I’m trying to get very accurate cuts and I’ve noticed that my 1/4" endmill isn’t really 0.25", the base is very close to that number but the cutting area is 0.232"
I’ve measured a few different end mills and it seems that’s the norm
Are you guys measuring each end mills cutter and then creating a custom tool for it?
Just for context I do lots of cuts in aluminum with multiple end mills / rest machining.
Any relatively quality endmill is going to be very, very close to the advertised diameter. Your .232 is close to .236", which is actually 6mm.
So, can you be sure that you do not have a 6mm cutter instead?
Aliexpress Endmills:
0.25 → 0.232
0.125 → 0.118
0.0625 → 0.052
Amana Tools
0.125 → 0.112
The shank itself is very accurate, not off but much it’s the cutter that’s smaller than advertised.
Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.
It’s pretty much impossible to grind a cutting diameter to the shank size barring tools with “land” (where part of raw ground shank is left in the flute). It’s why in high accuracy tooling you will see a +/- on tools with a diameter less than the shank size and a +0 -X on tools with diameters listed as shank size. I went more into it here if you are interested:
That said, how are you measuring the tool? It’s almost impossible to get a true measurement with something like calipers and you can pretty easily damage the tool (also went into that on the previous link). Best thing to do is cut a slot and measure the result. That will also include your runout and the like which give you the cutting diameter that more important.
That is pretty far away though (assuming a good measurement). Is it a new tool? you might also be seeing a reduction from use. Which is another issue that you have to deal with if trying to hit tight specs.
Hope that’s useful. Let me know if there’s something I can help with.
Hi thanks for the info.
All brand new tooling except the 0.25" that one has a lot of use.
If in my cutter library I have my tool as 0.25" and the cutter itself is 0.232" that’s definitely going to cause problematic results for me right?
Was adding an edit but I guess I’ll reply to this instead…
It depends on what it actually cuts to if it’s an issue. As an example (and one of the things I was going to add in edit). You extra can’t measure a single flute tool. The opposite side from the flute is ground down to prevent anything but the edge from touching the cut slot. Similar issue with any tool with any number of flutes where there’s not flutes 180° from each other. There’s also the issue in multi-flute cutters where you have tool runout. Where one of the flutes is “shorter” from the center than the others. The “largest” flute from the center will determine the cutting diameter.
All that said. What kind of tools are these and how are you measuring them?
Rob hit it on the head. Tha Aliexpress endmills are obviously metric sizes, that they are just calling out by the closest Imperial measurements.
Similar to acrylic sheet, where “1/4 inch” sheet is usually 6mm (.236") thick and “1/8 inch” sheet is usually 3mm (.118") thick.
To reiterate what other folks have said, import tools tend to be metric and advertise otherwise. I’ve even seen this with Amazon sourced imports.
To answer your question about size of tool in your library, yes it will absolutely impact your results. Your toolpaths are going to be dictated by your tool library.
Imagine you have a slot that is .5” wide. You have a tool in the library set for .25” but it’s actually .232”
Assuming a perfect world where you have 0 runout and perfect alignment everywhere, you’ll end up with a slot that is .018” smaller than expected.
In an aesthetic wood application, maybe that ~half a millimeter doesn’t matter but in metal with any kind of functional application it certainly will.
(Folks should feel free to correct me if necessary, I’m only 90% sure I didn’t screw up the logic somewhere along the way 
)
I have a mix of imperial and metric tools in my custom library. Only the collets care about the shank size, and the toolpath generator only cares about the flute size… As Tyler said, you will want to measure the actual flute size if you are doing close-tolerance work.
I could see this being true. However, I wouldn’t stop looking from the easy answer.
I think there’s something greater than just the usual mislabeled import tools going on as:
Again, I’m not saying it couldn’t be that but I’d confirm.
Your comment brings up the thought that the Aliexpress endmills might be regrinds of worn-out imperial endmills…
I don’t think I’ve ever worn out an endmill, but real CNC machinists who know what they’re doing likely wear out tools rather than snapping them… 
An undersized endmill is certainly going to cause problems if it’s outside the tolerance you need.
How are you measuring them? Is there an obvious step from the shaft to the flute?
The simple answer being… if it’s truly an undersized cutter, and you want to use it, then create a new tool in your library for it.
You have metric mills. I’ve bought a lot of cheap tooling and they are never that sloppy, off by 1 or 2 thousandths at most, and I’d bet most of that was my collet runout. The cheap tools don’t cut as well and/or wear out sooner, but they are never that far off when new.
All the info here is very good. I agree with the notion that it’s better to cut and see on test material if available. There’s so many factors like the material, direction of cut, deflections, runout and so on that affect the actual size of the cut relative to the cutter. I wouldn’t program a custom tool for minor variance until it was confirmed wrong with real world results. Unless its an obvious metric vs. imperial mis-match
I’ll add another tangent to also consider the accuracy of the measuring tools. I have several “prosumer” or “budget precision” grade from woodworking or industrial suppliers. I could measure thin stock with 3 pairs of 6" calipers and get three different measurements which add up to well beyond the stated .001" accuracy. It makes cutting tight fitting joints for assembly that much harder.