April 2009 / Volume 61 / Issue 4|
By LaRoux K. Gillespie
Burrs on parts worldwide cost an estimated $27 billion to $50 billion annually, not including lost revenue from rejected parts. Despite that high cost, parts manufacturers typically only have a superficial understanding of burr economics. Few are aware that there are at least 109 different deburring processes to choose from as well as a variety of burr prevention techniques.
Roughly 73,000 workers in the U.S. perform full-time deburring while almost 1 million machinists and toolmakers who cut metal or plastics also perform some deburring or deflashing. Practically everyone who works in a machine shop is familiar to some extent with burrs. Few enjoy removing industry’s little thorns.
Small shops generally do not closely track the costs of deburring, preferring to use a rule-of-thumb estimate that costs are some factor times their salary—for example, 2.5 times the salary of a deburring worker. That makes cost estimating and budgeting simple, but may not include the true cost of burrs. It does include the cost of deburring tools and power usage and factors in the observation that other workers also perform some deburring at their machines.Measuring Costs
There are several ways to look at the cost of burrs and deburring. First, there is the cost of the individual performing deburring. Next is the cost of deburring a specific part. The cost of deburring specific classes of parts is a different, larger issue. Beyond that is the total cost of deburring in the plant. Last is the cost of all burr and deburring issues. The latter includes both deburring costs and the cost of all other problems produced by burrs or inadequate deburring. That includes lost revenue due to inadequate deburring or edge control, such as the cost of recalls when a burr that wasn’t removed breaks away and causes problems.
Deburring costs for part families can vary widely. For example, aircraft bulkheads are much more expensive to deburr than steel road-grader parts because of the high precision required on aerospace edges to control stress-causing cracks and the extraordinary cost of the parts if deburring causes scrap. Microscopic deburring performed by very dexterous manual workers under 10× to 30× magnification requires more skill and time than tumbling parts in abrasive media.
There are essentially four classifications of deburring costs:
• Cost of deburring a specific part
• Cost of deburring a specific family of parts
• Total plant or department deburring costs
• Total plant cost of burrsTable 1: Tasks commonly performed by manual deburring operators.
Note: Only one row of this table refers to actual deburring, but most shop staff recognize the operators perform many of these.
It is important to use factored labor costs that include both the direct salary and benefits cost of the operator doing deburring. Operators do not typically see these “hidden” costs when they make cost estimates. Some companies will include the costs of simple supplies such as miniature rotary burs, abrasive products, knives and related burr removal tools in their estimate of hourly costs of a deburring operator.
Today, companies pay deburring workers a wide range of salaries, starting at minimum wage for a beginner. Skilled microscopic deburring workers command significantly more. Any deburring worker can consume $50 to $250 of consumable supplies per day.
Many small shops working on relatively standard parts estimate deburring costs as 3 percent of machining costs. Thus, if a shop has 20 machinists or machine operators making $20 an hour (including fringe benefits), the company estimates that deburring costs will be 0.03 × 20 × $20 = $12 an hour or roughly $100 per day. This formula may be appropriate for some shops but will fall short of real costs in others. In many shops the cost of operators or machinists in general is much higher so the cost of burrs may also be proportionally higher.Cost Factors
Within each deburring classification mentioned earlier, there are five main deburring cost factors: labor, machine depreciation, overhead, supplies and maintenance. Overhead can be expressed as floor space costs (cost per sq. ft.) calculated from rent, electricity, heating and cooling, insurance, a percentage of management salaries, percentages of the labor costs for fork lift drivers and others who deliver to and from the department or support the department, and direct supervision costs.
In short, overhead is the cost of doing business even if nothing is being deburred. It is often based on the amount of floor space a department uses. For a single deburring worker, floor space would be small, so overhead would also be small.Table 2. Elements of deburring costs.
Note: hr. x salary = yearly overhead; yearly costs = yearly salary plus yearly overhead; depreciation only goes into yearly costs; energy costs = hr. x overhead cost = yearly costs; deburring supplies = hr. x overhead cost = yearly costs; floor space only goes into yearly costs.
Let’s calculate, for example, the costs for deburring a zinc die-cast part via manual deburring. The part requires an output of 150 per hour on an 8-hour-day, 6-days-per-week and 50-weeks-per-year schedule. Annual output is 360,000 parts. Note that manual deburring would not normally be used for this situation in the U.S. except for running new jobs while waiting for the proper deburring equipment to be installed.
Manual deburring for this example requires 120 sq. ft. of floor space for workers. The cost calculations assume that the consumable deburring tools and the electric motor wear out and require $600 per year to replace. Labor and fringe benefits cost $20 an hour and manual deburring requires 2.8 minutes per part, which amounts to $0.93 per part. Overhead costs (the total of the six floor space costs) are assumed to be $25 per sq. ft. annually, which only covers the overhead for direct labor—not for shop accountants, managers and others (overhead at most shops will be much higher). The total cost per part for manual deburring totals slightly more than $0.95 per part. This cost was developed using a table developed by the author, which is available by clicking here.
