What Is The Hand Scraping Technique

MAXNOVO MACHINE Tips :  Hand Scraping is a special technique that is even more difficult than woodcarving. It is the starting point for precision tooling machines to have a basic level of accuracy. Hand Scraping precludes us from relying on other tooling machines and eliminates deviations caused by clamping forces and heat. Although nowadays the manufacturing technology is very advanced, but many machines’ beds still need to Hand scraping, this is why? The following is a detailed description of this artful way of craftsmanship. This is thanks to excellent lubrication. The spatula technician needs to know many techniques, but only experience will allow him to master the feeling of achieving that precise leveling.  

The metal scraper craftsmanship is considered one of the greatest assets in machine tool industry. To make the surface perfectly flat and smooth, we repeatively check and scrap by hand with time-proven techniques that passed on to generations.

Hand scraping is a manual process of truing and providing texture to mated surfaces in a machine tool. It is most often performed using a flat scraper, which is a hand tool with a flat-edged tip similar to a wood-carving tool. The person performing the operation holds the tip of the scraper firmly against the surface to be worked with one hand while grasping the tool handle with the other, thrusting the tool against the surface with powerful strokes using the body’s weight in order to create a pattern.

At MAXNOVO MACHINE, hand scraping is meticulously performed on every machine we build. That is because there’s no substitute for hand scraping when it comes to maintaining high levels of machining accuracy and sustaining a long, stable and productive life for your machine. We strictly enforced that all our contact surfaces must be scrapped with at least 20 spots per square inch and 40% of contact rate.

Shovelled rails are less prone to wear, and when you pass by a tool and machine shop and see the technicians doing the Hand Scraping by hand, you can't help but wonder, "Can they really improve these currently machine finished surfaces by Hand Scraping them? ( Would people be better than machines? )"

If you're referring purely to its appearance, then our answer is "no", we wouldn't make it any prettier, but why bother Hand Scraping at all? There are reasons, of course, and one of them is the human factor: the purpose of a tooling machine is to make other tooling machines, but it will never be able to reproduce a product that is more accurate than the original. Therefore, in order to make a machine that is more accurate than the original, we must have a new starting point, i.e., we must start with human effort, which, in this case, means Scraping by hand.

Hand Scraping is not a "freehand", "do-as-you-want" operation, it is actually a method of reproduction, a near-perfect replica of the parent body, which is a standardised plane that is also created by hand.

Hand Scraping is hard work, but it is a skill (a skill at the level of art); it is perhaps more difficult to train a shoveller than a woodcarver, and there are not many books on the market that discuss this subject, especially not on "why Hand Scraping". This may be the reason why Hand Scraping is regarded as an art.

Where to start ?

If a maker decides to grind with a grinder instead of a spade, the rails of his "mother" grinder must be more accurate than those of the new grinder. So where does the accuracy of the first machine come from? It would have to come from a more accurate machine, or it would have to rely on an alternative method of producing a truly flat surface, or perhaps it would have to be copied from a flat surface that was already well made.

We can illustrate the process of surface generation with three methods of drawing circles (although circles are lines and not surfaces, they can be cited to illustrate ideas). A craftsman can draw a perfect circle with an ordinary compass; if he traces along a circular hole in a plastic moulding with a pencil he copies all the inaccuracies of the hole; if he draws the circle with his bare hands the accuracy of the circle is determined by his limited skill.

Theoretically, a perfectly flat surface can be created by the alternating friction of three surfaces (Lapping). For simplicity, let's illustrate this with three rocks that each have a fairly flat surface. If you rub the three surfaces alternately in a random order, you will make the three surfaces flatter and flatter. If you rub only two rocks, you get a mating pair of one concave and one convex. In practice, in addition to using a spatula instead (Lapping), you will also follow a clear order of mating pairs, and the spatula maker will generally use this rule for the standard fixture (straight gauge or flat plate) that he is going to use.

To do this, the spatula maker applies a colour developer to the standard fixture and then slides it over the surface of the workpiece so that the area to be spatula'd is revealed. By repeating this action, the surface of the workpiece becomes closer and closer to the standard fixture, and the final result is a perfect replica of the standard fixture.

Castings that are to be roughed are usually milled to within a few thousandths of their final dimensions, then heat-treated to release the residual pressure, and then sent back for a clean surface grinding before being roughed. Although Hand Scraping is time-consuming and labour-intensive, Hand Scraping can be a substitute for processes that require high equipment costs, and if Hand Scraping is not an alternative, the workpiece must be finished with a highly accurate and expensive machine. In the final stage of the finishing process in addition to the high cost of equipment involved, there is a factor to consider, parts processing, especially large castings, often need to carry out some gravity clamping action, when the processing of a few thousandths of the precision, the clamping force often cause the workpiece distortion, so as to jeopardize the accuracy of the workpiece in the release of the clamping force after the accuracy of the workpiece; processing of the heat generated will also cause the workpiece distortion. Distortion of the workpiece. This is one of the many advantages of the spatula, which has no clamping force and generates almost zero heat. Cast iron is supported at three points to ensure that it is not distorted by its own weight.

