Wire EDM
Through cuts and skim strategy.
Wire EDMGlossary
EDM Spark Gap and Overcut on Injection Mold Steel
Programmed EDM paths are not final steel size. Plan skim passes and fit checks when inserts must seal or slide.
Definition
What Are EDM Spark Gap and Overcut?
EDM spark gap is the controlled clearance where discharges erode steel. Overcut or overburn is extra material removed beyond the programmed tool path.
Spark gap settings depend on material, electrode or wire size, and whether the pass is rough or skim.
Overcut makes final steel slightly larger than the nominal electrode or wire path. Insert fit and corner radius must account for it.
Skim passes use tighter gap conditions to reduce overcut before polish or assembly fit checks.
Wire corner radius ties towire EDMdiameter plus overburn. Sinker detail ties tosinker EDMelectrode shape and burn sequence.
Process
Spark gap and overburn on EDM paths
Programmed path is not the same as final steel size.
- Spark gap clearance sets how much material each discharge removes.
- Overcut widens slots and rounds corners beyond nominal wire or electrode size.
- Skim passes and fit checks tighten insert clearance after rough EDM.
Wire EDM
Overburn on through profiles
Programmed wire path is smaller than the finished slot or corner.
Internal corner radius is larger than wire radius because of spark gap and overburn
Rough cuts leave more overcut; skim passes tighten insert clearance and shutoffs
Start hole axis must align with wire path so threading does not add extra gap at entry
Sinker EDM
Gap between electrode and cavity steel
Electrode undersize and orbit-style motion compensate for gap on shaped burns.
Rough electrodes are undersized versus final cavity dimension to allow aggressive gap
Skim electrodes approach final land with tighter gap and less overcut
Deep ribs and shutoffs may need fit checks after skim before texture or polish
RFQ
Fit and tolerance notes
State mating inserts and clearance when EDM defines the final interface.
Call out insert clearance and parallelism when wire cut plates stack together
Note whether CMM or fit check after skim EDM is required before assembly
Cosmetic corners may need tighter skim than functional ribs on the same tool
Link wire start holes when buried paths affect entry gap at the start hole mouth
Go deeper
Related tooling terms
Gap and overcut connect wire paths, sinker land, and assembly fit.
Wire EDM start hole
Entry axis before threading.
Wire EDM start holeSinker EDM
Shaped electrodes and skim sequence.
Sinker EDMMold base
Plates and inserts that must fit after EDM.
Mold baseEDM overview
Family context.
EDM overviewVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Wire EDM spark gap overburn on mold insert profile
Wire EDM spark gap overburn on mold insert profile
Sinker EDM electrode gap above cavity steel detail
Sinker EDM electrode gap above cavity steel detail
CMM check of wire EDM insert fit after skim pass
CMM check of wire EDM insert fit after skim pass
FAQ
Frequently Asked Questions About EDM Spark Gap and Overcut
Why is my wire EDM slot wider than the wire diameter?
Spark gap and overburn erode material beyond the wire. Skim passes reduce excess on critical fits.
What is EDM overcut on mold inserts?
Extra steel removed compared with the programmed path. It affects corner radius, slot width, and insert clearance.
Can overcut be eliminated?
Not entirely. Shops control it with rough versus skim settings, electrode undersize, and fit checks.
Does spark gap affect shutoff seal?
Yes when shutoffs are wire or sinker cut. Skim EDM and stoning bring lands into seal range.
Should I dimension corners to wire size on CAD?
Use DFM-friendly radii and ask the mold maker for achievable corner after wire diameter and skim plan.
Next step
Quote tight clearance tooling
Upload CAD with insert fits, wire paths, and cosmetic corners so spark gap, skim passes, and fit checks quote correctly.