|Peter G. Shaw||24/05/2011 21:00:03|
986 forum posts
A little bit more ignorance coming your way, so please do not run away!
Right then, I've been having a go at scraping, and found it surprisingly easy - unless I am doing it all wrong that is, but we had better not go into that.
Now, my question is, with what should I apply the blue? I have used one piece of cloth which because of the coarseness of the weave left parallel lines of blue. I am now using a much finer cloth which does not leave lines. In both instances, it seems difficult to easily get an even spread of blue as it takes a lot of shifting to spread it. Furthermore, this stuff spreads and soaks into the cloth and eventually onto the fingers. The surface plate is either an old mirror or a lump of plate glass, both act the same.
Incidently, my scraping tool is, I think, an old knife sharpening tool, a bit like the circular ones the butcher uses, but this one is sort of triangular with a bevelled front edge. A bit of honing and it cuts remarkably well. I have also got a proper butchers type knife sharpening tool - circular, tiny longitudinal grooves, and slightly pointed. Stamped into it are the words "Cast Steel". I have no use for it as a knife sharpener, but I am wondering if it could also be used as a scraper. Thoughts?
Over to you.
Peter G. Shaw
|94 forum posts|
Engineers blue / micrometer blue can be applied with almost anything as the stuff is used to reveal the low spots by accumulating in the gaps between the reference surface and the part being scraped down to a good fit. The important thing is to be very - very - very economical in the application and to work the parts together well to spread the 'blue' so that pealing the two surfaces apart leaves the high spots a bright clean metal with the blue accumulated in the dips.
I know it is obvious but it is the bright parts that are scraped bit at a time with constant checking. I apply a little straight from the tube or with a match if working on very small parts. A little goes a very long way! It is also much easier to add blue than to remove it BUT you must use enough to cover all the lows. I suggest starting with a dab in the middle and observing how it spreads as you will very soon get the hang of it. The only difficult part is taking off very fine slivers from only the highest parts as you get near to perfection.
I made my scrapers from old files as I could get the teeth ground off on a surface grinder without drawing the temper of the files. The working end being ground to a gentle radius - estimated at around 100 mm and the surface honed to a good finish at right-angels to the flat surface of what was the business of the file. Fitted with a wooden handle for safety and ease of pushing away fine curls of metal. I also use one made from an 8 inch triangular file ground to one sharp edge for getting into corners along with two purchased from a tool merchant many years ago which are very nice for a final tickle. These files were very old and would have been cast steel since the aim is to shave away very small curls of metal at a time those groves in the butchers sharpener would have to be removed or you will leave loads of comparatively deep scratches.
I was taught to produce a flat surface by working three surfaces in simultaneously rotation to bring each one to a flat surface. The current trend is to use a reference surface against which the work is blued to reveal the high spots.
Just a note of caution I have hand problems due arthritis and no doubt scraping might have contributed. There is a thread on the CNC Zone concerning making a DIY power scraper out of a demolition / pendulum saw. These are available commercially but very expensive.
Just a point Peter are you going to be scraping the dovetails of the mill which I think is the subject of another thread on this site, If so are you absolutely sure the ground surface requires attention. The inclined surfaces of these machines are manufactured using a planning action and have loads of parallel groves running in the direction in which the way moves. These groves help with spreading lubrication. Improving the sloping surface finish will not IMO achieve an improvement in accuracy as it is the large flat area that carries the mating part that determines the accuracy of the way. The dovetail only guiding the surface and keeping the two surfaces in contact. Scraping was done as a routine when surface grinders were not common workshop equipment and when done properly is very good as the regularly mottled surface holds oil well. The modern approach is to surface grind the mating surfaces and use a low stick-slip oil with only large awkward bits being scraped either by hand or using hand held power tools to save wrist and finger joints of the poor sod doing the scraping.
Hope this helps. - Regards - Pat
Edited By Pat on 24/05/2011 21:58:17
|420 forum posts|
I used to use a bit of fine leather (chamois), wrapped around a champagne bottle cork. It is important that you drink the champagne first before you start scraping, then you feel not so embarrassed about the blue fingers...
