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The Search Engineer finds solutions to all your questions, problems, and dilemmas. Occasionally, he could be wrong. But he doubts it.
By Design News Staff -- Design News, January 10, 2005
Bill of material software . . .Control circuits . . .Riveting material . . .
Dear Search Engineer: I need the math process for the circumference of a helical trapezoidal thread (1) revolution. I must cut a pattern and need the exact length without using my CAD program. —S.L., DN reader
Dear S.L.: Merely "unwrap" the thread and the answer will be obvious. The length of the helix thread is really the hypotenuse of a triangle whose other sides are the circumference of the round part and the lead distance formed by the pitch of the thread. Therefore, the formula would be: helix length = square root of [(PI x diameter)^2 + (pitch length)^2].
Dear Search Engineer: After 10 years in industry, I have decided to start my own engineering company. I am currently looking for a Bill of Material software system. Something that costs $1,000 to $2,000—$2,500 max. The company that I have started is truly a manufacturing company that will make to order, make to stock, purchase finished goods, do the final assembly, and sell them at both the wholesale and retail levels. I need a comprehensive system that has great part name and BoM structure, global find and replace item number, indented bill of materials, drag-and-drop BoM creation, graphical interface (preferred), cost analysis viewing and printing, MRP features, quoting, customizable reports, and easy data import and export. I currently use Ezijobz and Jobshop Premier, and have looked at Alliance Mfg. (cost prohibitive) and some of Microsoft's answers, but they all seem to be cumbersome and not efficient. Any suggestions? —A.Z., Locust Valley, NY
Dear A.Z.: One respondent has been using Parts & Vendors from Trilogy Design (www.trilogydesign.com), which has most of the features you require and costs less than $500.
Dear Search Engineer: I am looking for a moderately priced submersible pump, like a bilge pump, but working at depths of up to 100m. Any suggestions?—J.E., U.K.
Dear J.E.: Look at Gould's line of submersible well pumps at www.goulds.com/. They are engineered for harsh conditions, all stainless models available, and they pump brine and other tough fluids. Also: Carr Lane, Rietschle Thomas, Gorman.
Dear Search Engineer: We are manufacturing a retractable step that is made mainly out of aluminum for corrosion issues. We are using stainless bearing shafts. In isolated cases, we are experiencing oxidation buildup from the aluminum although it is anodized. This buildup interferes with our DU bushing and sometimes creates binding. Any suggestions? —H.L., Irvine, CA
Dear H.L.: Could be you are seeing a little bit of galvanic corrosion between the aluminum and stainless steel. You may be able to add sacrificial zinc anywhere on the aluminum piece, or else isolate the aluminum and stainless steel pieces. Another solution may be to use a composite bearing such as Orkot Composite bearings (www.orkothydro.us).
Dear Search Engineer: I need a control circuit in which closing it activates a device and closing it again deactivates the device. In other words, push the button and it's on; push the same button and it's off. Do you have any ideas on how it can be done? —R.R. in CA
Dear R.R.: You may want to try using a relay where the push button is connected to an electronic toggle switch. The normally open terminals are between the device and supply voltage, and the coil is connected to the controlling dc voltage via the toggle switch.
Dear Search Engineer: Is anyone making low-cost small-scale PAL type devices in the new lower-voltage standard (i.e., 3.3V)? If not, then what is being used for simple glue logic in medium-volume designs? Have we moved back to discrete logic components? Any suggestions? —H.L., GA
Dear H.L.: Look at the Altera MAX 3000 family of devices. They are inexpensive, operate at 3.3V, and come with as few as 32 macrocells. If you are in need of even fewer gates, take a small step back and look at the overall design, and then look at the smallest PIC Controllers from Microchip. You can also try TinyLogic and similar families. Otherwise, it seems the way to go would be FPGAs, ASICs, and custom-programmed MCUs.
Dear Search Engineer: We manufacture sealed boxes for installation on ships. The environment is severe, with temperatures ranging from -30C to +60C, including sun loading. The pressure delta between inside and outside the box causes our seals to be breached. We need a way to equalize the pressure (both directions) without allowing ingress of the outside environment. We tried a gas permeable membrane, but it quickly became clogged with salt accretion from the salt spray. Can you help? —M.S., CA
Dear M.S.: Is there space available, or can total box volume be increased sufficiently to add a bellows? Imagine a bellows that takes up part of the interior of the box, with one end of it being the floor of the box. Let a small hole through that floor communicate between the outside world and interior of the bellows. As the functional volume of the box pressurizes, the bellows will be forced to contract, with no major rise in pressure at least until the bellows reaches its limit of compression.
