# When the surface plate is just too small ..



## graham-xrf (Nov 28, 2021)

The granite 18 x 18 in inches. The diagonal is about 25.4 inches. The need is for a straightedge 36 inches long. A good granite that size costs a bundle, and takes up lots of room in a small shop. Also, I already have the 18 x 18.

I get it that spotting 2/3 of the length is likely to get messed up by the overhang, and it gets mad confusing if one then spots it by shifting the work along to get at the 2/3 from the other end. What if there was a high spot near the overhang end? That would produce a whole new spotting pattern over the 2/3 already done.

I thought there may be a strategy, but also perhaps that this problem already has a well proven strategy that I just don't know about yet.




*1. Null the overhang.*
I am thinking to eliminate the overhang bow effect using something like in the sketch. The jack is set to carry half the weight of the overhang. This could at least allow a spotting pattern on the tested 2/3, but not any hinging moves.

*2. Getting at the overhang end*
The next move is to turn the work around to have the overhang now included in the 2/3, but resolutely ignore all results other than those from the previous overhang, plus a very little extra. Work only on what used to be the overhang. If there was a high spot, it will be seen. If there was a local low, then that too will be seen. This can inform a strategy on what to do about the initial 2/3, and how to bring it all into a common line.

*3. Getting a single spotting result over the whole work*
Suppose the condition of the overhang was (extreme case) all a bit low, needing the entire initial 2/3 to be scraped down to meet it. This too might be done by looking for a spotting pattern that has a middle third that repeats, even though it now has the initial overhang shared over the plate. I even thought one may be able to check with a 2-colour spotting, to see if one overlays the other in the middle third.

All of the above is just my first attempt at a plan. If experienced folk here say it is doomed, then plan B may be to start saving up for a bigger granite. Of course, it may be that a load cell and all that stuff is just not needed, and there is already a well know work-around. I sure hope so !


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## C-Bag (Nov 28, 2021)

Plan B. 

Or Plan C, save up for a nice 36" B&S camel back like I did. But I have a 18x24 granite.


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## graham-xrf (Nov 28, 2021)

C-Bag said:


> Plan B.
> 
> Or Plan C, save up for a nice 36" B&S camel back like I did. But I have a 18x24 granite.


It had crossed my mind, but the only one I saw was not only £££, it also looked like it needed to be put onto a surface plate, and given some TLC. I do keep an eye out! You got me looking for a B&S outlet in UK 

 Regardless, it would be nice to know if one can spot a straight edge in overlapping sections, and still get a sensible result


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## C-Bag (Nov 28, 2021)

graham-xrf said:


> Regardless, it would be nice to know if one can spot a straight edge in overlapping sections, and still get a sensible result


Connolly specifically warns against it many times, over and over as it's a waste of time and a rooky mistake.


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## graham-xrf (Nov 28, 2021)

C-Bag said:


> Connolly specifically warns against it many times, over and over as it's a waste of time and a rooky mistake.


OK - I hear it. Connelly maybe did not envisage going the way of load cells, etc.  I am in a field where I just don't know, and I mean to get to know some before I expend on discovering what others have already exhausted themselves in proving. When I read the accumulated wisdom, I do soak it up, but I am not a slave to it! There is a logic to observing that a reference flat surface has a line in it's plane that aims through space to continue it's reference some distance beyond it. We use the same principle to enable making gunsights, and theodolites. Seeking a (possibly new) practical strategy is surely OK.

With all respect to a fellow member, I quote @Richard King 2 from 2019 ..
"_Remember Connelly was not a scraper, he was a organizer of thoughts from several craftsmen, much of that book is wrong in my opinion_".

It occurs to me that where one should scrape - or not, might yield to something like illuminating the work in monochromatic light, or laser raster, in an interferometer  comparison to an optical flat, to display the deviations. Perhaps some third technique. I keep having all these sideways thoughts, but mostly I dismiss them on good practical or cost grounds. I think the present best route is keep looking for a bigger granite in good condition, and get either a blank, or a used SE with potential, and gain some of the skills that @Richard King 2  teaches right here. This, I guess, might be sometimes contrary to Connelly, but that's OK. I may be rookie, but you are witnessing me not blindly blundering into a mistake.

