Valve Grinding – Selecting Abrasive Materials
The quality of abrasive materials is difficult to judge by just looking at the product. For instance, your nearest DIY store will probably have a fine selection of abrasive materials. None of these will, in all likelihood, be suitable for valve grinding.
Abrasives on a paper or cloth backing are generally intended for use with traditional belt or disc grinders. In this type of operation, each individual grain is only engaged in the cutting operation for a fraction of a second, before moving away from the working area. It will have plenty of time to cool down before returning to work.
Grinding dust is removed from the area by the same action, keeping the grains clean.
Nearly all the heat generated by friction is removed with the grinding dust, contributing further to the cooling of the abrasive material.
All of this has a positive effect on the service life of the grains, helping them stay sharp for longer.
When using a valve grinding machine, the abrasive material, with its backing, is usually attached to a rotating metal disc, using double sided adhesive tape.
The size of the grinding disc is mainly limited by the available space inside the valve body, but technical considerations also play a part. This includes, for instance, the size of the disc bearings, affecting stability and flatness.
The size of the motor is another such consideration. A disc with wide diameter requires a larger motor, which in turn affects other parts of the design.
All these factors combined result in valve grinding machines having comparatively small grinding discs, restricting the number of grains on the available surface of the backing.
This means each individual grain has to work harder. The abrasive never leaves the working area, remaining in constant contact. The cooling is practically non-existent.
In addition, the dust generated by the grinding has nowhere to go and stays in the working area. This not only reduces cooling but also clogs up the abrasive material.
If possible, always use a grinding disc significantly wider than the seat. This will enable the abrasive to reach over the side and remove the dust. However, due to space constraints, this may not always be possible.
Valve grinding material – Three main components
Fortunately, when grinding valves, the grinding speed in is relatively low, which is the reason it still works as well as it does.
But it does place great demands on the abrasive material. Lower quality abrasives, such as those stocked by your local DIY store, are simply not suited for this type of work.
The abrasive has three main components that work together:
- The abrasive material or grit – This needs to be of right quality to cut through hard metal while at the same time absorbing heat without losing efficiency.
- The backing – The backing provides stability
- The resin – The resin holds the grains together and attaches them to the backing. Additionally, modern resins aid in the cooling process.
Developing abrasives to meet these requirements is costly and time-consuming business. Not surprisingly, abrasives suitable for valve grinding tend to cost substantially more than standard abrasives. But they could make the difference between the success or failure of a job.
LarsLap has chosen to work with two specific manufacturers of abrasives, 3M and a grinding material with the proprietary name Ceramix.
3M manufactures a product called Micron paper. The backing can be made from various materials – LarsLap mainly uses imperial micro finishing film. The abrasive is aluminium oxide. LarsLap mainly uses 100, 80, 60, 40, 30, 15 and 9 μm. The material comes with a self adhesive backing.
This type of abrasive is manufactured using grains with exactly the same size, spaced out equidistantly on the backing. The abrasive achieves an even, smooth finish and is suitable for dense surfaces.
Ceramix comes in two ranges of grit abrasives for industrial grinding, SPX and SPY.
X and Y indicates the material of the backing. Both are made from cloth, but Y is slightly more rigid and is used for coarsest grade.
Ceramix abrasives are originally designed for use with belt grinders and are manufactured without self adhesive backing. LarsLap adds the self adhesive backing and produces grinding discs in appropriate sizes.
The adhesive is formulated in cooperation with the manufacturer to withstand the heat generated. The abrasive is called CO, Ceramic Oxide, which Is a proprietary formulation from the manufacturer.
Grit vs. Micron
Grit abrasive is manufactured by attaching grit up to a certain size in an irregular pattern onto the backing. This results in very effective abrasion but a less smooth finish compared to micron abrasives.
The grit is made from aluminium oxide and ceramic materials. LarsLap mainly uses 40, 80, 120 and 180 grit.
Micron and Ceramix papers complement each other in the LarsLap system. The greatest issue to most users is the difficulty in comparing the two ranges.
Grit abrasives are arranged on a descending scale – a low number gives a coarse cut and high abrasion.
Micron, by contrast, uses an ascending scale – a low number gives a fine cut with little abrasion.
80 grit and 80 micron have roughly the same abrasion. But from that point onwards, the two scales move in opposite directions.
