Stripes after finish pass on pocket bottom - Router Forums
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post #1 of 15 (permalink) Old 05-04-2018, 01:29 AM Thread Starter
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Question Stripes after finish pass on pocket bottom

I've been struggling to get a clean surface finish on the bottoms of pockets with my CNC router. There are usually faint rows from the parallel finishing pass that don't appear to be bumps or cusps. (photo attached)

After roughing, I'm using a 0.25" diameter, cove-nosed end mill (radius 1/16") to skim the final 0.03" off the bottom with a step-over varying from 0.125 to 0.06 (doesn't seem to make a difference). 80IPM, ~15000RPM
Lastly, I do the contour of the pocket with the same mill.

Results:

Pronounced "corn rows" that run in the same direction of the grain. Basically it looks like stripes of high-sheen area where a clean cut was made next to stripes that look dull and abraded, as if sanded. The final profile that finishes the walls and cuts to the same depth as the floor leaves a really nice, clean finish, so I know the mill is capable of doing it.

Theories/Attempts

I've used a flat .25 end mill to finish the bottom of the pocket and this doesn't leave stripes that are as noticeable--it leaves a uniformly dull finish, like what you'd see from sanding with 200 grit paper. Contrasts with the clean profile left by the bullnose. Zeroing the two tools exactly the same is also a challenge, sometimes there will be 0.001 or less that's visible.

Tramming: I've trammed the machine using a 5" diameter tool to <0.001 about the y axis. I've had less success tramming about the x axis (the same axis as the wood grain) maybe 0.002 over that 5" span, but this error should go to nearly nothing when using a tool that's 1/20th the radius as the test tool.

Scraping/sanding/planing inside the pockets isn't an option--besides being time consuming, they're often too small to work inside. The products are home goods where surface finish matters a lot.




What bits do you use to cleanly finish a surface? Is it possible to get a surface as nice as a hand-plane?
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post #2 of 15 (permalink) Old 05-04-2018, 08:06 AM
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luclefleur,

I have had similar issues when pocket cutting if I used a v bit. Since that time I have used a Flat End bit of (1/8) (.125) This has solved my problem when making Pocket Cuts, similar to what you are showing. I have used the .25 end bit, but I recover much more material to avoid the rowed kind of finish you speak of. When I add to the Step-over for the .25 end Mill Bit, it cleans up great. The bull nose bits for this always give me trouble. I only use them for signs. One other issue, I have had some bits are just not as good as others. I have purchased a few cheaper. but in the end I have paid the price. The bits need to be in good shape and sharp, if you do a lot of this type cutting. Your moving a lot of wood. Depending on the depth of the cut, varies the amount of passes to be made. Vectric Software will usually set the number of passes needed to make the cut. I always add one more pass, to take a smaller amount of wood per cut. Now with all this said, I am quite new at the C N C Routing. So others here may have better solutions. There are a lot of very talented and smart Woodworkers here.
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post #3 of 15 (permalink) Old 05-04-2018, 08:10 AM
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Welcome to the forum! When you get a minute go ahead and complete your profile by adding your first name.

I don't know if it would make a difference but you're cutting 0.030" for a final pass whereas my final passes are more in the 0.005" range. Can you try the flat bottom bit to take the final 0.005" to 0.008" and see what that looks like?

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post #4 of 15 (permalink) Old 05-04-2018, 08:14 AM
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bit isn't 90° to the bottom...
bottom of the bit has a slope to it...
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post #5 of 15 (permalink) Old 05-04-2018, 08:23 AM
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What brand of bits are you using? It can make a difference.

Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.
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post #6 of 15 (permalink) Old 05-04-2018, 10:10 AM
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Is it possible the bit and surface are not exactly perpendicular to each other...?

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post #7 of 15 (permalink) Old 05-04-2018, 12:32 PM Thread Starter
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David, I'll try taking a shallower finishing pass. Also, not sure where to update the profile.

Stick & Nick, I've trammed the machine as closely as I can get (the axis in question requires shimming) using a dial indicator on a 5" circle, I get a difference of about 0.002. This should be less than a few tenths of error on a 1/4" bit. Do you think that little error could still cause finishing issues?

Chuck, using new Amana and Whiteside carbide bits.

What I don't understand is that the pass along the edge is glass smooth, like you'd get from a chisel, so I know the bit is capable of it. It's just finishing a broad surface without having to scrape or sand is a different issue.

Thanks,
Luke
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post #8 of 15 (permalink) Old 05-04-2018, 03:25 PM
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You say you are using a "cove-nosed end mill (radius 1/16")" to clear the bottom of the trays and using "a step-over varying from 0.125 to 0.06".

What that translates to is a 0.125" ball nose end mill and using a 48% to 100% stepover. That percent of stepover is more than I would use for a roughing pass. You are trying to get a finish bottom in these trays so if you want to use the 0.125 ball nose bit you should be using a stepover of around 8% to 10% for finish quality cuts. You are leaving a lot of material between passes of the bit.

Looking at the trays you are cutting I would be using a dish or bowl bit with a 30% to 40% stepover to cut and clear the bottoms and I don't see where there would be a problem sanding the bottoms of the trays. Of course this all depends on how tight your machine is.
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post #9 of 15 (permalink) Old 05-04-2018, 04:20 PM Thread Starter
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Hi Mike,

I might be misunderstanding what you're saying, but I'm not using a ball end mill. It seems like even at .125 stepover, a 1/4" diameter, 1/16" radius cove nose mill shouldn't leave cusps (theoretically). Wouldn't the flat middle 1/8" of the bit cover the radiused outer 1/16 portions?

That said, I ordered some dish cutting bits and plan to experiment with those.

Thanks,
Luke
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post #10 of 15 (permalink) Old 05-04-2018, 04:47 PM
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If you're on a computer it's easy, Luke, not sure on mobile platform. Go to the upper right and click on Account Settings. Once on that page look on the left under Your Profile and you'll see Edit Your Details. Your first name field is down the page a bit.

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