Pick up a new or barely used 1/2" chucking reamer and study the geometry of the 'chamfer' end of the teeth. That is the only part of the reamer that does the cutting, and that is all that you would normally touch up on the tool and cutter grinder.
It has been a while since I sharpened a reamer, but I think you'll need a flaring cup wheel to get proper access to the tooth. My recommendation is to get a CBN grinding wheel (for HSS reamers), as these will hold their shape and size for a long time if carefully used. At least, you won't detect wheel wear in the course of sharpening a single reamer, like you might with a soft toolroom wheel.
If your reamers are actually chipped, that is going to require quite a bit of grind back. It may not be feasible to grind between centers, because the chamfer will get so large. For this, you might need a very accurate workhead and collet arrangement, to grind the reamer without a center, because you'll have to butt grind the end and you would lose the center hole.
Anyway, setting the wheel and fingerstop height is a matter of patient practice. If the reamer
was cutting before it got dull, then you can pretty much copy the relief angles that are already on the tool. I usually apply some ink to the chamfered end of the tool, and eyeball the wheel height/fingerstop height so that it looks like the wheel will grind a clean facet right across the old facet. But on the first approach, you barely want the wheel to touch. Turn the grinder on and grind about halfway across the first tooth facet and withdraw and study how the new grind angle compares to the original. Adjust the wheel height if required and try again, on the next tooth if necessary. Once you get it right, then grind all the teeth.
Chances are good that there are two angles to be ground, primary and secondary clearance angles. Now a CBN wheel is pretty cool grinding, so the necessity of grinding the secondary clearance angle first is not so strict as with vitrified wheels that wear rapidly. Again, compare what you are grinding to a new reamer to get an idea of what it should look like.
The secondary clearance angle basically only provides heel clearance to the primary clearance angle. The primary clearance angle is the one that runs right up to and holds the cutting edge. The desired primary clearance angle is maybe 5 degrees, not more, or the reamer will chatter. The secondary can be whatever....10 degrees or whatever you can manage. Grind the secondary clearance only deep enough to leave a small land containing the primary clearance angle.
If you have a dial indicator with an extended needle point, you can indicate right on top of the primary clearance angle before you grind the secondary. Rotate the reamer so that the indicator rides up the facet and almost drops off over the cutting edge. You should see a definite rise to the indicator, showing you that you actually have ground in some clearance. Your eyes can play tricks, and sometimes you might think you have some clearance, but it may actually be negative if your setup was incorrect.
Use a bit of trigonometry to determine how fast the indicator needle is rising compared to the distance the reamer is rolling beneath the tip. This will give you an idea of the clearance angle better than trying to measure it with some kind of a protractor gauge.
11-09-2008, 08:23 PM
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