Engineer’s and toolmaker’s guide

EDM 102: An Engineer and Toolmaker’s guide to EDM Machining

Ó Mercatech, Inc., 2000

To get the most out of wire EDM capabilities, thorough preparation is essential. Careful attention to items such as, starter hole location, pickup hole or edge preparation, minimum radius and other requirements will simplify your job, reduce time and money in wire burning, and will insure finished part integrity.

The following items are suggestions we came up with. They are the product of experience and should not be seen as requirements in any way. Not every job will fit into an “ideal” wire EDM situation. But by using as many of the tips presented hereafter you will reduce the time and money needed in wire burning.

We want to make EDM work for you!

Material Preparation

All material should be clear of burrs and shavings.

All steels should be demagnetized.

Draw an outline of what is to be burnt on top of the material with a marker or a paint pen for easy profile identification. Take this opportunity to write some needed engineering notes where needed. Scribing on blue die chemical is also a valid option if surface finish is not an issue.

Wherever possible try to grind at least two sides, top and bottom parallel to each other.

Drawings

If you have CAD drawings available, please provide them to us.

Include all pertinent manufacturing information like clearances, shrink factors, etc. with your prints or CAD files.

Starter Hole Location

Wire EDM requires a starter hole. We can drill it when you send us the material or you can include it yourself. It is completely up to you.

When possible, the starter hole should be located:

A On a symmetrical centerline or a place that is easy to dimension

B .150 to the inside for dies or die-like profiles

C .150 to the outside for punches or punch-like profiles

D In a common relationship with the punch and the die

E On “One cut punch & die” applications, the starter hole should be placed so as to allow the entry line for blanking and piercing operations as follows: Outside the profile for blanking dies Inside the profile for piercing dies

F Where the “glitch” or wire starting point can be easily dressed or stoned

Starter Hole Condition

If used for datum location, the starter hole must be round and, either be absolutely square to surface or relieved to present only a small land.

If size is a constraint, the hole must be square enough to allow passage of the taut wire. Note that the most economical and widely used wire diameter is .010”.

Any scale or residue should be cleaned from the hole.

No matter how large the opening is to be cut, the optimum hole size should be no bigger than .125”. The additional material around the starter hole in the drop area aids in the wire EDM process by keeping the wire in the material.

Datum Accuracy

Wire EDM machines use the wire to gain location to a workpiece in a similar way a center and edge finder is used on a milling machine. Except that the wire is way more sensitive than the edge finder. The wire has to go completely through the whole surface of the piece when picking up a hole or past an edge, and will pick up any burr or imperfection causing mislocation. The three typical EDM pick up stations are: a through hole, an edge, a combination of the two.

Minimum Radius Consideration

The minimum radius that can be put on an inside corner is equal to half of the kerf on a single pass cut. The kerf being defined as the wire plus the spark gap. In order to get a smaller radius, you must either do several skim cuts, or use smaller diameter wire. Keep in mind that both these options are more expensive due to the increased burning time of multiple passes and the slower cutting speed of smaller wire. Our standard burning wire is .010” in diameter. It is important for you to consider what inside radius you consider acceptable for your project.

Punch From Die Technique

This technique works very well for short-to-medium run tooling. A punch can be pulled from the die with great economy. This is possible because the small kerf inherent to wire EDM. The angle at which the wire burns is adjusted to produce the required clearance between punch and die. Another advantage of this technique is that the punch and die can be pre-located for perfect alignment before burning.

Taper Relief

In the past taper relieving was a less-than-perfect technique, any kind of relief behind a parallel cutting land was acceptable. With wire EDM, accurate tapering is possible on all cut-outs. Current practice calls for relief between ¼ degree and ½ degree with no parallel land. The amount of dimensional change due to regrinding is negligible and is normally acceptable. If required, land and taper relief are possible with additional wire passes.

Accuracy

Generally speaking, +/- .001” is achievable in one cut. Tighter tolerances are possible by making a rough cut and doing multiple skims.

When cutting a punch or punch-like geometry out of a block, it is always better to use a starter hole than cut in from outside of the block, as the stress in the material will find in the lead line a place to relieve itself.

Whenever possible, it is best to leave between ½” and ¾” of material around all edges to allow for clamping and frame strength while cutting. Even though the frame is usually discarded after cutting, it is a rigid frame which insures finished part integrity.

Design

Because of the increased strength of one piece dies as opposed to split and ground construction, die designed for wire EDM can frequently be made more compact and with fewer stations. Keep in mind when designing components such as feeder plates that with our four axis wire EDM, one component can often take the place of previously several components. Examine your design to ensure that you have taken full advantage of all the numerous benefits of wire EDM machining.

Part-Specific Tips

Punches Plan for your punch blocks to have multiple punches nested in them. One starter hole can go along way in a well nested punch block.

