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Looking for tips on turning down a reamer

Hi michiganbuck:
If you need a REALLY nice hole, like in a mold cavity, you will always put scratches in it if you go down with a sharp cornered tool.
So your comment about putting a radius on the corner is good.

It doesn't need to be big.
But it needs to be able to cut only on the bottom half or three quarters.
If you make it able to cut all the way up to the side of the tool it will booger the sidewalls every time, usually from a chip you picked up that welded itself to the cutting edge.
At least that's what I found.

When you have to manually polish out every last scratch with an abrasive stone, you notice these things in a whole new way.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
My suggestion was to fire up the flat nose cutter well into the hole, and park it well into the hole..so with 1/4" corner/chamfer angle on the reamer only have the flat tool traveling a little more than the 1/4" when turning.
 
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Here's how it's done if you want a true flat bottom hole to size with a reamer HSS, HSCO or carbide that cuts to center. indicate parallel and perpindicular and grind off the chamfer and the center hole and get a nice finish on the end. Next gash in the cutting edges and make two at 180 degrees apart extend slightly past center leaving a .02" - 025" thick web at the apex of the reamer center. Set your work head to 9 or 10 degrees.Because reamer flutes are not semetrical use a snap finger under the flute to be ground amd start removing material. What you want to do is hair line the cutting edges leaving maybe a .003 to .005 wide " wear land" so it will take a few passes to sneak up on things. If your stop is correctly set for the center cutting flutes by the time you finish you will see with a loop that you have what looks like a lazy "Z" at the apex of the center web. On a comparator you will notice zero deveation to the perpendicular line.
 
If a flat bottom needs to go to the center and a drill considered one can flatten the drill held in a V block then gash the end with an angled wheel to bring the cutting edge to the center, so taking the web thickness away so the two end flutes come almost touching at the center. Many sharpener hands will make one flute go past center so to be sure of a full flat at the bottom.
Then do the back-off of adding clearance primary and secondary.
A well-oiled drill will do little damage to a reamed hole because the drill OD has a circle land, but best started in the hold near where it will cut so running in the hold for the shores time. A drill should be measured and be a few thousandths smaller that the reamer.
If the spec won't allow the few thousandths smaller at near the bottom then a flat bottom reamer may be needed. A reamer may need cut off of the center and end sharpening.
Using an end mill for a flat bottom may make the hole go oversize because end mills are sharp on the OD.

Usually, high tolerance spec at a hole bottom is poor/over engineering
 
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A reamer can not do a blind up sharp hole.
Reamers cut in the front only. The sides are circle ground with a very small back taper .. not like a drill or end-mill that have relief angles on the sides.
Yes you can go down to small front angle but problems, lots of problems.
 
Yeah you can grind a cutter to do it, but it often doesn't end well if it has cutting edges on both the side and the bottom face. There's always an element of sideways force when taking a heavy cut on the bottom face due to unbalanced forces. The cutters are never perfect. Better to either do most of the work with the drill point then nip the center out, use a shelf at the bottom if possible and not worry about what the very center looks like, or the exact opposite (finish center to depth flat at a slightly smaller diameter) and finish the bore diameter last.
 
Sorry for the confusion I created in my last comment. This only works if you have an existing hole to size. Lets say you want a .500" diameter reamed hole. I've had good luck by grinding a four flute carbide end mill .015" under size on the diameter with a 45 degree X .02 chamfer. ( .485" diameter ) Follow this with the .500" reamer and then follow the reamer with the flat bottom cutter. The hair line / wear land I mentioned will give you a clean and true flat bottom in Ti or stainless however as mentioned above getting good finish at the center apex is tricky. I've seen it done with a single flute gun drill but the cutting edge can't extend beyond the axel center line without creating a negative clerance issue.
 
Make a D bit from 20mm drill rod, straight grind , I’ve done it with an angle grinder before now. Harden in oil, don’t bother with temper, scrapes the hole flat
Cheap and easy , however flatting a twist drill is just as good as shown
Mark
 








 
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