Forrest Addy
Diamond
- Joined
- Dec 20, 2000
- Location
- Bremerton WA USA
In the thread "Pull Scraping," Richard King wrote of "signature pass," a distinctive style of finish scraping recognizable by the cognoscenti. Well done Richard. Very descriptive and helpful.
I've never addressed that aspect of the scraping business except to reference a personal style or to emulate the spot count, bearing,and texture found in unworn parts of the original scraped surfaces. It's an apt name for an essential concept and Richard's citing it stimulated me to recall another branch of the scraping trade based on recollections of yore. I think the following is fairly accurate but I may have missed some important details.
I recall Hunter's Point Naval Shipyard where a bunch of us traveled in 1971 to help with the workload. Their machine shop had no surface grinders worthy of the name so pump and turbine flanges needing fine consistent finishes for metal to metal pressure tight joints were precision scraped.
There was a specialized crew of scraper hands and I'm sorry to say I never got a chance to pump their brains while I was there. For one thing, I worked at the other end of the shop and got yelled at if I strayed. For another, these were old school depression era workers who guarded their secrets. They assumed curious visitors were interlopers out to steal their jobs and had to be driven away by curses and threats.
They scraped little cast iron but lots of cast steel, cast alloy steel, cast and wrought stainless of dozen of different alloys, bronze, Babbitt, aluminum, everything that you could imagine that required tight-fitting face to face sealing. It was all hand scraped in that section; all day, every day. Theirs was a high volume operation where small work - a steam chest cover for a reciprocating pump for example - was briskly done in minutes and the flanges of a big LP turbine case were swarmed by a team and scraped flat in a matter of hours.
They had all the parameters worked out for scraper end radii, edge angles, carbide Vs HSS Vs Stellite, Vs cast alloy... for every combination you could think of. They had racks and drawers of scrapers of every conceivable type. They had spray bottles of "Mystery Milk" they would mist over the printed surface prior to scraping that greatly reduced scratching and "drag" particularly in ductile steels.
They mostly hip scraped, that is, their long flexible scrapers were driven by powerful nudges from the hip as opposed to driven by arm, shoulder, or belly power. The body end of the scraper had a leather covered wood pad 4 to 6" in dia. When driven by a powerful thrust of the hip the scraper made actual chips. If Mystery Milk was in use every stroke was accompanied by a tiny puff of vapor; there was that much heat energy imparted to the chip.
Here's a video showing a strong resemblance to a HPNS scraper hand in action:
https://www.youtube.com/watch?v=ZTru...5TSv-IL19eJwVd
Imagine a dozen older Californian men dressed in blue bib overalls working as teams in a congested and grubby shop constructed during WW II and you'll have a good idea of the appearance and activity in the scraping section.
I recall they did some pull scraping mostly in areas where access was difficult or restricted to one direction, Their technique was much like seen here in previous posts but done with vigor and efficiency.
I recall their finishes were typically very smooth and carefully stoned to reduce the inevitable scraping ripple to a minimum. They did not scrape to high spot count as I recall - 4 to 8 spots per square inch sticks in my mind - and most of their work was intended for liquid tight and pressure tight boundaries, Babbitt and bronze bearings. No cast iron on cast iron sliding bearings at all.
Which brings us to signature scraping. Their large cast iron surface plates saw heavy continuous use and while they babied them, it was necessary to touch them up every few weeks. This was a process of lifting and inverting plate onto plate in succession, taking prints, and scraping to a definite texture. Their largest plate had scraped in it a HPNS logo which had proper bearing and looked no different up close than any other part of the plate. Stand back in the right cast of light and the logo flashed brilliantly or receded darkly from the flash of the rest of the surface. I puzzled over this for the longest time and finally concluded the last few passes of the scraper in the logo were at right angles to the last passes of the balance of the table.
Little extra time was expended in the creation of this refinement but it served as an undeniable statement of pride and competence of a very capable and tight knit group of craftsmen.
That was 44 years ago. Most likely all who worked in the scraping section are very old or dead and their vast store of specialized lore has gone with them. Too bad their feats were never recorded.
