hole popping small holes

Today we used .012" brass tubing to put holes in a 1" thick piece of D-2. We were skeptical to start with, as we were trying to keep the holes straight within .003 so that with overburn going to about .015 we could still wire EDM the hole to a finished size of .020 Any one have experience with this size / length? We have a Methods bridge type hole popper with a .0005 resolution DRO that has always been accurate from hole to hole. On the bottom side of the part we were measuring .0005 up to .008 of out of position (walk off of the electrode). Any reccomendations (including shops that specialize in this type of work)? This was a job for another shop that we were trying to do in a test part. He is still looking for a solution, I don't know if it will be me. Thanks, RJT

I trust you checked the tram of the guides on your machine and that it is straight & square. And that your table/work holder is true. We've done holes like that and usually can hold about .0015 to .003 straight, that's with a Current CT300 with a Kern multiguide. Can you drill half way from each side? Try different water pressure too, rarely, but sometimes that helps us.
Dave

Whats a Kern multiguide???? We use a single guide about 1/2 inch long. Are you using brass tube? Yes we have checked alignment. Holes are in position at the top and run away at the bottom.
RJT

RJT, Aside from guide alignment and quality of setup, spindle rpm and dielectric pressure are the two primary factors that will affect how much the tube will "walk" off location as it progresses through the work piece. These are almost equally important, since it is a combination of the two that makes the whole process work. If the flushing pressure is too low or the dielectric viscosity too high (Believe it or not, water is almost too viscous for this application. The .0030" diameter opening in a .012" dia. electrode is about 11% of the cross-sectional area of the resulting .0015" spark gap.), there is not enough volume of fluid coming back up around the outside of the electrode to keep it centered in the hole, and it cannot burn straight. Spindle rpm is equally critical since even with a "perfectly straight" electrode, variations in wall thickness will cause flushing to exit toward the thinnest wall section of the tube as the electrode wears. The resulting flush exiting at an angle to the rotational center axis of the electrode will cause deflection at the tip. This will cause "walking" if the spindle rpm is insufficient to compensate for this effect. This combination of factors is the reason you will often see "bouncing" when the electrode begins to break through at the bottom of the hole, since flushing pressure is no longer holding the electrode centered in the hole, and the deflection caused by flushing pressure makes the tube essentially "bore" its way through the bottom of the hole with no real benefit from flushing at the point of discharge. Your application has a depth/diameter ratio of 83.3/1, which makes this even more difficult. As a single burn, this application is hovering at the limit of "hole popper" capability, and even the most skilled operator would need to get everything "just right" to perform this operation consistently and predictably from piece-to-piece (keep in mind that at .015" dia. with "perfect" position at entry, a mere .0025" drift at exit gives you a bad part). As Dave suggests, you might find better success in burning half-way from each side, but none of us really want to make two setups for one start hole, do we? Ideally, to do this with a single burn, this application would be best suited to a machine which can provide a STEADY supply of a low-viscosity (ex: Iono-Plus 3000 ET) dielectric at around 1200+ psi and around 1000 spindle rpm for the most accurate and most consistent result. Granted, a machine with this capability is in an entirely different class than the average "hole popper", and is commonly referred to as a "fine hole machine". These machines are typically priced between $150K and $300K, depending upon the builder and machine configuration.

Do you know of anyone with this type machine that would do this? I have 3 parts with 25 holes each. 1 part 1" thick, , 1 part 1 1/8 thick, 1 part 3/8 thick.
RJT

RJT, I'll e-mail you some info.

Yes, we use brass tube for all steel, mostly Saturn and Belmont and some Current brand. The Kern multiguide is/was? made/marketed? by Kern EDM spplies. I can't find it on their website, so I emailed them. http://www.kernedm.com/kernedm/
It replaces the guide holder on the bottom of the machine and has three "jaws" similar to the precision chuck that Herman Schmidt makes. http://www.hermannschmidt.com/productcart/pc/viewCat_P.asp?ProdSort=19&idCategory=16&page=2&iPageSize=10
Kern's has saphire rods in the grip surfaces for wear and insulation. Tube range is .010 to .250 with good concetricity and location repeatability. We routinely pickup with one size tube and burn two or more sizes without re-pickup.
I'll post their answer for availability when I hear.

The Hermann Schmidt 3-jaw is a work holding fixture actuated by a screw mechanism.

The Kern multiguide is a spring loaded 3 jaw with saphire rods for guiding the tubes.

There was also a multiguide version built by Tech Mold in Tempe, AZ.

Try using a machine that uses dielectric oil. You can use mineral oil, kerosene, or number 1 diesel for lower viscosity. Mix a little old oil in with it if necessary and make sure you are filtering very very well. You can prefilter your fluid before putting it into the system.
You will probably need to make a special bushing (from brass or aluminum and drill it with a small drill on the lathe) to backup the rubber seal so that it can take the extra pressure that you will need. You cannot use the standard latex seal as the oil will toast them quickly. You may need to make a special mold to make some seals. Santoprene works very well for seals. I use a piece of .25mm brass wire from the wire EDM as a core pin to preform the hole in the santoprene bushing.
If you use the machine made for dielectric oil, you can adjust the settings for a rather small overcut which will eliminate the holes wobbling off course. That way you will not need a machine that only the insurance company or title company can afford to own.
If you wish to have someone else try the holes, please contact me.
[email protected]

Oh, I would try the previous suggestions about going in from both sides and aligning the guides. Going in from both sides will reduce the extreme wear that you will be getting from gong through such a long length. You can super glue a piece of shim to the top of the block and drill a very short hole to check hole location with a microscope and reposition if necessary to get your proper hole location.
I have noticed that some of the Charmilles hole poppers have excellent long guide systems, but you will have a longer unusable elcectrode to throw away.

RJT, If you want a shop recommendation Bengtson Tool & Die in NH could be your answer. I worked for this company for 23 years before going on my own last year. They have put in millions of holes in all kinds of materials. I would be willing to bet there are only a handful of shops out there with the experience and knowledge they have. Drop them an email ([email protected]) you will get fast courteous service.