EDM Tolerances

That’s a trick title. Most of the time design engineers aren’t concerned with the tolerances associated with EDM process directly and I will tell you why.

First some background. EDM, or Electrical Discharge Machining (thanks again Wikipedia), is a material removal process wherein a large amount of electrical energy is delivered to the workpiece, burning material away. Energy is delivered through an electrode, either a block of conductive material (known as sinker, cavity, or volume EDM), or a wire (known as wire EDM). In both types, tools are mounted on machines with CNC controls.

EDM'd Moldbase and Electrode
EDM'd Moldbase (left) and Electrode (right)
Image via Eroda Tools, Ltd.

EDM processes are typically very slow and not suitable for volume production. However, EDM is typically very accurate and very effective on hard materials making it a great process for making plastic, sheetmetal, and other types of tooling. This is why design engineers are not usually concerned with the tolerances introduced by the EDM process; they are usually most interested in the tolerances of the parts coming off the tooling.

In this ongoing discussion of part tolerances (start here) I wanted to bring up EDM before talking about injection molding, metal stamping, metal forming, and other tooling-dependent processes because EDM is where the tooling starts. And you know how I love the fundamentals.

Cross-Disciplinary Education of Design and Engineering Students

One of the best ideas I’ve heard in a while:

Cross-Disciplinary Education of Design and Engineering Students.

…an industrial design professor and an engineering professor decided to switch students for one quarter each year, each teaching their contrasting discipline and perspective. The engineering students are exposed to creativity techniques, user empathy, and visual communication. Industrial design students are experimenting with injection molded polymers, carbon fiber composite lay-up, thermoforming and materials science. The two groups later are combined into design teams to work on an industry sponsored project together. …

June Issue of Develop3D is Available

I just noticed that the June issue of Develop3D is available. You can download your free copy here.

Inside you’ll find some interesting information on the design process at Marin Bikes and Senz Umbrellas. There are a quite a few nuggets on MCAD software updates and even an article on establishing assembly constraints by yours truly. Have a look and let me know what you think.

Tolerances and Material Removal Processes

If you haven’t already, I recommend reading and understanding my previous post about tolerances before digging in here.

Material removal processes are often used to build tooling for other manufacturing processes such as plastic molds, dies and punches for metal stamping and forming, extrusion dies, EDM electrodes, and many others.  Understanding material-removal process capabilities will be invaluable in understanding capabilities of downstream processes.

Common material-removal processes include lapping and honing, grinding, boring, turning, broaching, reaming, milling, planing and shaping, and drilling.

Engineering Toolbox has a good summary of tolerance limits for different material-removal processes:

Tolerance Grades
4 5 6 7 8 9 10 11 12 13
Lapping and Honing
Cylindrical Grinding
Surface Grinding
Diamond Turning
Diamond Boring
Broaching
Reaming
Turning
Boring
Milling
Planing and Shaping
Drilling

ansi-standard-tolerances

As you can see, lapping and honing give the tightest tolerances while milling, planing and shaping, and drilling have wider tolerances.  Also notice that  the tolerances get bigger as the part size gets bigger, regardless of process.

This is great information, but it doesn’t tell you anything about cost.  In general, as tolerances get smaller the parts gets more expensive regardless of process.  That is, a milled part with a dimension of 1.00 +/-0.05mm will be more expensive than a part spec’d at 1.00 +/-0.20mm.  How much more?  It’s impossible to know for certain because there are so many other factors that affect cost.  The take-home message is simply that better parts are more expensive.

Another key piece of information you do not get from this table and chart is any indication of the applications for these processes.  If lapping gives me the tightest tolerances, why don’t I just make all of my parts by lapping them?  Well, lapping only works on flat surfaces.  I suggest clicking through the links above and checking out what wikipedia has to say about each process.  All of the overviews are pretty good.

We will discuss applications of these process as they relate to mechanical components in later posts.

On Form, Curvature, and Emotion

Gray Holland of Alchemy Labs has a great article up on Core77 about the relationships among form, surface curvature, and emotion. You can argue some of the technicalities around class-A surfacing and “C” versus “G” continuity definitions, but his insight into the fundamentals of form is quite enlightening.

He also has a great perspective on the now-decaying debate of “engineering” versus “design.”

When we speak of product development, we frequently look at the domains of Design and Engineering separately, evaluating them in different ways. Engineering, at its core, is a measurable process; Design, for the most part, is not. This gives the former an inherent advantage: engineering efforts are easily quantifiable, and this provides them with authority. Design is intuitive, working on the non-verbal levels of our experience, sometimes triggering our most subversive emotional states; this makes it difficult to evaluate empirically. Lacking an analytical vernacular, Design is labeled subjective, when it is actually the agent of universal truth through form.

Curvature Evolution

Tolerances

I’m writing this post to help those in the audience that aren’t familiar with detailed mechanical design. A basic understanding of tolerances is essential to follow subsequent discussions here.

First, a quick definition:

Engineering tolerance is the permissible limit of variation in

  1. a physical dimension,
  2. a measured value or physical property of a material, manufactured object, system, or service,
  3. other measured values (such as temperature, humidity, etc).
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge).

Thanks Wikipedia.

Every manufacturing process has some variation on dimensional output and a sound mechanical design needs to account for these variations. If you ask a machinist to make you a block that’s 1″ by 1″ by 1″ you might get a block that’s 1.012″ by 0.923″ by 1.103″. Is that close enough?

Could the machinist have done a better job getting closer to the 1″ target? Probably, but since we didn’t specify a tolerance, technically it’s close enough. If we wanted something closer to 1″ per side we’d need to specify how close. That’s the tolerance. We’d say 1″ plus or minus 0.010″, for example. The 1″ dimension is called the nominal value and the 0.010″ is called the tolerance.

Later I’ll talk about tolerances associated with different manufacturing processes and environmental conditions and how mechanical engineers and product designers account for them in their designs.

Getting Started

Before getting into details I’m going to post a few quick notes on fundamentals. I will be referring back to these posts as the discussion goes deeper. Some of what I have to discuss may not be too interesting to those of you with degrees in engineering. I’m hoping to grab the interest of non-engineers/DIYers in the audience along with the product design professionals. I’m also hoping other seasoned professionals will comment with their thoughts.