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absolute scraping beginner - some advice need re chatter and scratches

StrayAlien

Cast Iron
Joined
Aug 18, 2014
Location
Melbourne, Australia
Hi all,

I've watched maybe very scraping vid there is, but, time to tool up and get some practice in before loosing myself on a machine. When I'm happy, this old girl will get some treatment. She is running, but, to do the job for real, she needs a proper recondition.

https://www.practicalmachinist.com/...own-sharp-2-surface-grinder-back-life-363506/

My first attempts have not been great. Terrible actually, and maybe that is an understatement. Chatter and scratches. So, seeking some advice to help me get over this beginner's hump. The following pic has some random stuff and experiments but gives the idea. I am going one pass diagonal, then another at 90 degrees, and other stuff just to see what is what.

PXL_20201127_083947947.jpg

I'm using carbide in a hand scraper. 60mm radius, and also tried 90mm. I've jimmied up a slow speed grinder in the guise of a fine grit 6" diamond wheel set on a backing plate (which is a gear) on a shaft and in the lathe 3 jaw chuck. Seems effective.

PXL_20201127_084232225.jpgPXL_20201127_084344809.jpgPXL_20201127_084402790.jpg

I'm grinding into the carbide so as not to chip the edge, the wheel is lubed with wd40. About 300 rpm. I've tried 5 degrees and 8 degrees negative angle. The 8 degrees made some small improvement I think. The material is cast iron and, from what I understand, not too bad quality. It is a piece from the last scraping class in Melbourne a few years ago.

The carbide edge has no flaws that I can feel, and it'll shave my fingernail just fine. The surface on the carbide looks good. I've tried heavier scrapes, lighter, shallow angles and steeper angles, long strokes and short strokes.

I've not managed anything I'm happy with.

All comments and advice appreciated.

Greg.
 
The Sanvik scraper is an OK scraper, but it is stiff and if you were to mill it thinner .020" say 3/4 of the length and try it. Also you need to push it with your body and not your arms. Puta rubber pad on the end of the wood handle. A 4" rubber sanding disk screwed to the end works good. Many screw up by pushing with there arms and you can't control the length of the stroke and it rolls side to side and you get corner scratches. Once you have the rubber pad on the handle pull the scraper into your body and then push with your body. Use a scoop motion or lift the carbide end up after you scrape no longer then 1/2". I call the motion a tap tap style. Because when your doing it correctly you get a tapping sound. Start the blade just above the cast iron plate about .125", push forward and down and lift up after the 1/2 stroke. Then cut 4 cuts(scrapes) and move your feet over to the left or right about 1". Do not leave the blade on the cast iron plate. push lift move over so your gap is as wide as you scrape mark, the move over is when you lifted it up and pull it back. Relax your arms when moving back. Think about it being a shaper cuts going forward and lifts when going back.

Your scraping a cast iron plate aren't you? Looks like steel almost. What grit diamond plate are you using? It should be a minimum of 600 and 1200 or more for super fine tip finish. practice that until you get a checkerboard look. Oh the lines of the scrape marks need to be separated by a minimum of .125" so you get separated rows and scrape marks and after you cut the checkerboard pattern over and over you will start to see results. Also roll on some bluing or red or yellow highlighter so you can see what your doing. I will add some pictures in a bit.
 

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Richard, thank you. Some great advice there.

To answer your questions first: ultimately, I'll be looking to scrape in the lathe, shaper, surface grinder and tools, but, will start with some smaller things then go onto the surface grinder before I do the rest - I didn't pay any money for the SG and it needs it most. The piece I am practicing on is what I thought to be a lump of cast iron. Re the diamond grit - it is 3000. Seems to work just fine at 200-300 rpm. Nice steady carbide removal but not too fast. I've ordered a 320 grit and an 800 also.

I found the culprit for most of the scratches: the carbide had become magnetised so chips where hanging on to get underneath the next stroke - I haven't read about that one anywhere!

