Small smoothers: Inspection Efficiency

Deburring recommendations for small machine shops with tight tolerances.

Every machine shop faces the problem of removing burrs. While burrs cause 20 different problems in parts manufacturing and product use, the most frequent problem is parts simply will not fit together or function properly unless the burrs are removed. If burrs are left on parts, users also can slice hands and fingers just handling parts, and lawyers always enjoy the opportunity to meet these injured individuals.

Determining which deburring operations are appropriate for a particular machine shop is based on many factors, including quality requirements, cost-per-part targets, production volume and work materials. General-purpose machine shops typically require less aggressive deburring than specialized shops. Large corporations need different approaches to deburring than 50-man shops.

As an engineer and manager, I encountered monthly runs of 10 to 50 precision parts from 350 different designs. My investigation of deburring for these parts led to recommending the following approaches for small, precision machine shops with 20 to 100 workers making parts with tolerances tighter than 0.001 “.

This article focuses on what small shops need to do to control and remove burrs, including:

• manage the deburring process,
• understand the requirements,
• define shop requirements,
• train the staff,
• examine the economics,
• define the operational issues,
• determine the effectiveness of outsourcing,
• minimize and prevent burrs,
• deburr on the machine and during the machining cycle, and
• consider all the issues when cost- justifying deburring machines.

Deburring Management

Burrs require management. Every part quote must consider where the problem burrs will be, the deburring approaches most likely to be successful and the cost of doing so in terms of manpower, equipment and supplies.

That seems elementary, but many parts are quoted without reviewing deburring and other finishing requirements. For example, deburring a part with a 4µin. R_a surface finish requires a very different approach than one with a 128µin. R_a finish. Some deburring and finishing requirements are buried in specifications 50-pages long and cost estimators may overlook them.

Only a few deburring processes work just at the edges of a part, so deburring processes affect many part features, not just the burrs. There are five questions that must be answered before selecting a deburring process:

1. How high and thick is the burr? Bigger burrs typically take longer to remove. Thickness is a more important parameter than height in many processes, but it takes more time to measure thickness.

2. What is the allowable edge radius or condition after deburring? Taking off a 0.005 “-thick burr without breaking the edge more than 0.001 ” takes a different approach than allowing a 0.025 ” break or radius.

3. How much stock can be removed from part features while deburring? Vibratory finishing, for example, can quickly remove 0.0001 ” of stock from external features although the intent was only to remove the burr.

4. What is the surface finish before and after deburring?

5. Is inspection performed under magnification and are there other exceptional quality requirements?

The last consideration reflects the seriousness of every edge meeting all requirements. If a single point on a single edge will cause a product to fail inspection or fail in use, much greater care is required to assure continual and total conformance. Some edges must meet requirements when viewed under 400× magnification, although 30× is the strictest most machine shops usually encounter.

Some burrs are extremely difficult to remove because of their size. Large burrs on macro-sized parts are more difficult to remove than small ones, and average-sized burrs on microparts are very challenging. Few machine shops can define the burr thickness range they can remove, so it is incumbent on them to have some idea of how big the burrs will be and where they will be and design the deburring process around that knowledge before part making begins.

Train the Staff

One of the key aspects of managing a deburring program is determining if workers are fully capable of performing specific operations. Machine shops that meet ISO 9000 and related control certifications know they must document training received by all personnel—at least with rudimentary training details and logs. The first training issue to resolve is defining what the staff must know and do.

Running equipment requires one type of training. Making decisions requires a different approach. Remedial operations—such as removing broken taps, smoothing surfaces and removing material left in corners during machining—require additional training. Completing documentation requires yet another form of training.

Finally, the machine shop must validate that workers comprehend and can perform the operations for which they have trained. The fact that inspectors are trained to find burrs and have done so for years has nothing to do with whether or not their eyesight is still capable. Attending training is not the same as being functionally capable. Validation or certification is the real requirement. Keeping records up to date is another facet of training that can be easily lost or ignored.

General deburring training includes a list of all the steps the person is expected to perform, understanding the requirements and what inspection is looking for (not the same in many instances), how the worker is expected to validate his work and proof that the training was provided and was effective.

For machine shops with consistent production and limited part variety, an hour may be sufficient for such training. Most shops, however, produce a variety of parts and operate several deburring machines. Therefore, 2 days of training and a qualification period of a few weeks may be required.

Several years ago, training new employees to deburr microparts with 0.000050 ” tolerances, 4µin. R_asurface finishes and 0.001 ” maximum edge breaks on microparts inspected at 30× magnification took up to 9 months. Today, for shops with limited part variety, that same ability can be provided in 1 week of training due to the availability of better tools and processes.

Deburring Economics