# What is scraping?



## Pontiac Freak (Oct 1, 2012)

I see it here all the time but have no idea what it is or how it is done?  Any quick answers?


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## Rbeckett (Oct 1, 2012)

It is releveling and truing the ways and beds of machine tools.  It is a procedure that is done very slowly by hand to achieve a hand lapped flat and true surface for the machine to travel across.  I do not know exactly what tool is used or how exactlu to do it but there is a member on site who teaches scraping classes all over the place.  It is a skill that is a forgotten art almost anymore.  If you can scrape, you can recondition machine tools to better than new in some cases.  Hope this helps a little and that I didn't oversimplify the procedure and detract from the art form too much.
Bob


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## Bill Gruby (Oct 1, 2012)

Good Bob but let me add that it is not just for that. You scrape mating surfaces so that they come together perfectly flat.
I hope Richard King jumps in here, this is his specialty.

"Billy G" )


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## Rbeckett (Oct 1, 2012)

Thanks Bill, I couldn't recall Richards name when I replied to the thread.  He is well known and very good at it too from his reputation.  I too hope he jumps in and explains some of the finer points too.
Bob


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## Tom Griffin (Oct 1, 2012)

Scraping is also used for oil retention (technically flaking). The marks you see on Bridgeport ways for instance is not to make them flat, but to hold oil in place and keep the ways lubricated.

Tom


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## DMS (Oct 1, 2012)

Hopefully this is not too long-winded.

Scraping is a method for fitting two mechanical parts, those parts can have any of a number of shapes, including flat, cylindrical, or conical. Shaping is performed with a single edged cutting tool (the scraper) which is typically made from carbon steel, high speed steel, or tungsten carbide. Scraping can be done by hand, with a hand scraper, or assisted with a power scraper.

Typically the process is

1) apply a marking compound to your reference surface
2) press the part you are fitting against the reference surface
3) remove the part from the reference, and examine, marking compound will cling only to the "high" spots
4) if high spots are evenly distributed over surface, you are done. Otherwise...
5) Use the scraper to scrape off the high spots.
6) De-bur
7) Go to 1

Scraping is very time consuming, and removes very little material. Typically surfaces are trued by grinding or filing to get them "close" before scraping begins. 

The main benefit of scraping is that it produces a surface that is flat (tens of millionths of an inch over a lathe bed), but not mirror smooth like the surface that is produced by grinding. The benefit is that with two smooth surfaces, lubricant is forced out from between mating surfaces. As the surfaces move in relation to each other, the lubricant layer is re-established, but in the beginning, you are left with metal to metal contact, or a very thin (inadequate) layer of lubrication. With a scraped, because of the very small variations in the surface, oil is retained, and the lubricant layer is re-established much more quickly. 

Sometimes ground or lapped surfaces are scraped to improve oil retention, a process called flaking.

There are a number of scraping videos on YouTube, if you are interested I would recommend checking them out. Quite fascinating, and the results are superior to any other method I know of.


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## Pontiac Freak (Oct 2, 2012)

Ok, I think I understand the basics.  Looks like a lot of work!  Very nice results though.

Thanks Everbody!


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## Bill Gruby (Oct 2, 2012)

Yes it is a lot of work. It is also an art. There aren't many left that can do it correctly. Richard King is a pro at it. He makes it look easy.

" Billy G " )


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## Richard King (Oct 6, 2012)

Good Morning,

I have been reading the thread, but was to tired to respond as the last 2 weeks while I was scraping the top and bottoms saddles of a Okuma CNC lathe in IL and I was tired and didn't feel like jumping in.  Now I am home and can add some info.   One thing is that it is easy to grind one side of a way system flat, straight and a great mirror finish.  But impossible to match grind the mating part.   If you have two flat surfaces and do not have the high and low scraping marks, the parts will get high friction and pull up scores ad the part will be hard to move, that's called "stick slip". 

 If you have ever rung gage blocks together you understand what I am talking about.  Like a Hardinge lathe.  The bed is a steel plate that is ground to .0002" It has a flat and 2 opposing dovetails.  The saddle that fits it can be ground, but making the dovetail angles and flat to an exact fit is impossible and must be tweaked to fit 100%  overall contact on all surfaces.  Of that 100% contact there are scraped holes that hold oil so you have 1/2 of the contact area even with 50% high spots and 50% low scraped pockets that are .0002" to .0005" deep. Those 1/2 moon shapes you see on Bridgeport's on the way surfaces are extra deep oil pockets .002" deep to hold the oil in a mini reservoir and they are installed on a 45 degree angle so the oil flows evenly across the ways when the moving member is moved.

They are also used for cosmetics on the table top to make the machine look good.  They also let the table top last longer too as you approx 50% high spots wear into the .002" holes / pockets.   The high spots carry the weight of the or the sliding mated part or saddle on the Hardinge lathe.  Another thing about scraping is it can correct the errors created in the machine you machine the parts on.  Let's say you have a milling machine that is worn out and you try to machine a lathe saddle on it. The error in the saddle is .002" and you want to get it back to new machine spec's then you have to scrape it. If your scraping a compound on a lathe that has soft iron on both slides.  Well you need a "Master" straightedge to put the marking compound on and scrape the compound straight, flat and parallel.  We scrape the longer member first and that becomes our master to match fit it to the shorter member.

This is a long story and I will try to scan in an article from Do-All titled "Why are they Scraped" that explains it very simply.   I will do that soon. 

Scraping the ways of a machine may seem simple, but you need to equip yourself with many types of tools.  I always say it is easy to learn to scrape, but knowing where to scrape and how much to remove is The ART of scraping.  When your scraping ways you have to consider squareness, alignment, sag from weight of the members when they are installed, tool pressure, the number of points (High spots) I call them PPI or points per 1 square inch, POP  Percentage of Points in the 1 square inch and how deep tour low spot is.

One more thing, many companies have a Preventative Maintenance Program where they will take a machine that is still cutting straight, but they can see the scraping marks are disappearing and will dismantle the machine and 1/2 moon it.  This extends the life of the machine as the oil pockets were wearing down.
I hope this helps. 

 Richard King


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## Tamper84 (Oct 7, 2012)

Ive wondered myself what scraping is. Now I know!!! It sounds like a very handy skill to have if you deal with old machines. Something I would like to learn as well.

Chris


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