July 2012 / Volume 64 / Issue 7|
By Dr. LaRoux K. Gillespie
| 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:
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. Ra surface finish requires a very different approach than one with a 128µin. Ra 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:
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. Ra surface 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.
Removing burrs is one thing. Determining the cost of removing those burrs is another, and is equally important to successful deburring. The economics of burrs and deburring are defined in a 200-page handbook written by the author, but the main considerations can be summarized in two tables. Detailed costs are calculated with simple spreadsheets that allow quick review of several alternatives (Tables 1 and 2 below).
The first step is to decide the financial goal. Is the goal to define the typical deburring cost or the lowest cost for a single part design? A different goal is to define the minimum cost of a deburring department. Yet another economic goal is to take a total company approach and reduce costs on a companywide basis. Those three goals will produce different solutions to reducing the cost of burrs.
While most job shops perform deburring, a few excellent contract deburring shops in the U.S. can do the work as well. Some can perform it at a lower cost and at a higher quality level than a typical job shop. Some deburring shops are used for work overload situations.
Table 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 tasks.
Contract shops typically perform different types of deburring. Microscopic deburring requires a dedicated, talented staff that generally is not available in most machine shops. The author has identified a handful of deburring shops in the U.S. with this capability. Some cities have sheltered workshops of disabled workers who can successfully perform general manual deburring. Other contract shops across the U.S. perform mass finishing, blasting, microblasting, electropolishing, electrochemical deburring, extrude honing and thermo deburring. A handful of deburring shops also use mechanical processes such as robotic deburring, dedicated brushing machines and other edge-finishing machines.
Deburring shops are good places to investigate alternative processes and equipment. Shop owners are knowledgeable about their processes, and, from the quoted prices for parts and a detailed inspection of the quality produced, a machine shop owner can determine whether such equipment might be a wise choice for his shop. It is important, however, that the machine shop owner clearly define the requirements and expectations as discussed previously. Controlling consistency is essential and that is hard to do in someone else’s shop.
What deburring should a machine shop perform in-house? There is no easy answer, but most companies want to do it all inside because that simplifies manufacturing and saves time. However, a careful review of true economics may reveal that some processes should be outsourced. The ability to hire trained workers may influence the results of the review. The ability to keep those workers is even more important.
Another question is who in a job shop should deburr. The author was recently asked, “Tell me how much deburring should the machinist do vs. what the dedicated deburring worker should do?” There are several principles to consider. First, the machining center needs to do most of the deburring in-process before the part is removed. Modern machine tools can do an extraordinary job of removing burrs and smoothing edges in seconds. Hundreds of brushes are available to blend edges and even remove thick burrs on milling machines.
Table 2. Elements of deburring costs.
Note: hr. × salary = yearly salary; overhead × salary = yearly overhead; yearly costs = yearly salary plus yearly overhead; depreciation only goes into yearly costs; energy costs = hr. × overhead cost = yearly costs; deburring supplies = hr. × overhead cost = yearly costs; floor space only goes into yearly costs.
Automatic screw machines have live spindles in secondary positions that can quickly remove burrs. Turned parts also can be deburred on the machine. Cross-holes can at least be partially deburred on the machine soon after they are drilled. In most instances, they can be completely deburred.
The Consortium on Deburring and Edge Finishing (CODEF), based at the University of California, Berkeley, has worked with major automotive manufacturers to create toolpaths that reduce or prevent burrs when facemilling. That knowledge is readily available and the toolpaths can also be obtained. Deburring handbooks contain a wealth of knowledge on burr prevention and minimization.
In every instance, the goal is to produce the lowest-cost part. If the machinist has free time while waiting on a part to finish being machined and has the dexterity to deburr, he should deburr. If the part has to be burr-free before being loaded on the next machine, the machinist will often deburr it. For continuous production, some owners add a deburring machine to make a small cell, and parts leave the cell burr-free.
A dedicated deburring worker can be justified when cycle time allows it, the deburring job requires a special skill, the machinist is always busy tending machines, the probability of scrapping the part is low, the deburring must be done along with several other manual operations or when it is more economical. “More economical” includes factoring in overhead costs, not just direct labor.
Justifying Deburring Machines
Table 1 provides an in-depth look at total shop deburring costs. Calculating the time or costs shown in Table 2 will also indicate how much time a deburring worker actually spends deburring. That person may only spend 25 percent of his time removing burrs, so a deburring machine may not be the cost-effective answer. Once those costs are calculated, a shop can decide whether it may be profitable to use more mechanized processes. Deburring machines produce higher quality parts more rapidly and with better repeatability, and assure that parts can be delivered when manpower is hard to find.
There are 122 different deburring processes, and the author has compiled a list of 1,000 companies manufacturing or selling deburring items. (Manual deburring is considered to be one process, even though there are more than 10,000 tools that can be applied for this one process.) Deburring equipment manufacturers can provide detailed operational costs for most parts, and define return on investment. They can run sample parts to demonstrate finished parts. What they generally will not do is tell you if there is a cheaper or better deburring process available. Deburring shops, on the other hand, can offer several viewpoints that may be useful during decision making.
Success in buying new deburring equipment begins with a clear definition of what you expect from the equipment: different configurations, materials, target cycle time, cost target, daily volume, surface finish, stock loss, edge break and size of burr to be removed.
Establishing the probable length of a parts contract and the possibility of it being terminated early will also allow a shop to calculate a realistic expected ROI. Do you expect to make different part families in the next 2 years? If so, you need to know those part families’ characteristics.
Spend time with a user of the exact equipment you are considering. Some of the cheaper deburring machines shake themselves apart prematurely. Some will contaminate the next load of parts, so several tubs or holders are required as well as places to store them. If the equipment has a robot, will you or the supplier maintain it? Do your payback figures include all the supplies and maintenance costs? Do you have staff with the knowledge and skills required to operate the equipment? Once they learn will they stay with you? What is the impact if the equipment does not function as expected? Do you have a workaround plan if the equipment proves to be inadequate? What does a guarantee mean for this equipment?
Burrs are an amazing business. “They’re only burrs,” say some shop managers. “Don’t waste our time on that.” Others know they can grow their business by offering edge quality others cannot. Some let parts wait for 2 weeks or more until the burr bench can get to them. Other shops recognize that if they get their money 2 or 3 weeks earlier by finishing parts on the machine, they have profited or can reuse thousands of dollars more each year—just by having quicker burr bench turnaround. Burrs are an opportunity for all shops. CTE
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