Tool sharpness and when to say farewell

This funny thread got me thinking, even if some of my endmills are not dead, they must be slowly agonizing (I’m looking at you, original #201 that came with the machine). So just for fun and getting to use the cheap USB “microscope” I got, I did a test cut with this venerable #201 that has seen many hours of cutting, and then with a brand new 1/4" endmill, at similar chiploads, in oak (1mm deep slotting):

old guy:

new guy:

No surprises there, but now at least I cannot fool myself into thinking I can get “just a few more cuts with this old endmill” anymore. Needless to say, the second cut sounded much better than the first.

So anyway, considering endmills are consumables, how do YOU estimate that an endmill has had enough ? sound ? finish quality ? visual inspection ?


Yes I see the difference between some older endmill and newer ones. I had a few crashes last week where the CNC decided to drive the bit down to the wasteboard and I suspect that the endmill took a beating but looking at it without magnification, it is hard to tell if it is really damaged. I’m looking forward to finding a good way of determining the state of the endmill.

I have quite a few days to ponder this while I’m waiting for a replacement sensor and controller board.

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I also notice if a “burr” forms in the wood from the endmill cutting. As of the endmill partially wants to push the material rather than cut it. I’m pretty sure my 201 needs to be either sharpened or decommissioned.


Well where I’m having issues is when I cut wood and I get relatively nice chips but I also get stings forming at the top. For example, last week I cut quite a bit of cedar and while the cuts were nice and clean, there were strings forming at the top. I tried a new endmill but could not get rid of the strings so I don’t think the end mill sharpness was the issue but one can question if the endmill is working properly. It looks like soft woods have a tendency to naturally make strings when you use an endmill. At least a quick cleaning of the work piece with a sharp tool gets rid of the problem.

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This is where upcut for bottom edge, downcut for top edge and compression for a good compromise of both worlds come into play.

upcut bits will leave the strings as they are pulling up on the fibers of the wood. There is no resistance on these fibers to enable a clean cut, so the fibers bounce around the bit (like those big blow up wavy arm guys- saying you can’t touch me) leaving the strings behind.

The difference between an upcut router bit and a down cut router bit is the direction of the flutes. On an up cut router bit, if you hold the bit by the shank and point it straight down as if you were going to do a plunge cut into the surface of your work piece, you would be able to drill into the work piece by turning the bit in a clockwise rotation. With a Down Cut bit, you would need to turn it counter-clockwise to be able drill into the surface.
Another way to determine if a bit is an up cut or a down cut bit is to look at the direction the flutes move around the bit toward the tip. If the flutes twist right around the bit to the tip, then it is an Up Cut Bit. If the flutes twist left around the back of the bit to the tip then it is a Down Cut Bit.

An Upcut bit is very efficient in evacuating chips up and out of the cut. It will leave a very clean finish at the bottom of the work piece, but will leave a rougher surface on the top of the surface (or the side that the bit enters the work piece). A down cut bit does just the opposite. It is best to use a down cut bit for through cuts, as a down cut bit pushes the chips down into the cut. Down cut bits will leave a very clean cut on the top of the work piece, but may leave a rougher finish on the bottom.

Down cut bits are also great for cutting shallow dados, rabbets, and visible slots because of the clean finish it gives on the edge of the hole or groove. When cutting dados or grooves with a down cut bit, be sure to slow the feed rate to allow more time for chip removal.

If you need a clean cut on both the top and bottom surface, try a compression bit. They can be pretty pricey, but Compression bits leave a beautiful smooth finish on both sides of the work piece being cut.

A cheaper solution would be to use
a down cut bit for the first pass to keep the top edges clean, then you could change to upcut bit for rest of passes. leaving the bottom of your cuts just as smooth.


Rockler and others sell bit cleaner. Many bits just need cleaning. If you use your finger on the cutting end it will feel sharp if it is indeed sharp. If it feels dull it is dull. Your finger is an incredible instrument. You can feel microscopic differences with just your finger.

Clean your bits, then feel of them. The ones that are no longer sharp should be decommissioned. FYI the flutes only carry away chips are are not generally considered as “sharp”. If however they are damaged they too should be decommissioned.


The actual wood makes a difference in the cut - beech is truly awful about the strings. The harder the wood, the less this happens.


  1. Own a loupe. They’re cheap.
  2. Use it, look at your tools. Look for chips in the tips of the cutting edges, gunk built up on the edges.
  3. Carbide is not as sharp as HSS.
  4. Carbide lasts a long time but not forever, even on wood.

Yep, this happens regardless of sharpness, based on what @mjmike6988 said about direction, in what I see. However, it will become more prominent the duller the endmill gets. It won’t cut as nicely.

I’d like to add something here - wood descriptions are weird. “hardwood” is wood that comes from trees with seeds that have hard outer envelopes, softwood is from trees with soft outer coverings - so some “softwood” is actually pretty hard (rock maple, for example).


The definition we use here is deciduous trees are consider hardwoods while conifers (fir, pine, cedar, etc.) are referred to as softwood however, poplar a deciduous is very soft, softer than some conifers and white birch a bit harder but still pretty soft.

My point was that in some materials, up-cut perform very well leaving practically clean finish while in others it does not and one can wonder if it is the bit betting dull or just the material not cutting well.


I’ve always understood hardwood vs. softwood to be specifically biology — hardwoods are “angiosperms” (specifically dicots — flowering trees), while softwoods are “gymnosperms” (trees with needles/cones).

Maple is a hardwood by this definition, and usually quite hard. By way of contrast, so is balsa, and it’s quite soft. On the gripping hand, Southern Yellow Pine is a softwood, but I can still remember vividly a piece with a slow (winter) growth ring) which was among the hardest pieces of wood I’ve ever encountered (possibly exaggerated by youthful inexperience, and frustration at having to sharpen my knife more often than usual).

There are listings of woods by hardness:


Well I learnt something (confusing and interesting) today, thanks. Maybe this is why I’m hooked on this forum, the amount of experience and knowledge available here is incredible.

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Rocker sells “pitch and resin remover”, which sounds like it’s meant for wood. Is there something more appropriate for plastic and/or aluminum, which I mostly cut? I have about a dozen HSS bits that nobody wants to sharpen & I’d like to try cleaning them.

There are threads on cleaning endmill but one in particular regarding cleaning aluminum from endmill. The short answer seems to be using lye. Tips & tricks for cleaning up aluminium residue from endmill?


Yep, I use sodium hydroxide (lye), which works really well.

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