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)點接(jie)(jie)觸，此時(shi)外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)兩個(ge)測(ce)量(liang)(liang)面(mian)之間的(de)(de)距離，就是(shi)被測(ce)零(ling)(ling)件(jian)(jian)(jian)的(de)(de)外(wai)(wai)(wai)徑(jing)。所以(yi)，用(yong)外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)測(ce)量(liang)(liang)外(wai)(wai)(wai)徑(jing)，就是(shi)比較(jiao)外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)與零(ling)(ling)件(jian)(jian)(jian)外(wai)(wai)(wai)圓接(jie)(jie)觸的(de)(de)松(song)緊程度(du)，如圖(tu)1-6(b)以(yi)卡(ka)鉗(qian)(qian)的(de)(de)自重能(neng)剛(gang)好滑(hua)下為(wei)合適。如當卡(ka)鉗(qian)(qian)滑(hua)過(guo)外(wai)(wai)(wai)圓時(shi)，我們手中沒有接(jie)(jie)觸感覺，就說(shuo)明外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)比零(ling)(ling)件(jian)(jian)(jian)外(wai)(wai)(wai)徑(jing)尺(chi)寸大(da)(da)，如靠外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)的(de)(de)自重不(bu)能(neng)滑(hua)過(guo)零(ling)(ling)件(jian)(jian)(jian)外(wai)(wai)(wai)圓，就說(shuo)明外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)比零(ling)(ling)件(jian)(jian)(jian)外(wai)(wai)(wai)徑(jing)尺(chi)寸小。切不(bu)可將卡(ka)鉗(qian)(qian)歪斜地放上工件(jian)(jian)(jian)測(ce)量(liang)(liang)，這樣有誤差。圖(tu)1-6(c) 所示。由于卡(ka)鉗(qian)(qian)有彈性，把外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)用(yong)力壓過(guo)外(wai)(wai)(wai)圓是(shi)錯誤的(de)(de)，更不(bu)能(neng)把卡(ka)鉗(qian)(qian)橫著(zhu)卡(ka)上去(qu)，圖(tu)1-6(d) 所示。對于大(da)(da)尺(chi)寸的(de)(de)外(wai)(wai)(wai)卡(ka)鉗(qian)(qian)，靠它(ta)自重滑(hua)過(guo)零(ling)(ling)件(jian)(jian)(jian)外(wai)(wai)(wai)圓的(de)(de)測(ce)量(liang)(liang)壓力已經太大(da)(da)了，此時(shi)應托住卡(ka)鉗(qian)(qian)進(jin)行測(ce)量(liang)(liang)，圖(tu)1-6(e)所示。