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scraping a flat surface ..... - richardkell
richardkell
richardkell
scraping a flat surface .....
I have a disc of cast iron previously machined and scraped flat, it is eight inch diameter say one and a half inches thick and which I use as a stable surface to help assemble and check for parallelism of my richard kell honing guide stainless rods. But now it needs a re-vamp, looks a bit sorry for itself; an excellent excuse to do some engineers hand scraping, something only self taught (but that goes for most of everything).

The three hours flew by, it can become so absorbing, but i have utmost sympathy for men stuck on this work fulltime. Its actually quite strenuous and goodness knows how arm tendons and ones back would fare.

The real hard work would for instance have been to achieve the  'Whitworth three plate method'  in producing a verifiable flat surface or set of three master surface plates. What a clever man ....  https://en.wikipedia.org/wiki/Joseph_Whitworth  ... and my heart goes out to those men engaged on any work such as this.

The easiest example is here where I scrape a simple flat surface against a known reference ie the surface plate, but also for more sophisticated and difficult applications we could scrape to produce dovetail surfaces that would be our best chance of creating accurate and lasting sliding surfaces for machine tool bedways, creating a correct form of 'running in' and thus enabling the Bielby Layer to develop, a very thin 'bedded-in' surface layer measured in millionths of an inch that is extremely low friction as in (and is needed) in machine tool slideways. Correctly run-in surfaces are both so bright and low friction we read that the initial scraped surface helps hold the lubricant so that the Bielby layer can be developed. Something of this phenomena is shown at the end of the exercise when we reach a close approximation of flatness, the heavy CI disc 'floats' for a second or two when laid carefully back onto the surface plate.

I had previously machined the disc both length and crossways ie X and Y axis on the Bridgeport milling machine, these still show after the finished session of scraping but as they are undetectable to touch they must be less than say a quarter thou inch.

I use a fourteen inch hand file ground at its working end and honed on the arkansas stone to do the scraping. Heft and weight do matter a lot in this activity.  The M&W commercial scrapers sold years ago which my dad had a set (still here) are little more than over-priced de-burring tools, the 'three square' pattern is particularly offensive and useless for anything. I've blogged on this before, see tags at right. And often the best de-burring tools are ones you make yourself, with experience comes the intuitive insight into how tools cut, this applies to lathework, any machine tool and handwork. This finer point of insight (and craftsman development) can no longer exist when CNC rules universally.





Pics explain best  .....




Above, looking a bit 'grotty', an excuse to waste some time and rescrape .....







Machined slots at the side of the disc allow me to clamp it onto the Bridgeport table to resist the scraping pressure therefore I get 100% access to the surface in question. Here above at top we can see material removed.







This surface plate came from Robsons Marine Engineers at Seahouses in the late 1960's; my dads employers at the quarry group at that time were Thomas Tilling (previously Mark Appleby) Easington, Belford, Northumberland and had purchased the remaining tools and tackle that remained in the old sandstone building. At right is the fourteen inch hand file doctored to make into a scraper, a diabolical useless and not very good product bought new many years ago, its from Oberg Sweden as a handfile.  Diabolical because it 'skates' like mad over the surface and is most reluctant to cut when functioning as a hand file; will have cost me lots when i first started self empoyment !! Far better go to the scrapyard and see what they have to offer.

In the above theres far too much engineers blue but useful for a photo purposes and shown before I rub it thin into a more useful state (done with the palm of ones hand).







Heres what to avoid, far too much blue on the surface plate, very much false results, usually where beginners trip up from the little I've seen online. The tricky edges i try to scrape but its not easy, a dead smooth worn six inch file copes with it, procedure is to attend to all the blued 'high-spots' individually, better shown as per below.






Above, here we are proceeding nicely, with hindsight I should have taken a deeper cut for the first couple of 'all-overs' to break the old surface and eliminate the remains of the machining.







Thats the three hours done, need to get back onto production, it will do well enough for what I want; feels incredibly uniform despite the lines visible and has developed the amazing property (expected from previous experience) of floating on air for the first couple of seconds as it is laid face down onto the surface plate.

pics do not do it justice, in reality the plate sparkles and gleams as one moves vantage point .....




postscript  ..... But it bugs me that those machining tracks still show even though i cannot feel them from finger touch so as this even a couple of months later still 'bugs me' I shall repeat the process and get it to what I want, in other words greater rigour is needed. If I'd had someone do this for me i would not have let it pass.

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