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Homing switches accuracy
« on: April 25, 2013, 07:27:14 AM »
I have an old Bridgeport V2XT that I retrofitted.  I used almost all of the old mechanics in thsi case teh old Balluff limit switches.  X and Y axis have a housing with a limit and homing switch in them.  Each side of the axis has a dog that pushes down on teh plunger.  One side of each axis has an extra dog for the limit switch.  My problem is this, homing accuracy is at best .010".  I am currently using the limit switches ( low on inputs). My homing procedure runs at 60IPM.  The only reason I am reluctant to go with proximity switches is I am afraid that chips might throw everything off.  I'm mostly cutting steel and aluminum.  Anyways the existing switches are made in Germany, why are they not accurate?  

I need to be able to rely on the repeatability of my home cause I run plenty of the same part over and over again.  If every ten parts I take a break and let the machine home that would be wonderful.  Say I lose .0005 with every part on every axis, ten parts later this could be a problem.  I am very intersted in any suggestions anyone may have to improve my homing accuracy.  My last option would be a proximity switch wired in series to the lower limit switch of my Z axis ( that is housed in a watertight quill housing)  and two of those switches that a member here makes based on an optical system.  This would help me minimize on inputs.  But if anyone has a way to make what I have work, I am very interested.        
« Last Edit: April 25, 2013, 07:30:50 AM by Mountainman »

Offline Chaoticone

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Re: Homing switches accuracy
« Reply #1 on: April 25, 2013, 09:02:21 AM »
Are you sure the home switches aren't repeating or is it skipping steps in the run somewhere creating a cumulative error? 

I would slow the homing speed way down for sure.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Homing switches accuracy
« Reply #2 on: April 25, 2013, 05:05:51 PM »
My servo configurator program ( DG2S drives) and mach agree as to the number of steps that are being sent to the drive.  So that doesn't concern me.  MY NSK ballscrews have like .001 backlash in them. 

What do you mean by the switch might be repeatiing? 

I will definately be slowing down.  I'm thinking about getting a card to utillize the 3rd port through my ESS and that will free up some inputs so I can run teh homing switches through my main BOB.  I suspect the had additional homing switches because they are more accurate than the limit switches?   

Offline Chaoticone

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Re: Homing switches accuracy
« Reply #3 on: April 25, 2013, 05:11:14 PM »
Can you not home in the drive using your index pulse?

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Homing switches accuracy
« Reply #4 on: April 25, 2013, 05:29:21 PM »
No index pulse only A and B pulses
Re: Homing switches accuracy
« Reply #5 on: April 26, 2013, 06:40:27 AM »
I been thinking, basically what you are talking about is inputting the index pulse into the break out board and rewriting the homing macro so that after the limit is hit, only when the index pulse is sensed will that position be deemed home.  I'll have to splice something into the encoder cable.  Any macro help out there?   

Offline Hood

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Re: Homing switches accuracy
« Reply #6 on: April 26, 2013, 08:35:11 AM »
You are not going to manage that successfully via a macro I am afraid. It will be far too slow. Ideally it needs done in the hardware and the SS/ESS does not support Index homing, in fact very few controllers available for Mach do support this.
On my machines with SmoothSteppers I do the index homing in my servo drives as they support this function. On my other machines I use the CSMIO controllers from CS-Lab and they support Index Homing.

Your best option would be enclosed optical switches, they are very accurate and being enclosed you dont have to worry about much or coolant. You cant buy them however they are relatively easy to make.

Hood
Re: Homing switches accuracy
« Reply #7 on: April 26, 2013, 09:26:35 AM »
CS lab, I would have been more than happy to dish oput the extra dime for what they make.  OK, I will more than likely be using the old homing switches.  You think there is an advantage to using 5VDC for for homing over 24VDC?  5VDC more susceptible to VFD noise, 5vdc I believe needs better "closing" on the contacts to make a circuit. 

Offline Hood

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Re: Homing switches accuracy
« Reply #8 on: April 26, 2013, 01:26:25 PM »
24v has all the advantages rather than 5v.
Hood
Re: Homing switches accuracy
« Reply #9 on: April 26, 2013, 02:42:27 PM »
Isn't 24V more likely to jump say a .005" gap than 5V?