Scrape to the point: Drilling Performance

The debate about roller-type linear guide ways vs. conventional box ways has raged for decades. Linear guide ways are fast, accurate and relatively easy to install in both high-end and low-cost, commodity machine tools. Proponents say linear guide ways create less heat and friction and don’t have the stick-slip action associated with box-style ways, which should make them more accurate. Linear guide ways are commonly found on many high-speed machining centers and Swiss-style lathes, where lightning fast rapid-traverse rates and light cuts are the rule.

On the other hand, old as the Industrial Revolution itself, box ways are known for their ability to carry heavy loads and take heavy cuts. They have superior vibration damping ability, but require a master craftsman to install. Compared to linear guide ways, box ways are less likely to be damaged in a crash, and, in most cases, offer longer service life. They, too, can be extremely accurate and are often used in jig boring, grinding and other machine tools that require submicron precision.

VIDEO

View a hand scraping video supplement courtesy Okuma America.

Big Business

While the argument about which is better won’t be settled anytime soon, both styles of guide ways should be appropriately sized for the machine tool, of sufficient quality for the application and properly installed. Poorly designed or improperly installed guide ways—box or linear—will doom even the best-designed machine tool, regardless of technological merits. For machine builders and service companies alike, one aspect of guide way selection is especially important: Box ways can take some serious elbow grease to scrape and substantial skill to install.

“For a large machine, it can easily take several weeks to scrape the ways,” said Scott Ashworth, president of capital equipment sales and service company KRC Machine Tool Services, Independence, Ky. KRC is rebuilding a boring mill, with an approximate 8′-long (2.44m) table and two bearing surfaces that are each 6″ (152.4mm) across. “Even with two guys tag-teaming the work, it takes a long while to complete a project this size.”

Knowing where to scrape and how much to remove takes years of experience. Image courtesy KING-WAY Scraping Consultants.

Ashworth said a substantial percentage of his business is the remanufacturing and retrofitting of old, heavy-duty machines, such as horizontal boring mills and vertical turret lathes. Many have nonfriction material, such as Turcite or Rulon, on the way surfaces. These materials increase the weight capacity of machine tools while reducing the tendency of precision load-bearing surfaces to “stick” together.

Ashworth explained that hand scraping creates small pockets on the surface to hold oil and provide lubrication, but, even with a properly scraped surface, some metal-to-metal friction occurs. Because of this, when the ballscrew is first turned, there’s a small amount of “windup” until the machine starts moving, at which point it tends to jump forward. “Granted, this is a very minute distance—microns, perhaps—but it can be an important factor with very fine, detailed machining and heavy loads,” he said. “This is why many builders opt for Turcite.”

Turcite and comparable nonfriction way surfaces are made of polytetrafluoroethylene (PTFE), a thermoplastic that is often blended with bronze and other additives before being “skived” into sheets 12″ to 24″ wide × ¼”, or less, thick (304.8mm to 609.6mm × 6.35mm). It is glued to the cast iron or steel substrate, then scraped in the same manner as metal box ways to create oil-bearing surfaces. This ubiquitous material offers a frictional coefficient roughly 20 percent that of scraped metal-on-metal ways.

A technician (top) power scrapes the high spots on a metal way surface. A finished way (below) is ready for installation. Images courtesy Busch Precision.

A Lasting Legacy

Whether metal or plastic, hand scraping requires application of a flat, carbide-tipped blade, to make the way flat, and then “flaking” the finished surface. The following six steps describe the process:

1. Make an initial “cross scrape” to clean and prepare the surface.

2. Remove any loose chips and dust, then apply a small amount of mineral oil and “stone” the surface to remove burrs.

3. Apply a thin layer of bluing paste to the surface.

4. Rub the way on a surface plate, which removes the colored paste from the high spots.

5. Scrape the high spots, then repeat steps 2 through 4 until the surface is completely flat and the machine’s geometric requirements are met.

6. Flake the surface to create oil pockets, then clean and stone it one final time.

Bob Behnke, a machinist at Busch Precision Inc., a Milwaukee contract manufacturer and machine repair company, said scrape marks are like fingerprints of the person who worked on the machine. “Everybody has their own individual pattern. Over time, you look at enough of them and you’re able to recognize who did the work. We have people who scraped here 25 years ago, and I can still pick out which machines were theirs.”

Behnke said the amount of scraping, the depth of each individual mark and the distance between them (points per square inch) depends on a number of factors. A very worn machine could require grinding or machining to true the geometry, followed by a complete scrape job. A light rebuild may only need to be “freshened up” to make surfaces flat and create the proper amount of oil retention. Machines that will see heavy loads call for deep scraping, where light-duty machines can be effective with shallower scrapes. In each case, the craftsman follows the same basic process, but makes technique adjustments based on skill level, experience and the application needs of the machine tool.

“There’s a lot to it,” Behnke said. “The length of time spent scraping any particular surface or machine comes down to how many high spots there are to begin with, how much resistance is acceptable when the machine tool traverses back and forth, the amount of lubrication that’s needed and the size of the way surface. It takes a lot of experience, but it’s also physically demanding. You have to push on the blade and really dig in if you’re going to get a deep scrape, which is the best way to assure longevity in the machine tool.”

Sticking to the Plan

Aside from strong forearms and a steady grip, successful scrapers have a solid game plan. “You can’t just go at it randomly,” Behnke said. “You have to be very meticulous, scraping first in one direction, then the other. That breaks up the lines and gives a nice, uniform pattern.”