Rotary CNC sinker

I'm looking at quoting some 3 inch diameter by 2 inch wide parts that need to have a pattern EDMed around the periphery. Not sure if I would rotate the part and make a flat electrode, or how I would do it. Anyone have experience with this kind of thing?

Hi RJT:
Is there any chance it could be done by vectoring a multi pronged electrode around the part.
Here's a link to a page on my website that shows a tiny tube done that way and shows the electrode too.
Implant Mechanix – Design & Innovation » EDM
The tube was stood up in the worktank and the electrode vectored in 6 times.
It's a method that's suitable only for a limited kind of work, but for stuff it's good for, it's very good indeed.

A variant of this method, suitable for more complex trodes is to index the trode for each orientation of the burn you want to put around the periphery of the part, but it requires a C axis on your ram, and you need to be pretty accurate with your trode mounting and pickup.
However, it saves you mounting a rotary axis or indexer and mounting the part to that, so you need to decide which is the least painful way to mount the work and trode; I've done it this way mostly when the workpiece is awkward to spin and the trode is very simple.

If you're talking about EDM cutting a pattern that is continuous like the engraving on a print roll, you need to make a flat trode and traverse it past the workpiece that's rotating in synch with the traverse of the trode just as a rack can be made to traverse past a gear.
I doubt there are many sinkers that can make the necessary coordinated motions and still be able to move appropriately in response to spark gap conditions, so you might have to fake it with a program that makes the coordinated traverse and rotate motion in small increments followed by a conventional Z axis burn after each rotation and traverse increment.
If you've got a sophisticated CNC sinker though, it might have a canned cycle that can do roll engraving right out of the box.

The third way, of course is to split the pattern into quarters, machine your trode with the features around the inner surface of a circle and index the part four times with four trodes.
This will only work if you have shallow features and can accept the distortion such an approach will create if the features have no draft.
That, of course is also true for method #2, but less so as the indexing increment is small with method #2.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining

The pattern is too fine and too small to do any other way than rotating. Similar to a knurl pattern. I have an EDM applications engineer that is going to take a look at it. Making the electrode whether it's flat or circular will be a challenge.

Hi RJT:
Is it possible to post a picture or PM it to me?
I love manufacturing puzzles.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining

3r used to make an add on "c axis" for sinker edm's....matter of fact I have a sinker with one on it thats operational. Not talking a drilling attachment but a full 4th rotary "spindle" you can turn while cutting if need be.

Hi WILLIEO6709:
The problem is the complexity of the multi axis motion the machine needs to be able to make whenever it needs to reverse in response to the spark gap conditions.
A C axis on a sophisticated machine can rotate forwards or backwards in response to gap sensing inputs to do a swept burn, but I'm not sure they can coordinate with another one or two axes, especially another rotary axis, while they do so in order to do a rolled burn.
Imagine the motions you need to be able to do in both directions in response to gap sensing inputs and you'll see how difficult the problem is.
Sodick recently put a super multi axis machine on the market, and it was showcased in a little video on this site, but I doubt the average machine has that kind of capability.
So probably the best way forward is to index in very small increments while doing the coordinated motions between the rotary axis and the longitudinal direction of motion of a flat trode, then stop that motion and do a conventional Z axis burn to depth, then roll a little more, then do another Z axis burn etc etc until you've made it all the way around your part.
The indexing increments can be a fraction of a degree if you want to, since every index is accompanied by a linear move to keep the electrode coordinated with the rotation much like a rack and pinion, but the two motions need not even be simultaneous because the electrode is not cutting while the rotary motion and it's accompanying linear displacement occurs.
The motion required to do the burn and sense the spark gap conditions becomes a single axis, just like any conventional burn.
If the rotation increments are kept small enough, there is very little distortion of the form.
The biggest pain is the programming; if you can run a subroutine it's not too bad, but sinker EDM controls are often limited in what they can do in this regard; certainly my cheapie Hansvedt cannot be coaxed into doing this.
The next challenge as RJT alludes to is making the trode or trodes.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining

Any chance the pattern could be done with a laser? Just a thought. Would be interesting to see what type texture/pattern is needed.

NDA apply to this, so I can't post drawings or pictures. I'm going to play with our C axis and see if it will coordinate with X or Y. We usually use it to just spin the electrode or index so I'll have to experiment with it. I know Erowa and 3R sell 4th and 5th axis units for wire EDM, but I've never seen one for a sinker. I'm sure it's big bucks. I've seen 4 axis laser work, this wouldn't be an application for it.

Hi RJT:
OK, since you have a C axis but no rotary table for the sinker, here's how to do it.
Mount your round workpiece to the C axis, clock it in, and reverse the polarity of your burn.
Mount your flat electrode in the worktank aligned with the X axis and the engraved surface in the XZ plane behind the workpiece (in the Y+ position) so you can see easily what you're doing and you can direct your flushing wands more easily.
Write your code as I've described; make a 1 degree index (or whatever you decide is reasonable) with C, then make a corresponding X axis shift to bring the trode and the rotated workpiece into proper alignment, then do a burn in the Y+ direction.(there is no need to have any of these moves occur simultaneously with any other axis; they can happen sequentially)
Back up in Y when that burn is done, index again in C, advance the trode in X, and do another burn in Y+.
Rinse and repeat for one complete rotation of the C axis.
It'll work slicker than snot and won't cost you a dime in new tooling other than what you may have to cobble together to mount your job on the C axis.

With regard to making the trode; here's a neat trick for cutting shapes that are tough to make with rotating cutters:
If you have spindle orient on your VMC you can use it as a big CNC shaper to engrave patterns like knurls and other features that are repeating patterns of a constant cross sectional profile.
Suppose your pattern is a 30 deg diamond knurl.
You make a single point shaper tool with the proper cross section, orient it properly using spindle orient, and then write a simple bit of code to whistle it across your workpiece in your repeating pattern without rotating the spindle.
Then re-orient to the other cutter alignment and whistle across the workpiece in the other direction to get the cross pattern.
A simple subroutine will let you make these trodes pretty easily
I've used this tactic to shape tiny profiles for lighting fixture molds that were impossible to cut any other way, and if you get the top rake right on the tool and lube it; it will cut a beautifully smooth feature, especially if you make copper trodes instead of graphite trodes for this job.
Cheers

Marcus
Implant Mechanix – Design & Innovation - home
Vancouver Wire EDM -- Wire EDM Machining