Most fittings on standard pens are press fit into the brass tube that forms the core of the writing instrument. There are several tools that can be used to press in the fittings. Important qualities are pressing straight in with large but controlled force. Pressure must be applied evenly around the barrel of the pen blank. Any uneven pressure can crack the pen barrel. At this point in the process the pen barrel is turned to it's finished size and is very thin.
Note: Before assembly, make absolutely certain that no glue is inside the brass tube. Any glue left in the tube will cause the barrel to crack.
When pressing in the transmission for Slimline and other Cross style pens the transmission is pressed in until the pen ink point is at exactly the correct extension. Some folks use a jig for Slimline pens. Others use press and test fit, remove ink, press and test fit, remove ink, until the pen point extends the correct distance when extended.
A jig can be made to make this process easier. The jig is a complete blank with a V-groove in it. The length should be 3.95" (~100.33mm). Press the nib of the pen into the lower barrel, then start the transmission into place. Lay the pen in the groove and press the transmission into the lower barrel until it is stopped by the jig.
Quick Grip Clamps
Quick grip clamps can be used, but the clamp faces must be even.
Install a bolt into the chuck of the drill press with the bolt head down. Glue a small piece of wood over the bolt head. Use the feed lever to advance the bolt head and press the fitting into the pen blank. Make sure to provide a slightly resilient base for the pen barrel to rest on.
There are several specialized pen presses made just for this purpose. They work well, but specialized tools mean you will have more tools that you know what to do with! Always think what else can I use this tool for?? Multi-tasking
A woodworkers vise equipped with hard wood jaw faces works well. Some of these have a quick release and you can open or close the jaws without having to turn the lead screw. Alignment marks on the jaw faces allow you to exactly align the pen during assembly.
A bench vise has the same advantages as the woodworkers vise. But usually has a diamond pattern on the jaw faces so it is VERY important to make wooden jaw faces for it.
A one ton arbor press is a good strong pen press. You may need to modify the stage and presser head with a slightly resilient material.