Seems the site has a problem with the piccies, I see none.
|Peter G. Shaw||24/05/2011 22:45:05|
986 forum posts
Yes, it is all part of doing up the mill. At present I am making new gibs and am scraping the bearing side because I cannot mill a decent finish. Well, I can, but due to the curvature on the table, long pieces do not end up flat.
Your point about the longitudinal grooves on the dovetails is valid and taken as that is exactly how mine appear. However, I do want to ensure that the bearing surfaces are satisfactory with no lumps in item.
What I am doing is to spread the blue onto the flat surface, then gently rubbing the gib (in this instance) onto the blue, lifting off and scraping all the blue spots on the gib surface. This seems to be the exact opposite of what you are saying, yet it does seem to work as I am reasonably easily managing to correct and presumably flatten the surfaces. Flatten to the glass, that is.
I have made one edge each of three 150mm rulers straight by the three surfaces method. If nothing else, it got rid of the hollows in the middle of the edges.
Fascinating stuff this is. Well, maybe not to you, but it is to me as I have never done anything like this before.
I have not put up any pictures with this thread.
If I can find some chamois leather, or even if I can find some artificial chamois leather, I will try it and see if it is any better.
Point taken about the champagne, but I assume I should drink it at least 24 hours before using the cork, otherwise.....
Blue fingers. These clean up ok with a nail brush and soap or Swarfega, or both. But it is still messy. But then, if I had wanted clean hands, I would have taken up bridge, or dancing, certainly not engineering.
Peter G. Shaw
|32 forum posts|
Hi Peter, Sorry I cant help you with your scraping or bluing problem. but there is an
article in model engineers workshop issue June/July 1991by the late Bob Loader.
I dont know if you can still get the early issues of m.e.w , and I dont know the first
about computers so I couldnt email it to you, but there must be some one on this site
who would. I wish you luck with your scraping.
|Gordon W||25/05/2011 08:51:20|
|2011 forum posts|
Use a finger to apply the blue. I never realised there were other methods 'till reading this thread.
|94 forum posts|
I don't like the sound of this scraping off the blued bits. The 'blue' accumulates in the hollows i.e. gaps between the surfaces being rubbed together leaving the high parts clear of 'blue'. Thus you need to be removing the high spots so that you are achieving a uniform 'blue' which is indicated by a uniform mottling. If you have scraped proficiently the blued surface will look a bit like freshly oxidized galvanized sheet.
Are you rubbing the parts together hard enough? I ask this because it would be possible to use a thin coating of 'blue' to touch contact the mating parts with out any rubbing action to reveal the high spots. This is a valid technique for things like pipe flanges which only go together one way round but is not valid for scraping sliding surfaces flat. This is because you are interested in getting the surfaces flat over a length not just a mating pair that do not slide one on the other.
The final scrapping of the way would be to leave a pattern that is zig zag across the width to encourage to lubricant to be evenly distributed. Do not leave parallel marks to the direction of travel as these encourage scores to develop in use.
Hope this clarifies the use of 'blue' in both fitting of non sliding and sliding parts. If it is not clear try it using two bits of scrap.
Regards - Pat
|420 forum posts|
Hmm, some different opinions, and each one is a bit right in some sense.
In the actual issue of the (American) Home Shop Machinist magazine just started an article about scraping. It is fully clear the author talks about scraping the blue spots. I am also the proud owner of that famous Connelly book, and the same story there. just imagine what happens when you put a hollow, non-straight gib strip on a surface plate. The ends are touching and become blue; the hollow center does not.
However, when the surface becomes almost perfect, the picture changes and what Pat describes happens. I hope it is no crime to cite a sentence from that HSM article:
"(This photo) also shows an alternative spotting technique. Here, blue was applied to the work and the work rubbed on a clean area of the surface plate. Like a photographic negative, the results are reversed, with the high areas showing as bright spots...sometimes...easier to see, especially when doing fine work"
(Italics by me)
maybe I have to state also that I use a real surface plate (scraped cast iron), and have no experience how a slick glass plate works.
Btw. Peter, do you push the scraper or pull? I absolutely have no control when pushing, all I have done was with the pulling method.
362 forum posts
I am just trying to get my head around the method you have explained, I always assumed that you would scrape the blue though I have not done any scraping. I would have done the same as Peter describes.
