Hand scraping uses simple tools to introduce fine texture to machine ways
Hand scraping is a practice that has lost its luster for many machine builders. Because of the associated costs in time and maintaining skilled labor, it’s an art that has largely been abandoned.
So why do many high-end machine tool makers still use the technique to introduce fine texture to surfaces that contact sliding components? Because many industry experts feel that there’s no substitute for hand scraping when it comes to maintaining high levels of CNC machining accuracy in a long-life machine tool.
Why hand scrape?
Whether the work moves and the cutting tool is fixed, or vice versa, machine tools must use platens or tables travelling along precision ways. Straight, smooth ways are the key to close tolerance machining and all the control software in the world can’t compensate for a tool or table that won’t index repeatably.
Despite major advances in CNC grinding technology, hand scraping still sets the foundation for productive CNC machining.
Here’s a video from Okuma that shows scraping in action:
There are several reasons to hand scrape:
Accuracy
Scraping is done to align components within millionths of an inch, allowing for consistently-held, tight tolerances.
Flatness
Eight to ten contact points per square inch are created to prevent rocking, add balance when tightening, and to allow for true flatness in parts.
Oil Retention
Some machine surfaces are lubricated by oil spray or pressurized systems. Scraping creates oil pockets that hold oil on the surface and allow for gliding motion. Without these pockets, very flat, smooth surfaces will “cold weld” and restrict movement. Even worse, they’ll react erratically to applied forces, with high initial resistance to movement followed by a rapid breakaway, which is difficult to control consistently.
Appearance
The finishing touch of scraping is aesthetic. Once accuracy, flatness, and oil pockets are handled, parts are “design scraped” to achieve an attractive textured finish. A part of this is steeped in history. The highest quality machines have always showed scraped ways so it has become a traditional measure of craftsmanship.
Excessively flat and smooth surfaces inhibit motion and create inconsistent parts
Hand scraping introduces carefully placed high and low spots on the mating surfaces of the CNC machine. While your natural instinct may lead you to believe that flat surfaces would be better for machining accuracy, this is in fact not true. Tight tolerances can only be achieved by adding high and low spots that hold oil and allow for gliding, rather than sticking. Introducing more contact points via the high and low spots also allows for balance and therefore perfect mating. With flat surfaces there is only one contact point, which creates a state of imbalance.
Tools for scraping are simple, but it’s a highly skilled task
For moving parts, introducing high and low points allows the two surfaces to glide together without sticking. For bolted parts, hand scraping creates a tighter fit, which is less likely to separate as a result of the material expansion, contraction and flexing that occurs during CNC machining processes.
Big castings can’t be fixtured for machine grinding
One machine tool builder, Okuma, uses extensive hand scraping in primary machine tool castings. This corrects uncontrollable process variables in casting such as slight changes in base materials, mold release agents, temperature and environmental conditions that cause each component to have unique characteristics. And, while metal behaves in similar ways from one casting to the next, once it’s released from a constrained position it can move and flex in unpredictable ways. Hand scraping corrects fluctuations arising from these variables.
Hypothetically, if a mechanical process were used to perform hand scraping, it would necessarily be based on predictable base material behavior. That’s the nature of machining processes. However, due to the variables described above, each component must be handled with consideration for its unique characteristics. This can only be done by experienced artisans who can see, touch and feel where and how, the scraping must be applied.
Hand scraping of gibs, cross slides, saddles, headstocks and tailstocks are performed on all components of Okuma machine tools that involve mating surfaces.
When working with large castings, it’s often impractical or impossible to mount the part into a surface grinder or mill, making hand scraping the only available method. With low cost tools and high portability, however, hand scraping is also used frequently when machine techniques are available.
It’s perhaps the last bastion for the blacksmith’s art in a CNC world, and it’s not going away any tine soon.