
Hood
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #17 on: October 16, 2012, 10:29:42 AM »
Why not just make up some optos in a sealed box, you will have very accurate homing and wont have to mess about fitting pseudo encoders.
Hood
I read your description of that earlier in the thread but I must be even more dense than usual today.  (Too much blood in my coffeestream?)  Got any dusty ol' .jpgs lying around somewhere to help edumacate me?
Milton from Tennessee ya'll.

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #18 on: October 16, 2012, 10:47:16 AM »
Here is a picture of the inside of the original ones I bought from the USA, there is a flag that is attached to the rod and in the pic it is blocking the opto. When the axis reaches one of the ends of travel the rod hits the stop and pushes the flag out and thus the opto is no longer blocked.
These ones are overly complicated as there are actually 2 optos, one for each end of axis when in reality only one is needed.

Hood
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #19 on: October 16, 2012, 10:56:27 AM »
Ahh, I see (said the blind man)  Thanks, that looks very doable!
Milton from Tennessee ya'll.