Ours is a trade handed down through generations from master to apprentice and much is lost if there is even a small gap. It's a pity we have no tradition of written record. If we were scientists we'd have written stores of past knowledge ready to enlighten the delver but alas we as craftsmen can only look back at patchy history often chronicled by the well meaning ignorant.
I've never addressed that aspect of the scraping business except to reference a personal style or to emulate the spot count, bearing,and texture found in unworn parts of the original scraped surfaces. It's an apt name for an essential concept and Richard's citing it stimulated me to recall another branch of the scraping trade based on recollections of yore. I think the following is fairly accurate but I may have missed some important details.
I recall Hunter's Point Naval Shipyard where a bunch of us traveled in 1971 to help with the workload. Their machine shop had no surface grinders worthy of the name so pump and turbine flanges needing fine consistent finishes for metal to metal pressure tight joints were precision scraped.
There was a specialized crew of scraper hands and I'm sorry to say I never got a chance to pump their brains while I was there. For one thing, I worked at the other end of the shop and got yelled at if I strayed. For another, these were old school depression era workers who guarded their secrets. They assumed curious visitors were interlopers out to steal their jobs and had to be driven away by curses and threats.
They scraped little cast iron but lots of cast steel, cast alloy steel, cast and wrought stainless of dozen of different alloys, bronze, Babbitt, aluminum, everything that you could imagine that required tight-fitting face to face sealing. It was all hand scraped in that section; all day, every day. Theirs was a high volume operation where small work - a steam chest cover for a reciprocating pump for example - was briskly done in minutes and the flanges of a big LP turbine case were swarmed by a team and scraped flat in a matter of hours.
They had all the parameters worked out for scraper end radii, edge angles, carbide Vs HSS Vs Stellite, Vs cast alloy... for every combination you could think of. They had racks and drawers of scrapers of every conceivable type. They had spray bottles of "Mystery Milk" they would mist over the printed surface prior to scraping that greatly reduced scratching and "drag" particularly in ductile steels.
They mostly hip scraped, that is, their long flexible scrapers were driven by powerful nudges from the hip as opposed to driven by arm, shoulder, or belly power. The body end of the scraper had a leather covered wood pad 4 to 6" in dia. When driven by a powerful thrust of the hip the scraper made actual chips. If Mystery Milk was in use every stroke was accompanied by a tiny puff of vapor; there was that much heat energy imparted to the chip.
Here's a video showing a strong resemblance to a HPNS scraper hand in action:
https://www.youtube.com/watch?v=ZTru...5TSv-IL19eJwVd
Imagine a dozen older Californian men dressed in blue bib overalls working as teams in a congested and grubby shop constructed during WW II and you'll have a good idea of the appearance and activity in the scraping section.
I recall they did some pull scraping mostly in areas where access was difficult or restricted to one direction, Their technique was much like seen here in previous posts but done with vigor and efficiency.
I recall their finishes were typically very smooth and carefully stoned to reduce the inevitable scraping ripple to a minimum. They did not scrape to high spot count as I recall - 4 to 8 spots per square inch sticks in my mind - and most of their work was intended for liquid tight and pressure tight boundaries, Babbitt and bronze bearings. No cast iron on cast iron sliding bearings at all.
Which brings us to signature scraping. Their large cast iron surface plates saw heavy continuous use and while they babied them, it was necessary to touch them up every few weeks. This was a process of lifting and inverting plate onto plate in succession, taking prints, and scraping to a definite texture. Their largest plate had scraped in it a HPNS logo which had proper bearing and looked no different up close than any other part of the plate. Stand back in the right cast of light and the logo flashed brilliantly or receded darkly from the flash of the rest of the surface. I puzzled over this for the longest time and finally concluded the last few passes of the scraper in the logo were at right angles to the last passes of the balance of the table.
Little extra time was expended in the creation of this refinement but it served as an undeniable statement of pride and competence of a very capable and tight knit group of craftsmen.
That was 44 years ago. Most likely all who worked in the scraping section are very old or dead and their vast store of specialized lore has gone with them. Too bad their feats were never recorded.
Ours is a trade handed down through generations from master to apprentice and much is lost if there is even a small gap. It's a pity we have no tradition of written record. If we were scientists we'd have written stores of past knowledge ready to enlighten the delver but alas we as craftsmen can only look back at patchy history often chronicled by the well meaning ignorant.
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