# What is the end result of scraping in a machine?



## Wobbles42 (Feb 12, 2021)

This may seem obvious, but I haven't seen it clearly explained.  Aside from being a good learning opportunity, are there real performance gains to be had by scraping in a lower end machine?

It seems many people have undertaken this project.  I've found a lot of info on *how* to do this, and I understand the advantages of scraping as a process, and most people indicate that it makes a difference, but it isn't clear what that difference is.

I am curious what kind of changes people see in the machines themselves after improving the ways geometry.  What sort of improvement is observed?

A little Sieg X2 is what I have access to, and I am curious what potential exists.  People joke that these machines can be "casting kits" more than finished machines -- with a little love can they be turned into a more refined tool?


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## markba633csi (Feb 12, 2021)

On older machines it's done to compensate for wear and return the machine to factory specifications. On new inexpensive machines it's sometimes done to correct manufacturing defects as received from the factory, or get closer to an ideal, with the end user performing the dual role of quality control and rework department, in most cases for zero money LOL
-Mark


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## benmychree (Feb 12, 2021)

In a word, ACCURACY, where is did not exist before.


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## Ulma Doctor (Feb 12, 2021)

increased bearing surface
when properly scraped and flaked, the wear surfaces use the film of oil, to float the assemblies they carry- with little friction due to large surface area to support the load 
wear surfaces that don't share the capacity to trap oil, wear away quickly due to increased friction, coupled with the very few areas of contact supporting the entire load, lends to rapid wear


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## C-Bag (Feb 12, 2021)

All the above are very true. In my case with the compound on my 9x20 it reduced to zero stiction while fixing the inaccuracy of the way the dovetaiI was machined. I was not aware of this stiction until I read the Connelly book although I’d felt it in mechanisms before. It seems counterintuitive but if you get two smooth surfaces together with a fluid between them like two pieces of glass, they will stick. You can overcome it, but it makes mechanisms “sticky”. So not only is the goal to increase the precision of the fit it also reduces stiction which like in the case of my lathe carriage (which needs to be scraped) it would smooth the feed and make for more consistent finish.


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## Wobbles42 (Feb 13, 2021)

Thanks everyone!  That's exactly the info I was looking for.  I just didn't know what to look for.  I think I am going to try the project.

I have an extra saddle assembly for the mill, so it will be interesting to compare (and I have one chance to get it wrong if it comes to that).


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## Hellkell (Feb 13, 2021)

It seems counterintuitive that you’d scrape something flat then flake it to allow oil to get in. Doesn’t that reduce the flatness again? negating the scraping. 
Wouldn’t a less than flat surface (non-scraped) do the same thing (allowing oil to be trapped)?


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## markba633csi (Feb 13, 2021)

The average flatness would remain the same after flaking if done evenly over the whole surface- the overall accuracy would remain.  However, the fit between two parts may be way off requiring re-milling and scraping
-M


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## Ulma Doctor (Feb 13, 2021)

Hellkell said:


> It seems counterintuitive that you’d scrape something flat then flake it to allow oil to get in. Doesn’t that reduce the flatness again? negating the scraping.
> Wouldn’t a less than flat surface (non-scraped) do the same thing (allowing oil to be trapped)?


The flaking is done on the bottom surface of the top sliding member. The voids created by the flaking keeps the oil trapped rather than allowing it to be squeezed and run out, creating a bearing surface. The inability to compress a trapped liquid is the very same principle that floats the assemblies, rather than have them grind together.


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## C-Bag (Feb 13, 2021)

My impression from the Connolly book was flaking was more ornamental than functional as scraping is. I’ve seen more and more especially on these crazy priced eBay machines where there is flaking with no underlying scraping. And truly effective scraping would not be on an up facing surface, like my ways, it would be on the bottom of the carriage. This where you would want the cushion of oil trapped, not necessarily on the ways where it would also attract dirt and grit.