 Edit: [ Looking about, I find £340 + £30 P&P gets a somewhat used scratched and dinged 36" SE with some rust pitting. A little 12" raw unmachined castiing is £150. That last one is maybe reasonable, but the present need is for 36" or 48". I pass over anything bigger because they are just too heavy and unwieldy]


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## rwm (Nov 28, 2021)

I understand why large surfaces and surfaces on machines get scraped. For a straight edge, why would you not just have it surface ground it? Or are you specifically trying to do this yourself and learn scraping?


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## C-Bag (Nov 28, 2021)

graham-xrf said:


> With all respect to a fellow member, I quote @Richard King 2 from 2019 ..
> "_Remember Connelly was not a scraper, he was a organizer of thoughts from several craftsmen, much of that book is wrong in my opinion_".


And with all due respect to you too,  RK2 has referred to Connelly a lot and is still his selling book. I know from my time as a mechanic you ask 10 of them how to do something, and you’ll get 11 answers.

So I refer to what makes sense not “historical engineering“(we’ve always done it this way). The most used and crucial check for me is hinging the work. And Connelly said in the passages I remember that having more than a couple of inches sticking out on both ends of your reference is going to make it really sketchy to hinge anything properly.

I get it’s really easy to come up with an alt solution and go with it. But I understand this one parameter and without it I see no way to properly do a very basic check that will keep from down the wrong track. Which once you get off just goes further into the weeds.

But I’m certainly nobody and not an expert so shouldn’t have responded in the first place.


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## rabler (Nov 28, 2021)

With load fee etc, I was able to acquire a 3'x4' surface plate for less than US $200.  I'm sure I'll spend more than twice that having it calibrated.  I think there is an aspect of intellectual pursuit to your method, rather than just purely pragmatic progress.


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## Weldingrod1 (Nov 28, 2021)

Cast iron levels are pretty cheap to buy. You could do the three plate method with them and originate some straights...

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## graham-xrf (Nov 28, 2021)

rwm said:


> I understand why large surfaces and surfaces on machines get scraped. For a straight edge, why would you not just have it surface ground it? Or are you specifically trying to do this yourself and learn scraping?


I want to fix up the ways of a lathe, perhaps eventually on both, and I have nothing to lose in attempting some scraping. Of course, I do not intend to have the lathe ways be the very first thing I scrape on, and up to now, I have only scraped on some small things. A straight edge capable of being useful for the later scraping seemed logical, but I am open to a short cut if I come across a reasonably affordable one.

I do have a "straight edge", which is too short, but is of the type used with feeler gauges across engine blocks. About 5mm thick, 600mm long, with a beveled edge down to about 2mm. So far, it has only been used to rule lines with a pencil for cuts on woodwork!


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## graham-xrf (Nov 28, 2021)

rabler said:


> With load fee etc, I was able to acquire a 3'x4' surface plate for less than US $200.  I'm sure I'll spend more than twice that having it calibrated.  I think there is an aspect of intellectual pursuit to your method, rather than just purely pragmatic progress.


Wow! A real score! Would that be granite type?
Mine was acquired as a freebie, because it had been replaced by a new, larger one, and it had stood around for a couple of years unused. My pals even delivered it for me 




In the work I did at the time, I would have the surfaces of satellite Earth Station tracking dishes tested. The surface would be a optically shaped not-quite parabola polynomial in carbon fibre. It is quite difficult to get a 6.1m or 7.3 metre diameter Cassegrain dish, with its subreflector optics surfaces made and positioned all to within 0.25mm (0.0098 inch). The measure method was photogrammetry, where the surfaces are covered in accurately made little circular decals, and various bar-codes. Also, an invar bar covered in barcodes is placed in the dish.  A camera on a pole is used to take a series of images that are then processed by the software, adjusted to get all the corresponding decals to register. The software is then capable of fitting the whole lot in comparison to a CAD model, and publishing coloured contour plots of deviations. One can see immediately where the departures are or where the cure in the mold had a distortion.




