Questions about speeds/feeds for inventables' 1/32" fishtail downcut spiral bit

Hello everyone, I’m reaching out for some recommendations on cut settings. I’m using a piece of 1/8" thick baltic birch plywood and have a 4"x4" design that is kind of intricate. My goal is to cut the entire design through the wood, so I’ve set my depth of cut to 0.125". In Easel it only shows that I can cut it when using a 1/32" bit. I’ve ordered Inventables’ 1/32" fishtail downcut spiral bit, it should be arriving today. My machine is a CNC 3018 pro, the brand is MySweety and the motor goes at a maximum of 10,000 rpm. I’ve been doing tons of reading and research about this topic and can’t find anything exactly like what I’m looking for. I’m wondering if anyone has had experience doing a similar cut and what feed rate, plunge rate and depth per pass settings you would recommend using? When reading about the specific inventables 1/32" bit, it doesn’t list any information about these settings. Thanks in advance!

The downcut bit drives material downward and is not as efficient as an upcut for clearing the material. You will want to make sure that you do what you can to keep the cutting path clear of material; vacuum or a can of air.

Next, your material is an eighth of an inch thick and you say that you have set the cut depth to one eighth. If you want to cut all the way through, typically you might consider cutting deeper than the thickness of your material and have a layer of waste material below your plywood to cut into. That will make sure that you cut all the way through.

Be mindful that if you cut all the way through and your workpiece is intricate, you may have cut out pieces that come free. You may want to consider tabs to hold those, or try the double tape method to hold everything down. Also be sure to cut from the inside to the outside where the holding clamps are. If you cut a major interior section free it may move around on you.

Typically, one does not cut clear through a workpiece in one pass. Usually, you pick some depth per pass such as 1mm or 2mm so that it slowly cuts away and does not bind the bit or try to mill through more material than the machine is capable of removing. However, this has its drawbacks too. The thin 1/32 bit gets something of a lever action placed on it as the spindle moves forward and the workpiece resists at the very tip of the bit. I had a carve I did through 1/2 inch oak and broke an army of cheap Amazon bits finishing the project.

Thank you so much for your reply, and the fruit bowl looks amazing! Yes, I’ve definitely realized I should have bought the upcut bits. :flushed: I plan to stand nearby with a vaccuum to continually clear away debris. Thank you for the suggestion on double sided tape, I think I will definitely be doing that to help secure the workpiece to the wasteboard, as well as the clamps. I was thinking to put my settings at 400mm/min feed rate, 120mm/min plunge, and 0.3mm depth per pass. Using the 1/32" bit on a piece of 1/8" thick baltic birch. Does this sound reasonable? I guess that means it would take 10 or 11 passes!

One of the things that I did with my oak to figure out a feed rate was to set the the spindle running at the depth I intended and then I manually pushed a piece of wood into the spinning bit. I listened to the sound of the spindle to see if it was getting too loaded and I watched the bit to see if it was deflecting. I measured how far it cut over a period of time and this gave me an idea of the natural feed rate.

With wood such as oak, there are grain and knots and other things that affect the bit. Although the CNC is pulling the bit in one direction it will tend to still try to follow the grain. For this reason, you may want to go a little slower than the bit seems to be capable of cutting. Baltic birch shouldn’t be as bad as the oak was.

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That’s a really good idea, I’ll have to try that with my cnc; to try and find the natural feed rate. How did you measure how far it cut? Thanks again.

I pushed the stock into the bit for a set amount of time (for example 30 seconds) and then measured the distance with calipers. Since it’s harder to cut across the grain than with the grain I went across the grain.