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## Buffalo21 (Feb 13, 2021)

benmychree said:


> In a word, ACCURACY, where is did not exist before.



if done correctly, I work for a company, that paid huge money to have a Wells-Index milling machine done, when the repair company got done, it wasn’t scraping that occurred, what they had done turned the mill into scrap. The company had to buy a new mill, with a large percentage of the purchase price, paid by the repair company, who soon after went out of business


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## C-Bag (Feb 13, 2021)

Connolly spends probably 40% of the time, maybe even higher, trying to drive home scraping is not something that just comes naturally. He talks over and over about the common mistakes and how a reasoned and methodical approach if not strictly adhered to will cause failures. And if not understood and corrected early can lead to disaster. Just starting in on a machine you want to ultimately use without some study and deep practice of the fundamentals is folly.  It so reminds me of the first film we watched in auto class as a freshman in highschool where its an old guy and a young apprentice and a car comes in with a problem. The apprentice makes a snap judgement and immediately starts tearing it apart where the old guy spends time talking to the customer, getting symptoms, checking them against what the car is doing and goes right to fixing it. Meanwhile the apprentice has half the car torn apart and still hasn't doesn't really know what's wrong.  I observed this with the guys I got to work with. Lots of them smoked and they always kinda ambled up, listened to what the customer said, then got out their smokes and took their time messing with them but all the while they were thinking about the whole problem. Learned a LOT from those guys. Mostly engage brain before mouth and hands.


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## Wobbles42 (Feb 13, 2021)

C-Bag said:


> Connolly spends probably 40% of the time, maybe even higher, trying to drive home scraping is not something that just comes naturally. He talks over and over about the common mistakes and how a reasoned and methodical approach if not strictly adhered to will cause failures.


This is good advice, and I have seen those chapters in Connelly, and have taken them to heart. I bought a second copy of the 'bottom half' of this mill specifically to use as a learning vehicle, partly for scraping and geometry correction.

I also bought this to use in some motion control experiments without taking my "working" mill offline -- I have a mind to do a CNC conversion and intend to do the firmware and controller for that from the ground up (Embedded systems like this are how I actually make a living, so that project at least has a reasonable chance of success -- it's mostly a chance to indulge perfectionism that I am forced to temper in a professional context). The assembly can thus serve it's intended use even if I mess up the geometry, provided that I don't mess up so badly that the screws get bound up for unloaded movent.

My first step will indeed be tons of thinking and research.  Truly understanding the how and why of making the machine more precise is honestly my real end goal here.  Just imitating some procedure from Connelly without really understanding it -- even if it could be successful at producing a better machine -- would not accomplish that end goal.  The power of using our minds plus simple tools to make a precise complex tool is what drew me to this hobby in the first place.


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## Dabbler (Feb 13, 2021)

@Hellkell This will seem counter-intuitive, but the physics is non-negotiable.

You are confusing flat with contact area.   You can grind a surface to be nice and bright and shiny, having a lot of apparent contact area, but it will wear out within months of use.  You need a balance between contact area and valleys to hold oil.  the surfaces will actually 'float' on the oil if the balance is right and the oil choice is correct.  

This is why you must use a specialized way oil, and flake after scraping. You want somewhere around 30-40% contact area, with the rest being where the oil lays. osmotic pressure is a wonderful thing - it only works fro short distances on machine tools.  This is what you will see when someone scrapes to around 20 points of contact per square inch.  

@Richard King 2 can give you more detail on how this actually works in practice.  I am only repeating what my mentor has taught me.


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## Wobbles42 (Feb 13, 2021)

Dabbler said:


> @Hellkell This will seem counter-intuitive, but the physics is non-negotiable.
> 
> You are confusing flat with contact area.   You can grind a surface to be nice and bright and shiny, having a lot of apparent contact area, but it will wear out within months of use.  You need a balance between contact area and valleys to hold oil.  the surfaces will actually 'float' on the oil if the balance is right and the oil choice is correct.
> 
> ...


