Reaming is the process of refining the size of a hole to the required tolerance, and it is used during the case where teh standard drill bit dont allow for the necesary precision in creating the press fit. A machinist uses reaming to achieve a specific diameter for the hole, and it focuses upon the refinement of the bore rather than the cutting of the metal to create the bore. Reaming is a process that a machinist performs with care, as it is a precision task that are different from drilling.
In order to ream a hole, the machinist must decide which reamer to use. Hand reamers are used in the machining of a few part only, whereas machine reamers are used with a drill press or lathe; the machine reamer is made for faster removal of the metal. If the part to be machined contains shafts with tapered fasteners, the machinist will use a taper pin reamer.
How to Ream a Hole
For large bores, a shell reamer may be used instead of replacing the entire arbor for the reamer; this saves the shops money over time. The amount of material that is left behind for the reamer to cut will impact the quality of the bore. If too much material are left behind, the reamer will have to remove too much metal, which will create chatter marks in the metal in the form of an oval shaped hole.
Conversely, if too little metal is left behind, the reamer will skate over the metal without cutting it, preventing the reaming process altogether. The amount of material that is left behind will increase with the size of the bore; small bores will have to leave behind a smaller amount of metal for the same reason that small bores is more prone to deflection. The spindle speed for a reamer is different than that used for drilling.
Using the same RPM for reaming as drilling will cause the reamer to overheat from the high speed. An overheated reamer will produce an oversized hole. The machinist calculates the spindle speed using surface feet per minute (SFM) when machining; the SFM will change depending upon the type of metals (aluminum or stainless steel).
For reaming, the speed will be slower than for drilling, and it’ll have to be maintained at a slow and steady speed to avoid dulling the edge of the reamer. The material used to make the reamer will impact the way in which the tool is used. If the user uses high speed steel to make the reamer, it is strong and the user can sharpen it.
However, carbide material for reamers allows for higher speeds and tighter tolerances; however, carbide is brittle and will shatter if it isnt mounted correctly on the arbor. In some cases the fluid used for reaming can impact the surface of the hole if the correct fluid arent used. For instance, water based coolants used on aluminum will weld the aluminum to the cutting edge of the reamer.
For cast iron, the reamer can be run dry or with an air blast, as the metal naturaly contains graphite to act as a lubricant for the process. Using the wrong fluid will scratch the bore of the part. In removing the reamer from the hole, care must be used to ensure that the rotation of the tool is not reversed; this will dull the cutting edges of the reamer and create a rough bore finish.
The reamer should continue to rotate in the same direction in which it was cutting to remove it from the bore. By following these rules and using the correct tolerance classes for the reamer, the machinist will achieve a precise and proper press fit.