The opposite takes place when the pressure is reduced due to cooling of other conditions. The total pressure excursion is reduced, thus cutting differential across the seals. If the bellows can be made with its neutral position at +15C, roughly halfway between full contraction and full expansion, it will do the most good even if it cannot be large enough to eliminate the pressure change entirely. Putting the hole in the "floor" provides some natural protection from rain, snow, dust, etc., and may even provide some self-cleaning.
Material choice might start with electroformed nickel or similar. A more esoteric version would use an evacuated and fully sealed bellows (not communicating with the outside), possibly with a spring arrangement to set "neutral" as desired. Vacuum should be adequately compressible or expandable. This means it can never fully compensate, however, since it uses the increased or reduced pressure to expand or compress the bellows.
Dear Search Engineer: I have a 120-cavity mold that produces small parts, each with a dome with a 0.5-mm radius in the bottom of a tapered recess. I need to check all 120 parts from one shot to verify the radius and height of the dome relative to other surfaces on the part. This measurement exercise needs to be repeated every time maintenance is done on the mold. I could try to grind each part down exactly to the center line of the dome and measure under a microscope, but this would be very laborious. Is there a service available that can take these parts and then run through some sort of precision profile measuring machine to check them? —T.M., Cork, Ireland
Dear T.M.: A roundabout way to solve this problem—rather than directly measuring dimensions—is to ask: "Is the mold wearing?" Measuring the change in volume might be the easy way to do this. Simply fill the mold with a fluid and measure how much will fit. Or have a "go/no-go" gauge fabricated, which would basically be a thin piece of hardened metal with the centerline (or other critical aspect) cross section profile(s) of your part ground away, allowing for such tolerances as you define. This gauge can then be used at your site to test each shot group, or as an ongoing quality check, testing parts selected from the production output. The part is placed on a flat, true surface, then the gauge is placed on the part to test for fit into the gauge profile.
Dear Search Engineer: We manufacture electronic energy meters and use brass screws with tin plating and a nickel undercoating. These screws are engaged with brass cage clamps, which are also tin-plated with nickel undercoating. This assembly is used to fasten electrical cables. We have a problem with the brass screws breaking while fixing the cables when fastened at a torque of 1.2 to 15 Nm. We did not have this problem when the screws and cages were not plated. Can you suggest a solution? —E.M., Udaipur, India
Dear E.M.: Check your screw threads before and after plating. Chances are, your threads are closing up after the plating process. If you are manufacturing the cages, you may need to open up your threads before plating to get the required dimensions after plating. You will also want to re-evaluate your screw torque. You may have reduced your coefficient of friction with the nickel plating and thus will require a lower screw torque. Another question to ask: Has the screw vendor changed? I have seen a great deal of variation between domestic and offshore brass screws. Some are hard and very brittle while others are like peanut butter. I would do the dimensional checks first as you will have more control over those variables.
Dear Search Engineer: I'm looking for a clear, breathable film material with a permanent pressure sensitive adhesive on the back. Any idea where I can find it? —D.D., in NY
Dear D.D.: There is such a film available in the medical industry known as bio-occlusive bandages. If you are prototyping, this material may be a useful place to start.
Dear Search Engineer: We are looking at riveting together two pieces of 16 gauge G90 galvanized material. We are using 1/4-inch diameter rivets in an orbital riveter. During the riveting process, some of the rivet zinc coating is removed. Is there a potential for galvanic corrosion? Would you recommend using zinc-plated or stainless steel rivets? If stainless steel, what type? —N.R., DN reader
Dear N.R.: In response to your first question, I think the answer would be uncertainty of the potential for galvanic corrosion. There is no mention of the media to allow the corrosion to take place. Also, no mention is made if this is in a severe environment or sees extreme conditions. I think the best thing would be galvanized rivets. The steel is chemically more compatible with the base material, and the potential difference between materials should be minimal. You do not mention if the steel has been galvanized after the rivet hole is drilled or if the material is galvanized and then drilled. The exposed steel of the sheet will be a source of corrosion as well as the mandrel from the rivet. Check out this site for an overview: http://rbi.ims.ca/4385-511.
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