The shaft is the part of a gear drive that buyers most often underestimate. A gear can be cut to a perfect profile, but if the shaft that carries it is out of tolerance, the assembly rattles, overheats or fails within weeks. Drawings use compact symbols like g6, k6 and H7, and they are not hard to read once you know what they mean — understanding them is what separates a smooth inquiry from a week of clarification emails.
Machining a gear shaft follows a standard sequence. The blank is turned on a CNC lathe to create the steps, diameters, undercuts and end faces; then the part is heat-treated if the application needs strength or wear resistance; then the bearing and gear seats are ground on a cylindrical grinder to reach the tight tolerance classes; finally keyways, splines or threads are cut, and the shaft is checked on roundness, cylindricity and runout. Each step leaves material for the next, which is why a shaft drawing with no tolerance still cannot be quoted "by the sketch" alone.
Tolerance classes are the language of fits. In a drawing, a dimension such as Φ35H7 describes the bore of a gear or pulley, while Φ35k6 or Φ33g6 describes a shaft seat. The letter gives the position of the tolerance zone and the number gives its grade. H7 bores are the default for press and transition fits, k6 shaft seats give a light interference suitable for gears that transmit torque, and g6 shaft seats give a controlled clearance for bearings. When the same letter and number appear on both parts, a machinist can tell immediately whether the pair will assemble by hand, by press or by heating.
Grinding is what makes tight classes achievable. g6 allows only a few microns of deviation across the whole diameter, and cylindrical grinding is the only practical way to hold that on a production basis. Grinding also corrects the distortion caused by hardening, and it is where roundness, cylindricity and runout are actually controlled. A shaft that is turned but not ground can rarely meet g6 or k6, so the presence of a tolerance class on the drawing tells you the process chain in advance.
The questions buyers should ask are the questions that catch problems early: which seats are bearing fits and which are gear fits, whether the shaft is hardened and to what depth, which keyways or splines are functional, and whether concentricity between the gear seat and the bearing seats matters. Giving the supplier a drawing with these marked, or simply stating the fit classes, removes almost all guesswork from the quote.
At our factory the whole chain is in-house — CNC turning, machining centers for keyways and splines, cylindrical grinding for g6 and k6 seats, and full dimensional inspection before dispatch. Send us your shaft drawing with the fit classes marked, and we will confirm the process chain and give you a quotation within 24 hours.