Is there a reason to not lap instrument surfaces (straight edge, etc) after scraping ?

[max.levesque]

I understand that for sliding surfaces (dovetail, etc.,) the pockets left by scraping are desirable, because they allow oil to remain longer between surfaces.

I was wondering if, for instruments like straight edges, it would be a good or bad idea to lap the surfaces after scraping.

For example, if I have a angled straight edge that I use to correct a machine dovetail, will a straight edge with a lapped surface transfer the prussian blue more uniformly ?
Or do the scraping pockets on the straight edge "help" in any way (ex. because they contain more prussian blue )?

 

[Mark Rand]

When scraping a straight edge or the like, one does not use a heavy hand on the scraper and does not leave deep pockets. One is also scraping to >=40 points per sq in, which is unnecessary for machine ways.
Lightly stoning the surface can improve the contact area without removing much metal if the pockets are a tenth of a thou or less. Such surfaces transfer blue very well.

 

[Richard King]

I disagree. You scrape a straightedge to 40 PPI and deep. High to low depth so the SE lasts longer. High spots wear, even with bluing they wear. Good examples are or were scraped to 40 PPI. Sip, Moore, Dixie jig bores were scraped to .0.0005 " per 12 and 40 PPI. If you own one now, 50 years since they were made they are as accurate as they were on day 1. You scrape a SE to 40 PPI and a depth of a minimum of .0002" and maximum of .001".

In my classes I use the example of 2 men lifting a 500 pound table vs 10 men lifting a 500 pound table. The more the longer they can lift it.
A ground SE won't be anymore accurate then the grinder it was ground on. If it is stoned or lapped and the depth of the low spot is less then .0001" it will wear out faster and get stick slip.
I have been making cast iron SE's, machining, scraping and using them for over 55 years. Lapping or stoning a scraped SE will defeat the purpose of scraping.

 

[MCritchley]

One thing I’ve noticed hen bluing up a straight edge on a granite surface place is suction or wringing. Smooth on smooth will wring and pull a concave straight edge flat and give you false print. lapping is bad as it will increase the suction.
Moore talked about this phenomenon in their book, they preferred a scraped plate. Let’s say you had a 24” long straight edge that was concave, I would predict you could pull the center down a few tenths with bluing applied to a granite plate. Depth is a good thing, it prevents wringing. It’s hard to explain, I met Rich 12 years ago and am still trying understand the finer points. It’s best to experiment and try different things so you can develop an understanding of how metal behaves.

Most machinists cannot grasp metals behavior until you try to scrape a surface flat to a high degree of accuracy. Metal is rubber, embrace the fact and outsmart it.

 

[dcsipo]

I understand that for sliding surfaces (dovetail, etc.,) the pockets left by scraping are desirable, because they allow oil to remain longer between surfaces.

I was wondering if, for instruments like straight edges, it would be a good or bad idea to lap the surfaces after scraping.

For example, if I have a angled straight edge that I use to correct a machine dovetail, will a straight edge with a lapped surface transfer the prussian blue more uniformly ?
Or do the scraping pockets on the straight edge "help" in any way (ex. because they contain more prussian blue )?

When you scrape a piece to 40+ PPI, the pockets will be shallower than at 20- PPi. Still, you might change the geometry if you lap the surface to the bottom of the pockets. You will be chasing your tail. Also, the surface you will use to lap may impart errors more considerable than the accuracy of the master you scraped to. The lapped surface offers no real advantages in metrology or when using it as a spotting master. A 40+ PPI surface is indistinguishable from a precision-lapped surface for the purposes above.

 

[TGTool]

In the early days of the industrial age they did indeed lap surfaces. However, they learned by experience that scraping was better. With lapping you don't have good control of where material is being removed. With scraping you remove it precisely where it needs to be removed.

If you lapped an already scraped surface, you're more likely to degrade the accuracy than improve it.

 

[johansen]

You can do both. Scrape something to 20PPI at 50% contact and then lap it to get your last couple tenths out, if too much contact percentage, get back to scraping.

 

[JST]

Does not running over the scraped surface with a stone have somewhat the same effect (but less of it) as lapping would? "Some" material is always removed, you can often see the effect, even with minimal stoning.

Nobody says you have to lap the daylights out of it. Might just be a little more controlled version of stoning after scraping.

 

[johansen]

Does not running over the scraped surface with a stone have somewhat the same effect (but less of it) as lapping would? "Some" material is always removed, you can often see the effect, even with minimal stoning.

Nobody says you have to lap the daylights out of it. Might just be a little more controlled version of stoning after scraping.

Yes, but its not much..you wont remove tenths, more like millionths, with generous lubrication.

I used a dremel and a small grinding wheel to get at least 20 PPI on various things, such as the hardened steel feet of a height gauge, using the surface plate to identify the highspots by polishing them. This method was basically as fast as any other. (I used bluing compound to get to around 5 ppi, but beyond that, just grinding the polished high spots was rather easy)

I would not suggest anyone abuse a good surface plate as a polishing lap, but rather the cheap 12x18 or 9x12 plates.

I later found i had a couple microns convex rocking on said height gauge. (About 4" by 3" base), everywhere on my surface plate.
So it ain't perfect.

 

[dcsipo]

Does not running over the scraped surface with a stone have somewhat the same effect (but less of it) as lapping would? "Some" material is always removed, you can often see the effect, even with minimal stoning.

Nobody says you have to lap the daylights out of it. Might just be a little more controlled version of stoning after scraping.

I know this is an older thread, but it just popped into my mind; Richard was talking about scraped gas seals that would require enough lapping that the highs create a contiguous gas barrier. Sort of a hybrid of a lapped and scaped surface.