Suppose I was scraping a gib strip and using a flat surface plate as a reference. The way I understand what you are saying is that I would blue the gib strip, rub this against a clean reference surface and the blue would be moved to the low points, therefore I should scrape the clean metal showing through. Is that right? Or does it matter which is blued, the reference surface or the gib?
As I am writing this I am beginning to think it does not matter which you blue or even if both are blued, so long as the blue is applied very sparingly the high spots on both will rub and clean away the blue from said high spots. The trick must be in having the blue applied very sparingly.
So the same would apply to a bearing?
|Gordon W||25/05/2011 16:16:34|
|2011 forum posts|
The way I was taught, is do not put blue on the bit that needs scraping. Put a very thin trace of blue on the reference surface, plate or shaft or whatever, rub both together, then scrape of the high spots as shown by the blue. Just checked my tiny tin of blue, it's Micrometer brand, and over 50 years old, so you don't need much.
|Peter G. Shaw||25/05/2011 20:33:56|
986 forum posts
I have two books which both briefly desribe scraping: both say to put the blue on the reference plate, apply the work to be scraped, and then scrape of the blued parts onthe work.
I have also looked up Bob Loaders article in MEW5 (thanks Colin - I don't understand why I didn't think to look) and his repeat in MEW105. Again both give the method above.
The tool itself has a front about 1.5mm, maybe 1.7mm deep and about 10mm wide which is curved, and roughly at right angles to the bottom surface. I say roughly because somewhere I have seen a suggestion that the angle between the front edge and the bottom surface should be slightly less than 90°. Although there is a second cutting edge above which is on the bevel, I do not use this.
I have found that by altering the cutting angle and/or the pressure, I can alter the amount removed. Reducing the pressure and the angle results in a finer cut.
The tool, despite its rather unconventional look, certainly appears to work. It is noticeable that when I first start, I usually only get blue coverage at say the middle, or the two ends. Gradually, the blue coverage extends to cover the full area and does not seem to take very long. In fact the major problem is that I find it very tiring standing still for long periods of time, so I am having to do this over a few days.
I am scraping alternately diagonally.
I am pushing the tool. To be honest, I cannot see how pulling the tool would work.
Peter G. Shaw
|94 forum posts|
The scraper is pushed forward with under firm pressure at a very low angle of attack. The square end of the scraper being only a few degrees off vertical. (positive rake) With a well honed front edge the scraper will remove fine curls if it is then worked with the other hand with a slight side to side motion. Brute force is not required if the edge of the scrapper is well honed and the material being removed is cast iron or brass. If the material has been induction hardened and left hard that is going to make for difficulties. This method is for fine removal of material from a surface that is nearing being perfectly flat. Scraping curved surfaces as in white metal big end bearings is a different ball game and requires shaped scrapers to stop excessive shavings being removed instead of controlled curls.
On small parts an on bearings the scrapper is often pulled using both hands and the thumbs to guide the motion relative to the edge. Difficult to describe and a technique that has died with the use of shell bearings and high load ball bearings other than for some specialized scenarios.
Over the last few years the tendency is for small machines to have induction hardened and ground ways. The surface of these should be respected and not touched with a scrapper or diamond lap or abrasive unless they are suspect. With a small machine the surfaces will have been ground at one setting of the machine and hence will be both flat and parallel. Yes it is possible that one side might have been done and the wheel changed without redoing the first side but this would be rare in a well supervised workshop.
The method of applying the engineers 'blue' uniformly over a surface and noting where 'blue' is transferred to the other part as a single touch is my experience only used when the surfaces are a long way from being flat or in the very rare cases where the surfaces can not be wiggled together. The detection sensitivity is very much reduced.
The method I have outlined will enable three surfaces to be flatted together as a set to a very high degree of accuracy with no other tools than a bad of blue a well honed scraper and a few rags. I was taught this by a crusty old fitter some fifty years ago.
Hope this helps as the process is simple and satisfying but hard on the hands if you do too much at one go. Regards - Pat
Edited By Pat on 25/05/2011 20:43:48
Edited By Pat on 25/05/2011 20:47:38
|94 forum posts|
In a word yes. Minimal amount of 'blue' surfaces well rubbed together and the lows will be revealed as blue patches and the highs as clean metal. It does not matter which surface is blued or if the coating is uniform but you must make sure that the 'blue' has had a chance to color in all the lows. a safety net is provided by only removing the high bright spots what are enclosed as islands in a blue sea.
