Granite mounting plates serve as the benchmark carrier for ultra-precision equipment. Apart from flatness indicators, the machining quality of various positioning holes, mounting through-holes, threaded holes, counterbore holes and special-shaped holes directly determines the alignment accuracy of complete machine assembly. Many people mistakenly believe that granite plate processing only involves grinding and flattening. In fact, hole-making on granite belongs to ultra-precision machining of hard brittle materials. Each working procedure carries unique risks, and this is also the core dividing line separating ordinary stone processing factories from high-end ultra-precision component manufacturers.
Natural black granite features high brittleness and tiny internal grain boundaries. During drilling, reaming, tapping and counterboring, defects such as edge chipping, corner breakage, microcracks on hole walls, hole position deviation and hole taper tend to occur easily. Once invisible internal microcracks are generated, the plate will slowly release stress under alternating temperature and continuous equipment load, resulting in dimensional drift of hole positions and ultimately damaging the long-term positioning accuracy of the whole set of equipment. This is the most fundamental and fatal difficulty for granite hole-making, which is essentially different from CNC machining of metal parts.
The first difficulty lies in coordinated control of hole geometric accuracy and geometric tolerances. For a group of mounting holes on precision granite mounting plates, it is not enough to merely guarantee qualified bore diameter of individual holes. Strict control of hole spacing tolerance, perpendicularity between hole axis and plate surface, and parallelism among multiple holes is required. Ordinary stone processing only checks bore size while ignoring the perpendicularity of hole axes. After machining, lateral stress will be generated when bolts are tightened, causing tiny deformation of the whole benchmark plate and destroying the finished flatness of the plate surface. Especially for large-size granite mounting plates, the heavy self-weight of the plate leads to uneven clamping force. Slight deformation of the plate during machining will bring hole position deviation. To ensure the positional accuracy of multi-group hole arrays, positioning and machining must be completed in a low-vibration constant-temperature environment to avoid errors caused by clamping stress.
The second difficulty is suppression of edge chipping and microcracks during machining. Granite is formed by mineral crystal aggregation, with high hardness yet high brittleness. Chipping is most likely to happen at the two end faces where the drill bit cuts into and exits the plate. Cutting heat generated by high-speed cutting is also a hazard. Sudden local temperature rise produces thermal stress at grain boundaries and induces internal microcracks. Improper conventional water cooling may cause thermal shock and accelerate crack propagation. To avoid such defects, special diamond cutting tools must be matched, feed speed and rotation speed optimized, a progressive layered cutting process adopted, cutting temperature controlled, and vibration monitored throughout machining to prevent impact cutting. Many low-end manufacturers simplify processes. No damage can be seen visually after machining, but internal microcracks have already hidden risks of long-term precision failure.
The third difficulty refers to machining of special hole types and subsequent hole wall treatment. Counterbore holes, stepped holes, blind holes and pre-embedded threaded holes are common types on granite mounting plates. Direct tapping cannot be performed on granite itself. Threaded holes generally adopt metal thread bushing pre-embedding technology. The depth of bushing pressing, coaxiality and preload control are extremely strict. Improper assembly of bushings will result in loosening and tilting after long-time locking, directly changing the equipment assembly benchmark. For counterbore holes, the bottom surface of counterbores must stay parallel to the plate surface with tight tolerance on counterbore depth. Uneven depth will generate local stress when fasteners are tightened and pull the plate to deform. Such composite hole machining involves a long process chain, and tiny errors in each step will be superimposed and amplified.
The fourth difficulty is stress release and re-inspection calibration after machining. Hole cutting introduces machining stress around holes. Even if the appearance remains intact, stress will be released gradually and cause changes in hole positions and flatness. Therefore, products cannot be delivered right after machining. Aging treatment is required to wait for stress release, followed by multi-dimensional re-measurement and calibration. Re-inspection should not only check bore diameter, but also re-verify plate flatness, hole coordinates and hole axis perpendicularity. Once machining stress is introduced during hole-making, the originally qualified plate flatness will be damaged, and previous grinding work will be wasted. That is why ultra-precision granite components must be machined, aged and inspected in a constant-temperature, anti-vibration and dust-free environment.
Material, cutting tool, environment, clamping, aging and inspection are closely linked, forming the technical barrier for granite hole machining. Through long-term project practice, UNPARALLELED has built a dedicated machining process system targeting granite hole-making pain points to avoid microcracks, edge chipping and stress deformation. The finished granite mounting plates can maintain nano-level benchmark planes as well as meet geometric tolerance requirements of all hole arrays for assembly of high-end equipment including semiconductors, optical inspection machines, CMMs and linear motor platforms.