Edge finder plastic vs. metal

[homebrewed]

Something I've become curious about recently is the relative accuracy of two different kinds of edge finder when it comes to working against a very slick material like teflon or UHMW polyethylene.

I have three different types of edge finders. One uses an LED touch sensor scheme so it's not useful for plastics. The other two are based on slightly different designs. One uses a snap-in ball in a collet like this one. The other uses a spring-loaded scheme like this. The former relies on friction between the tip and the work to kick the stem out, while the other uses a different approach that should be less affected by the type of material it's in contact with..

While machining some UHMW polyethylene, which is quite slick, I wondered if the two approaches might produce different results, since (perhaps) the snap-in ball scheme relies on friction between the tip and work for its action. A quick experiment showed that the two produced very similar results, but they weren't EXACTLY the same, while they individually were pretty consistent. I tend to think that the spring-loaded approach is going to be less dependent on the type of material.....but I don't know that for certain. I note that the "wiggler center/edge finder" is made by Starrett; but, on the other hand, Starrett makes the other style as well.

Are the different styles interchangeable? Is one generally better than the other?

 

[GeneT45]

I used to prefer wigglers, and still do for locating punch marks, but mostly use the regular edge finder (spring-loaded) because I can back off and hit the surface 2-3 times to check for consistency. Did you try your experiment against metal to see if the offset was systematic or truly due to the material? I might try this later today (with steel - I have no UHMW PE on hand) as it's something I've never thought to test.

GsT

 

[Bone Head]

Hard to go wrong with Starrett. Both have their uses, but I would go with the first.

 

[atunguyd]

I am of the opinion that the wiggler does not rely on fiction but rather work on the exact same principle at the spring loaded one. The workpiece initially pushes it into perfect concentricity but as soon as you go part that the forces multiply and force you the "kick out" on both types.

Again going on a hunch but I think it has more to do with gyroscope precession than friction of the surfaces. I did some internet searching but cannot find it the science behind them. Now you really got me interested. How do they work and why?

Sent from my SM-S908E using Tapatalk

 

[RJSakowski]

As you approach the edge, the tip is forced into concentricity and forces are balanced. As you continue, the forces become unbalanced and centrifugal force flings the tip out. Kind of like what happens with an unsupported bar in a lathe.

 

[homebrewed]

Maybe one way to think about the wiggler is as a linkage. If while spinning the stem is pushed out of alignment with the body it will spin around with a bit of an angular offset, then contact the surface at a slightly higher point, have the misalignment imposed again and as a result of repeated contacts like that climb up the side of the piece. Maybe -- if applied literally, what does it say about the approach-before-centered behavior?

I just saw RJ's comment. One reservation I have with the centrifugal force theory is that the wiggler stem can start out wiggling around but it doesn't fling itself out. I _can_ see that happening at high-enough RPMs.