Methods of thread inspection range from lasers to fixed and variable limit gaging. While no one method is best, optical comparators can visually show many of the key thread parameters. By profiling the magnified thread, the flank angle, which is the straight side that connects the tip or crest of the thread to its root, can be checked by a comparator. The minor diameter, the lowest point of a thread that touches the root, and the root's radius are two other parameters that can be inspected. The thread surface can be checked for burrs and imperfections with surface illumination.

Fastener companies such as MS Aerospace (Sylmar, CA) measure thread parameters using optical comparators in combination with other technologies. The company, which has been in business since 1992, supplies fasteners to the aerospace industry. More than 21,000 of its fasteners are currently holding together components of the International Space Station that currently is in orbit above the earth. These heat-treated fasteners, made from a variety of materials, have the tensile strength to withstand 280,000 pounds per square inch of force.

Six on the floor
The company uses eight optical comparators, and six of them are on the production floor. Two of them are in the inspection area, says Artak Dovlatian, quality manager. In all, five major thread characteristics are checked. The major diameter of the thread is inspected with a micrometer. The pitch diameter is checked with a thread gage. The company uses optical comparators to check the minor diameter, root radius and the flank angle.

"We measure this way because the optical comparators are the simplest, most accurate and reliable way of measuring those three specified thread units," says Dovlatian.

Traditionally, companies use plastic overlays for thread inspection, and MS Aerospace is no exception. These overlay charts, which are often made from a mylar material, are clipped to the comparator's screen and a part's image is projected onto the screen and aligned to the shape on the chart to see if it is in specification. There are standard charts for most threads and custom charts can be made for specialized threaded products.

Dedicated thread charts can be used in cases in which high volumes of one or two thread types are being produced and the chart does not need to be changed often, notes Tad Davis, comparator product manager at vendor Optical Gaging Products Inc. (Rochester, NY). "Typically, a combination chart is used for multiple thread forms," he adds. "This requires less frequent setups."

Environmental factors have little effect on comparators, and because of this, they can be placed inline on the shop floor. This can speed inspection, which is especially important in a go/no go scenario when there is not time to transport the part to a metrology laboratory. "Comparators will tell you why the thread is wrong," says Peter Klepp, president of Dorsey Metrology (Poughkeepsie, NY). "You can be making parts that are failing but you won't know why unless you go to a comparator."

At MS Aerospace, six 24-inch Gage Master optical comparators from Quality Control Solutions (Temecula, CA) are placed in strategic locations along the production line. The company also uses two L.S. Starrett Co. (Athol, MA) comparators in an inspection area for final inspection. "In-process inspections are done by our operators as they verify the parts they made. Each operator is an owner of his own process," Dovlatian says. "He verifies the dimensions that he is generating as the parts are produced so that out-of-tolerance parts are detected quickly."

Rooting out problems
To check flank angles, minor diameters and root radii, an optical comparator should have helix capability. Helix motion, accomplished by pivoting the worktable on the Y axis, allows a true profile of the thread form to be imaged.

"You will want to be able to angle the threaded piece so that the lens can view it correctly," says Russell Ureel, president of Genx Corp. (Kalamazoo, MI), an optical comparator vendor. "If you are looking down into an angled V, situate the part so that you can see each facet or area on that thread." Ureel adds that many machines have a swivel table that will rotate as much as 180 degrees for optimum positioning of the portion of the thread that needs to be inspected.

When used as a go/no go gage, or -- as in the case of MS Aerospace -- as a pivotal inspection tool on the production line, an optical comparator can stop bad parts before they proceed on to other machining steps, or ultimately being shipped to the customer.

According to Bob Larzelere, president of the comparator vendor Deltronic Corp. (Santa Ana, CA), the comparator will often give clues to problems that are occurring during the manufacturing process.

For instance, some screws begin as a plain cylinder and material is removed to make the thread. "The optical comparator allows you to take a look at the groove that was cut and make sure the flanks are straight and smooth and that the roots are sharp enough," says Larzelere. "Often, these kinds of problems are related to the breakdown in tools. The roots might not be sharp enough, and that could mean interference when the parts are assembled."

On other occasions, other thread measurement tools might expose a problem, but not detail what the problem is. Louis Todd, president of Quality Control Solutions, says that compared to a contact method of thread inspection such as thread rolls, only a comparator presents a complete picture. "With an optical comparator, you are able to see all three thread forms and see what is actually taking place."

However, to ensure that the errors are being made on the shop floor and not by the comparator, a regular schedule of calibration is vitally important, says Wally Wardzala, precision measuring instrument manager for Mitutoyo America (Aurora, IL), another comparator vendor. "If a thread is magnified and thrown up on the screen and you are referencing the thread pitch against the overlay chart, then what is important is the calibration of the machine, the movement of the stage and things like that."

At MS Aerospace, Dovlatian has a mantra for shop-floor use of a comparator: Keep it clean. "Cleanliness, that's the key as far as the production floor goes," he says. "It is an oily atmosphere. We have a grinder, and other machines and all of them create abrasive dust. Our calibration technicians make sure to clean them monthly."