When a tooling machine's spade track wears out, it can be reconditioned by re-sharpening, which is a big advantage over throwing the machine away or sending it to the factory to be dismantled and reworked.

When a tooling machine's rails need to be resurfaced, this can be done by the factory's maintenance staff, but we can also find someone locally to do the resurfacing. In some cases, both manual and motorized resurfacing can be used to obtain the final geometric accuracy required. If there is a set of table and saddle rails that have been shoveled flat and the accuracy has been met, only to find that the table is out of parallel to the spindle (which would take a lot of work to correct), can you imagine the degree of skill required to use just a single shoveller that can remove the right amount of metal in the right place without losing flatness and with proper correction of alignment errors?

This was certainly not the original purpose of the spatula, nor should it be done as a method of correcting large alignment errors, but a skilled spatula maker can accomplish this type of correction in a surprisingly short period of time. This method, although it requires proficiency, is much more economical than machining a whole bunch of parts to great accuracy, or going to some reliable or adjustable design to prevent alignment errors.

Improvements in Lubrication

Practical experience has proved that spade track can be reduced by better quality lubrication, but there is little agreement as to why this is so. The most common opinion is that the low points of the spade (or more specifically, the pits that are cut out and the extra pockets of oil made for lubrication) provide many tiny pockets of oil that are scraped out by the many tiny high points around them.

Another way to logically put it is that it allows us to maintain a film of oil consistently, allowing moving parts to float on top of it, which is the goal of all lubrication. The main reason why this happens is that these irregular pockets of oil create a lot of space for oil to stay, making it less likely to run off The ideal situation for lubrication is to maintain a film of oil between two perfectly smooth surfaces, but then you have to deal with preventing the oil from running off, or it needs to be replenished as quickly as possible. (Oil grooves are usually made on track surfaces, with or without spade, to aid oil distribution).

Such a statement would call into question the issue of the effectiveness of the contact area. Hand Scraping reduces the contact area, but creates an even distribution, and the distribution is the point. The flatter the two matching surfaces are, the more evenly distributed the contact area will be. But there is a principle in mechanics that says "friction is independent of area", which means that it takes the same amount of force to move the table whether the contact area is 10 or 100 square inches. (Wear and tear is a different matter; the smaller the area under the same load, the faster the wear and tear.)

The point is that we are looking for better lubrication, not more or less contact area. If the lubrication is perfect, the track surface will never wear. If a table has difficulty moving as it wears, it is probably related to the lubrication, not the contact area.

How is Hand Scraping done ?

The purpose of this section is not to teach the art of Hand Scraping, but to give you an idea of the process of Hand Scraping. Although the actual operation is difficult, the concept behind the operation is quite easy.

Before finding the high spots that must be removed, apply the colour developer to a standard fixture (a flat plate or a straight gauge fixture for V-rails), rub the standard fixture with the colour developer on the surface of the track to be shaved, the colour developer will be transferred to the high spots on the surface of the track, and a special shaving tool will be used to remove the high spots that have developed the colour. This is repeated until the track surface shows an even and consistent transfer.

Hand Scraping is performed on six (or more) Machine Components.

A Hand Scraping craftsmen must, of course, be proficient in various techniques. Here, I'll discuss two of them:

Firstly, before performing the highlighting action, we typically use a blunt file to gently rub the surface of the workpiece to remove burrs.

Secondly, the surface should be wiped with a brush or by hand, never with a rag. Using a cloth to wipe may leave fine linen threads that could mislead during the next highlighting operation.

Hand Scraping Craftsmen themselves will compare their work with standard fixtures and track surfaces to check for accuracy. The inspector only needs to inform the craftsmen when to stop the operation; there's no need to worry about the shoveling process. (The craftsmen can take responsibility for the quality of their work.)

We used to have a set of standards as to how many high spots there should be per square inch, and what percentage of the total area should be in contact; but we have found it almost impossible to check the area of contact, and it is now left to the spatula maker to determine the number of dots per square inch that should be in contact. In any case, the spatula maker generally endeavours to achieve a standard of 20 to 30 points per square inch.

Nowadays, some of the levelling operations in the Hand Scraping process are done using electric shovellers, they are also a form of manual Hand Scraping but can eliminate some of the strenuous work and make the Hand Scraping operation comparatively less tiring. There is no substitute for the feeling that Hand Scraping produce when you are doing the most delicate assembly work. 

Hand scraping adds some cost to the machine building process. However, by fine-tuning the machine so that it operates in balanced harmony rather than creating wear and tear on itself and on parts, more, higher-quality parts are produced, and the machine lasts longer. At MAXNOVO MACHINE, we do not embrace a “disposable parts, disposable machine” philosophy. We do hand scraping because we know that lifetime cost of ownership and cost-per-part are reduced when you produce more, quality parts over a longer time frame.

To learn more about hand scraping, welcome to contact us or visit the MAXNOVO MACHINE website at www.maxnovocnc.com