An EDM surface finish chart translates VDI 3400 texture grades into Ra roughness values. VDI 12 is about Ra 0.4 µm (16 µin), VDI 24 is about Ra 1.6 µm (63 µin) and VDI 36 is about Ra 6.3 µm (248 µin). With those conversions in hand, you can match a drawing callout to a burn setting, a texture plaque or a profilometer reading.

The chart is only useful if everyone reading it means the same thing, so this guide also covers units, what Ra and RMS each measure, how to write a finish callout that cannot be misread, and why every step toward a finer finish adds burn time.

VDI 3400 to Ra Conversion Chart

VDI 3400 is a logarithmic scale. Every 6 steps up the scale doubles the Ra value, which is why the numbers climb slowly at the fine end and quickly at the coarse end. The Ra values below follow the widely published VDI 3400 to Ra reference table, with microinch values calculated at 39.37 µin per µm.

• VDI 12: Ra 0.40 µm, about 16 µin
• VDI 15: Ra 0.56 µm, about 22 µin
• VDI 18: Ra 0.80 µm, about 32 µin
• VDI 24: Ra 1.60 µm, about 63 µin
• VDI 30: Ra 3.2 µm, about 126 µin
• VDI 36: Ra 6.3 µm, about 248 µin
• VDI 45: Ra 18 µm, about 709 µin

A quick way to sanity-check any chart you find: VDI 18 should read about twice VDI 12, and VDI 24 about twice VDI 18. If a chart breaks that pattern, treat it with caution.

Watch the units

Multiply micrometers by 39.37 to get microinches, and divide to go back. Unit mix-ups are a common source of scrap. "Ra 32" on a US drawing almost always means 32 µin, a fine finish near VDI 18. Read as micrometers, Ra 32 would be rougher than anything on the chart. Confirm the unit before choosing a burn setting.

Older drawings and N grades

Legacy prints often use the ISO 1302 N grade system. N6 is Ra 0.8 µm, N7 is Ra 1.6 µm and N8 is Ra 3.2 µm, so N7 lines up with VDI 24. ISO 1302 has since been replaced, so if a print shows only a symbol and a number, confirm the standard and units with the customer before you burn.

What Ra, RMS and Rz Each Tell You

All three numbers describe the same surface profile in different ways, and they are not interchangeable.

Ra is the arithmetic average of how far the profile strays from its centerline. It is the most common callout and the easiest to compare between shops.

RMS (also written Rq) squares each height before averaging, then takes the square root. Because of the squaring, one deep crater raises RMS more than it raises Ra. A common rule of thumb puts RMS at about 1.1 times Ra for a typical random EDM texture, but treat that as an estimate. Older US drawings often specify RMS, while newer ones use Ra.

Rz and Rmax describe peak-to-valley height. Two surfaces can share the same Ra and have very different peaks, and in EDM a single overburn crater or debris streak can push Rmax well above anything an Ra-based estimate would suggest. If peaks matter on your part, such as a seal face or a fatigue-loaded slot, call out Rz or Rmax directly and measure it.

Writing a Finish Callout That Works

"EDM finish" or "fine finish" on a drawing leaves too much to interpretation. A useful callout states:

• The parameter, such as Ra, Rz or a VDI grade
• The units, µm or µin
• The maximum value, or a range if both limits matter
• Which faces it applies to
• How and in which direction it will be measured

Recast and surface integrity are separate requirements from roughness. If the part needs them controlled, list them separately rather than hoping a fine Ra will cover them.

Measuring the result

A profilometer drags a stylus across the surface and reports Ra, Rz and other values. Set the cutoff length to suit the texture, since coarse VDI grades need a longer cutoff than fine ones or the reading comes out low.

Comparator plaques show what each VDI grade looks and feels like. They matter for cosmetic approval, because two surfaces with the same Ra can look different under light. For mold work, use both: numbers for the drawing and plaques for the customer's eye.

Picking the Right Finish for the Job

For moldmaking, the cavity texture transfers to every molded part. The same VDI grade looks glossier on some resins than others, so ask for samples molded in your actual material and color before approving a texture. Deeper textures also need more draft, or parts will drag on ejection.

For functional surfaces, the right finish is the coarsest one that still does the job. A seal face, a sliding fit and a surface that will be coated each have different needs, while a non-mating pocket wall needs very little. Calling for VDI 12 where VDI 24 would work can add hours of finishing passes without improving the part.

Why Finer Finishes Cost Burn Time

Smaller sparks make shallower craters, which is how EDM reaches a finer finish. They also remove far less metal per spark, so reaching a fine surface means burning away the deep craters left by roughing one light pass at a time.

On a sinker, finishing uses lower current, shorter on-times and orbiting, and the electrode's own surface transfers to the part, so electrode preparation belongs in the finish plan. On a wire machine, finishing comes from skim passes along the same path, each removing a thin layer. Either way, count only the passes the part needs. How quickly a machine can step down from roughing to finishing depends a great deal on the generator, a trade-off we cover in our article on power supply settings for speed versus surface finish.

Struggling to hold a finish your sinker used to reach without trouble? Our engineers can look at your machine, power supply and setup to find what has drifted. Talk to an Engineer.

Confirm the machine can hold it

Before quoting a fine finish, confirm the machine can deliver it today. Worn servos, poor flushing or an aging power supply can keep a sinker from reaching the finish it once could. Run a test burn on the actual material, measure it, and keep axis accuracy and servo response in check with regular EDM calibration. EDM Zap's calibration service includes laser interferometer verification, so you know the machine is accurate before you promise a customer a number.

Specify the Finish the Part Needs

VDI 3400 gives toolmakers and customers a shared language for texture, and Ra gives a number you can measure. Use the chart to set a target, state the parameter, units, faces and inspection method on the drawing, call out Rz directly when peaks matter, and choose the coarsest finish that meets the part's function. Then prove it with a test burn on your own machine and material.

If finish targets are pushing your sinker past what it can hold, our team can check calibration, power supply condition and setup and help bring the burn back into spec. EDM Zap has supplied EDM equipment since 1971, and our engineers respond to inquiries within 24 hours. Request a Quote or call (630) 852 1699.

Frequently Asked Questions

What does a 3.2 surface finish mean?

It usually means Ra 3.2 µm, about 126 µin, which sits at VDI 30 and N8. It is a matte finish often used on roughed EDM surfaces and textured mold walls.

Is VDI the same as Ra?

No. VDI 3400 is a texture scale with its own grade numbers, and Ra is a measured roughness value. Each VDI grade corresponds to an Ra value, and every 6 VDI steps doubles that Ra.

Can the same chart be used for sinker and wire EDM?

Yes. Ra and VDI values mean the same thing for both processes. The texture can look different at the same Ra, because sinker finishing uses burns and orbiting while wire finishing uses skim passes.

Can I convert Ra directly to RMS or Rz?

RMS can be estimated at roughly 1.1 times Ra, but there is no exact conversion. Rz depends heavily on the texture, so measure it directly when it matters.