Electronic capabilities
In addition to overlay charts, many comparators rely on state-of-the art electronics that bring powerful software capabilities to the traditional inspection tool. Some optical comparator companies, such as Deltronic, Micro-Vu and Quality Control Solutions, use digital readouts (DROs) that they produce, while other companies use DROs from companies such as Metronics Inc. (Bedford, NH), makers of the popular QuadraChek products, and Fagor Automation (Elk Grove Village, IL) which has just released its new NVP200 QC series.

DROs have capabilities ranging from simple display of X-Y coordinates to complete, programmable, geometric processing with a computer-numeric control worktable. When data points are entered, the DRO can calculate the thread's geometry and keep that data on file to measure future parts against. These points can also be used to generate data for statistical analysis. Also, some comparators allow computer-aided design models to be imported into the DRO, which creates an electronic overlay on a computer linked to the comparator.

Much like using gages, operators can use overlays and DROs in tandem, according to Bob Frieze, vice president of S-T Industries (St. James, MN), whose machines use Metronics' DROs. "You can use the overlay and see where there is something wrong with the thread. Then you can go back and use the QuadraChek to get the actual reading to tell you how far off the thread is," Frieze notes. "You can do this with an overlay, with its plus or minus tolerance line, and it can tell you how far it is from tolerance. But DROs are more accurate. With them, you can get down to 50 millionths of an inch."

TECH TIPS

• Optical comparators inspect the flank angle, minor diameter, root radius and surface of threads.
  
• Comparators can use overlay charts to compare the magnified image against a template.
  
• Digital readouts have given added capabilities including statistical analysis.

What features do you need?
Q: What should potential end users require of a comparator?
A: You need good clear optics, a nice bright light source and ease of adjustment on the table. Does it have readouts? Does it have micrometer heads? Does it have the full-blown read out with the geometric functions in it that allow you to automatically calculate diameters of circles, distances between circles, distances between lines? Adjust for skew of the part. If you have a part that's a little tilted, can you adjust for that. -- Timothy Allen, vice president of manufacturing, Suburban Tool (Auburn Hills, MI)

Q: For measuring threads, which is better -- a horizontal or a vertical comparator?
A: Most of the time, threads are inspected on horizontal comparators because the stage has to helix. When you look at a thread form, the thread is on a helix angle and it is normally below 15 degrees, somewhere around 7 degrees, so you have to twist your stage or twist your part so that you are looking straight across the thread and it is not up and down.-Peter Klepp, president, Dorsey Metrology (Poughkeepsie, NY)

Q: Can thread inspection be done without helix travel capability?
A: Companies that are looking to inspect threads say they require helix capability. Most vertical machines don't have helix capability. Some have a helix fixture, which is a compromise. You can measure threads in one little area. There is a fixture that can go on and twist the part. But you can't move the stage while you helix. So you can only check the stage in one location without moving it around and readjusting. -- Peter Klepp, president, Dorsey Metrology (Poughkeepsie, NY)

Q: What should users know about lighting?
A: Because threads are measured with projection lighting, the intensity and uniformity of the illumination and affect measurements. Collimated through-the-lens on-axis illumination is best. Light that is not directly on-axis and collimated can cause shadows, increasing measurement error. -- Tad Davis, comparator product manager, Optical Gaging Products Inc. (Rochester, NY)

Q: There are comparators that project different images. What are they and which is better for most end users?
A: Depending on the comparator, there are three types of images that can be presented. One is the single reflex projector, in which the image on the screen is produced upside down and backward. Another is the erect image version, where the image is correct top to bottom, but reversed left to right. The third is the fully corrected image, where the image produced is correct top to bottom, but reversed left to right.
You would think that you would want the fully corrected image but, ironically, this requires additional optics and mirrors and reduces the actual image quality. Each time the light bounces off the mirror some of the light is lost. The best image is the simplest optical mechanical design, the single reflex. Seeing the thread magnified right side up is simply a matter of fixturing it so that the correct view is on the screen -- Louis Todd, president, Quality Control Solutions (Temecula, CA)

Q: What do users need to know about screen magnification?
A: The more a part is magnified, the better the resolution, but the field of view will be smaller. For example, in a 14-inch screen diameter with a 10X lens, the largest section of the part that can be seen at one time measures 1.4 inch. Because of this small viewing areas, parts that are longer than the field of view will have to travel on the stage along the X and Y axes. -- Bob Frieze, vice president, S-T Industries (St. James, MN)

Q: Can optical comparators be used to inspect internal threads?
A: The other side of the thread is the nut. Typically, what is done is to make a casting of the thread out of a silicone rubber type of material. It is allowed to harden and is then carefully lifted out and put on the optical comparator, and the thread form is inspected. -- Bob Larzelere, vice president, Deltronic (Santa Ana, CA)

A: Yes they can be. There are three ways to measure internal threads -- each with limits of effectiveness and accuracy: One is destructive: cut the part in half and use surface illumination. The part must be carefully deburred and carefully cut exactly in half. Two, trace the part with a stylus and follower. The stylus must be properly sized to trace all the details of the thread, usually an expensive and time consuming method. And Three, mold -- usually with a silicone rubber material -- must be poured precisely into a very clean thread and extracted with care to insure all features stay intact. -- Tad Davis, comparator product manager, Optical Gaging Products Inc. (Rochester, NY)