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T Long
03-15-2004, 03:52 PM
Hey guys,

I have a very basic novice question regarding my 827A Starrett edge finder.

My confusion come in as to what part of the edge finder should come into contact with the work.

As the edge finder is spinning in the spindle, at about 1000 rpm, where do you want the edge finder body to conact at?

Also, the bottom of the edge finder is radiused, is this where the contact takes place?

Sorry for the basic questions, my search in the archieves did not answer my question.

Thanks in advance.

ERBenoit
03-15-2004, 04:06 PM
The edge finder should contact the work with the small (.200")diameter. I use a max spindle speed of 700 rpm. when edge finding. The contact occurs along the length of the .200" dia. contact. Unless there has been a change in design, the end should be square to the contact body. I have six 827A edge finders, none of which have a radius on them. Is the edge finder brand new?

[This message has been edited by ERBenoit (edited 03-15-2004).]

winchman
03-15-2004, 04:10 PM
I've never used one like that, but it looks like it would be best for it to touch at the top of the probe (the section with the 0.200" diameter). However, it should work OK if it touches anywhere on the probe.

The joint line between the upper and lower parts will disappear as the probe touches the work, and reappear if you go too far.

Roger

winchman
03-15-2004, 04:15 PM
It looks like the part that goes in the chuck has the radius. The end of the probe looks like it is flat.

http://www.littlemachineshop.com/products/images/827B.jpg

Roger

Ironland Spline
03-15-2004, 04:46 PM
Hey,

I'm not familiar with that particular model, but I'll describe a couple common edge finder configurations, and how they work and see if this helps out.

I have one Starrett edgefinder that is a simple two-piece one. The long end is the shank, and the short end with the smaller diameter is the part that comes in contact with the workpiece. The short end on this model has a diameter of .200". With your spindle on, slowly bring the edge of the work piece up to the smaller diameter of the edgefinder until you make contact. After you make contact continue to move the workpiece into the spindle and watch for a step to form at the joint between the small diameter and the shank. Usually you'll see the small contacting diameter become more and more concentric with the shank the more that you bring your workpiece into the spindle(after the small end has contacted the workpiece).

Keep bringing the spindle into the workpiece until you see a step quickly form where the small end of the edgefinder meets the shank. As soon as that step forms, stop moving the table. You are at the zeroing point. Now zero your table screw or DRO. Back the spindle away from the workpiece a little 'till you see the small end of the edge finder "running out"; then slowly bring it in again 'til the step forms again. What you are doing now is making sure that you didn't go too far beyond what was necessary to find the edge with the step. Watch carefully as you bring the work piece into the spindle and as soon as the step forms (snaps) again where the small part of the edgefinder meets the shank, stop moving the table and doublecheck your zero position. Now the spindle centerline is 1/2 the contact point diameter away from the edge. Use that information to either put the centerline of the spindle on the edge of the piece (if the contact diameter is .200" then you are .100" away from having the centerline of the spindle on the edge of the workpiece!), or to go somewhere else. I usually start my CNC mill a standard increment outside the workpiece after finding the edge. Just so I know where the centerline is, then I can program or otherwise plan and produce my toolpath.

I also have an edgefinder that has a point on the end. This is actually a combination edgefinder/holefinder. This point is for picking up small holes. You don't have to try to indicate small holes in if you have one of these. Especially threaded holes. If the hole is chamfered these pointed edgefinder/holefinder things work pretty good. The point on the edgefinder works like the cylindrical smaller end, but the difference is that with the point you use the very tip of the edge finder and bring it down into the hole until a step forms and then you move the table around (making sure that the edgefinder is having full contact with the hole) until there's no step in the edge finder. You've then located your hole!

I've also seen other combination ones where you have a smaller cylindrical portion terminating into a ball on the end. This can be used for both finding a hole (with the ball) and finding an edge (with the cylindrical portion- or the o.d. of the ball). I haven't seen one yet with just a radius on the end. But if yours has a straight cylindrical portion beyond the shank, that is the portion that you use for finding an edge. And you don't have to contact the whole surface area of the contact portion for it to work. Just use maybe 1/2, or 1/4 of the length of the contact portion; it will suffice.

Hope this helps,

Howard

spope14
03-15-2004, 05:17 PM
The pointed edge finders can be used to find center punched locations on parts to a high edgree of accuracy - to almost exactly the opint of the center punch location (then error from the punch mark takes over). I generally do this with the edge finder lightly seated in the punch mark, and with the spindle not running (binding will cause havoc on the spring). Same goes for holes.

The same goes with 4 jaw chuck work in a lathe, where you can do a light center punch mark on the work at the centering point, and use the edge finder to help get very close in the ball park with your jaw setting. I turn the chuck by hand, and seat lightly to prevent bind. The edge finder goes in a drill chuck, and your accuracy is very close but for error of the punch mark, and possible error of the drill chuck.

Always measure an edge finder end. i have had more than my share of the "cheapos" that have measured in between .199 and .197 diameter on the .200 end. Also check for end burrs. These negate any gain from edge finding.

Just a suggestion for extreme accuracy. Work off center of a part, and indicate the part to center, or indicate edge with a parallel set-up. Quite a challenge to a newer person or even an experienced person with less experience on this practice, but the results are outstanding.

SGW
03-15-2004, 05:38 PM
The idea is to make work contact with the 0.200" dia. portion. When it "kicks" (when you move past center), the spindle is 0.100" from the edge. Since most machines these days have 5 tpi lead leadscrews (one turn = 0.200"), set the graduated dial at 100. Then when the dial reads 0, you'll be at the edge of the work.