Setting Power Feed Stops
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  1. #1
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    Default Setting Power Feed Stops

    Hi all....I'm just a hobbyist with a milling machine. I am milling a channel in a work piece, I have a power feed on the X axis and a DRO. How can I easily set the power feed stops without resorting to trial and error? I miss the hard stops that my old machine had.

    It would be very convenient to move the end mill into position, via the DRO readout and adjust the stop while observing a test light to indicate accurately when the power feed will disengage. Any suggestions?

    Thank you.

    John

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    I have never tested the accuracy of repeatability of a power feed stopping from its limit switches. Aside from your DRO having the capability of seeing PLC stuff, I wouldn't necessarily trust the accuracy of it.

    What DRO and what power feed are you talking about?

    Jon
    H&W Machine Repair

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    You don't mention what machine you are using but the power feed stops on my Bridgeport aren't repeatable enough to give DRO level accuracy. Probably inherent with any system that relies on a spring held over centre mechanism that flips when things are pushed just past centre. The Bridgeport system springs aren't exceptionally strong and there are a fair few parts with significant friction involved. The single tooth dog clutch in the feed drive of my P&W lathe is thou or so accurate but the spring loading is much heftier and it works with a considerable bang.

    On the Bridgeport I set the DRO zero to the end of cut and use the power feed to get close and finish a quarter or so turn of the hand wheel. Works well but you do need to exploit the multiple zero memory set up of the DRO on most jobs. There are times when I'd kill for a simple remote switch to click through half a dozen or more presets.

    I've long considered re-working the stop to have a micro switch and micrometer adjuster so the feed drive is powered off directly when the stop hits rather than via the lever. Which should be accurate. But a good reliable system is harder to engineer than it looks at first sight. Usually works is pretty easy, every time dead on the money not so much. Usually works won't do for me as after the third or forth scrapped job it likely that human powered Bridgeport injection into orbit will be successfully demonstrated.

    Realistically best way would be via a DRO having multiple zero position memories with a logic output at zero to control a relay in the feed motor power line. I know of none such. In the UK EMSi offer one with a logic output at zero operating on six presettable zero positions but its not really user friendly.

    Clive

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    You might get away with it using a big old machine with a geared feed in the knee and stops, but it'll take some trial and error to see how close it will actually kick out on multiple repetitions. May hold a couple thou, may hold 20. Don't thing it will ever work on a Servo type add on feed drive. Just too many variables... motor wind down, loading, spring tension, etc...

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    Set stop for switch a wee bit short, hard stop to hand feed to (maybe 1/4 to 1/2) turn

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    Hi Jon,

    It's is a Grizzly GO796 with SINO SDS6 DRO and a ACLASS Power Table Feed. You're right, it is not repeatable even at the same feed rate and obviously every time you adjust the feed rate, you have to reset the stops. I didn't have any experience with a power feed and I guess I was expecting too much.....I'll will simply get close with the power feed and then manually crank it to where I want it.

    Thanks!

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    Hi Clive,

    Yes, your method is exactly what I'm going to do now........I was simply over optimistic about it's capabilities.

    Thanks!

    John

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    Hi Mike,

    Yes, I have abandoned my optimism of having my big clunky manual machine accurately and repeatedly cut off within a few thousandths of an inch.

    Thanks!

    John

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    Hi Doug,

    Yes sir.....exactly!

    Thanks

    John

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    "How can I easily set the power feed stops without resorting to trial and error?"

    Simple answer, you can not.

    More complicated answer would be to mount those switches with a micrometer adjustment. But you will still need to take a trial cut and adjust from there. Having the switch easily adjustable with a real scale just makes that process easier.

    BUT, virtually no electric switch is very well suited to this task. I have pursched many switches over many years and I have never seen a single one of them that even had a spec on the repeatability of the mechanical location of the switch point, much less a tight one. Even the Micro switches that are commonly used for limits are simply not designed with any real level of precision in mind. In fact, the ones with a lever arm are made in just the opposite maner so that the inherent accuracy of the switch mechanism itself is made many times less accurate by that lever. Precision? +/-0.005" or worse!

    Mechanical limit switches on a machine tool are there for protection of the tool, the accessories, and the work piece when the operator gets distracted or falls asleep. They are not intended to be operational controls.

    Can you rig up something and make it work? Perhaps. But this is not a good way to do things. A DRO with the limit being set in the digital hardware or software is a far better idea.

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    Yes sir....thanks for your input!


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