Sounds like too much leads to increased wear and (possibly?) sticking.  When we start there process though the contact area is much *less* than that 30-40% in the case of these import tools.

What is the downside of too little contact area?  Does that show up as measurably reduced effective stiffness of the machine (because you aren't "squishing" that inadequate contact spot)?


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## Richard King 2 (Feb 13, 2021)

Here is the simple way I explain it in my class work booklet.

On conventional machines like a lathe, knee mill, etc.   we want 20 high spots  or points per inch  (PPI) and 40 to 60% contact.  The high points or spots carry the weight and the low areas hold oil pockets.  The uneven low spots lets the oil to adhere to the surface  .    A surface grinder, straight edge, Jig Bore we shoot for 40 PPI 40 to 60%.   The more points the longer the ways lastas long as the depth of the scrape mark is a min of .0002" to .001" deep.   To much contact the ways lap together and get stick slip like wringing gage blocks together.   Low points and percentage and the ways wear super fast,  as the 1980's Taiwan "disposable machines"   and the modern Rung Foo  cheap machines


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## Wobbles42 (Feb 13, 2021)

Richard King 2 said:


> Here is the simple way I explain it in my class work booklet.
> 
> On conventional machines like a lathe, knee mill, etc.   we want 20 / 40 to 60 % contact points per inch  (PPI) .    A surface grinder, straight edge, Jig Bore we shoot for 40 PPI 40 to 60%.   The more points the longer the ways lastas long as the depth of the scrape mark is a min of .0002" to .001" deep.   To much contact the ways lap together and get stick slip like wringing gage blocks together.   Low points and percentage and the ways wear super fast,  as the 1980's Taiwan "disposable machines"   and the modern Rung Foo  cheap machines


Thanks!  I didn't expect to hear straight from Richard King.  I just got done reading six months worth of posts over on PM as you helped someone try and scrape in the exact casting that I am starting at right now.

You set me straight on the top ground surface of the mill base (the one that contacts nothing)  I had assumed, like the other inexperienced guy, that it was intended as a datum plane to build the machine off of... I'm still mystified as to why it exists, but I was about to head down a wrong path.

I really appreciate all the time took on that thread.  I learned a lot from your posts.


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## Richard King 2 (Feb 13, 2021)

Here is another pamphlet I have inside my class work booklet. Its a simple way to explain why we scrape instead of grinding.  It may cost you something.









						Why Are They Scraped Booklet | PDF | Machine Tool | Machining
					

A basic intro to hand scraping machine ways and why grinding is not an option




					www.scribd.com


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## Richard King 2 (Feb 13, 2021)

Also a GREAT book is at the top of this forum....  The Foundations of Mechanical Accuracy


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## Wobbles42 (Feb 13, 2021)

Richard King 2 said:


> Also a GREAT book is at the top of this forum....  The Foundations of Mechanical Accuracy


I have a hard copy of this one.  I bought it years ago.  Understood only a little the first time I read it (I had never even touched a machine before), but after a year of messing around in my garage I reread it and got a lot more out of it.  It really is a wonderful book.


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## Richard King 2 (Feb 13, 2021)

I maybe teaching another class in Olympia WA fall pf 2021   so you can learn why in person


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## Wobbles42 (Feb 13, 2021)

Richard King 2 said:


> I maybe teaching another class in Olympia WA fall pf 2021   so you can learn why in person


Oh, thanks for the heads up.  I'm only a couple hours from there.  I'll keep an eye out -- Corona gods willing that would be awesome.


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## Richard King 2 (Feb 14, 2021)

Here is a scientific paper written about scraping by some Taiwanese Engineering University Professors.
Take a look at the last page bottom left under references  



			http://140.112.14.7/~measlab/download/2011/3-D%20Measurement%20and%20Evaluation%20of%20Surface%20Texture%20Produced%20by%20Scraping%20Process%20Measurement%20.pdf


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