[Jonnygexter]

I'm cutting thin polycarbonate... milling in the X and Y is going great, but I notice an abrupt knock sound each time the bit plunges down into the material.

I tried reducing the plunge rate to 30mm/min and the depth per pass to 0.2mm, but no change. My machine is the DIY kit CNC 2418. I'm using a 1/8" single edge (flute?) bit from https://www.amazon.ca/gp/product/B07...?ie=UTF8&psc=1

I was under the impression the single flute is the best for polycarbonate to avoid melting. But now I'm reading that they aren't good for plunging, and it would make it really difficult for me to plan all my cuts from the edge if I can't solve this. I fear this knocking sound is bad for the machine and could break the bit... so I don't know what to do. Any advice? Thanks.

And gosh darnnit, I was mislead by the title of that Amazon product to think thank all the bits are carbide... but re-reading the description I see now that the single edged bits are steel. Could the weaker steel have anything to do with it?

[mactec54]

I'm cutting thin polycarbonate... milling in the X and Y is going great, but I notice an abrupt knock sound each time the bit plunges down into the material.

I tried reducing the plunge rate to 30mm/min and the depth per pass to 0.2mm, but no change. My machine is the DIY kit CNC 2418. I'm using a 1/8" single edge (flute?) bit from https://www.amazon.ca/gp/product/B07...?ie=UTF8&psc=1

I was under the impression the single flute is the best for polycarbonate to avoid melting. But now I'm reading that they aren't good for plunging, and it would make it really difficult for me to plan all my cuts from the edge if I can't solve this. I fear this knocking sound is bad for the machine and could break the bit... so I don't know what to do. Any advice? Thanks.

And gosh darnnit, I was mislead by the title of that Amazon product to think thank all the bits are carbide... but re-reading the description I see now that the single edged bits are steel. Could the weaker steel have anything to do with it?

No the material of the cutter will not affect what you are doing, can you do a video of it make the noise, there are many thing that could be causing this

If you look at how the end of the cutter and it is Ground at an angle on the end then it should be fine to plunge, if it is flat across the end then that is not suitable for plunging

[Sterob]

Do you have any percievable end float on your spindle? You should not have any.
If you do, your spindle bearings may be shot or not shimmed correctly?

Old tradesmens trick:Try using a long screwdriver as a stethoscope against the spindle as it plunges ( I know it will be difficult ) to listen for clues.
Note: Blunt end against your ear....lol

[cyclestart]

I tried reducing the plunge rate to 30mm/min and the depth per pass to 0.2mm, but no change.

30mm/min is way slow for polycarbonate, you want to be making chips not heat. If the tool path suits ramping do that instead of plunging. Ramping can be helical or sloping deeper as the tool follows a contour or slot.

I fear this knocking sound is bad for the machine and could break the bit... so I don't know what to do. Any advice? Thanks.

I don't know the source of the noise but if a tool is punching through instead of cutting the sound isn't pleasant.

... but re-reading the description I see now that the single edged bits are steel. Could the weaker steel have anything to do with it?

I've read HSS works well in plastics, something to do with sharper cutting edges compared to carbides. It's not a theory I've tested, carbides work fine for me.

[Jonnygexter]

30mm/min is way slow for polycarbonate, you want to be making chips not heat

I thought that as well, but then I read this from Bantam tools... for a 1/8" flat end mill they recommend
Feed rate: 600 mm/min
Plunge rate: 46 mm/min
Spindle speed: 12,000 RPM
Max pass depth: 0.25 mm
(https://support.bantamtools.com/hc/e...-Polycarbonate)

The knock sound is like it was punching through... but reducing the plunge rate further to 25mm/min and pass depth to 0.1 eliminated the sound entirely. And there are plenty of chips, but now I am worried that perhaps it can be making dust/heat that I'm not aware of while plunging, and maybe the chips on the bed are from milling? Can I can test for chips by plunging with no milling and then checking for chips?

Do you recommend increasing the plunge rate? Note, my motor only gets to 7000-8500 RPM, so I figured my settings could be a little slower than the recommended from Bantam.

[Jonnygexter]

If you look at how the end of the cutter and it is Ground at an angle on the end then it should be fine to plunge, if it is flat across the end then that is not suitable for plunging

You can see the ends in the Amazon picture...
https://www.amazon.ca/gp/product/B07...reative=330641
can you confirm? Because it seems to me that the 4 in the middle have an angle (and the bigger ones on the left are the same although I think the image is cut off).