Smoothing edges: General Industry Coverage

Courtesy of L. Gillespie

These microparts from Glen Mills, including micropins in the upper right-hand corner and stainless steel pivot gear blanks near the thimble, were finished with a Turbula shaker/mixer.

Four deburring methods for mass finishing microparts.

Deburring parts smaller than ⅛” via mass finishing poses several challenges. Part manufacturers that are used to working exclusively with larger parts need to confront these challenges when deburring microparts.

In addition to the difficulties posed by part size, lightweight surface tension and electrostatic charges can retard the motion of microparts and deburring media. The biggest impediment to effective deburring, though, is tumbling media that’s too large.

Three rules should be followed when mass finishing microparts: Keep the burr thin, use a process and media targeted for microscale parts and optimize the separation of parts and media.

Consider Figure 1. The data is typical of all mass-finishing operations, although finishing time and results will vary by application. In this example, it takes 30 minutes to remove a 0.003″-thick burr from a stainless steel part. As the burr is removed, a radius forms on the part’s edge and grows to about 0.003″ (76.2µm) after 30 minutes. The surface roughness improves during finishing, with the outer part surfaces being reduced by 0.0001″ (2.54µm).

Burr thickness determines removal time, and that time determines the final edge radius. Making thinner burrs shortens cycle times and lessens the impact on part features. If burr thickness can be kept to 0.000050″ (1.27µm) or thinner, a light tumbling action and short cycle will remove it.

Because the part features are microsized, it is essential to use tumbling media that are smaller than the part. As a rule of thumb, use tumbling media that are at least one-fourth the size of the feature to be deburred. However, when the media are very small, time becomes a major consideration.

Studies have been conducted on ½”-dia., 1018 steel cylinders that were centrifugally tumbled in ¼”, plastic, pyramid-shaped media. The cylinders developed a radius according to the formula R=0.00281t^0.33, where t is time in minutes, while 0.115″ (2.92mm) cylinders tumbled in No. 14 aluminum oxide (0.053″, or 1.35mm, particles) had a radius defined by R=0.00076t^0.41. That is a factor of 3.7 slower radii generation when using smaller media on a smaller part. When even smaller parts and smaller media are involved, the time requirements can be 10 to 100 times longer than for ¼ ” or larger parts.

The reason for slow cutting when using small media on microparts can be envisioned with the numbers in Table 1 (page 83). As seen in the table, a 0.001″-dia. pin weighs 1/61,600th as much as a ¼”-dia. pin. So, the force between a deburring particle and the part is far less than for a ¼” pin. The cutting ability is actually much different than that because in a conventional tumbling operation, the fine grains of deburring media support each other, and the part tumbles as if in a glob of mashed potatoes.

Presuming the burrs are kept to a thickness of about 0.001″ (25.4μm) or less, there are four reasonably good choices for removing burrs: centrifugal barrel deburring, conventional vibratory deburring in palm-sized containers, media mixers and shakers, and magnetic-abrasive finishing.

Centrifugal Deburring

In centrifugal barrel finishing, a small barrel containing the parts and tumbling media rotates, causing the particles to rub against the part edges and remove burrs. The gravitational force between parts and media when centrifugal deburring is 10 to 25 times greater than when vibratory finishing, because the barrels are rotating on a long arm that is also rotating, which provides additional centrifugal grinding force. More force equals faster deburring.

Courtesy of L. Gillespie

Figure 1. Four changes in part finish imparted by mass-finishing processes. Arrows indicate properties that can be achieved in 30 minutes. For example, a 0.003″ burr can be removed in 30 minutes (top chart). During 30 minutes of deburring, the other three characteristics will also change as indicated by the arrows.

The key is to find centrifugal machines that use small barrels, so that separating parts from media can be accomplished more quickly. For microparts, the barrels typically should be pint- or cup-sized.

Figure 2 illustrates a small, 303 stainless steel pin that was centrifugal-barrel-finished for 20 minutes with No. 14 fused Al₂O₃ as the media at a finishing force of 15 G. The initial height and thickness of the burrs are shown. After centrifugal finishing, the part was burr-free at 10× magnification and the radius produced was less than 0.005″ (127μm). The cut-off burr is not removed in deburring. Most microparts require 15 to 20 G, and, because the microparts will be smaller than the part used in Figure 2, will need No. 24 or smaller media.

Smaller media will require longer deburring times if the burrs are the same size as shown in Figure 2. A No. 24 particle has an average size of 0.0270″ (0.69mm); a No. 80 has an average size of 0.0065″ (165μm).

Most centrifugal machines are designed for larger parts and much higher volumes. But at least two companies—Bel-Air in North Kingston, R.I., and DB Importers of Blanchester, Ohio, which distributes Tipton machines—provide barrel sizes of about a liter, which is appropriate for most microscale work. The equipment is simple to run and most machines will last for decades.

Vibratory Finishing

Vibratory finishers vibrate a container of parts and media in such a manner that the mixture rolls. This allows the media to rub over the parts and remove burrs—much like waves and sand that slowly smooth the sharp edges of driftwood and stones on a beach.

Table 1