Edge finders

[MichaelP]

I'm still waiting for an explanation how a friction (mechanical) edge finder neutralizes TIR.
I hope we all agree that an electronic one doesn't. Now what's special about a mechanical finder?

Again, I'm talking about a single edge indication, not a case when you need to find a center between opposing edges.

[npalen]

I'm still waiting for an explanation how a friction (mechanical) edge finder neutralizes TIR.
I hope we all agree that an electronic one doesn't. Now what's special about a mechanical finder?

Again, I'm talking about a single edge indication, not a case when you need to find a center between opposing edges.

The probe of an electric edge finder is not free to find its center (axis) of rotation. The kind that I'm familiar with doesn't require rotation although it would be more accurate if readings were taken at various points around its periphery and then averaged to account for runout.

[npalen]

Click on flatted edge finders is dependant on speed and strength of the spring.

-D

So are you saying that an audible click is only experienced when using a "flatted" edge finder? (I'm still working on the definition of this "click", as you may perceive.)

[npalen]

May I introduce the idea that radial runout does not effect the edge finder accuracy
but angular runout seems like it would have a larger effect and cause accuracy issies.

-D

Good question! I would think there would be an effect but not sure whether it would be of any consequence.

[QSIMDO]

Question I've had, is the finder not on the edge when the tip stops spinning?
Why does it have to "kick off" to the side?
Just asking 'cuz I know...years of use says that's the way to do it.

[mikey553]

Question I've had, is the finder not on the edge when the tip stops spinning?
Why does it have to "kick off" to the side?
Just asking 'cuz I know...years of use says that's the way to do it.

Because it is easily visible when it kicks off to the side. You cannot tell accurately when the end stops moving around.

[Doozer]

Question I've had, is the finder not on the edge when the tip stops spinning?
Why does it have to "kick off" to the side?
Just asking 'cuz I know...years of use says that's the way to do it.

Maybe you should use a wiggler.
The concept you are asking about
is clearly illustrated when playing
with a wiggler. Just be careful not
to prick your finger. (That's why no
one uses a wiggler any more).
Get your fingers tangled up in it for
the first time, and you'll say fuk that.
But it is a usefull tool to have to fuk
with the new guys.

-D

[MichaelP]

The probe of an electric edge finder is not free to find its center (axis) of rotation.

OK, let's see. The tip of a mechanical one is, indeed, free to find the center of rotation which is the spindle axis. But unless the spindle axis and finder axis are concentric, the tip is not going to be flash with the shank of the finder when the spindle axis is above the edge. The tip will be pushed in (closer to the spindle axis). When we use a mechanical finder, we rely on the moment the tip and shank are flash with each other (or the first moment they suddenly shift from being flash). So we'll need to bring the table away from the edge to the TIR distance to make it look flash.

Do you see any inconsistency in my thinking?

[npalen]

OK, let's see. The tip of a mechanical one is, indeed, free to find the center of rotation which is the spindle axis. But unless the spindle axis and finder axis are concentric, the tip is not going to be flash with the shank of the finder when the spindle axis is above the edge. The tip will be pushed in (closer to the spindle axis). When we use a mechanical finder, we rely on the moment the tip and shank are flash with each other (or the first moment they suddenly shift from being flash). So we'll need to bring the table away from the edge to the TIR distance to make it look flash.

Do you see any inconsistency in my thinking?

I assume that when you spell "flash" you actually mean "flush"? Would you be willing to run the simple test outlined above in post number 16?
If not, I can only assume that you are here simply to troll the discussion.

[MichaelP]

I got it. You cannot explain how it works, and therefore I am a troll.

P.S. Thank you for correcting my gramma. English is not my native language, so I appreciate this kind of pointers. In return, may I suggest to put a comma in your first sentence?

[npalen]

It's very simple and I will explain it once again. The probe goes off center when it can no longer stay on the center of rotation. The center of rotation is not the shank (upper part of the edge finder), it is the center of rotation of the spindle bearings. Two different things that you apparently are unable to comprehend.

[Randy]

Talk about complicating a simple subject...

Boy, ain't that the truth. Starting a thread is like a box of chocolates. You never know what you're gonna get. Sometimes a useful answer out of left field, sometimes just an argument.

All I wanted to know is, can I get one with a 3/8" shank. Surprisingly, djc actually answered, but the merchant is in Hong Kong, so I don't think I'll go for that, although it seems that shipping from China can be surprisingly cheap. I think I'll get one from a US merchant and grind the shank down to 3/8".

Or maybe get a 10mm collet. That's perfectly reasonable. My collet rack is full, and I don't want to make a bigger one, but I very rarely use my 7/16 collet and could stick it a drawer.

Some asked, why not use a drill chuck? I do, if I'm drilling. If I'm milling I use a collet. Whichever requires the least raising or lowering of the knee.

