I have a 1 1/2″ straight bit that I want to round the corners over on so that I get a small radius (~1/8″) curve on the inside of the groove it makes. I made a first attempt with a combination of the diamond stone, grinder, and a waterstone. The result led to some spectacular burning on my test piece of cherry!
Anyone have any advice on how to do this properly? It’s a carbide bit, so it’s darn hard to reshape. I need to make the two rounded over corners sharp, as well as exactly the same. Also, my router is a variable speed unit, I wonder if I should run it slower with the 1 1/2″ diameter bit?
Please spare me the suggestion of buying a router bit with this profile already!
Thanks,
Dan
Replies
Dan,
Sounds as though you did not grind any back relief. Carbide is tricky to grind freehand and I would advise against doing this unless you have a green wheel for your bench grinder and the essential diamond dresser. A diamond wheel is preferable.To get the green wheel to cut takes some pressure-there is a knack to it and you should set the tool rest to maintain the required angle.
For the amount of radius you require it is not serious if the two edges are not quite the same, but there must be sufficient relief to avoid burning.I would not want to grind a bigger radius than what you talk of- it is better to send the cutter to a specialist who has the machinery to do it properlt
What is the top speed of the router? A cutter of that size should be running at about 15000max I would think, which would be less than the top speed of most routers. So it should definitely be run slower. Better to start with a slower speed and speed it up until you feel it is cutting sweetly.The wood type also has a bearing on speed selected.
I would do the groove in a minimum of two passes, with the last cut being small to get a nice finish.
I have done it, with no problem, but I do have a diamond wheel on one of my grinders.
It sounds like you likely have the angle off a bit, or maybe rounded over a little too much at the cutting edge, which makes it hit behind the edge. Remember that the blade also needs clearance from the bottom as well as the side. If it is ground properly, there should be no need to change the RPM because of this.
I usually agree with Philip, but I tried the green wheel before getting the diamond, and I would advise you to not waste your money on one. Maybe I didn't get the right one, but it left ####horrible edge that would take a lot of honing to dress back up.
To elaborate on What Philip said about relief angles, please see the attached drawings.
-Steve
Hi Dan,
If you don't have a big production run, try a high speed steel cutter. It's more easily shaped and takes a keener edge.
Paul
There are router bits available with the small radius you require.I believe they are called dish bits.Not sure if you can get one with an 1 1/2" radius, but a smaller one would still work after using a straight bit .
mike
Thanks guys, I appreciate all the good ideas. I think my grinding work fell into the 'really bad' category! A non carbide bit would have been more appropriate to try and regrind I agree. I wound up taking it down to a local sharpening shop, they said they could grind it properly for me no problem. Thanks for the tip on the bowl bits, I see that they would do the job too. The larger ones generally have a 1/4" radius on the roundover though, I'd have to go with a smaller size to keep the 1/8" radius. But it would work still, I'd just have to creep up on it. Dan
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