用巳在鋼直尺上取好尺寸的外卡鉗去測量外徑時，要使兩個測量面的聯線垂直零件的軸線，靠外卡鉗的自重滑過零件外圓時，我們手中的感覺應該是外卡鉗與零件外圓正好是(shi)(shi)點接(jie)觸，此(ci)(ci)時外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)兩個測量面之(zhi)間的(de)(de)(de)距離，就(jiu)是(shi)(shi)被測零件的(de)(de)(de)外(wai)徑。所以(yi)，用外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)測量外(wai)徑，就(jiu)是(shi)(shi)比較外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)與零件外(wai)圓(yuan)(yuan)接(jie)觸的(de)(de)(de)松(song)緊程度，如圖1-6(b)以(yi)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)的(de)(de)(de)自(zi)(zi)重能(neng)剛好滑(hua)下(xia)為合適(shi)。如當卡(ka)(ka)鉗(qian)(qian)(qian)(qian)滑(hua)過外(wai)圓(yuan)(yuan)時，我(wo)們手中(zhong)沒(mei)有(you)接(jie)觸感覺，就(jiu)說明外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)比零件外(wai)徑尺寸大(da)(da)，如靠外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)的(de)(de)(de)自(zi)(zi)重不能(neng)滑(hua)過零件外(wai)圓(yuan)(yuan)，就(jiu)說明外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)比零件外(wai)徑尺寸小。切不可將卡(ka)(ka)鉗(qian)(qian)(qian)(qian)歪斜地放(fang)上工件測量，這樣有(you)誤差。圖1-6(c) 所示(shi)(shi)。由于(yu)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)有(you)彈(dan)性，把(ba)外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)用力壓過外(wai)圓(yuan)(yuan)是(shi)(shi)錯(cuo)誤的(de)(de)(de)，更不能(neng)把(ba)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)橫(heng)著卡(ka)(ka)上去，圖1-6(d) 所示(shi)(shi)。對于(yu)大(da)(da)尺寸的(de)(de)(de)外(wai)卡(ka)(ka)鉗(qian)(qian)(qian)(qian)，靠它自(zi)(zi)重滑(hua)過零件外(wai)圓(yuan)(yuan)的(de)(de)(de)測量壓力已經(jing)太大(da)(da)了，此(ci)(ci)時應托住卡(ka)(ka)鉗(qian)(qian)(qian)(qian)進行測量，圖1-6(e)所示(shi)(shi)。