What is the proper technique for using an edgefinder in a vertical milling machine? For this example, I'm using one with a 0.200 inch diameter projection, thus 0.100 inch offset from true centerline. Prior to making contact, the 0.200 extension is off center, but a soon as contact with the work piece is made, it seems to be now rotating true (machine is running under power about 150 rpm). Ok, now set x or y graduated dial to zero. BUT, what happens if I advance the table, say another couple of thousands? How can I be sure that the first contact is correct if I can still move the work a tiny bit closer to the indicator without anything happening? Tool made in India (no instructions); Machinery's Handbook says nothing about edgefinders. HELP!
Wind in the slide until the finder runs true, by eye.
Continue winding in until the 0.200 projection moves to an offset postion and stops running true. At the instant it does, your spindle axis is
0.1 inch from the edge.
Just creep up on it, and keep re-zeroing the dial or DRO until the edge finder snaps off-center.
Jim
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Some suggestions to add. If you are setting up on a Bridgeport mill or other brand where one turn of the leadscrew gives .200 of movement set the dial to 100 when the edge finder kicks. That way when you move the table .100 you will be over the edge of the part AND the dial will be at 0 instead of being at 100. This assumes a .200 end on the edgefinder.
Also, run edge finders at about 1000 RPM for better results. Check and recheck after setting the dial. On a Bport it is easy to move the table slightly when tightening the knurled collar. Better to recheck than wish you had!
If using a double ended edgefinder in a collet be VERY careful when removing it! It is easy for the .500 end to get hung up in the collett and if you pull it hard the spring inside gets very long. Don't ask how I know this. I always (now) remove them from the collett by removing the collett from the spindle and pushing the edgefinder out with a slender rod.
If you have a DRP always be in the habit of setting your dials to zero when setting up. If the power blinks out and you haven't done so you may be up the creek. When I did industrial modelmaking for a living often times the pick up point would be machined away and without a good set up I would have been lost after a power outage.
Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech
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If using these in an NC machine it pays to remember it's there in the spindle. At the former night job, they had several edge finder-shaped divots in the aluminum vise jaws!
Hmm. I may be old, but I ain't drping yet....
Jim
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Thanks to all for good comments - the link to the article by Mike Rehmis entitled "Edge Finder" was right on target. Also, his comments about a cheap tool being useless (which is what I was using) made perfect sense. So, I immediately bought a Starrett #827A which is much better. But, all these comments evidently assume that we are locating a straight surface from the spindle centerline. What about locating a curved, internal edge of a hole? Maybe that was my problem. I was attempting to locate the edge of a 0.402 inch diameter hole with the 0.200 tit on the finder. The "kick" now depends on the direction of rotation of the tool and whether the curved surface being contacted is coming nearer or further away during the "kick". Thanks once again. Dave
"David Anderson" wrote in news:Pxhsb.3122$ snipped-for-privacy@newssvr30.news.prodigy.com:
David, You need a centering indicator for finding the center of holes. This mounts in the spindle, the dial is rubber-banded to some projection on the quill housing to keep it from rotating. The indicator tip (much like a last word) is swept in the hole using the spindle rotated by hand. Kind of like indicating a part in a lathe...backwards, the indicator is spinning instead of the part.
[Note: I corrected "DRP" in the text above to "DRO" to avoid further confusion.]
That's a good tip Errol. Another one is to buy an Uninterruptable Power Supply (UPS) for your DRO. This is also commonly known as a battery back-up power supply. This serves several benefits:
If the power blacks out (or browns out) briefly, you lose nothing. The DRO can remain powered on.
If the power blacks out (or browns out) for an extended period, you have time (using the battery back-up) to position your table back to the origin (zero/zero) and lock it down. Then you can shut off the DRO and wait for the mains power to return.
Quality UPS's have superior AC line filtering, thus protecting your expensive DRO from the line noise and transients that are common in a shop (due to motors and such).
It used to be that a decent UPS was very expensive, but with the proliferation of personal computers, that is no longer the case. A DRO typically doesn't draw much power, so you don't need a large UPS. I highly recommend the UPS's made by APC (American Power Conversion). No affiliation, etc., but I have used them extensively on servers and workstations for many years with great success, and now have one powering my DRO. Also, APC offers a lifetime warranty on the UPS *and* the protected equipment from any damage caused by electrical surges, spikes, or even lightning.