You can look up the technique, and I am sure there will be stuff on YouTube. This stuff is widely used in the aerospace industry, and the software is available. There are even free open source versions, and various ways of calibrating digital cameras. You can, of course, buy the whole kit, but it comes expensive. Applied to smaller stuff, like say a thing like a lathe bed, I just don't know the kind of accuracy it is capable of. I had a hard enough time getting the work to under 0.25mm (but the thing was huge)!  The measure kit seemed accurate to better than 0.1mm (0.004"), even on that scale.  The servo kit behind has resolution to point to within about 300 milli arc-seconds. It has to find a moving target satellite position accurately, but also at an exact celestial time.

The clinometers are set up on a granite surface plate, but that is only good enough for an approximate start point. They are then set to track a sunspot, to discover construction leveling correction and North axis. Finally, known satellites are tracked, and the numbers adjusted to achieve true horizon and zenith calibration.

From what I learned among the guys who do this, I do suspect that traditional scraping just might be augmented by some newer tools, and different technology techniques. Now away from the rat-race, I am quite happy to scrape away on my own stuff, with some reference to Connelly, but not exclusively, when I want to try something of my own.


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## rabler (Nov 28, 2021)

graham-xrf said:


> Wow! A real score! Would that be granite type?


Yes, 6” thick.  Stand included but I may make a new one with wheels, of course 3 point support.  FWIW, 4’ x 3’ x 6” of grante is just about 1000lbs.

Those satellite dishes and barcodes are an interesting technology.  Thanks for sharing.


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## Richard King 2 (Nov 29, 2021)

I just saw this and my name was mentioned.   I am to tired to respond in full now.   I do use the book as a reference all over the net and in my classes  them to time time plus I sell them on ebay . I have a lot of respect for it as much in valuable but much isn't. I don't know any professional rebuilder the needs it, but rookies do.  Lance Baltzley and Adam Booth (friends and students) have done You Tube shows on YT and they don't use it and teach how to lap plates.   
My Dad met Connelly.  I invited Connelly to lunch once but has had onsets of dementia then, so we never met in person, just talked briefly on the phone.  This was approx. 45 years ago.  good night.


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## graham-xrf (Nov 29, 2021)

Richard King 2 said:


> I just saw this and my name was mentioned.   I am to tired to respond in full now.   I do use the book as a reference all over the net and in my classes  them to time time plus I sell them on ebay . I have a lot of respect for it as much in valuable but much isn't. I don't know any professional rebuilder the needs it, but rookies do.  Lance Baltzley and Adam Booth (friends and students) have done You Tube shows on YT and they don't use it and teach how to lap plates.
> My Dad met Connelly.  I invited Connelly to lunch once but has had onsets of dementia then, so we never met in person, just talked briefly on the phone.  This was approx. 45 years ago.  good night.


Thanks Richard.
Citing you in the context of a conversation on the Connelly publication is, I hope, taken as something of an honourable mention.


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## rwm (Nov 29, 2021)

That is a very interesting idea to try to use tech to replace a surface plate. My guess is you will spend a lot more money and work harder for the same result? How would photogammetry be applied to such a precise surface? Would a digital camera have enough resolution?  What about something like interferometry? Maybe that would be too sensitive?


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## graham-xrf (Nov 29, 2021)

rwm said:


> That is a very interesting idea to try to use tech to replace a surface plate. My guess is you will spend a lot more money and work harder for the same result? How would photogammetry be applied to such a precise surface? Would a digital camera have enough resolution?  What about something like interferometry? Maybe that would be too sensitive?


As I understand it, get close enough with the right lens, enough to get images from different angles, the limit is probably in wavelengths of light. Some have experimented  with blue light, and microscope cameras. The resolution is decided by the images you take. I have not explored the trick extensively, being that I designed the surface, but did not have to make it. I did see one of the guys scan a real car, with it's front door taken off, and overlay a CAD wireframe image onto it, to exactly design a new hinge location arrangement. It included the animation of the door being swung open.

