description other objects features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings in which fig1 is a perspective view of a preferred automatically retractable stop apparatus according to this invention mounted on a machine tool table and interengaged with a part to be machined fig2 is an exploded perspective view of the stop apparatus fig3 is an elevational end view of the stop apparatus with the stop bar in an extended operational position fig4 is an elevational side view of the stop apparatus in its retracted condition the collar is removed to illustrate the retracting spring used by the apparatus fig5 is a view similar to fig4 of the stop apparatus with the stop bar in its extended operational condition fig6 is an elevational view of the opposite side of the stop apparatus with the cap of the pivot axle removed to illustrate the interengagement between the movement limiting dowel pins the apparatus is illustrated in the retracted condition fig7 is a view similar to fig6 but illustrating the apparatus with the stop bar in the extended operational condition and fig8 is a plan view of a pair of opposing stop mechanisms positioned on a machine table top on respective sides of a clamp vise there is shown in fig1 an automatically retractable stop apparatus that is operably mounted to the platform or.