When you do a trigger job and you bring single action down does double action always follow along? Is there a way to seperate the two? Meaning bring the double action down without affecting the single any further?

Single action trigger weight pull is a function of the geometric/mechanical interaction of the sear and hammer plus the force necessary to overcome the rebound spring. Double action weight is a function of the mainspring hammer force and the rebound spring force.Decreasing single action by changing angles on the hammer or sear will have no effect on double action. Decreasing double action will decrease single action somewhat particularly if you change the rebound spring.

If you are talking about reducing single action and double action pull weight, the answer is a qualified "yes". SA pull reduction strives for reducing the sear engagement without the danger of "push off".

DA pull weight is based on mainspring and rebound slide spring tension. SA pull weight is largely determined by mainspring tension. Both interact to the degree that mainspring tension is the biggest factor for both, however, there is a balance between DA pull and reliability (trigger recovery, primer impact).

What most action jobs accomplish is remove as much DA roughness as possible without reducing reliability. Trigger pull weight is reduced, but the greatest benefit is making the action smoother and keeping maximum ignition reliability.

As an action job smooths most interior parts of the gun both SA and DA operation are normally going to be affected, particularly if springs are changed or modified. The rebound spring strength directly affects both pull weights. The same is true I believe of mainspring strength or tension.

A good gunsmith can apparently compensate for this if desired by changing the angle of the single action sear on the hammer. This is talked about in the S&W Kuhnhausen gunsmithing book. It also warns that the sear can be ruined if too much is removed, removing the case hardened surface of the sear.

Since the single action sear engagement on S&Ws is about 5 thousandths of an inch, I keep my paws and tools off them. But generally, if you change spring tension on either mainspring or rebound spring, you affect the weight of both pulls.

Removing the machining burrs and polishing friction surfaces (BUT NOT SEARS!)on the moving parts can make the DA seem lighter/smoother without spring changes.