You are right in that a main aim is to use software and optical tricks to dispense with the need for a physical reference surface. To stay with the reference flat for a minute, it is already possible to use a CNC router-style assembly, fitted with an electronic transducer indicator, to measure over a surface plate, making a subtraction mapping calibration array, to effectively "zero out" all the deviations in the machine. The structure might have all sorts of inaccuracies, but the quality of the reference surface is in effect "captured" in the stored numbers of the calibration map. From then on, accurate measurements can be taken on a real object put under it. This method was explained to me by a work-experience student. It is the way software is used to calibrate all sorts of stuff, from the noise in an astronomical image dark current, to a full scale metrology machine.

Take it one step further, using the machine to measure it's own ups and downs by using a mounted mirror, and calibrating from a laser interferometry arrangement with an optical flat. The calibration mapping gets stored as before. Quite novel was the idea of measuring a surface with the machine, and then printing the result contours onto the surface with a mounted robotic inkjet printer head. Predictably, all those guys were mad keen on CNC, and 3D printing, and knew all about ball-screws. I freely admit, I was getting out of my depth!


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## Bi11Hudson (Nov 29, 2021)

Much (most) of the discussion is beyond me. I know enough to follow but nowhere near the skill, or even desire, to pursue the matter. My post is only in response to the original question of trueing a straightedge to a surface plate. In the history of things, it actually is the surface plate trued to a straightedge. 

The origin of a straight edge is well known with the use of three "bars" that are worked together to an "exact" match. How close the word "exact" is to true is an exercise for the user. In fiber optics work, devices are measured in microns.  To make that level of accuracy over a three foot distance is a matter of the type of marking "fluid" used. Nothing more except a *lot* of scraping.

.


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## Richard King 2 (Nov 29, 2021)

Here is an interesting / educational magazine article about Granite Plates and testers. Repeat-a-meter.
I am going to draw a picture like Graham did teaching you how to scrape long parts on a short part, be it a surface plate or a straight-edge. Tonight) 
 All these "new methods" are not new idea's as they have been done for years.  People are always trying to re-invent the wheel.  What I teach was passed down from Journeymen Scraping masters for generations.  I didn't just dream them up or guess at them.  Master scrapers have passed them down for years.  Don't you think all these new idea's have been thought of and tried hundreds of times before?  What has changed in test devices.  I did a race years ago at General Motors.   They had a 20' tall surface broach laying on it's back and I raced a new Hamar laser and my King-Way level and Starrett 199 level.  The test was to see who could level the machine to .0002" / 12" the fastest.  I won the race as setting up the laser and letting it warm up to use took 20 min. longer then me doing it the old fashion way.  


			Quality Digest Magazine


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## rwm (Nov 29, 2021)

Richard-That is a most interesting and educational article! After reading it twice, I am still fuzzy on the difference between flatness and repeatability? What am I missing? It it because flatness is indicated at single points and repeatability is based on a larger area of the plate?


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## Richard King 2 (Nov 30, 2021)

rwm said:


> Richard-That is a most interesting and educational article! After reading it twice, I am still fuzzy on the difference between flatness and repeatability? What am I missing? It it because flatness is indicated at single points and repeatability is based on a larger area of the plate?


Watch this and you will understand vs. me telling you.  Tom Lipton is also one of my scraping students.


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## Richard King 2 (Nov 30, 2021)

Here is another one. Your question is answered around min 15 -


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## Richard King 2 (Nov 30, 2021)

rwm said:


> Richard-That is a most interesting and educational article! After reading it twice, I am still fuzzy on the difference between flatness and repeatability? What am I missing? It it because flatness is indicated at single points and repeatability is based on a larger area of the plate?


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## graham-xrf (Nov 30, 2021)

Wow - I never thought the thread would so quickly become the filtered pure wisdom from the most experienced guys we know! It's now about getting the best from the primary reference one can have in a shop if we aspire to some precision.

