# How Do I Tell When ....



## Transformer (Apr 27, 2015)

an end mill is dull?  I am really on my first project, working on steel on my Grizzly mill using HSS end mills.  As I was playing around with feeds and speeds and depth of cut I realized I do not have the knowledge or experience to tell whether my setting is off or I have a dull end mill.  At this point I am pretty sure my new end mill is fine, but how do I tell when it is getting dull?
Dennis


----------



## Karl_T (Apr 27, 2015)

The first sign is heat. You need more coolant and it starts steaming more.

The corner of the EM is the first spot to go. Hold a light to reflect. A sharp EM will break from one surface to the other with no reflection. As they dull you see light reflecting off the dulled surface.

You can also feel the sharp edge with your thumb. Some use a fingernail.

This is just an experience thing. For some jobs you can push them till they burn up. others, the mill need to be perfect. small mills need to be sharper.


----------



## smoa (Apr 27, 2015)

Another indicator is your surface finish will deteriorate. Your machine will also be under a larger load, or working harder.


----------



## JimDawson (Apr 27, 2015)

Great question!

What everyone said above is correct.  In more extreme cases of dull, the machine will actually shudder on a heaver cut (or sometimes on a lighter cut).  As you gain more experience you will be able to tell pretty quickly when the end mill is done.  When starting out with a new, sharp end mill, it will pretty much cut the same for some period of time then all of sudden everything will change, you can feel it and hear it.  It's at that point you have to decide to continue cutting or change the end mill.

End mills have a finite life and normally, under optimal conditions, will last about 80 minutes.  Excess speed is the biggest killer, as is too much or not enough feed.  Too much speed and/or too much feed = excess heat.  Too little feed causes the end mill to rub rather than cut, also  = excess heat.  There is a sweet spot for any job you are doing, and you will be able to find it pretty quickly as you gain experience.


----------



## Tony Wells (Apr 27, 2015)

For me, other than the feed pressure if I am hand cranking it, I watch for a large burr to be pushed up at the edge of the cut. Sharp end mills leave smaller burrs. When it gets noticeably harder to push, and I see a large burr.....it's done.


----------



## chips forever (Apr 29, 2015)

Transformer said:


> an end mill is dull?  I am really on my first project, working on steel on my Grizzly mill using HSS end mills.  As I was playing around with feeds and speeds and depth of cut I realized I do not have the knowledge or experience to tell whether my setting is off or I have a dull end mill.  At this point I am pretty sure my new end mill is fine, but how do I tell when it is getting dull?
> Dennis


Here is a trick you might want to try.  It does help to keep end mills a tad sharper.  Take a very fine grit stone and carefully angle it to match the flute and draw it across the flute.  Do it for each flute.  It is not so much to sharpen but to remove burrs that form thus keeping the end mill a tad sharper


----------



## kingmt01 (Apr 30, 2015)

Good question. Glad you asked. 
I'm trying to teach myself. & when doing so you don't actually know what to ask ahead of time until you get yourself in a corner. Reading the questions of others helps avoid this.


----------



## David M (Apr 30, 2015)

Speed/feed charts aren't hard to find, either look around on this site or an internet search. Get you in the ballpark anyway.  A dime store magnifying glass can come in handy for a good look at the pointy end of your tooling too.


----------



## Bill C. (Apr 30, 2015)

Karl_T said:


> The first sign is heat. You need more coolant and it starts steaming more.
> 
> The corner of the EM is the first spot to go. Hold a light to reflect. A sharp EM will break from one surface to the other with no reflection. As they dull you see light reflecting off the dulled surface.
> 
> ...



The thumb nail trick I learn from a older tool maker.  I used it to test reground die sections for sharpness.


----------



## Lood (May 1, 2015)

Thanks to all from me also. I'm brand new to milling and I just learned a lot from this thread.


----------



## RVJimD (May 1, 2015)

Thumbnails for fish hooks rule was set the hook point down on your thumb nail and if it would glide off easily it was dull, if it grabbed and left a scratch mark it was sharp.

Jim


----------



## brav65 (May 2, 2015)

Great question and great answers. Having been a woodworker for too many years I can tell instantly when a tool is dull just by how it is interacting with the material, what the machine is doing, and what the cut looks like.  I usually notice the first two and then confirm with an inspection of the cut.  I think that all of us rookies need to put some chips under our belt and experience will be our teacher.  A retired machinist gave me a bunch of used tooling some of it actually no good and told me to try them all out on scrap to see what the results were. It has been a great learning experience for me. I can see what a chipped mill does vs. a dull mill vs. a bent mill...  I spent most of one day trying them out at different speeds and feeds and materials. When I  took out a new high dollar 1/2" 4 flute to try on some steel is was amazing to feel the difference.  Looking at your cuts under magnification is also a good experience.  

I spend a great deal of time at least skimming posts here as you never know what little tip one of the experienced guys will mention, or a short story.  This is an awesome group that has thought me so much as I dive into the world of metal.


----------



## Alphawolf45 (May 3, 2015)

I seldom use HSS. Good to start with. Yesterday I was using a 3/8" endmill that looked new but I knew it was going dull on me .Finally it starting pulling out of the collet and cutting deeper than desired...That could be disastrous on some projects...Another thing to watch for- milling slots will want to cut to the side opposite endmill rotation...A really dull endmill will mill a slot well over to the side as the endmill flexs away from the cut.


----------



## itsme_Bernie (May 3, 2015)

+1 on this fantastic forum where I learn from threads I was just perusing for the heck of it!  Ha hah


----------



## C&I Harry (May 3, 2015)

brav65 said:


> Great question and great answers. Having been a woodworker for too many years I can tell instantly when a tool is dull just by how it is interacting with the material, what the machine is doing, and what the cut looks like.  I usually notice the first two and then confirm with an inspection of the cut.  I think that all of us rookies need to put some chips under our belt and experience will be our teacher.  A retired machinist gave me a bunch of used tooling some of it actually no good and told me to try them all out on scrap to see what the results were. It has been a great learning experience for me. I can see what a chipped mill does vs. a dull mill vs. a bent mill...  I spent most of one day trying them out at different speeds and feeds and materials. When I  took out a new high dollar 1/2" 4 flute to try on some steel is was amazing to feel the difference.  Looking at your cuts under magnification is also a good experience.
> 
> I spend a great deal of time at least skimming posts here as you never know what little tip one of the experienced guys will mention, or a short story.  This is an awesome group that has thought me so much as I dive into the world of metal.


I agree with the magnification tip. Myself, I am very new in this game but decided up-front to get myself a good magnifying glass. After every cut I inspect the tool to see how it is doing, and either change my process or my tools to suite the next time I have to do a similar cut. I quickly learned from these observations that its sometimes much beter to increase the feed rate than to stay on the safe side and dull the tool through rubbing.
During my very first cut on the lathe I chipped the first carbide insert while trying to cut the thread off a high tensile bolt. After the break the cut went much smoother and I could not understand why. Under the magnifying glass, the reason became evident. The break resulted in a large negative angle, which was much better for the interrupting cutting required to remove the thread!


----------

