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Thread: Bit for Best Finish

  1. #1
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    Bit for Best Finish

    I have to make a few 10" bowls and I'm wondering what would be the best bit for a final pass to get a nice finish on the inside of the bowl. I'd rather not be sanding for hours.

    Thanks!

  2. #2
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    Surface finish is a combination of cutter diameter and angle for the ball nose bit you select and step-over for your passes...and "how much time do you have". There will always be a compromise, but generally speaking, your sanding will not be much different than it would be if you were turning manually outside of the "tool marks" being a lot more regular coming off a CNC machine. On the holiday ornaments I'm producing using a 1/8" ball nose with 9% step-over, there's minimal surface issues and hand sanding takes care of that really quick. On a traditional lathe, most folks sand while spinning the work (at reduced speed) which makes quick work of the job. I'm not clear on how you're producing these bowls and if sanding that way is possible, but if you have to sand off the machine, using an angled drill like the Souix or Milwaukee with a soft pad and the "start" type abrasive pads will make quick work of getting the tooling marks off and then you can finish with hand sanding.
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  3. #3
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    Thanks Jim! I need to make 6 bowls for someone and I thought why not use the CNC. I know it will take a while, but probably no longer than turning them. I have designed what I want in Fusion for a 2 sided piece. Inside first, then the outside. Each bowl will take about 2 hours, but I can work on other projects while this is going on. Your response got me thinking that I could add a tenon on the back of the bowl and put it on the lathe for sanding after the CNC is finished. I'm thinking this will work very well!

    As for the bit. Would a larger ball end (1/2) leave a better finish? Or a smaller one (1/8)? In my head I think a larger one would be better, but I could be wrong, I don't make a lot of curved parts like this.

    Thanks again.

  4. #4
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    If the 1/2" inch bit will fit use it.

  5. #5
    What is the shape of the bowl? Is it a conventional bowl shape with fully radius sides? Flat bottom? I think an aggressive turner that is fairly proficient may argue that its a waste of time but if its a conventional bowl shape I would think a rough hoggin pass on he CNC (scrap any close stepover finish passes) and then leaving your tennon to put it on the lathe and quickly lathe turn to a near sanded finish and conventional lathe sanding to wrap it up would be a solution. But 2 hours of machine time for a bowl regardless other work (plus the drawing time, setup and fixturing) would seem like a pretty spendy bowl.

    A ton would depend on the shape and design of the bowl but I would be wondering if to speed the process you couldnt hog the material off some other way (forstner bits in the drill press and band saw on the exterior) and conventionally turn faster.

    When you were asking about bit selection I was thinking maybe you were talking a large flat bottom bowl with shallow radius' edges which would make more sense for the CNC. I do have an old 1" radius ball nose brazed carbide bit (1/2" shank) that I use a lot for odd work. You can really hog off material pretty fast and use a fairly coarse stepover with a heavy depth of cut. Takes material away super fast.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  6. #6
    I like using the CNC to do things I can’t do on other tools. Why make the bowl round? Go for oval, or tri-lobed or ?
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  7. #7
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    @Mark - It is a conventional shaped bowl, small flat bottom and about 3" deep and 10" round. Usually the lathe is a fun thing for me, I like to make bowls for myself and friends and family, but this is a run of pretty boring bowls for someone else, so I would rather have the CNC doing it while I do something else. No really concerned with the time it takes. So, I want the CNC to be able to do as much of the work as possible, hence the question about the bit that will give me the best finish.

    @Richard - Like I said above, I love lathe work, but this is for someone else, so I don't get to pick what the bowl looks like.

    Basically I want to just get these done and collect my money :-) That being said, they still need to be quality, so I'll do whatever I need to do, but I sure would like for the CNC to do most of the work!

  8. #8
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    Your problem will be finding a long enough bit if depth of cut is somewhere close to 3". For the final pass around the perimeter I find the low helix bits can give you the best finish. Terrible for dust collection because of the helix, so that would be a finishing pass. Maybe something like this: https://www.vortextool.com/4280.html. If you want minimum finishing on the bottom of the bowl, then a bottom surfacing bit works well. They have a very small radius at the tips of the cutters. That radius helps reduce the reference line we all know and love. This would be a final pass as well, because the cut depth is very small for most of these. I do not see any long enough for this bowl, but you can take a look at: https://www.vortextool.com/wood-tool...g-cutters.html

    If you want cheap and chearful, I would think about a good old straight bit. It will do a good job, but finding on long enough might be a challenge.

  9. #9
    As long as the side walls are not too steep, you will find that you do not need a bit as long as the depth of cut. You just need to make sure that the collet and spindle body clear as you work your way down. You may also find you have to remove your dust shoe as some point. Create a simple cross section of the bowl, and hold it up to your machine with bit installed and you can verify the clearance. Among my bits, the spiral type ball end are longer than a core box type bit, and I usually get a nicer finish.
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  10. #10
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    Brad, there are longer ball nose bits available for sure...Amana has them and I suspect Vortex does, too. While they are often pointed at working with thick HDU, etc., they could certainly be used for the OP's desired type of work if necessary, although there needs to be enough Z-axis height available to get them in and out of the workpiece without damaging anything. Of course, I wouldn't want to start out with such a long cutter in wood...the hollowing would have to be more gradual.

    (I actually bought a longer, reduced shank 1/4" endmill that's 4" long for cutting deeper into HDU and there's a matching ball nose version. The 1/2" version is 6" long)
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    The most expensive tool is the one you buy "cheaply" and often...

  11. #11
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    Jim, I have looked at some of those longer bits of Amana's for cutting rigid insulation and I have seen extensions too. I would be concerned about bit deflection of a small bit if my bit protrusion is 3" +. I would want something 1/2" diameter or greater to cut a hardwood unless the cut depth is very small.

  12. #12
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    Yes, the longer cutters in, say... .25" diameter...could be subject to deflection which is why I mentioned that I wouldn't want to "cut a bowl" with them in wood directly. They are more optimal for HDU and other foam products for sure. So I absolutely agree, that I'd want the 1/2" version or larger for such long lengths in harder material as well as the machine to move it effectively with minimal effect to the gantry/spindle assembly.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  13. #13
    I'd be heading for something like this:
    https://www.toolstoday.com/v-4990-45...AaAjVVEALw_wcB

    Its got enough radius to allow you to get the nose of your spindle down into the bowl but wont work with a large spindle for any depth of cut beyond the collet nut depth. but I'd guess with a touch of stickout you'd get pretty darn deep
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  14. #14
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    Thanks for all of the advice guys! I've been thinking about the best way to do this and I think I'll hog out as much of the bilk as I can with a forstner bit, just to make it easier on the machine and the cutter. If I can't reach the bottom, I'll have to finish up on the lathe. I really don't want the cutter sticking too far out, except maybe on a finishing pass. I have a 1/2" 2 flute straight bit that is 2 1/2" that I'll use for everything but the finishing passes. I really need to do a test to see how far down I will be able to get before I decide on a finishing bit.

    Thanks again!

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