The times that a split-in-the-middle edge finder might be useful, it seems to me, is when I want to pick up an edge that's well below the top of the part. Like finding the center of a 2" rod by picking up front and back, where the action is hard to see with a comventional edge finder. Or maybe ther's no significant advantage, I don't know. I'll give it a try. It's worth the small price to find out.

Metric probe diameters are not a problem. Just enter an offset into the DRO of .079 or .197 rather than .1 or .25 if I'm using inches.

None of my edge finders click audibly, but I have a spare Starrett 827A, and I plan to grind a flat on the probe like the Fisher Audible edge finder just to see (hear) if it works.

And for those who just can't understand Paul's and npalen's explanations of how a rotating edge finder can be self-correcting for a small runout, replicate npalen's experiment. Seeing it happen will make it clear, or at least force you to accept the truth, even if you don't understand it.

[Doozer]

Sometimes I need a long shank edge finder.
I have used a 3/8" piece of ground or chrome shafting
of whatever length, like 6" or 8", whatever you need.
Chuck it up or put it into a collet. Having perfect run-out
does not matter. Now ink up the lower end with a Sharpie
marker. Start the spindle. Run it kinda slow. Now bump
the surface on the part that you want to edge find.
Because there is always some run-out, you will see a
partial bit of the marker rub off as the ground rod spins.
Keep moving the table until the marker rubs off completely
around the circumfrence of the rod. The rod is bending slightly
and that is what you want. A full rub means the end of the rod is
running perfectly on axis. Now you can debate perfect, that's fine.
But anyhow, subtract half the diameter of the rod, and that
is your center. This method has got me to .001" of being
accurate. Don't overshoot minimum full rub, or all bets are off.
But this works if you have a difficult to access surface to pick up.
Kinda similar to your electric LED edge finder that blinks until
it is in position, only you look for full ink rub all the way around.

-Doozer

[npalen]

Boy, ain't that the truth. Starting a thread is like a box of chocolates. You never know what you're gonna get. Sometimes a useful answer out of left field, sometimes just an argument.

All I wanted to know is, can I get one with a 3/8" shank. Surprisingly, djc actually answered, but the merchant is in Hong Kong, so I don't think I'll go for that, although it seems that shipping from China can be surprisingly cheap. I think I'll get one from a US merchant and grind the shank down to 3/8".

Or maybe get a 10mm collet. That's perfectly reasonable. My collet rack is full, and I don't want to make a bigger one, but I very rarely use my 7/16 collet and could stick it a drawer.

Some asked, why not use a drill chuck? I do, if I'm drilling. If I'm milling I use a collet. Whichever requires the least raising or lowering of the knee.

The times that a split-in-the-middle edge finder might be useful, it seems to me, is when I want to pick up an edge that's well below the top of the part. Like finding the center of a 2" rod by picking up front and back, where the action is hard to see with a comventional edge finder. Or maybe ther's no significant advantage, I don't know. I'll give it a try. It's worth the small price to find out.

Metric probe diameters are not a problem. Just enter an offset into the DRO of .079 or .197 rather than .1 or .25 if I'm using inches.

None of my edge finders click audibly, but I have a spare Starrett 827A, and I plan to grind a flat on the probe like the Fisher Audible edge finder just to see (hear) if it works.

And for those who just can't understand Paul's and npalen's explanations of how a rotating edge finder can be self-correcting for a small runout, replicate npalen's experiment. Seeing it happen will make it clear, or at least force you to accept the truth, even if you don't understand it.

Here is one that may not be quality because of the low price. I checked the shipping and it shows $4.25, same as the price of the edgefinder.

https://www.ctctools.biz/3-8-edge-finder-d61/

Edit: I went to order one and then the shipping showed $12.00. Think I will pass.

[npalen]

Wouldn't a person have to note the difference in position from where the shafting first touches the work to where it contacts all the way around the shaft, and then move half that difference plus the measured radius of the shaft?

Edit: Sorry, just being ornery.

Sometimes I need a long shank edge finder.
I have used a 3/8" piece of ground or chrome shafting
of whatever length, like 6" or 8", whatever you need.
Chuck it up or put it into a collet. Having perfect run-out
does not matter. Now ink up the lower end with a Sharpie
marker. Start the spindle. Run it kinda slow. Now bump
the surface on the part that you want to edge find.
Because there is always some run-out, you will see a
partial bit of the marker rub off as the ground rod spins.
Keep moving the table until the marker rubs off completely
around the circumfrence of the rod. The rod is bending slightly
and that is what you want. A full rub means the end of the rod is
running perfectly on axis. Now you can debate perfect, that's fine.
But anyhow, subtract half the diameter of the rod, and that
is your center. This method has got me to .001" of being
accurate. Don't overshoot minimum full rub, or all bets are off.
But this works if you have a difficult to access surface to pick up.
Kinda similar to your electric LED edge finder that blinks until
it is in position, only you look for full ink rub all the way around.

-Doozer