Gordon if you rub the parts together the 'blue' accumulates in the gaps. Removing the bright spots will then take you further away from being flat not nearer. Some where along the line the spot contact and the rubbing methods have become mixed.
Spot contact is for parts that can not be rubbed or wiggled and you remove where the 'blue' touch points.
On surfaces that can be rubbed or wiggled better results can be obtained by using the 'blue' to indicate the hollows.
The end of my scraper is slightly rounded in plan and the edge is 90 degrees to the shaft. It is ground and polished on all faces. The triangular and curved bearing scrapers have 60 degree edges and are honed and polished on all faces.
Regards - Pat
|The Merry Miller||25/05/2011 21:05:31|
484 forum posts
After reading Gordon W's post I had to dash into the workshop to check out my little pot of blue.
Made or sold by "Giffen's (London) Ltd. NW10 "
Instructions for use printed on the outside of the little tin pot are:
"Apply blue very thinly and evenly with fingers or cloth to surface and rotate to determine high spots. Scrape and repeat until surface shows entire blue field"
This little pot of mine Gordon I had when I started my time in 1953, how time flies!
|94 forum posts|
Regards - Pat
Edited By Pat on 25/05/2011 21:22:00
|Jens Eirik Skogstad||25/05/2011 21:39:36|
375 forum posts
Hi, see these nice scraping method..
Edited By Jens Eirik Skogstad on 25/05/2011 21:45:11
|94 forum posts|
The high spots and the blue spots need sorting out. My background is in precision mechanisms and optics. Here the highs pots are found by rubbing the parts together and it is very obvious which is which and very little has to be removed from ground surfaces ex a surface grinder.
I think it matters not provided the high spots get less with each successive session - in which case you know which is which!
The videos which Jens has posted appear to be removing loads of metal as well as removing the parts that have collected color. Are the parts being scraped a long way from being flat? In which case why are they not machining the excess metal away? The heavy scraping is also leaving what looks to be a heavily pocked surface.
Yes a very light non parallel scraped pattern helps retain lubricant but in my experience a specialist lubricant of the anti stick slip sort on fine ground surfaces is so good I would not be inclined to apply a scraper just to make some oil pockets for oil that in my opinion is far too thick. This is not just a small machine issue as on bigger machines the introduction of forced oil or air between the ground surfaces used to be common were precision movement was required.
Sorry to be so persistent but I was made fifty odd years ago to make a surface plate using nothing other than some more chunks of cast iron - 'blue' - and a big scraper and a smaller one. How I hated that guy! It must have taken me less than a week as he would have held back my pay and that I would remember!
Regards - Pat
|Keith Wilkinson 1||03/06/2011 20:03:45|
|4 forum posts|
I am a retired engineer I served my apprenticfship on machine tools where scraping was the bedrock of accuracy ,The blue can be applied to the master surface and the mating component rubbed well ,the hardest wearing blue sections where the colour has gone almost black are the areas to scrape first and to the greatest extent then less so the paler hades where contact has been made.Wipe of the very light coloured bits of blue and re rub .Carry on doing this until the required surface bed is achieved.Then mottle or curl the surface using the scraper.Scrape in a different direction after each rub.
The flat scraper can be made by extruding the end of an old file then grinding to a good Keith Wilkinsonpolish to about 1/16 thick at its tip then grind at a ( v) and hone.
|94 forum posts|
Yes this is relatively easy provided you have a white wall and a darkened room. The laser can simply be obtained from a cheap laser pen pointer or on the internet. Obtain the appropriate goggles and ensure no visitors or provide them with suitable eye protection. Make it the rule never ever to look into a laser source no matter how low powered. All you need is to make or obtain a suitable reflector. The problem being to get the reflecting surface optically flat. A surface aluminised optical flat or a prism is ideal but a good quality hand bag mirror will get you a long way but will give a double image which will make marking the centre of the spot a bit more difficult. Also not being perfectly flat you must take care to keep the beam on the same area of the mirror throughout. (Using the same are of the reflector is just good practice.) Mount the reflector on an very solid jig that can be moved down the way. I have used a standard 'v' block for this in the past with success. Simply shine the laser at the reflector keeping the beam as near right angles to the reflector as possible. Mark the centre of the blob of light where it hits the wall. Now move the rig up the way. As it tilts assuming the way is not perfect the beam is deflected. The amount of deflection indicates both the direction of the tilt but also the magnitude. I have done this using a throw of 3 meters as a check on a very nice cast base when the proper test equipment way away for calibration. This method is also good for checking for level when combined with a simple water gauge when no fancy equipment is available.
