Screw, M4x6 Flat Head Phillips Machine 

In your instruction manual, this part appears on the parts list as:

Item No	Models
12	Harbor Freight 47158 Central Machinery Micro Mill Clarke CMD10 Metalworker Micro Mill Various X1-US SIEG Micro Mill
148	Grizzly G0463 Small Mill Harbor Freight 93885 Central Machinery Small Mill Various X3-US SIEG Small Mill Woodstock International M1110 Shop Fox Small Mill
48	BusyBee CX611 Craftex Small Mill Grizzly G0619 Grizzly Small Mill Various SX3 SIEG Small Mill Travers Tools OT2225SX3 OTMT Small Mill

Click your model to see the complete parts list.