View Full Version : Scraping / Grinding / Lapping: Serious Discussion

Paul Alciatore
01-02-2011, 04:41 PM
These are three different ways to get a better, more accurate surface. As I understand it, each has it's advantages and disadvantages.

The general opinion in the machining community is that scraping is the most accurate. It is also perhaps the most labor intensive. And the one that requires the most skill.

Grinding, as in surface grinding, is perhaps the easiest to do, but requires a fairly expensive machine. From what I have seen, it appears to be capable of the same or at least close to the same surface accuracy as scraping produces over a small area. The results over larger dimensions could be somewhat less, depending on the accuracy of the ways in the machine used. They may not be completely straight or parallel.

On the other hand, lapping seems to be the step-child process here. It does exist and is used, but not so much for large surfaces, like machinery ways. But lapping is the process that is used to produce the best optical surfaces, although that industry sometimes calls it grinding. These are perhaps more accurate than any that are produced in a surface grinder and certainly more accurate than those produced by scraping. One possible drawback to lapping is the possibility of leaving some amount of abrasive particles embedded in the surface being finished. This is minimal in optical work as the material being finished is very hard and not suceptable to penetration. But on softer materials, like steel, it can perhaps be a problem. I would greatly like to hear some discussion on this and exactly how much of a problem it really is.

So, what I would like to begin is a full and deep discussion of the merits, problems, and consequences of using these three and any other methods of precision finishing. I know there is a difference between surface finish and the accuracy of the surface. A piece of jewlery can be highly polished and have a mirror finish, but not be at all accurate. Machinery ways can be highly accurate but not be polished. On the other hand, a Jo block has both accuracy and a mirror finish. And perhaps the most accurate surfaces also must have at least a degree of good finish.

Looking for discussion of these processes.

01-02-2011, 05:18 PM
Lapping is done over huge areas... Granite surface plates are lapped to get their flatness..Cast iron surface plates are scraped to get their flatness..

Forrest Addy
01-02-2011, 05:34 PM
Here' about lapping as applied to machine tools. The problem is not mere flatness. The problem is flat, linear surfaces, parallel to and/or interecting with other plase surfaces and parallel and suqew to other axes of traanslation and rotation.

Lapping glass optics isn't nearly as big a deal a lapping metals. For one thng the usual shapes for glass are flat or spherical (don't hassle me on aspheric lenses etc. What Zeiss does with elaborate custom made equipment in production is hardly transferrable to a home shop) and glass takes far less energy per unit of volume to remove. For another except for astronomical apparatus glass optics are less than a foot across. Scale and sytem energy and a large installed industrial base is a huge advantage for manufacture of glass optics via lapping.

The naive may think by extended logic that lapping metal to accurate geometry can be accomplished by smearing abrasive paste and rubbing the work to a tool of some kind. Not so. Lapping to accurate geometry is very tooling intensive and the tooling requires constant machine touch up when the tooling wears. The accuracy and geometry is generated on the tool by machining and scraping and the tool laps that accuracy and geometry onto the work; kinda like real slow and expensive three dimensonal carbon paper.

If you wish to study the application of lapping to machine tool manufacture read the relevent pages in Wayne Moore's "Foundations of Mechanical Accuracy." Needless to say Mr Moore would not have resorted to such an expensive and time consuming process as precision lapping if it was not absolutely necessary to the success of his product line

Then there is uncontrolled lapping where a shade tree bumpkin smears lapping compound under a tailstock casting and runs it back and forth on a lathe bed. He finishes with a smoooth matte finish, abrasive everywhere, and the problem of tailstock quill bore's concentricity amd alignment with the spindle axis unsolved.

I've done accurate lapping for geometry off and on for most of my working life and machine tool rebuilding for the final couple years of my career. In my professional experience lapping for machine tool accuracy and geometry is beyond the resources and skill level of any shop not posessing the resources to verify linearity, alignment, lapping tool manufacture and refurbishment, and personnel posessing the skill, experence, and judgement to accomplish it. Lapping machine tool working parts is a far more skilled and time consuming an undertaking than scraping or grnding way surface.

The problem is boredom and temptation. Lapping is tedious, repetitious, time consuming and deceptivey simple. Even the best and most knowledgeable mechanics are tempted through mind numbing boredom to take shortcuts that later lead to errors reversible only by investment of double or triple the time of doing it tight the first time, by the boring numbers.

01-02-2011, 05:36 PM
... and small areas. Glass reagent bottles often have lapped stoppers for a very close fit.

