[Andy Graves]

I am cutting some soap dish grooves with a 3/8″ ball nose bit. In the center of the cut it leaves a white line.

Is there a special bit I can use to eliminate that line? It is difficult to sand and if I slow down the feed, I can get a cut so smooth I wouldn’t have to sand it at all.

Andy suggestions would be a big help,

Andy

[John Christensen]

Andy,

I think that because it is a ball bit and it is rotating on an axis, the center point on the ball will have virually no cutting geometery at that point.  I don’t think there is a way around the line issue with a round bit.  If you can live with a slight flat bottom in your groove then the bit would have at least a minimum of flat area and room on the bit to have a cutting edge that swings a small distance away from the center.

When I have had to make round bottom grooves I always get a bit of a line.  I found it quite tedious to sand the line away.  I now use a cabinet scraper that I have cut the end at about 20 deg.  then the acute angled corner gets sanded to about 1/4″ radius.  I use that to take long scraping strokes to cut away the line leaving minumal sanding.

Kowboy would use a butterfly drill  bit and grind a radius on it.  Same Idea.

When these scrapers get dull, they are easy to touch up on the disc sander.

Johnny C 

[KCWOOD]

Andy, does your ball nose bit have 2 flutes? 
I have cut grooves into a plastic lighting fixture housing years ago, with a core box bit, 2 flute Ekstrom Carlson bit and do not remember lines in the center.

Give them a call…  800-981-8927  they are the big boys with CNC tooling , they make everything!

[Andy Graves]

It is a single flute, spiral bit.

I will look into those two ideas.

Thanks for the help,

Andy

[Steve Lefebvre]

Get a 3/8 cove bit, type with a bearing. Remove the bearing and grind the bearing post away. Look for a bit with the smallest dia bearing. Note that you will need to ramp down to finished depth. You could also hog with a straight bit leaving .020 Z for finish pass.

[Thomas Marino]

Andy,

We do as John mentions above and run the pass with a slight offset, routing inside a rectangle .020 wider than the bit vs. profiling along a line with very good results. We also use a 3 flute solid carbide ball mill from Vortex and the end result needs little sanding once you get the feeds and speeds right.

Good luck,

Tom in PA

[Andy Graves]

Thanks for the info.

Do you have any idea what speed and feed you’re running?

[Len Smith]

I’d like to see a sketch or photo of what Thomas is suggesting. For some reason I am not getting a mental image.

[Andy Graves]

Len, it seems you would end up with two lines instead of one, but would probably be easier to sand with the flat bottom.

[Thomas Marino]

Andy,
We run the groove in two passes, a rough and a finish. Rough leaves .020 material for finishing. We run 14Krpm rough at 150ipm and 18Krpm at 100ipm for finishing. With the 3 flute and the offset we get only a slight line that comes out easy.

Len,
Imagine machining the inside of a pocket that is .02 wider than your tool vs. running the tool along a line. Using the ‘pocket’ method I can climb cut on both sides of the groove leaving a better overall finish.

Tom in PA

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