Let’s now review comparative data for applying vibratory finishing to these parts. In this case, vibratory operations yield 400 parts per load in a 3-cu.-ft. bowl machine that draws 3 hp of energy. In each cycle, the operation runs for 2 hours, rotating 1,600 times, followed by another 30 minutes, rotating 1,100 times, and 30 minutes are needed to load and unload the parts. In this instance, a slower speed is used for the finishing cycle to polish the parts without the aggressive action needed for deburring. The process uses plastic media that wears 0.3 percent per hour and costs $1.20 per lb., which equals an hourly cost of $0.61. The bowl holds 170 lbs. of plastic media. Liquid compound, at a cost of $9 per gal., is fed at a rate of 0.05 gph, for an hourly cost of $0.45. The total hourly consumables cost is $1.45.
The vibratory machine, hoists, media hoppers and screens cost $14,200. The annual cost is $2,028.57 when divided by the 7 years of depreciation allowed. The machine requires 30 sq. ft. of space and bowl liners must be replaced twice each year at a cost of $4,400 each. Total maintenance cost is $4,600 a year, including liner replacement. In addition, the annual floor cost space for vibratory machining is $750 and the machine hourly rate is $2.76 when used for 2,400 hours per year.
The resulting cost estimate for vibratory finishing is only $0.04 per part. In this instance, the operator who loads and unloads the machine does not work a full day on this operation. The rest of the time he is available for other shop needs. (A table available by clicking here explains the cost calculation used to determine this cost per part).
This type of analysis provides some insight into costs per year, cost per hour, cost per load and cost per part. Armed with this detail, it is easy to develop rule-of-thumb numbers for part families. With a little more effort, once can capture more accurate estimated costs using this format. This approach compels users to address each issue based on specific part requirements.
Deburring departments may not represent a large investment for shops, but parts manufacturers must take every opportunity to improve productivity, such as using automated deburring equipment. Cost per part provides part of the picture, but cost to operate a deburring department provides a different picture.Deburring Cost Insights
Table 2 provides insight into the total costs of deburring for a department or for an entire plant (most plants only have one deburring department, but cellular operations will require a look at each cell as a “department”). The subtotal at the bottom provides a complete cost. Users can add to the table as needed to represent approaches unique to their plant, but the items shown reflect the major costs.
The total plant cost for burrs includes the lower rows in Table 2. For example, automotive transmissions fail when too many burrs break free and jam or score gears. Warranty work is thus a cost of burrs. It is not identified necessarily as a cost going back to burrs, but it is a direct reflection of the existence of burrs or poor deburring quality.
The author has participated in multimillion dollar lawsuits alleging inadequate deburring. Defending such suits and the expense of an unfavorable judgment can add vast costs. Also, the cost of lost work because customers are unhappy with delivered edge quality is rarely considered, and it is clearly hard to measure. However, having that line in a cost table such as Table 2 compels a company to consider potential lost revenue.
Table 1 shows that many deburring workers perform tasks other than deburring. It is clearly erroneous to state that a person spends 8 hours a day deburring when, in fact, he is performing tasks that have nothing to do with burrs, such as removing broken tools, sanding surfaces, fixing damaged features and correcting dimensional defects.
The economics of deburring typically drives companies to apply better deburring processes and prevent or minimize burr formation. The author’s books, “Deburring and Edge Finishing Handbook,” “Mass Finishing Handbook” and “Hand Deburring: Increasing Shop Productivity” provide guidance on how to improve deburring processes.Burr Prevention
One of the key principles of burr technology is that if you do not make a burr, you do not have to remove it, reducing deburring costs to zero. There is no reason that faced and turned parts should ever have burrs (provided they have no internal features). Modern precision part processing encourages programs that generate a small edge radius or chamfer on each edge.
Because chamfering can also produce burrs, a radius is a sure way to eliminate burrs. Use of form tools on lathes also will reduce the number of edges that have burrs, but it will not eliminate burrs unless the tool wraps around both ends of the part. It only takes seconds or fractions of seconds to put these extra touches on a part and those seconds save minutes of time later in the process and eliminate a host of other shop problems.
Preventing burrs is more challenging on milled and drilled parts, but even there major burr reduction is possible. University of California, Berkeley, Professor David A. Dornfeld’s CODEF consortia has developed a program that allows facemilling users to nearly eliminate burr formation on many edges while placing others at easier-to-remove locations. Three major automotive companies in the U.S. and Germany have used the program, which took years to develop.
Similarly, brushing edges with stiff wire or abrasive-filled nylon brushes while parts are still on the machining center greatly reduces the cost of deburring for some applications. The parts can come off the machine with no visible burrs. Again, it adds a few seconds of machine time, but can eliminate the need for secondary operations.
Dornfeld and others have also published a number of studies of minimizing burrs produced by drilling. Cody Hellstern at the Georgia Institute of Technology is finishing work on minimizing burrs in drilled aluminum laminates, and Peter Stringer at University College Dublin (Ireland) is exploring minimizing burrs on drilled automotive parts.
Calculating the real costs of burrs is not difficult, but requires an understanding of all the elements. A complete analysis of burr costs lets the user analyze the best way to minimize the cost of burrs through deburring, burr prevention or a combination of both. CTE
Editor’s Note: This article is based on a book by the author, “The Economics of Deburring,” to be published later this year.About the Author: Dr. LaRoux K. Gillespie is a retired manufacturing engineer and the author of 11 books on deburring and more than 200 technical papers and articles. E-mail: firstname.lastname@example.org.
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