Reaming Practices

Reaming practices
Reamer (re 'mer) n. Any of various tools used to shape or enlarge holes.In machining, the reamer is used to obtain a hole diameter with a better finish and tolerance than obtainable with a drill.
A better finish and tolerance is also obtainable with boring, honing, grinding, lapping, wire edm, ball sizing, and so on, but these methods are all more expensive than reaming.
The problem with reaming, one is usually a little apprehensive of the outgee until the hole is finished. Understanding what makes a reamer do what it does helps, as well as having quality tools and using good machining practices.
Hole Tolerances: The tolerance one can expect to hold regarding hole size can be broken into three categories.1. An average operator pushing for high production can be expected to hold a tolerance of about 0.0022. A good operator, using a good machine, can hold a tolerance of about 0.0013. A tolerance of 0.0005 is about the best you can expect, and this takes care and skill.I have seen closer tolerances held, but not in production.Obviously, the closer the tolerance, the higher the cost.
Reamer Tolerances: All the major tool manufacturers hold a tolerance on the reamer diameters of 0.0002/-0.0000 for sizes up to 1/2", and 0.0003/-0.0000 for sizes 1/2 thru 5/8".If you are going to need to depend on this tolerance being held, it is important to buy from the reputable manufacturers, such as Morse, Cleveland, etc. Take care with the imports and U.S. made economy tools. Some of the cheaper tools have very rough grind finishes, and they are not within the tolerances mentioned above, as well as the chamfers being off center..The grind finish on the flutes needs to be a good finish, and not feel rough. The chamfer needs to be concentric (even all around). If the chamfers are ground off center, even a little, you are guaranteed an oversize hole. This is another reason to know your supplier. This chamfer is very difficult for the average shop to measure.
Reamed hole finish: The finish in the reamed hole is determined by several things.1. Sharpness and quality of the grind finish of the tool.2. The type of material being reamed.3. The skill of the operator4. The setup.5. The coolant. (if used)6. Cutting speed and feed
Application: The tool needs to be turning at a lot slower rpm than a drill of the same size would. For example, a 1/4" hss drill, drilling low carbon steel such as 1018 would turn at about 1500 rpm. A 1/4" reamer in the same material, taking about 0.005 per side stock would turn at about 1000 rpm maximum.The feed rate of a reamer has a large control over not only the hole size, but also the finish. Obviously, a slower feed will improve the finish, but it will also dull the reamer, which in turn starts causes it to produce oversize holes.Too fast a feed can cause oversize holes and give a rougher finish. So what is the correct feed?To start with, the reamer must be cutting at all times. A feed to just acgeplish this and no more is optimum. A good start point is around 0.005 inches per spindle revolution. (ipr). Keep in mind, there are anywhere from 4 to 6 flutes, and each tooth on the reamer needs to be taking some stock. The od of the reamer also cuts. It's function is to finish "shaving the hole", bringing it to size. But it will not remove grooves in the hole sidewall left by galling and tearing.If you are feeding the reamer by hand and not with a controlled power feed, the best feed, is when you feel the reamer just beginning to "take a chip", try to hold this feed all the way. If you are trying to hold very close reamed tolerances, resist the temptation to extract the reamer until the depth has been reached. If you have no allowance for chips, this cannot be avoided, so stop the spindle, pull the reamer out of the hole, remove the chips accumulated in the hole, and feed the reamer back down to a point near where it will start cutting again, and then restart the spindle to continue reaming the hole. When trying to hold very close tolerances (.0005 or less), it is important to not ream the same hole twice.Also, straight fluted reamers will not usually produce the same finish as the spiral fluted reamers.Better size control can be obtained by stopping the spindle when the reamer reaches depth, and with the spindle stopped, extract the reamer. This prevents it from cutting twice.The quality of the drilled hole will have a major influence on the quality of the reamed hole. A reamer will not fix a bad drilled hole, it will just follow it. If the drilled hole is tapered or bell mouthed, it will adversely affect the reaming process.With a little care and practice, you can produce holes within specification with a reamer.