Die Plates We do not have any problem burning dowel holes in die plates, they do, however, add some EDM time to your plate. Convenience and accuracy vs wire time is something worth debating.

Mold Pockets The usual way to construct those pockets is to locate them off the center of the mold base. The best way to pick them up with wire EDM is to provide a starter hole near the center of the mold base. It eliminates extensive and costly four-sided edge pick-ups from all four sides of the base.

Sub Plates & Die Shoes It is preferable not to rough out a sub plate prior to wire EDM. Should you want or have to do it for reasons like handling weight. Please keep in mind to leave between ½” and ¾” material to allow for the wire to be inside the material at all times. Let the wire burn the clearance for you.

Surface Finishes

Surface finishes are measured by instruments known as roughness meters. Experience proves that measurement of surface finishes by visual and tactile comparison with a standard results in errors not exceeding 2 CH classes.

The roughness criteria are Ra (Europe) CLA (UK) AA (USA) Mercatech uses the Charmilles scale of roughness. Here is how it relates to the standards.

VDI	Ra		Rt max.	Class	VDI	Ra		Rt max.	Class
3400	CLA		Approx.		3400	CLA		Approx.
	AA		*8Ra**			AA		*8Ra**
CH	Micro mm	Micro inch		ISO 1302	CH	Micro mm	Micro inch		ISO 1302

0	0.1	4	0.8	N3	23	1.4	56	11.2
1	0.11	4.4	0.88		24	1.62	63	12.96
2	0.12	4.8	0.96		25	1.8	72	14.4	N7
3	0.14	5.6	1.12		26	2	80	16
4	0.16	6.4	1.28		27	2.2	88	17.6
5	0.18	7.2	1.44		28	2.5	100	20
6	0.2	8	1.6	N4	29	2.8	112	22.4
7	0.22	8.8	1.76		30	3.2	125	25.6	N8
8	0.25	10	2		31	3.5	140	28
9	0.28	11.2	2.24		32	4	160	32
10	0.32	12.8	2.56		33	4.5	180	36
11	0.35	14	2.8		34	5	200	40
12	0.4	16	3.2	N5	35	5.6	224	44.8
13	0.45	18	3.6		36	6.3	250	50.4	N9
14	0.5	20	4		37	7	280	56
15	0.56	22.4	4.48		38	8	320	64
16	0.63	25.2	5.04		39	9	360	72
17	0.7	28	5.6		40	10	400	80
18	0.8	32	6.4		41	11.2	448	89.6
19	0.9	36	7.2	N6	42	12.6	500	100.8	N10
20	1	40	8		43	14	560	112
21	1.12	44.8	8.96		44	16	640	128
22	1.26	50.4	10.08		45	18	760	144

Definition of roughness criteria No. CH = 20 log (10 Ra) (micro mm)

Mercatech as a general rule allows a tolerance of +/- 2 CH for surface finish measurement.

Recast Layers

EDM is a burning process that causes the melting away of the metal surface generating a new hardened layer of martensite on the cut surface, this layer is an unstable structure that needs tempering.

This “recast”appears as a thin white layer measuring from .0002” to .003” and is composed of metal that was molten during the EDM burn and resolidified when flushed by the dielectric fluid. It can, therefore, possess a higher carbon contents.

Tests have been performed on such surfaces and have discovered that the hardness of this new layer often measure to a 70 Rc or higher. This explains why tools made with EDM generally will wear slower than tools made by traditional processes. The main problem encountered by some industries is that this layer is always full of minute cracks.

Under this first recast layer we can find a second white layer that will usually measure a hardness of 65 Rc.

A third layer can be found under these, it is a gradient layer of metal that has been overtempered, drawing down the actual hardness level. This layer gets harder as it goes deeper in the metal away from the first 2 layers.

These layers have received many names over the years. The main ones are recast, white layer, heat affected zone (HAZE).

Now what to do with those layers? The answer depends on the application the tool is needed for.

If you want to use the EDMed tool in a wear application such as a stamping die for example. It is then recommended to immediately perform a 250 to 300 degree Farenheit temper to stabilize the fresh martensite layer. After this a very light stoning of the cutting edge will provide you with a long lasting tool.

For high pressure applications like extrusion and injection molding. Both white layers need to be removed either by grinding or lapping. If this is not done the cracks in the outer white layer will cause the tool to fail prematurely.

For all aerospace applications another way of removing the layers may be through the use of chemicals.

Recommended reading on the topic would be: “Heat Treatment, Selection and Application of Tool Steels” by Bill Bryson. ISBN 1-56990-238-0

Finally….

If in doubt about any aspect of preparation, give us a call!

Our many years of experience in the field will be used to answer even your toughest questions.