01-02-2011, 06:38 PM
The general opinion in the machining community is that scraping is the most accurate.

I'm not sure if that's really true any more. At the time of Moore's Foundations of Mechanical Accuracy (1971) it was, but all ultra-precision machine tools have ground beds. Even the contemporary ultra-precision lathes of the era: Monarch 10EE, Schaublin 150, Hardinge HLV-H, had ground beds.

On the other hand, lapping seems to be the step-child process here. It does exist and is used, but not so much for large surfaces, like machinery ways.

Lapping is very common in machine tools, even in the modern era. Spindle bores are lapped, surface plates are lapped, so are gage blocks, cylindrical squares... Bearing balls are lapped round. But lapping is just a specialty version of grinding in my mind.

IMHO, the value of scraping is that a home-shop guy with a lot of time, energy and patience can restore an old beater machine to as good or better than new alignment.

01-02-2011, 07:57 PM
Grinding is no easier than scraping, it just looks that way.
It is just as accurate and more so given a good machine and an operator that actually knows how to use it. In almost 30 years of running a grinder in mold shops I have seen maybe 2 dozen guys that were actually really, really good at running a grinder, especially small 6"x12" or 6"x18" surface grinders.

01-02-2011, 08:02 PM
It was talked about before and there was a thread where someone wanted to do it. If a shop guy set up his own system using a smaller grinder and was able to grind his own ways to within we will say the same tolerances you could get with scraping which way would be better?

01-02-2011, 08:09 PM
I wonder if it would be useful to include the context for which process is best. In my home shop lapping and scraping are best, and grinding is all but unachievable, for example.

So some contexts would include cost, who does the work (hired out or in-shop), new manufacturing vs restoration/repair, scale of the job (does it warrant grinding when an hour of scraping will suffice). There are surely others but without a context of any kind any valuation of what is best is entirely subjective.

01-02-2011, 10:08 PM
These people (http://www.kellenberger.com/english/jig-grinders/hauser-s-45-400-s-55-400.html) are still scraping their high precision jig-grinders.


01-02-2011, 11:13 PM
Something I haven't seen mentioned yet is that the process of lapping is usually targeted for working a soft material against a hard material. The soft material is actually the tool and the harder material is the workpiece. What happens is that the lapping abrasive embeds in the soft material and abrades the harder material. For most applications the abrasive is placed on the soft tool and woked into the surface with ahardened tool or roller. Then the tool is cleaned to remove the loose abrasive and leave the embedded abrasive. Then the tool is used with lubricant on the workpiece to essentially hone off material.

A really good example of unintended lapping was with the Washington DC subway and light rail system. It was originally installed with steel rails and rubber wheels to keep down noise levels, but within two years they had to start replacing rails. Dust and grit was embedding in the wheels and grinding away the rails at a high rate. They finally gave up and went to steel wheels to reduce rail wear.

If you put abrasive between two materials of the same hardness, such as a lathe bed and tailstock, the abrasive embeds in both surfaces and they continue to wear each other even after the surfaces are cleaned, even if the lapping compound is supposed to break down after a while.

I have used fixed abrasive laps, such as nickel plated diamond or plastic lapping film. These work, but are still less effective than scraping.

As far a s surface grinding, you can get the work as accurate as the machine you are using, but it's difficult. Between heating, machine accuracy and release or introduction of residual stresses, it is hard to make scraping accuracy by surface grinding.

I have used lapping frequently to make and restore accuracy in granite squares, straight edges and hand flats. In those cases I used yellow pigment to spot and either a nickel plated diamond or a babbitt lap charged with diamond lapping compound. For this sort of work, annealed cast iron, annealed copper or babbitt metal work very well. A machined surface of hot rolled low carbon steel works so-so.

Paul Alciatore
01-04-2011, 12:17 AM
Food for thought here. I am going to have to read the responses several times before even beginning to ask further. Thanks to all.

Forrest Addy
01-04-2011, 12:37 AM
Obfiscation 'R' Us.

01-04-2011, 02:41 AM
Obfiscation 'R' Us.

Aye up Forrest, I find that spelin confusing & a bit obscure :D


Ian B
01-04-2011, 03:00 AM
A bit on how slip blocks are made:



01-04-2011, 11:25 AM
A bit on how slip blocks are made:


Neat link! Especially the section ""Manufacturing Gauge Blocks" where they describe Johanssonís method of mass-producing gage blocks by selectively sorting and lapping 8 blocks at a time.