Electrode Fabrication Question

[NYMike]

Hi,

I have a question regarding "overburn" in electrode fabrication. Is this the amount of material added to the electrode dimensions to allow for shrinkage in the molding process? I know a lot of you guys out there cut electrodes everyday and I would like to pick your brain. Any other electrode info would be greatly appriciated. Thanks for your time.

Mike

[chico]

Hello NY,

 

Overburn really has nothing to do with shrinkage. An electrode is machined with o/b to account for the spark distance(sometimes referred to as spark gap)that is emmitted when the electrode comes, I'll say in contact with the workpiece, although the electrode never really physically touches the work piece.

 

The more power that the electrode is given the faster the w/p material is removed, however, the rougher the finish will be on the w/p. As you get closer to the size you are tring to achieve power to the electrode is cut back. Hence, less power, less spark distance, better w/p finish. The electrode will need to move closer to the w/p because your spark gap (overburn) has decreased.

 

In short overburn = spark gap.

 

Hope this helps.

[FUSION]

Hi NY,

 

The term "overburn" as applied in the mold company I work for has to do with the amount of overlap the electrode has to the w/p or another trode.For example if a cavity trode burns right to parting line you dont want the finish detail on the trode to end exactly at parting line as this can be detramental to the parting line shutoff, As well as difficult to machine.If two trodes burn in the same area say on a contour, if the trodes do not "overlap" you may get a witness line between the two. Spark gap in this shop has always been referred to as Spark Gap.Our company just had its 25th anniversary, pretty good for a mold shop.

[mbuster]

overburn you put on a electrode is for the amount of power that you are going to use while burning.

i come up with my overburn by the size of the electrode and in 3-d causes by the minium radius sizes on the contour....

one thing to remember is that when using graphite for your electrode you cant burn with more than 50 amps per square inch.

[Chuckles the Clown]

Overburn allowance depends on materials and machine on close tolerance work. We usually run tests before going into production, but our edm guy is pretty close most of the time in his estimates.

What material are you burning, and on what kind of machine?

[Lucky]

Due to the spark gap (overburn) and the orbital motions used during finishing (if your machine is CNC), the electrodes must be cut undersize. This is best accomplished by applying a negative "stock to leave" in your toolpaths. This will undersize all features by a fixed amount. Search this forum for a number of topics related to the do's and don't of using negative stock allowances.

 

Compensating for shrinkage is a separate issue. This is handled by scaling the part model an amount appropriate for the material being molded.

 

Order is important! First, scale the part model to account for shrinkage. This will give you the mold cavity needed. Then offset this geometry to allow for electrode undersize or better yet simply cut the scaled geometry with negative stock to leave.

 

This technique results in an electrode that is equally undersized in all directions (3D). Obtaining a correct cavity requires the use of a spherical orbit on the CNC Sinker EDM. If you must use a different type of orbit or spherical is unavailable you will have to remodel the part (oh no!!) Examples of this type of orbit are "circular" and "vector".

[NYMike]

Thanks to everyone for the education.

Regards, Mike