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Top of part bigger than bottom of part

tvalen1432

Aluminum
Joined
Nov 21, 2014
Location
Pomona
Scenario: I wire a rectangular slot for a punch in 2" steel material @ 0.2 x 0.4456. using 0.010 wire. I have a Mits. MV2400S that has straight comp that is supposed to adjust for straightness when you set your Z1, Z2, Z5 and also you have to go into your EPAC's for the second and third passes and adjust the T and CC settings for the actual thickness of the part in mm. Then once I get that close(the bottom to a snug tap fit at least), I wind up dealing with the slot smaller in the middle or hour glass affect. So then I go back into the first and second skim EPAC's and raise the VG .5 ish. I still have a slight taper per say. My tolerance is very small like + 0.00005 -0.00000 per side. The fits like firm tap fit, light tap fit, and slip fit are how they classify them. I also read to adjust the FA and VO as well. By the time you are done some what, now you have to alter passes and offsets. :willy_nilly: I mostly deal with dies, punches, punch holder blocks, all with dowel holes as well.

Does anyone have a magic wand or solution to getting a straight hole or slot or is this normal? I get the whole wire deterioration as it cuts part but there has got to be a way around that. What is tipping the wire? Is that for taper cuts or angled holes? I don't understand how or when you would do that and for what purpose, but would doing this help?

If the people who create the EPAC tech already know that the part will have a drum, hour glass, or bigger at top than the bottom happen, then why don't they already have all the tweaks and adjustments already adjusted? This would be greatly appreciated and save me money on antacids, Extra Strength Tylenol, and slow down the aging process immensely! :wall:

What am I missing or doing wrong? This is ALWAYS an issue!

Thanks for any and all input,
Tvalen1432
 
Scenario: I wire a rectangular slot for a punch in 2" steel material @ 0.2 x 0.4456. using 0.010 wire. I have a Mits. MV2400S that has straight comp that is supposed to adjust for straightness when you set your Z1, Z2, Z5 and also you have to go into your EPAC's for the second and third passes and adjust the T and CC settings for the actual thickness of the part in mm. Then once I get that close(the bottom to a snug tap fit at least), I wind up dealing with the slot smaller in the middle or hour glass affect. So then I go back into the first and second skim EPAC's and raise the VG .5 ish. I still have a slight taper per say. My tolerance is very small like + 0.00005 -0.00000 per side. The fits like firm tap fit, light tap fit, and slip fit are how they classify them. I also read to adjust the FA and VO as well. By the time you are done some what, now you have to alter passes and offsets. :willy_nilly: I mostly deal with dies, punches, punch holder blocks, all with dowel holes as well.

Does anyone have a magic wand or solution to getting a straight hole or slot or is this normal? I get the whole wire deterioration as it cuts part but there has got to be a way around that. What is tipping the wire? Is that for taper cuts or angled holes? I don't understand how or when you would do that and for what purpose, but would doing this help?

If the people who create the EPAC tech already know that the part will have a drum, hour glass, or bigger at top than the bottom happen, then why don't they already have all the tweaks and adjustments already adjusted? This would be greatly appreciated and save me money on antacids, Extra Strength Tylenol, and slow down the aging process immensely! :wall:

What am I missing or doing wrong? This is ALWAYS an issue!

Thanks for any and all input,
Tvalen1432
The Taper Comp doesn't necessarily guarantee your part is going to be straight. What it is doing is comping the U and V axis for the drag in the wire and trying to optimize the cut. Given your part is square and everything is just so on the set up and machine, yes it can be effective and helpful.

More or less if you are getting an hour glass shape, I would try seeding up your wire, give it more tension and adjust your flushing. Watch your cuts too, if your second and third passes seem a little sluggish, you might need to make some offset adjustments as well.
 
The Taper Comp doesn't necessarily guarantee your part is going to be straight. What it is doing is comping the U and V axis for the drag in the wire and trying to optimize the cut. Given your part is square and everything is just so on the set up and machine, yes it can be effective and helpful.

More or less if you are getting an hour glass shape, I would try seeding up your wire, give it more tension and adjust your flushing. Watch your cuts too, if your second and third passes seem a little sluggish, you might need to make some offset adjustments as well.
On top of that I'd do a wire alignment before starting, probably 2 or 3 times. With those tolerances you want to take any error out of the head.
 
I can see the hourglass effect, but the bigger on top than the bottom thing kinda makes me suspicious.