The reason for this is that the Micron system indicates the size of the grain. A low number indicates a small grain size. The grit system indicates the number of grains within a certain area. Larger grains give a lower grain count. 40μm is a very fine abrasive, while 40 grit is very coarse.
|Very fine grades|
Breaking the paper in
The choice of material and grit depends on the seat material and the degree of seat damage.
When using grit paper, the larger grains will be the first to start cutting. Excessive force in combination with new, coarse paper can cause deep scratches that damage the valve seat. To avoid this, the paper can first be broken in by using it on a cast iron surface.
But before starting with a coarse grade of paper, it is wise to begin with a finer grit to avoid making deeper marks in the surface than those that are there already, potentially making more work for yourself and shortening the life of the valve. Once the finer paper has been used for a short space of time, it will be easy to see if there is a need for a coarser grain size,
It is important to change the paper at regular intervals and to use the correct abrasive for the seat material. Otherwise, as the paper wears, it will ride on the worn-out grains and dust, preventing the fresh grains from cutting the surface.
Grit paper for valve grinding
Ceramix works particularly well on hard seat materials, such as Stellite and hard alloys. When using Ceramix, the grinding must be monitored very thoroughly, as this is very effective at removing material. Ceramix can be frequently be used with good results in many cases where, in the past, diamond or CBN was required.
Micron paper for valve grinding
When using Micron paper, all the grains are exactly the same size. They will start cutting simultaneously and wear uniformly. Micron paper is only available in medium to fine grain sizes, making it perfect for grinding a fine surface finish or even lapping of very fine surfaces.
Micron paper is effective for use on all seat materials. However, for the hardest and most wear resistant alloys, the Micron paper may not be the first choice, as the aluminium oxide tends be less effective at cutting through surfaces harder than 48-50Hrc. Instead, excess friction is created, causing heat that risks loosening the adhesive at the back of the paper. Micron paper can also be used for wet grinding.
Diamond and CBN abrasives
Comparison of the hardness between different materials.
For hard seat materials, diamond grinding material can be particularly effective. Diamond material is available as permanent grinding plates and heads as well as disposable adhesive paper and grinding paste. It is available in uniform grits, each grain being exactly the same size.
If a mirror finish on the seat is required, special adapters can be used for lapping the surface with a diamond liquid or paste.
In particularly demanding applications, cubic boron nitride (CBN) abrasive may be the answer. CBN can achieve results in applications where even diamond fails to make an impact.
As both diamond and CBN are extremely hard materials, they are suitable for materials that can be difficult or even impossible to grind with conventional abrasives. Both materials have the same structure, but diamond is made up of pure carbon and CBN consists of boron and nitrogen.
Diamond valve grinding wheels
Diamond is extremely hard. It is particularly well suited for the following materials:
- All carbide metal grades
- Oxide and non-oxide ceramics
- Hard facing alloys
- PCD / PCB
- Fiber reinforced synthetics
- Sapphire, glass
- Precious and semi-precious stones
Steel has a high affinity to carbon. This means diamond is not suitable for machining steel, as it consists of pure carbon. Due to the high temperature generated during grinding, the steel will absorb carbon atoms from the diamond and erode the grit.
CBN valve grinding wheels
CBN is made up of boron and nitrogen. As it does not contain any carbon, it is suitable for machining steel. CBN is better for the following:
- Hardened steel over approx. 54 HRc hardness
- Nickel-based special alloys
- High-speed steel (HSS)
Low temperature for diamond
Diamond is sensitive to high temperature, so the working area should be kept cool. The easiest way to achieve this is to apply only light force. This reduces excessive wear of the grinding material. Diamond is much harder than the surrounding metal and it does not take much force for it to be effective. Pressing harder will not make the grinding faster. Too much force only risks loosening diamond grit from the tool.
Also, select a grinding tool size that allows the grit to leave the work area. Any build up of dust in the grinding tool will increases the temperature through increased friction. For instance. If the seat is 25mm (1”) wide, a Ø50mm (2”) grinding tool or larger could be used.
CBN less sensitive to heat
While CBN is less temperature sensitive than diamond, the grinding force is still important. This should still be light – the force is not related to the grinding performance. To achieve the best performance, the grinding procedure should be the same as for diamond, as should the selection of grinding tool size.
CBN also has an advantage in its thermal stability. Diamond loses a lot of its hardness at around 700°C, but the hardness of CBN remains unchanged at well over 1000°C.
Finally, to achieve a high surface finish, lap the surface using abrasive suspension, paste or powder.