Please help me understand milling bits

[dave_the_nerd]

I've read the docs and I understand the types of different end mills, and that they're used for different things.

I'm having a harder time finding info about expected lifespan, and whether some types or brands will last longer than others. I intend to do mostly wood and maybe a little HDPE, which I know are easier on the bits than metal. But how is bit-life measured? (cutting hours? inches? volume of material removed?)

Also, I'm having a harder time understanding why there is such a big price variance between different sellers for what seem like pretty similar bits. (Say, the $25 10-pack of single-flute upcut carbide bits from Inventables vs. individual 2-flute uncut carbide bits going for $10+.)

Is there a recommended strategy wrt quantity vs. quality? Am I better off buying one $20 bit vs. 4 $5 bits? Or visa versa? Can they be easily resharpened or are they best treated as disposable?

TIA

[WillAdams]

The bits are resharpenable, but the shops which do this have a fairly large minimum order.

Please don't dispose of them --- they should be recycled.

In general, the more expensive / higher quality bits seem to last longer and cut better for more of their life, but that's hard to quantify w/o someone doing some extensive testing and sharing the data.

[DanMc]

If you are just starting out, might make sense to go with the more economical ones.

The best thing I've found so far for checking wear is an inexpensive 10x loupe, it's amazing what you can see in terms of damage/wear on the tips. Get some cheap bits and keep an eye on them.

[WillAdams]

DanMc has a good point --- bits have to be considered a consumable item, and when you're starting out, it's easy to damage them inadvertently (I snapped one off just moving my gantry once).

Start cheap, replace as needed, then buy the good ones when you're certain the investment will pay off in terms of increased life or better cut.

[dave_the_nerd]

Okay, thanks. Cheap first, awesome later. Got it.

[TDA]

Bit life is usually measured in linear inches. This is really the only fair way to measure it as the time cut or volume of material is too variable. Even using linear inches though is fairly inaccurate as many things effect the life of a tool that will be different on every machine. Runout would be a good example. The more runout that exist in a spindle/router the worse your tool life will get as you are basically slamming the edge of the tool into the material over and over.

Differences in price of tools has many causes. Tool geometry can significantly effect the tool cost as the more advance designs usually take longer and consume more of the the grinding wheel. Adding primary relief as an example will keep the tool sharper as it wears but requires an additional grind. Material cost for the blank is also a huge issue. The differences in hardness and transverse rupture strength (load you can put on the tool before it breaks) can be quite large. This is true even for sub-micro grain carbide within the same grades. As an example a very good Chinese carbide YG3X has a HRA hardness of 91.8 and a TRS of 1.35 GPa. The same grade (industrial C4, ISO K01) of Mitsubishi MF07 has a HRA of 94 and a TRS of 3.9 GPa. These differences will make large changes in the tool life and breaking point of the finished tool.

If you want more specific information let me know.