PXL_20201128_004654520.jpg

I bodged up a quicky degausser from 4 rare-earth magnets - following 'presso' here DIY Demagnetizer Tool (That Works!) - YouTube. I worked well enough to keep me going:

PXL_20201128_051027258.jpgPXL_20201128_051033837.jpgPXL_20201128_051042568.jpg

I haven't used the mushroom on the end of the scraper as yet, but I have found a way to use my body to do the work and, yes, it makes a large difference to controllability. Thank you.

But, I think you are right in that my practice piece may not actually be cast iron. No pepper grinds coming off it at all, just shavings. So, I switched to a small ancient derelict vice I had about and immediately that showed what a different it makes to scrape actual cast iron! Like ... soooooo easy compared to my practice piece. Butter.

PXL_20201128_055212023.jpg

Not trying for checkerboard as yet, just the motion, balance, and control. Your instructions re scrape spacing, stance, movement all make sense - thank you.

Now I have to try and find a larger piece of cast iron to practice on (not as easy in Oz as in the US)

Thanks again Richard.

Greg.
 
Great work on that surface grinder! Clearly you have patience and would do well on the next stage of the project. Sounds like you are doing it all right - starting with test pieces for practice scraping, and eying the machine that 'needs it the most' i.e. can likely only be improved by your efforts. When I started playing around with scraping that was a great comfort to me - knowing that with scored and badly worn ways, even less than perfect scraping would only make it better.

My only advice (from my novice perspective) would be to time some of your final practice scraping efforts. Figure out how much time it takes to remove a certain volume of cast iron as well as final scraping to get to a certain PPI. Then survey your machine and estimate how much material you will need to remove - whether it is taking the bow out of worn ways or re-aligning ways to be parallel or perpendicular.

It not always fun to do a 'honest assessment' of time spent, but it might be useful for you since your surface grinder is reasonably large. I scraped a toy shaper (7") by hand, and then decided to get a power scraper before doing anything bigger. They do cost a small fortune, even used, but they can also save a lot of time and strain.
 
Looks like your test piece is ductile iron not grey which is a real pita to scrape, also I recognize that scraper! Ralph brought that up to to the goldcoast when he came to visit some time back. personally I prefer the mushroom pattern handles, great for roughing off the palm or the shoulder

edit: looks like octagon scraper handles is just a weird Melbourne thing and that's not ralphs scraper

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LAMachinist, thank you. All advice appreciated. :+1


Marcus. Hello there! I do indeed have a few items from the "Ralph A collection of metrology wonders" to assist me on the scraping journey (we used to work together). Including a straight edge he did at your class in Brisbane. I'll be touching it up if need be.


The hand scraper was made by (or made for) Phil Fehring (machtool) in Melbourne. I got it plus the practice piece from him a few years ago when I visited his shop. Maybe that explains why octagon handles are popular in Melbourne. :-)


Odd re the practice piece being ductile. It came gratis (hand scraper did not) so no complaints though he did say it was what his students used. I'm guessing some small mix up reaching into the scrap pile on a busy day! But, you sure are right. It is a PITA to scrape. I'm happy to have found that out early. One good thing though is it made me try a bunch of different things and also bodge a nice functional slow-speed carbide sharpening setup.


Greg.
 
no offense to PDW who cast that straight edge but the straight edge is one of his early ones and appears to also be ductile iron not grey. Andre brought the same straight edge to the meetup and gave up scraping it in lieu of a camelback he brought which was double the length but was a breeze to scrape in comparison as it was a beautiful soft grade of grey iron.

The slow speed grinder is a must, while the carbide scraper blades last a long time when lapped properly you'll find the dulling of your blades happens so slowly you wont easily pick up on even though its hampering your work. the proper glendo wheels last a lifetime compared to the cheap and nasty ebay ones but I must admit I have a backing plate to suit the Chinese ones on ebay and use them prolifically, gasweld has a slow speed grinder for about $100 AUD which is easily adapted but any old slow speed grinder can be modified. there is really no need to invent the wheel.


it is dangerous to use a 115-135mm radius blade when starting out unless you have a bit of cast to practice on but it is well worth getting comfortable with a flatter blade for roughing. no matter what people tell you, the factory sandvik grind is useless for any real scraping work and won't allow decent PPI or depth of scrape but when applied with extreme downforce and a low approach angle you can very effectively rough without the risk of digging holes.
 








 
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