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Guidance for High Quality Surface Finish from EDM

newtonsapple

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May 16, 2017
I am looking into options for production of a small mold type component. It is a small zero draft hardened 440C cavity used for casting a specialized polymer. We make these in a bunch of different sizes from a bit smaller than the one I have attached a cross section photo of, to around .25" ID. The cavities are currently EDM'd with the smaller through hole being jig ground to achieve the tighter tolerance (we are aiming for +/-.0002"). We are working with a shop that runs older EDM equipment, but does a pretty good job. In house we carefully polish the cavity, finishing with a linearly drawn mirror surface finish.

The polishing process is very time consuming due to the small part size and occasionally disasters happen. I am looking into options to bring the EDM process in house while reducing the amount of time spent polishing. We produce only 200-500 of this type of part each year, generally 1-3 of a single design. The desire do this work in house is based only on lead time reduction. The goal is to be able to fixture 3-5 of these at a time and have the machine work unattended, producing either the final surface finish or something close too it. It would be great to be able to eliminate the jig grinding as well.

What have your experiences been using Mikino's HQSF option or any similar types of equipment?

Has the extra machine cost, effort in very nice electrodes, and consumables cost been worth it? How good of a surface finish have you been able to reliably produce? I've read a bunch of articles, but I am interested in real world experiences.


polishing.jpg
 
Hi newtonsapple:
Newer sinker EDM equipment will give you a better surface finish compared to older sinkers assuming equivalent quality electrodes, so you can reap some gains there, but they will be small gains.
The reason is that the finish straight off the EDM will always be a finely pitted one; there will always be some amount of cut debris in the gap sparking randomly against your cavity walls, and your electrode will always deteriorate from the use and abuse of sparking.
Also the finer the finish the more important the processing details become, and the more time it all takes.

Having said that; the Makinos are very fine machines indeed; if anything will get you close to your goal a first class machine like this will certainly help immensely but if you hope to get a SPI A1 equivalent finish right off the machine consistently, I think you'll be disappointed.
I've seen some magnificent finishes from high end sinker EDM machines; so you may well halve your polishing time but I doubt you'll eliminate it.
Also you NEED the draw polish in my opinion, and no EDM machine will make that for you.
Fortunately you can flush well because of your through hole, and you can rotate and orbit your trode, so you have a good chance to do better than the pore size of your trode (assuming you're using graphite), and for something like Poco Angstrofine graphite that's pretty damn good!
You might do even a bit better with something like silver or copper tungsten.

So yeah; I'd do it for the quantities you're talking about and a Makino would be right up there among my top tier choices, but I'd demand a demo for sure, and I'd ask them to supply a turnkey solution that's the best they can do with their brand of equipment.

Make sure that turnkey solution covers everything in detail including stock allowance electrode processing, machine settings, fixturing....everything from soup to nuts.
Don't go cheap on anything or you'll wipe out half your gains instantly.
At this level they're hard gains to make and hold on to; one process deviation and you're back to polishing like crazy.
Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com
 
Hi newtonsapple:
Newer sinker EDM equipment will give you a better surface finish compared to older sinkers assuming equivalent quality electrodes, so you can reap some gains there, but they will be small gains.
The reason is that the finish straight off the EDM will always be a finely pitted one; there will always be some amount of cut debris in the gap sparking randomly against your cavity walls, and your electrode will always deteriorate from the use and abuse of sparking.
Also the finer the finish the more important the processing details become, and the more time it all takes.

Having said that; the Makinos are very fine machines indeed; if anything will get you close to your goal a first class machine like this will certainly help immensely but if you hope to get a SPI A1 equivalent finish right off the machine consistently, I think you'll be disappointed.
I've seen some magnificent finishes from high end sinker EDM machines; so you may well halve your polishing time but I doubt you'll eliminate it.
Also you NEED the draw polish in my opinion, and no EDM machine will make that for you.
Fortunately you can flush well because of your through hole, and you can rotate and orbit your trode, so you have a good chance to do better than the pore size of your trode (assuming you're using graphite), and for something like Poco Angstrofine graphite that's pretty damn good!
You might do even a bit better with something like silver or copper tungsten.

So yeah; I'd do it for the quantities you're talking about and a Makino would be right up there among my top tier choices, but I'd demand a demo for sure, and I'd ask them to supply a turnkey solution that's the best they can do with their brand of equipment.

Make sure that turnkey solution covers everything in detail including stock allowance electrode processing, machine settings, fixturing....everything from soup to nuts.
Don't go cheap on anything or you'll wipe out half your gains instantly.
At this level they're hard gains to make and hold on to; one process deviation and you're back to polishing like crazy.
Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com


Marcus, thanks for the input. I agree that I will very likely have to do some polishing, but the hope is to be able to reduce it significantly and improve final tolerances. I will definitely have Makino run samples do demonstrate capability before moving forward with anything. Who else should I be looking at? Makino seems to be the best at pushing there high end finishes and offering them in smaller footprint machines.