I would try just running it without the taper comp and just run as many skims as possible. It sounds like you know the trick of bumping up your VG to get rid of the hourglass. When going for a nice fit like that I'm generally bumping the VG up to straighten out the middle and tweaking my offset a couple tenths for fit. I was always generally able to do this pretty consistently across jobs with just those two.
 
What about flushing? Can that cause a dimensional difference top and bottom? Could material flexing play a part in it as well? Would running in Optimum state create less of a problem?
 
What about flushing? Can that cause a dimensional difference top and bottom? Could material flexing play a part in it as well? Would running in Optimum state create less of a problem?
In my experience flushing is mainly going to cause your rough cut to be slower. If you're skimming multiple times the difference would be negligible.
 
I even upped my tension 2 notches as well. This last part I ran was a 2 inch thick punch holder block with 14 rectangular slots for the punches to sit in. Each slot ran EXACTLY the same way yet some were tighter than others with the top of slot being slightly bigger than the bottom of slot. I ran the skims straight through with out any M0's after pulling all slugs first. IDK. Maybe I need to kick the VG up higher.
 
I even upped my tension 2 notches as well. This last part I ran was a 2 inch thick punch holder block with 14 rectangular slots for the punches to sit in. Each slot ran EXACTLY the same way yet some were tighter than others with the top of slot being slightly bigger than the bottom of slot. I ran the skims straight through with out any M0's after pulling all slugs first. IDK. Maybe I need to kick the VG up higher.
Turning your VG up will only your hourglass shape one way or the other. Also turning your tension up isn't really going to make a difference unless it was waaay too low in the first place.

I still recommend trying it without the taper compensation and run the most skims your epack will allow.
 
I even upped my tension 2 notches as well. This last part I ran was a 2 inch thick punch holder block with 14 rectangular slots for the punches to sit in. Each slot ran EXACTLY the same way yet some were tighter than others with the top of slot being slightly bigger than the bottom of slot. I ran the skims straight through with out any M0's after pulling all slugs first. IDK. Maybe I need to kick the VG up higher.
You do realize that there will be hand work even with edm.
 
...Bead blasting, lapping, honing; many way to skin a cat!
Yup...no getting around it. I have my process down pretty well but I still have to fine tune the parts sometimes. Took a lot of cutting and measuring to get the proper tool size to use with overburn for both 10 and 12 wire. I very rarely use skim passes and hit sub .0005 numbers.
 
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How old are your diamonds? Sounds like your guides could have a bit of wear that is affecting your tolerances. That would account for a small amount of taper at top or bottom, but probably not hour glassing. Could also possibly be worn contact pieces not giving constant current flow to the wire? (I assume mits uses them) We run Sodicks so I'm not familiar with the hardware on your machine.

Chris
 
In general if you get hour glass side walls. The servo speed federate over ride etc is too fast on maybe all your passes or just too much feedrate during skims. Can mic the slug to see belly or taper on rough cut. But really if you slow down the skims by cutting tech federate you should be able to dial out over or undercut in the middle of side walls. Slower should take out tight in the middle. If you end up with the opposite you would want lower servo control or increase your feed rate. If taper top to bottom can program a counter taper , use a uv adjustment stratify like the mv has or if just a tenth or few lower/increase wire feed. Less wire feed tighter at bottom in general. Also some factory edm settings are sensitive if flushing intention dosent match the cut your doing. Could lower flush pressure if you suspect deflection on rough cut. Can also see if changes at all on slug.
 
You were all right as a whole. I figured I had made so many different changes in variables that I needed to start all fresh again and begin with the basic steps to do before running a job. Beginning with cleanliness, alignments and so on. First, I forgot about the "head away" tech as the fixture is 0.5 but I can get the upper head close. Needed to clean guides, power feeders(and plunger contact point), slow down 1st and second skims(for hour glass), etc. Got it to +/-0.0001 ish difference top to bottom in size and was good enough to pacify boss. I also suggested we switch to 0.00000 place for programming if they want tighter tolerances held.
Also I verified my Z1, Z2, and Z5 values were good. The other thing I tweeked was I made my radius 2 knotches smaller in the machine settings to make sure the radius wasn't what was hanging it up. Then Walah, perfect fit! :cheers: Got the high five from the boss and went on to the next project to perfect. LOL
Hope this helps some one else.
Tvalen1432
 








 
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