What Jim said, but I'll add that the cheap imports I've seen (like those from India) don't seem nearly as crisp to use as a good domestic like Starrett.
For hole location you might want to investigate a co-axial indicator. Blake makes a good one for around $150-200 or there are imports available at half that price. They are also handy for aligning the TS on a lathe and can be used to locate a cylidrical projection on a workpiece. They are graduated to 0.0005" and probably accurate to 0.001-0.002", maybe a little better.
I had the same trouble a few months ago and decided to machine a block of steel to a known accurate surface, then using digital readouts use a centre-finder to see if it would correctly indicate the centre. It varied by about 0.2mm ( about 8 thou ) through about 10 trials. First I used the methode of slowly creeping to wards the edge until the end went still ( concentric ) and then tried going past this point until it suddenly flicked out. Both methods didn't seem to offer any better reliability. So I went back to my old ground steel bar held in an autolock chuck. That never lies.
If you think about what is actually happening, it's easy to work with. Mind you it's far easier with a DRO than without, but recall that edgefinders have been around a LOT longer that DRO's. I have found that when you start the spindle up to find an edge the tip of the edgefinder should be "flicked" with a finger so as to have a quite definite "wobble". I know you can't "see it" as so, but this eccentric wobble will tend to act like a fixed lobe on a cam, in that it will tend to remain in one place on the edgefinder relative to the edgefinder body, so to speak. As you approach and the tip of the edgefinder contacts the edge of the work, the tip will get pushed over to become less and less eccentric and more and more concentric, until it's impossible to visually see any eccentricity. But the eccentricity is still there, ever so slightly. Your moving the table is pushing the tip closer and closer to being centred, and it does so because it "can" get less and less eccentric. But as soon as the edgefinder does reach concentricity, that is it becomes centred, then any motion past that point will cause eccentricity to occur again. But this "new" eccentricity cannot remain in one place relative to the edgefinder body as it did on the approach. The over-centering causes the eccentricity to occur in one place relative to the work face instead, so the edgefinder seems to "kick" over immediately. That is the point you are looking for. That ever so slight movement of the handwheel causing the kick. Accurate to at least a thou, and it is also in a dynamic mode, so accounts even for a non-concentric spindle, or collet, or holder, or drill-chuck, too. That's why it is best to chuck the edgefinder in whatever holder or drill chuck that you intend to use for cutting.
As to finding the centre of a hole, it can be tougher to see, but really all you have to do is get a rough location, move the table so you get kick on one direction, then traverse only that axis in the opposite direction until you get kick again, note the difference in the two points and halve it. Move the table to this "halved" point, then do the other axis the same way. Now the hole should be fairly close to centre. Then do them both again, then once again, and this last time the halving will now give you the centre. This is only easily possible with a DRO. With a bit of math, you can even determine how close you are by the motion distance required to see the kicks, as it should be hole diameter minus edgefinder diameter divided by 2. For example, presuming you know that the hole diameter is say .375, and the edgefinder tip is .200, then when starting correctly centred on either axis the kick point should occur at [ (0.375" -
0.200") / 2 = 0.0875"] of motion in either direction along either axis. You can even reverse this theory, and find the centre then note the motions and that will give you a really good indication as to hole diameter, and even as to concentricity, in that all movements from centre to kick point should be equal.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXX On Wed, 12 Nov 2003 03:11:43 GMT, "David Anderson" wrote:
For locating holes at school we mount a dial test indicator in the spindle, then move the table around until the dial reads 0 all the way around the hole when the spindle is rotated.
For finding edges we have a beeping edge finder - when the ball on the edge-finder makes contact with the workpiece it completes a circuit or whatever and makes it beep. I've used it a few times, but mostly I've been using a 20 thou feeler gauge and bringing the cutter in (with the power off!) until the feeler gauge is just barely snug between the cutter and the workpiece. Add 20 thou plus half the diameter of the cutter and the middle of the spindle is over the edge of the workpiece. I'm not sure how accurate this method really is, but I haven't had any problems so far.
chem
David Anders> Thanks to all for good comments - the link to the article by Mike Rehmis
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