Finding Edge of Finished Surface

[Lavcat]

As each month (and year) goes by I am getting closer to having a metalworking setup for precision chassis drilling. It seems each small paycheck gets me closer to the next tool and a new "opportunity" that must be solved.

The current question is how to locate an edge of a finished surface, in my case, powder coated extruded aluminum. I assume a traditional rotating edge finder will ding the powder coat. A safety concern is the high minimum speed of my drill might launch the head as a projectile. And unless I'm mistaken an electrical contact edge finder wouldn't work on powder coat. The only practical method I can find is a laser edge finder:

Welcome to Laser Center/Edge Finder

Of course, the laser units are expensive (though one trashed enclosure would "pay" for it), and even when held in a collet some say the laser alignment is not that accurate. However maybe plenty accurate for my purpose.

Any thoughts?

[tvrgeek]

I just cover the surface with plastic film and do my layout on that. The film is thick enough that a point in the chuck only touches the film, not the surface. I never cut on a finished surface as a milling may spin around the bit and scratch the finish.

The mechanic who made the crankshaft for the Write brothers flier used a cold chisel and a file. It ain't the machine. It's practice. Practice with conventional tools on scrap.

[Lavcat]

For making the Wrights' crankshaft Charlie Taylor used a hammer and a chisel but he also used a drill press and a lathe! It's a bit disingenuous to imply he did not use power tools for the project. I'm pretty sure he would have used a mill if the Wrights' bicycle shop had had one.

'The Wright Experience' : founder Ken Hyde

I'm not old enough to have been around when Taylor built the crankshaft, but I was born before his description was written. It was touching to read the Wrights gave him a micrometer. I had a friend who was motoring around North Africa in the first years of the last century. She told me at one point her driveshaft broke, but fortunately in the nearest town there was a German machinist who turned her a new one, and she was on her way. So, yes, people can do good work with limited tools.

I have had training in metalworking but that was many years prior to when even the old-fashioned mechanical edge finder was invented, let alone the newfangled electronic kind. In earlier years I've done some nice (if I don't mind saying so myself) work with a hand held drill, ruler, and a center punch. But I'd like to learn what is best practice in 2012, not 1902.

From what I've read in the literature from aluminum producers, powder coat is designed to be machined after it is finished. It thus makes sense that Hammond and others sell powder coated panels and enclosures. I'm pretty sure I'm not the first person who wanted to find the edge of a powder coated surface.

[vinylkid58]

Originally Posted by Lavcat

From what I've read in the literature from aluminum producers, powder coat is designed to be machined after it is finished. It thus makes sense that Hammond and others sell powder coated panels and enclosures. I'm pretty sure I'm not the first person who wanted to find the edge of a powder coated surface.

Powder coating is pretty tough. I doubt a standard rotating edge finder would leave a mark. You could mask the edge with tape, and subtract the thickness, but not sure it's worth the bother.

jeff

[pinkmouse]

I may be being dense, but why do you need to have the spindle rotating at all when edge finding?

[vinylkid58]

Originally Posted by pinkmouse

I may be being dense, but why do you need to have the spindle rotating at all when edge finding?

Watch the video:

Using an Edge Finder - Tormach CNC - YouTube

jeff

[tvrgeek]

Taylor was no more just a local machinist that the Wrights were just bike shop owners. Glad his story is told.

Current best practice is a ruby ball tip on the multi-axis measuring arm built into the multi-million dollar cnc.

Power coat does scratch from a drill shaving that has not left the bit. Been there, done that.

[max426]

Originally Posted by Lavcat

As each month.....
Any thoughts?

do not tun the spindle on
use the solid shank of the edge finder(or dowel pin)
slide a piece of paper between the part and edge finder
move the paper back and forth while moving the table toward the part
until you feel contact
do the math
diameter of edge finder/2 + paper thickness


jim

[Lavcat]

Originally Posted by tvrgeek

Current best practice is a ruby ball tip on the multi-axis measuring arm built into the multi-million dollar cnc.

How does one insure a known displacement of the ruby ball to the center of the spindle? If I could do that I could probably find the edge by using a dial gauge. The advantage of the laser is that it is supposed to be concentric with the spindle.

Edit: I'm thinking you may mean a device that has the ball concentric with the spindle? One consideration unfortunately is cost, but I have not seen such a device small enough for my equipment. My drill can accept only up to 6 mm in a collet, and there is not enough clearance to the work for the length of this sort of indictor.

Originally Posted by tvrgeek

Power coat does scratch from a drill shaving that has not left the bit. Been there, done that.

Yes, I would think so. However I am expecting that any marks close to the holes will be hidden.

[Lavcat]

Originally Posted by max426

do not tun the spindle on
use the solid shank of the edge finder(or dowel pin)
slide a piece of paper between the part and edge finder
move the paper back and forth while moving the table toward the part
until you feel contact
do the math
diameter of edge finder/2 + paper thickness


jim

Mind that I have never done this myself, but I thought to use the paper method for finding an edge that the tool had to be spinning because of runout?