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SeymourDumore

Diamond
Joined
Aug 2, 2005
Location
CT
Ok, here is a question for the "old timers"

I have a fixture I've been using on a production part for quite a while now. It is made of 6061 AL.
This morning I've set the job up, and during a momentary lapse of sanity loaded the wrong program, thereby cutting the fixture in half.
Ok, no problem, let's make a new one but this time out of say.....303SST.
Done. Loaded the fixture, set everything up the way it's always been....
Wire gets to the part, no spark, short circuit.
Did it again... same thing.
Ok, not enough ground, quickly hooked up a piece of copper wire, attached to the part.. same thing.

No matter what I've tried to do, could not cut the damm thing.
Back to the drawing board, made a new fixture, this time out of brass.
Loaded, started, all is well. Except the cycle time is 15% longer. 7min 20 sec. instead of a 6 min 10 sec.

Can some of you fine fellas shed some light on this please?
 
Brother HS50A.


Mega D wire, .012 .

Never really paid attention to cutting speeds related to the fixture material. I have literally 100-s of them, some of stainless, inco, brass, but ovewhelmingly aluminum.
I guess just made the fixtures once from of one material, and never re-done it out of something else.
 
Had the same problem once too. I took the whole machine apart, couldn't find the problem. Reassembled the whole thing again, attached a grounding wire to the fixture, still nothing. Then I took the lower wire guide apart and found a sliver of the fixture stuck in the lower guide, which caused the machine to short out. After that, no problem. By the way, that was on an Agie Sprint 70.
 
Rolf

The machine is fine, no foreign matter or goofy stuff anywhere.
The 303 fixture did not cut, the brass, and now a new aluminum one does.
For the heck of it I did put the 303 back and tested again, still nothing.
The cycle times are still the same with brass vs. AL.

Just as an info, the stainless was machined on all sides and cleaned before put in the tank. The contact between fixture and table seems fine.
 
Just a thought 303 or 303Se both are non magnetic. I would check the chemical proptries and weldability. Generaly stainless cut good. Just a quess.
Muskrat.
 
Is it just THAT SS fixture, or any SS fixture?

Just finished a job that ran in all three of our Robofil machines and our pair of HS-70A's. All the fixtures were made of 304SS, they worked very well. We use .01" MegaD wire.

When we've had contact problems with fixtures it has almost always been because of surface corrosion on aluminum fixtures.

Have you checked your fixture for conductivity with an ohm-meter?

How's the conductivity of your water?

Have you cleaned the area around your brushes?

Remember that electricity follows the path of least resistance.

On our old HSC-300 I have seen the swarf build up around the lower brush to the point it leeched off most of the power. I have also seen the conductivity of the water high enough to do the same.

It sounds like your power may be finding an easier path to ground. It still works OK with brass and aluminum because their conductivity is better than SS.

Just a guess.
 
Just this particular one.
The fixture itself cut fine, as I had to make a couple of small wire start indents in it, and also squared it up in the machine by skimming the face.
Yep, the machine is clean and properly set.
This is a production part, made well over 3000 in the last 3 or so years. Cutting times are within 10-20 seconds of what they always are.

Stumped me....
Back to the AL fixture now and all is well, but still curious....
 
You didn't mention what material the part was and what method is used to hold it in the fixture...

IF the fixture cut fine on the EDM, but the part does not when it is mounted in the fixture, there is something in the mounting of the part that is isolating it from ground. I might try and (if possible) dusting off the top and bottom of the fixture.

Except the cycle time is 15% longer. 7min 20 sec. instead of a 6 min 10 sec.
Is the fixture an "exact duplicate" of the original? Same thickness etc? Probably need to know the nature of the profile... Could be simply due to slot thickness in the fixure not allowing as much flushing to get through.

One last thought, stainless does not conduct quite as well as brass or aluminum. You might have a conductivity problem in your spark cables. Cable could be worn, not allowing as much power (or ground) to come through to the gap. I am assuming with your cycle time that the parts are quite small. If a spark cable is getting "tired" it could also account for the increase in time. I know this is a stretch, but EDM is a funny animal


BTW, LOTS of machine tables were made from 303 stainless... So I doubt the material selection has anything to do with it.

Good luck!
Greg
 
The fixture shape is the same on all, cut with the same program.
The part is 3.6" tall inco718, stable as it gets.
Mounting is done with a 4-40 screw, unfortunately no change in that is possible due to the shape.
Funny animal.... Well yes. After 8 years all I know for sure that what I know may or may not be so.
Interestingly, strictly conventional guys can't quite understand that. For them if it cuts, then it cuts. For EDM... well, let's see..... maybe... or maybe....how about....or perhaps.....
Ahh. the hell with it, just git'r done.
 
try insulating the fixture from the table then run the jumper wire. Try insulating all the different fixtures from the table, run the wire then check the times.
 
Willy, if I had the time I'd do a bunch of other things too, but for now I could use 3 more wires just to catch up.
 
303 SS has a lot of sulphur in it. I had a problem a couple years ago welding some blow needles for a friend. I thought he used some weird coolant. I had
welded 304,308,312,315 with no problems. 303 is different animal.
 
Here's a question Seymore; what did you make your fixture on? Was it machined on a chipcutting machine, or did you cut the fixture out with the wire?

303 is a poor welding material, not because of magnetism or conductivity, but because the sulfer content contaminates the weld. It should EDM fine as do all austentic stainlesses.

If anything, I would say the lack of cutting is due to surface contact area. I dont' know how big your parts are, or what shape they are, or what contacts the fixture, but is it possible the aluminum deformed slightly to give a better contact area; whereas the stainless fixture isn't making a perfect contact? You mention the use of a 4-40 set screw to hold the part, that implies it's a small part to begin with.
 
Mits

I've squared up and drilled the fixture mechanically, and once it was on the machine I'v ecut the mating shape into it. IOW the part locates on EDM-ed surface.
The surface area is 3.6" x .310, in an ideally flat part would be 1.116sq" max. In reality it's much much less than that. but not less than parts I hold in a SST V-block. Also, on the AL fixture there should be some arcing marks if the contact was insufficient. After a gazillion parts I see none of that.

Sidenote: Funny how you guys, doing EDM for a living, calling a part held with 4-40 screws as small/ I also do this for a living (along with conventional machining as well), but for me this is one of the larger pieces.
I guess it ain't a 500 pound diestock with a 200 pound slug cut out of it. Literally takes me 4-5 months to fill up a few buckets with slugs to make a worthwile trip to the scrapyard. Sure, a bucket of my scarp usually worth upwards of a $1000 or so.
No offense taken, just funny.
 
Seymour, I have customers make parts on their machines that DWARF a 4-40 set screw!

Very weird that you coudln't get it to cut. Doesn't sound like anything that should be a problem.

Recast on your fixture from EDMing the mating shape? :confused:

EDIT: I should have said their parts are dwarfed by 4-40 screws.... oops!

[ 05-08-2007, 02:08 PM: Message edited by: MitsTech ]
 








 
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