CNC Bit Shrill Sound

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HandyAndyInMtAiryNC
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CNC Bit Shrill Sound

Post by HandyAndyInMtAiryNC »

Hello Everyone,
I'm using an Amana 46292-K CNC bit for cutting thru pine planks. During the entire cut with this particular bit, the bit is making a shrill scream. This is the only bit in my entire Amana bit collection that does this. I use this bit for final cutouts since it is long. I make passes that are shallow and many more passes than is recommended minimum. For this material, it is .741" I use 8 passes. I have tried slowing the feed rate, increasing the feed rate, decreasing the rpm, increasing the rpm. Adding to the number of passes, I once set the number of passes to 24. Still the same thing. I have added a picture of the bit specs that I have entered into the VCarve Database.

Any information, guidance or comments are greatly appreciated. Thank You in advance.

Andrew
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46292 Amana CNC Bit.jpg

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adze_cnc
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Re: CNC Bit Shrill Sound

Post by adze_cnc »

I'm baffled by this cutter. If you'd have shown me the image of it and asked "what's this good for?" I'd have said milling metal. Four flutes, shallow cut angle, etc.

Plugging 4 flutes and 18,000 rpm into a feeds and speeds calculator shows a feed rate of 360 inches/minute needed to get the recommended chip-load of between 0.004 and 0.006 inches (5 thousandths in my case) for softwood / plywood.

Yet, Amana's suggested feed rate at 18,000 rpm on their site confirms what you have: 0.0006 (6 ten-thousandths). According to Wolfram Alpha https://www.wolframalpha.com/input/?i=.0006+inches that's about 1/7th the thickness of copy paper or 1/2 the length of a human skin cell.

Rather than cutting through the material it almost seems that it's burnishing its way through.

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HandyAndyInMtAiryNC
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Re: CNC Bit Shrill Sound

Post by HandyAndyInMtAiryNC »

Yes Sir, that it does. Sounds like that also.

I'm using that bit because I need a 1/8" cutout that can reach all the way thru the 3/4" material. I'm looking for another bit, something with less flutes for certain but with good length. I have some 5/4 material that I will need to carve also. That bit came with a set. I am ordering some different bits today. Any suggestions?

Thanks
Andrew

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TReischl
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Re: CNC Bit Shrill Sound

Post by TReischl »

Long 4 flute bits scream, no doubt about it. My theory (absolutely no proof whatsoever for this theory) is that the webs get downright thin and the result is a bit that can flex, vibrate, and make lots of noise in the process. I have quite a few around here and no amount of tinkering around with feeds and speeds has quieted them down. They make a decent cut, so I just put on the heavy duty ear protection and turn up the tunes.
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Re: CNC Bit Shrill Sound

Post by Leo »

Usually that means to high a RPM and too low a feedrate.

Now - 4 flutes should almost never be used on wood - especially a softwood. Of course there is always an exception to the rule, but I don't know of any.

You chipload of .0006, is not even close to the ballpark. Forget that - it is not even of the correct coast. The chipload should be somewhere around .003 - .006

Then the length and small diameter of the cutter just exasperates the condition.

I use a 1/8 extra long cutter for the same reason you do. Yes it is noisy, but I use proper speed and feed. Make sure the amount of cutter sticking out of the collet is only a small amount (1/8 or so) longer than the depth you are cutting to. Never have the fluted inside the collet. You need to use a 2 flute cutter, and adjust your speed and feed in order to get at least .002 in the chipload box. I also use about .062 for a depth of cut. These values are somewhat conservative. You can adjust as you see fit for your machine.
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Re: CNC Bit Shrill Sound

Post by adze_cnc »

HandyAndyInMtAiryNC wrote:
Wed Mar 24, 2021 1:12 pm
I'm using that bit because I need a 1/8" cutout that can reach all the way thru the 3/4" material.
But are you sure you need to do the cut out exclusively with one bit?

How about:
  • using a 1/4" cutter to cut down to the depth of the top of your tabs (thickness - tab_height). Use a small "Allowance offset" (say 6 thousandths of an inch).
  • using a 1/8" cutter to shave off the "Allowance offset" and to get into the nooks and crannies that the 1/4" cutter can't get into. With this you can use a generous "Pass Depth" (say 1/8" or maybe even 3/16")
Fine, you're doing do two full depth cuts but together they can be almost as quick as one pass with a smaller cutter.

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