The method I have described with the use of scrapping and engineers blue I have personally carried out on a large surface plate using no external references or tools other than the scrappers - blue - and the other two plates. All three are brought to the same degree of flatness by rotating the pairing of the surfaces and testing for truth. In this method it is only the areas where the blue is transferred from the all blued to the one to be scrapped in the wiggle that are removed. Obviously the removal of the high parts could be indicated as the bright cleaned part if the roles of the plates is reversed.
The method of referencing to a know good surface plate by excessive rubbing of the parts is OK only if you can rely on the flatness of the reference and are some way from true flatness / even finish. Much of the information on scrapping of machine beds is also aimed at generating a random pattern of highs and lows to encourage an oil film between the bed and the slide. IMOH this can be better achieved with ways that are as flat and well finished as you can get them and then to use either compressed air / oil or a mix injected under sufficient pressure to cause the table to start floating Or to use one of the lubricants specially developed to ensure thin film lubrication. (Rocal Ultra Glide is readily obtainable in the UK not to be confused with the thick oil often sold by too merchants as slide way lubricant ).
Have a go - Regards - Pat
Edited By Pat on 06/06/2011 21:54:31
|Clive Hartland||10/06/2011 14:35:35|
2476 forum posts
Working in the Instrument repair business I have only used the Laser as an alignment tool.
Mainly aligning the graticule of an optical system to the laser so that the beam can be projected to a distant point. Actually using the laser as a surface/level device is very crude and not accurate at all. Even in industry it only works to about 2.5mm tol. over 50mtrs. Laser beams are highly diffracted and hard to determine the centers. If the beam hits the graticule it will diffract and special grats. with the no markings in the center are used.
Examples are boresights for setting up gunsights and aligning the weapon to the mount in an aircraft or fixed mounting.
Another example is for Zenith and Nadir optical plumbs for mines and lift shafts.
Also used for the run of drainage beds to allow for the fall of the drainage.
Mention is made of Autocollimation which is accurate and the readings are half the reflected image.
Lots of applications use autocollimation such as a long line of rollors in a mill and alignment of structures in ship building. Specially made Theodolites with stiff axis are used and they can be mounted sideways/rightangles on a bulkhead.
Personally I have used Auto collimation to survey the bed of a ships engine that threw a con-rod, this distorted the crankshaft and that then distorted the bed.
I measured the bed every 300mm and also did it across the bed and was able to determine that the bed was shaped like a banana with a max. error of 237Thou. dip in the middle and that one side was 180Thou. higher then the other.
The Insurance paid out for a new bed and a new crankshaft.
There is a long story about this engine and the ship and when it happened.
The simplest method is a theodoilte with an autocollimation eyepiece fitted and a Titanium mirror fitted to a levelled mount or cube which is absolutely square and can be put in any position.
Examples of this are aligning Head up Displays in aircraft and some aircraft and ships weapon systems.
Back to scraping, part of my training was to scrape Inspectors Clinometers, this entailed scraping the base and then using a ground square, scraping the end face. The material was Bronze and it was easy to overscrape and make it worse. Then I had to stone the cam that rode the underface of the drum and this was all checked on a master Clinometer with a long swinging arm.
Later I was faced with levelling the trunnions of Theodolites, this I did on a face plate and lapped them in until there was no Tilting Axis error. This faceplate had little quarter inch squares and was loaded with diamond paste.
Later I lapped another surface plate to it and trued it up and I still use that plate today.
Optical flats are for determining the quality and flatness of a surface, can be used to see if the anvils on a micrometer are square to each other. They are easily scratched and can soon become useless. I only used them for inspection purposes.
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