Why Scrape? • Elegantly simple technique that provides results that are difficult to match – Still used today on high end machines to fit sliding members to way surfaces – Scraped surfaces retain oil in the fine scrape marks in a way that ground surfaces cannot
• A must, on large new and rebuilt machine tools – Take the tool to the work – A straight edge can be a fraction of the weight of the tool
• Inexpensive Tools
Example, Finely Scraped Straight Edge
Reflection off of a fine scraped surface
Applications • Machine Tool Rebuilding – Any slide way surface – Precision mating surfaces
• Easy way to true up two mating surfaces if a surface grinder is not available or practical – Cylinder head to block – Bearing block halves – Flattening parts that are impractical to clamp or hold for milling
• Quick way to accurately correct damage to flat surfaces – Dings – Edge Impacts – Gouges
Basic Procedure for Scraping • Start with a known flat surface – Surface plate – Ground bed way
• Apply a spotting compound to flat reference surface • Apply work piece surface to be scraped to the inked/dyed surface (or vice versa) and move slightly back and forth • Separate and observe spots transferred to work piece • Scrape off metal from any locations with spots • Repeat until spotting produces a uniformly distributed set of spots through out the work piece surface
Flat Plate, Early Stage
Flat Plate, After Several Cycles
Flat Plate After More Cycles
Lathe Cross Slide, Early Stage
Lathe Cross Slide, Later Stage
Types of Surfaces to be Scraped • Any flat surface – Size constrained by size of reference surface or straight edge – Long surfaces can be successfully scraped with a shorter straight edge • • • •
Requires careful attention to spotting indications Start at one end and scrape until level Progress towards opposite end while overlapping portion of previous section Scrape new section until contact on previous section is restored
– Metals as well as some specialty plastics such as Rulon and Turcite
• Dovetails – Usually requires some method to measure distance between opposing dovetails – Precision ground rod in dovetail opening provides an easy and repeatable measurement point
Challenges, Spotting • Spotting Compound Application – Heavier during initial aggressive scraping – Lighter and thinner as surface progresses closer to final condition
• Accurate Spotting – Consistent and even application of compound – Consistent and even application of pressure between work piece and reference surface • Gravity is always best when possible, can be augmented with weights • Must be aware of how center of gravity acts on work or straight edge • Manually applied pressure can work but must be consistent
• Reading the spots – For heavy scraping; if its blue, scrape it – For light scraping; scrape light areas surrounded by blue
Fine Spotting
Challenges, Scraping • Basic technique is to apply pushing & downward force on scrapers • Careful scraping – Avoid gouges by controlling scraper angle and pressure – Developing an ability to scrape with required aggressiveness • Heavy metal removal when surface is far from desired flatness • Light metal removal when surface is close to desired flatness
– Avoid damage to edges
• Patience – The results can be slow but the reward is worth it
Control • Scraping “Straight Down” removes metal in a homogenous manner anywhere across entire surface – Work piece progresses as angle remains fixed as surface becomes “True”
• Techniques can be applied to actually tilt or skew a surface as need – Two surfaces on different planes can be worked to be parallel – Commonly done when working with dovetails and flat ways to compensate for uneven wear – Leaving one end un-scraped while continuing to spot and scrape all other sections will rotate work piece down from the“unscraped” edge or point
Tools Needed •
Flat Reference – Surface plate – Straight Edge
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Spotting Compound – Prussian Blue – Water based Compound, Canode #2243 from Dapra – Almost any substance that will leave a fine film and that will transfer • Lipstick • Paint pigment
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Ink Roller
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Scrapers – Carbide (best over all) • Commercial units available • Inexpensive inserts can be brazed to handles and sharpened as required
– Old files (will work but requires frequent sharpening)
Tools, Continued •
Clean flat file to debur surface before each spotting – More critical when doing heavy metal removal
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Paint brush to clean away scraping residue Sturdy holding device – Keep work piece from moving when applying heavy scraping action – Flexibility to move work piece around for best light and tool access
