The importance of precision lapping for valve plates in a free abrasive machining process cannot be overemphasized.
For instance, it can provide parallelism, flatness, thickness identity, and surface finishes to tolerances that are extremely demanding.
In the same vein, precision lapping can ensure that finished pieces are created without the introduction of heat and stress damage that are prevalent in other processes.
As a result, your finished product can benefit significantly from it and have cyclic fatigue resistance, physical strength, and high impact even at an economical rate.
It’s worthy to note that if valve plates have this high-level quality, they are reliable enough and won’t malfunction easily. In turn, it will not decrease the functionality of your compressor.
How are these made possible?
Let’s take a closer look.
What is Lapping?
A machining process where two surfaces have an abrasive between them while they are rubbed together slowly using a machine or hand is called lapping.
There are two forms of lapping and these are grinding and cutting of a harder material using a softer material.
1. Hard Material
In grinding, glass or any other brittle material is rubbed against another surface which could still be glass or iron.
The material which is rubbed on is called the grinding tool or lap.
The movement of glass on the iron or glass is carried out with an abrasive between both surfaces, which includes:
- Silicon carbide
- Optician’s rouge
- Emery
- Diamond, etc.
As a result, microscopic conchoidal fractures are produced due to the movement of abrasive between the two surfaces and the removal of material from either material.
2. Soft Material
The second form of lapping takes advantage of a softer material such as a ceramic or pitch which will be rubbed on and in this case, is the lap.
The material is charged with abrasive to enable it to cut a harder material.
What are Valve Plates?
Valve plates are used in compressor applications in a wide range of operating temperatures and gas compositions.
They serve as seals in various pressure zones within the compressor which has also earned them the name valve sealing elements.
Valve plates open as a result of the attainment of differential pressures between zones. They enable the high flow of gases from high-pressure regions to low-pressure regions during the compression cycle.
Precision Lapping for Valve Plates
A unique technique of precision lapping for valve plates is the free abrasive machining process.
In this process, an abrasive consisting of various compounds is mixed into a precision film layer of slurry before it is used to coat the lap plate.
The work part is then placed on the top of the lap in fixtures.
Next, the lap, as well as the work plates, begin to rotate which creates the cutting action at a controlled rate.
The components as they move do not come in contact with the lap’s plate surface and using powered lap plate rotation, cutting energy is transferred by the abrasive particles within the slurry layer.
In the end, the microscopic chips of material are removed from rubbing surfaces.
The process above is quite unique and many more can be used to ensure that the finished product is not susceptible to heat or stress damage. This damage would’ve been part and parcel of other processes including grinding.
Factors to Consider During the Lapping Process
When it comes to creating a high quality finished piece, great care has to be given to the selection of abrasive, speed of machine, pressure, and shape of the lap.
1. Type of Material
Great consideration has to be given to the type of material being processed.
These materials are components that will take part in lapping and they can ensure
- Surface flatness
- Precise size tolerances
- Surface roughness
Some of these materials are As-Fired 99.6% Alumina, Lapped Aluminum Nitride, Lapped Fused Silica, Polished 99.6% Alumina, amongst others.
2. Speed of Plate
The selected lap plate speed depending on the component’s aspect ratio must not cause the component’s surface to vibrate or oscillate the lap plate surface.
On the contrary, the surface that needs lapping must always sit firmly to the polishing plate surface.
3. The Flatness of Plate
The measurement of the lap plate’s flatness across its diameter can be dissimilar from the measurement of the component finished flatness specification.
The latter can be attributed to the fact that the component aligns to a small span of the geometric spherical radius of the lap plate.
The span of the component is defined by its diameter or the greatest length across its lapped surface.
As a result, a four-inch diameter component will be lapped less flat than a one-inch diameter component even when the same lap plate is used.
4. Pressure on Work-piece
Depending on the amount of applied pressure on the components that are lapped, it can affect the resulting surface roughness.
This effect is evident in the slurry film thickness being altered.
Its also worthy to note that a higher pressure can bring about a thinner film thickness but a greater chance of wiping the components on the lap plate.
5. Size and Type of Abrasive
Abrasive compounds such as polycrystalline diamond, mono-crystalline diamond, SiC, and B4C feature a crystal structure.
Due to this, their structure determines their:
- Shape
- Hardness
- Material’s friability
- The number of cutting edges
SiC, for instance, which is classified among harder compounds, has a greater depth of penetration on the component material under similar conditions of PSI load, than softer compounds.
In the same vein, there is fewer number of sharp cutting edges on a flat hexagonal crystal shape which also penetrates the component material less deeply. A tetrahedron shape with several protruding cutting edges cuts more deeply.
6. Abrasive to Vehicle Slurry Ratio
There is a need to select the correct ratio of the film strength and thickness during lapping in order to obtain the same surface roughness.
A film thickness that is too low will cause the component material to wipe more closely to the lap plate.
Another effect is that most of the cutting energy will be transferred to the component material through the abrasive particles. This could lead to severe scratching, greater penetration, and higher surface roughness.
Conclusion
The importance of precision lapping for valve plates are numerous and as such, great care has to be given during the machining process.
It begins with giving certain factors as outlined above your utmost consideration. This will ensure that your final valve plates have the best quality.
Also, it’ll be free from severe scratching, high surface roughness, and other unwanted elements.
Moreover, reliable precision lapping is one way to achieve a long life and low cost as a compressor manufacturer through the use of a compressor valve plate whose high thermal and dimensional stability is high.
