Hi Peter

 

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

Use a finger to apply the blue. I never realised there were other methods ’till reading this thread.

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.

Hi Hansrudolf

 

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

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!

Hi, see these nice scraping method..

 

 

 

Hi

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.

Hi Michael

 

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

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.

 

Clive

Posted by Keith Wilkinson 1 on 10/06/2011 16:46:41:
I return to this subject again .The machine tools i worked on were large precision lathes 6 to 8 foot centre lathes up to a 100 feet or more in length each section of bed had 4 or more slideways so had to be accurate across the bed as well as along it.The surface plates we used were lifted and moved by overhead crane the surface finish acceptable was 10 to 12 spots of blue per sq inch as accepted by the inspectors.The surface retention of oil and the finiished no of Spots per Sq In in the final assessment were achieved by Mottling or curling with the scraper

 

I’m with you on this Keith. We were always taught to blue the face you weren’t scraping because that’s the standard you are working to. The blue is transferred to the high spots on the part you are scraping and you scrape to remove the blue. You shouldn’t have to rub something that hard it squeezes the blue out or for that long it wears it away; you’d never make your bonus if you did!!

Scrapers should be honed with a fine slip stone on a regular basis; depending upon the material you are scraping they do sometimes lose their edge fairly quickly and then become more difficult to control.

 

John.