It is known that heat checking occurs when grinding under the following conditions: spindle speed of 4000 rpm, wheel diameter of 10 in., and depth of cut of 0.0015 in., and a feed rate of 50 ft/min. For this reason, the spindle speed is to be kept at 3500 rpm. If a (new, 8-in.-diameter wheel is now used, what should be the spindle speed before heat checking occurs? What spindle speed should be used to maintain the same grinding temperatures as those encountered with the existing operating conditions?