[hankdog]

hey all, long time woodworkers, new to cnc.what is the consensus on 1/8" vs 1/4" bits ? I'm used to using 1/2" shank whenever possible but I understand cnc doesn't have as much side load on bits. sooo 1/8" vs 1/4" shank diameter? I assume it's job/cut dependant, but just don't want to be snapping a buch of bits off thegetgo. any input appreciated

[awerby]

A good compromise is a tapered ball-nose bit. You get the point for detail, but the shank is thick enough to keep it from breaking too easily.

[peteeng]

Hi - Why doesn't CNC have as much side load? If your jobs need 1/2" bits then use them if the collet is big enough. Peter

[hankdog]

I am speaking of a hand held router vs cnc spindle. hand held you are hogging off full bit depth in a single pass where cnc is minimal on single pass. If I had a 1/2" collet I'd just use what I have. looking for working knowledge of 1/8" vs 1/4" bits on cnc. Thanks

[6061billet]

ummm, still not sure what type of answer you are looking for. My CNC has 100x more side load than a handheld router would ever see.

The strength of the bit is the cube of the diameter. So a 1/4 bit has 25% of the strength of the 1/2 bit. But the 1/8" bit is 16x weaker, it's only 6% of the strength of a 1/2" bit. The material removal rate is also 32x slower. So a 4 minute job with a 1/2" bit will take several hours with a 1/8" bit. You are cutting narrower and shallower at the same time. But costs go the other way. You can buy 1/8" bits for around a dollar a piece and a good carbide 1/2" bit could be over $100. The smaller the bit the less material it will remove before it is dull.

There is also a depth to diameter consideration. This is why everyone ends up needing far more bits than they expected.

If I have a big part and I really need to hog out material then 1/2" bit is way to weak and slow. I end up using 3/4" shank bits or even 2" carbide insert mills. But you need some serious workholding to do that. Other times a 1/2" bit is too aggressive and would just throw the part so I need to go smaller. For anything smaller than a shoebox I'm usually using 1/4" bits for most of the work and 1/8" bits for detail. But I have a bunch of 6mm, 4mm, and 3mm bits because the metric sizes are so cheap.

[peteeng]

Hi Hank - Hobbyists are tentative with their tools. Your description of "hogging" with a hand held is nothing like "HOGGING" with a machine. Hogging on a machine means very serious material removal. You need to learn about chip load and chip thickness per tooth or feed per tooth (Fz). Look them up. Tool suppliers have charts for various cutting conditions for each tool. You use the biggest tool for the job possible and the deepest DOC possible to maximise tool life and MRR. Since you mention timber workers I generally use 0.2mm feed per tooth (timber plastic and plywood) and use single tooth bits most of the time. Doubt I could do that by hand... and finishing cuts would be around 0.05 to 0.1mm feed per tooth.. Peter