What is the end result of scraping in a machine?

[Buffalo21]

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

 

[C-Bag]

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.

 

[Wobbles42]

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.

 

[Dabbler]

@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.

 

[Wobbles42]

@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.

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)?

 

[Richard King 2]

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

 

[Wobbles42]

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.

 

[Richard King 2]

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

 

[Richard King 2]

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

 

[Wobbles42]

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.