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 For my next CNC build I am going to make a PCB router, based upon a Proxxon KT70 compound table, and a dremel rotary tool.
I need to ensure that the steppers I select do have enough torque to turn the leadscrew, and therefore needed to find the minimum torque needed to turn the hand wheels. But the torque wrenches I could find in nearby tool stores where not suitable to measure such low torque values, so I needed to find the torque in a more practical way. Here is the description for my "Torque Approximator" setup.
 Torque is defined as the product of force and distance , τ = F * d (Illustration image: Craigs website). So to find the absolute minimum torque needed from the stepper motors, we need to know what force must be applied at a certain distance to make the lead screw turn.
Start by cutting a 20 cm length of a rigid material. For my setup I cut a piece from a 5mm aluminum tube. Mount this arm on the hand wheel, aligned with the center of the arm at the center of the lead screw. You now have a 10 cm arm to where we will apply the force. As the arm has the same length on each side, the weight of the arm is balanced out. This ensures that the weight of the arm will not interfere with our measurements. A very simple variable weight is then made by cutting a plastic bottle in half, and making a handle using thin wire.  Now we can adjust the weight just by filling the bottle with water. Insert a Nail into the tubing and hang the bottle from this Nail. Again we need to balance out the weight of this nail by inserting an identical nail in the other end of the tubing. If you make the arm from a solid bar, make the arm a little longer than 20 cm, drill a hole at 10 cm from the lead screw center, and thread the bottle handle trough this hole. Now gently pour water into the bottle until the arm moves, and the hand wheel starts to turn. Note: When this happens, it will turn over very quickly. So hold your hand directly underneath the bottle at all time to stop it from falling on the floor and spill water all over. You now know what weight is needed on a 10 cm arm, to turn the hand wheels. On my KT 70 a weight of 190g was needed. The Force applied by a weight under influence of gravity is given by F=m*g, where m is the mass of the object and g is the gravity of earth. So we have F = 0.19Kg*9.8m/s2 = 1.86 N. The Torque is defined by τ = F * d = 1.86N * 0.1m = 0.186Nm. This mean that a stepper with a torque of at least 0.2Nm would be able to (barely) turn the hand wheels, when there is no load on the table.
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