This has been a tricky part as most of the best sinker edm shops are focused on traditional injection mold type work and these parts don't come close to the billing targets many of them are looking for. It doesn't help that I generally want them really fast. Bringing this process in house is a fairly big/long term project. I need to really develop an understanding of the relationship between in machine surface finish vs cost/time/finickiness.

I am looking for real world experiences because I have seen all the advertising, but I haven't found many shops actually running the HQSF type of machines and willing to work of small projects like mine. I am skeptical when I don't see other people putting it to use.
 
Hi newtonsapple:
The names in sinker EDM are:
Agie
Charmilles
Sodick
Makino
Mitsubishi

Those are the high end ones I can think of without even trying, and they're in no particular order.
Agie and Charmilles have come together.
They were once upon a time the premier brand, and from what I hear are still very good but others have closed the gap.

Sodick was sort of first in the field with linear motors on sinkers and offered a machine with an additive once upon a time that was supposed to do magic on very fine finishes, but I never heard anything about them again after the initial fanfare so who knows if it was a success.

Mits drivers seem to love their machines too; I have never driven one so I can't say how good or not they are.

Makinos seem to enjoy a particular reputation as the go-to machine for particularly fussy applications, so I concur with your choice and I'll be jealous as Hell if you get one!!

As regards the relationship between finishes and cost/time/finickiness, yes as you suspect it's close to an exponential one, and at the high end, consistency of process is the place where the magic happens.
Once you've dialed in the process, you want to eliminate randomness in it, and the management of the dielectric is probably the most important of those.
So excellent filtration, temperature control, suction flushing, fluid maintenance; all these variables will have a profound impact.
The quality and consistency of the electrodes and the material choice for these will be the second big variable.
The machine is probably lower on the list. (They're all good if you manage the other variables well!).
Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com

Edit:
I just noticed, Makino's superfinishing strategy uses an additive as well.
Here's a snip from Manufacturing Engineering magazine:

"our HQSF [High Quality Surface Finish] additive technology. Traditionally in sinker EDM, as your electrode surface became greater, it became very hard to stabilize the gap and produce a surface finish across a very large area that was consistent. You'd get hot spots, because of debris. By using what we call a µSC additive, we're able to stabilize the gap in very large surface areas for producing exceptional surface finishes. With graphite, we're able to achieve surface finishes down to 2µm."
 
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Thanks for the additional info. I will have to spend a bunch of time talking to the EDM makers to figure out the best path forward. The potential exponential expense/effort for the gains is a concern. Spending a ton of money only to find out that it only works when the moon is just right seems to be a real risk. I've read enough EDM equipment repair horror stories to know to be scared (Replace a $30K board only to find that it's the other board with a problem).

What we get now works reasonably well. I would still like better accuracy than what we get from outside shops. We typically are limited to +/-.0005, and getting to +/-.0002 would be helpful. I wonder if in house consistent, accurate, and good, but not amazing, surface finish EDM would be a more reasonable route.

Automating some of the polishing process might be a more efficient route. I will have to look if there are any options there.
 
Thanks for the additional info. I will have to spend a bunch of time talking to the EDM makers to figure out the best path forward. The potential exponential expense/effort for the gains is a concern. Spending a ton of money only to find out that it only works when the moon is just right seems to be a real risk. I've read enough EDM equipment repair horror stories to know to be scared (Replace a $30K board only to find that it's the other board with a problem).

What we get now works reasonably well. I would still like better accuracy than what we get from outside shops. We typically are limited to +/-.0005, and getting to +/-.0002 would be helpful. I wonder if in house consistent, accurate, and good, but not amazing, surface finish EDM would be a more reasonable route.

Automating some of the polishing process might be a more efficient route. I will have to look if there are any options there.



Newtonsapple,

It appears that you have a pretty challenging work piece! From the Makino prospective, we would not recommend the use of the HQSF (High Quality Surface Finish) technology based on the small size of the detail and also the desired tolerances. The HQSF process does provide significant enhancement to the achieved surface finish and part metallurgical quality, but this technique is most commonly used for larger and more free-flowing geometry details. Simply put, HQSF technology will provide excellent surface quality, but it will not produce the best possible accuracies that Makino Sinker EDM machines are capable of.

Do you have and use the Sinker EDM process now in your shop, or do you outsource all your EDM work? The small size of your work piece detail creates some unique challenges, but these can all be overcome with a proper manufacturing process. Is your desire or thoughts to process these parts 1 at a time, or do you have a large volume of these details to produce? It is possible to achieve a mirror-like surface finish on these parts (at the expense of cycle time), but Copper electrodes will need to be used.

The biggest challenge I see from an accuracy standpoint is consistent electrode preparation, so do you have equipment in-house to reliably prepare 0.040” diameter electrodes? To achieve best accuracy, I recommend manufacturing the electrodes on the tool holders that will be used to EDM the work piece (no unclamping of the electrode). It is also possible to use the Sinker EDM machine to produce the final electrode on the EDM machine using a discharge dressing process, but this will add to the total cycle time. There are many things that can be done here, but a more detailed engineering discussion is needed to determine what is “practical” in the real-world for your application.

-Brian
 








 
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