It does not take much (distraction) to come across Robin Renzetti's Repeat-O-Meter, and it looks like something many of us might aspire to try and make. We know our effort would have to go some long way to approach Robin's extraordinary skill, and we might have to adapt to using one of those millionths dial guages in place of the electronic transducer, but it looks possible. There are other examples of DIY repeat-o-meters to be found on YT.






Since a Repeat-O-Meter does not measure flatness, there is also his auto-collimator. That kit, I agree, is probably out of reach for most of us, but then again, I am constantly surprised at what folk here can do! Robin does share all about his design and construction.






Without the benefit of autocollimator optics, there is always Connelly Section 9.12. which can be done with more simple equipment. One can use a straight-edge, which need not be perfect, but nor can it be "any old bit of iron". So long as one end stays constant relative to the other, and there is enough of reasonably good parts to spot on, there is a method to use it anyway. Basically, the flatter it is, the fewer spottings are needed.






[Edit - I have just seen from the description that @Richard King 2 helped ThunderDog do this check on his surface plate].


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## graham-xrf (Nov 30, 2021)

C-Bag said:


> I get it’s really easy to come up with an alt solution and go with it. But I understand this one parameter and without it I see no way to properly do a very basic check that will keep from down the wrong track. Which once you get off just goes further into the weeds.
> 
> But I’m certainly nobody and not an expert so shouldn’t have responded in the first place.


I disagree .. you are well ahead of me in your accumulated experience of working with this kit, and I do value your opinion.


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## C-Bag (Nov 30, 2021)

graham-xrf said:


> I disagree .. you are well ahead of me in your accumulated experience of working with this kit, and I do value your opinion.


Being an old guy who spent a lot of time with my grandfather when young I got used to taking seemingly simple ideas to heart. Some guys when an old guy tells them you shouldn't do something because it's a waste of time and won't work see it as a challenge. Most of the old hands I've learned from(not scraping unfortunately) spent more time telling me what not to do, than what to do. They knew the pitfalls of the noob. Those pearls of wisdom were not discounted or wasted on me because of my special connection to my grandfather. That's not to say I took everything as gospel. I asked a lot of questions to make sure their wisdom had a foundation and they understood why they were telling me it. That's why it took longer than how to do it.  It also made it "stick".  This is what I most got from Connolly. He spends so much time telling and explaining what and why not to do something.  I have very little actual time scraping but I've approached it like I do most things, I read and study long before I actually "do".  Being an autodidact has it's pitfalls in that you have to be careful to not learn bad habits because they are the hardest to break. 

So I'm not sure I have more accumulated experience than you do, but thanks for not completely discounting me.


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## graham-xrf (Dec 1, 2021)

OK - I have shelled out for a used Windley camelback, 48" x 5". I might have tried for something narrower (lighter), but this one I got for £340. It looks OK to use as is, so far as my needs go, but there is a scratch to one side, near one end. Actual detail condition I will only discover when I get it here.

To use, I will have to contrive some sort of hoist. The business of actually setting a thing like this down on a lathe ways without dinging anything is a little practical detail usually glossed over in the texts, which are content with a compact phrase like "spot the surface ".

No question - the nice 18 x 18 surface plate is looking too small to have much role in helping this SE along - unless the method @Richard King 2  mentioned lets us up work our way up to a reasonable, possibly improved somewhat, straightedge. I reckon, in the future, I will likely try for a granite about big enough on the diagonal to check it against.

I think, for a while, I am going to get back to XRF electronics - that will be after I finish fitting polyurethane insulation boards to the metal garage door. It is mundane stuff, I know, but it snowed on Monday, and I absolutely have to plug the heat leaks in this place. At least the new brush-strip draught (draft?) excluders have worked well. Not cozy, but reasonable comfortable. I am sure the surface plate will like it!


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## Weldingrod1 (Dec 1, 2021)

You will like the garage door insulation: it makes the door much quieter and minimizes what your neighbors have to listen to!

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