Choosing an end mill for the job

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

Moderators: Harold_V, GlennW

Post Reply
User avatar
tornitore45
Posts: 1568
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Choosing an end mill for the job

Post by tornitore45 » Wed Oct 17, 2018 5:02 am

For the same SFM, number of flutes and chip load an end mill 1/2 the diameter can turn twice as fast cutting twice as many chip per minute. It would seem it would be able co complete one pass in 1/2 the time.
How to choose the chip shape? Axial DOC X Radial DOC
Ideally one flute should always be cutting to avoid vibrations, for a 4 flutes tool that means Radial DOC > R, a tall order for an end mill. Still the smaller the diameter and the larger the Radial DOC the larger the tooth cutting arc, presumably reducing vibrations.
Of course smaller end mill are more fragile and deflect more.
Suppose one has to cut a rabbet of Depth X Width, how one goes about choosing the tool diameter, and the DOC in both directions?
Mauro Gaetano
in Austin TX

John Hasler
Posts: 964
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: Choosing an end mill for the job

Post by John Hasler » Wed Oct 17, 2018 9:29 am

Increasing the cutter diameter increases both the thickness and length of the chip so chip volume increases faster than diameter. This means that material removal rate increases with diameter at constant surface speed.

User avatar
tornitore45
Posts: 1568
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Choosing an end mill for the job

Post by tornitore45 » Wed Oct 17, 2018 11:24 am

increasing the cutter diameter increases both the thickness and length of the chip so chip volume increases faster than diameter. This means that material removal rate increases with diameter at constant surface speed.
Top
I do not see the geometry justification of that.
True the chip is longer because the the cutting arc is longer but arc angle is smaller. In other words the cutting Duty Cycle is lower for a larger tool.

Larger tools have a longer "Entrance" + "Exit" travel at reduced traverse speed.

I am sure that regardless of our theoretical single factor considerations 2 centuries of machining have tuned up the optimal approach by people whose livelihood depend on.
Mauro Gaetano
in Austin TX

User avatar
Dave_C
Posts: 951
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Re: Choosing an end mill for the job

Post by Dave_C » Thu Oct 18, 2018 4:41 pm

For the same SFM, number of flutes and chip load an end mill 1/2 the diameter can turn twice as fast cutting twice as many chip per minute. It would seem it would be able co complete one pass in 1/2 the time.
Ok, If the tool is only half as wide, then it has to take twice as many passes to remove the same material.

However, smaller end mills can't take as deep a cut as say the 1/2" end mill because of flute depth. The smaller end mills will pack the flutes so chip removal really isn't a constant such as you are applying in the hypothesis above.

Edit: I forgot to add in the "step over" factor. IF you use the same % of step over for each end mill then your chip length is very different even though the chip thickness remains constant. HP requirement then changes as well to make the cut with the larger end mill. So if you have the HP to make the cut, go with the larger end mill, turn it to the SFPM you want and go for it!

Four things to figure here: Cutting depth [how much of the tool length is cutting material], chip thickness [cut per tooth], chip width [tool step over into the material] and then the HP to make the cut.

Dave C.
I learn something new every day! Problem is I forget two.

Magicniner
Posts: 453
Joined: Thu May 30, 2013 4:40 am

Re: Choosing an end mill for the job

Post by Magicniner » Sun Oct 21, 2018 3:50 pm

tornitore45 wrote:
Wed Oct 17, 2018 5:02 am
For the same SFM, number of flutes and chip load an end mill 1/2 the diameter can turn twice as fast cutting twice as many chip per minute. It would seem it would be able co complete one pass in 1/2 the time.
Given that you should be running the correct rpm for optimum use of the cutter with the work material you will not have that option.
If you're not looking that stuff up you are wasting time ;-)

User avatar
tornitore45
Posts: 1568
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Choosing an end mill for the job

Post by tornitore45 » Sun Oct 21, 2018 5:08 pm

tornitore45 wrote: ↑
Wed Oct 17, 2018 5:02 am
For the same SFM, number of flutes and chip load an end mill 1/2 the diameter can turn twice as fast cutting twice as many chip per minute. It would seem it would be able co complete one pass in 1/2 the time.

Given that you should be running the correct rpm for optimum use of the cutter with the work material you will not have that option.
If you're not looking that stuff up you are wasting time ;-)
The correct RPM is the one that gives the correct SFM which is stated as a constant for both end mills.

If you are not reading the entire post you are not adding a useful reply. :wink:
Mauro Gaetano
in Austin TX

User avatar
Dave_C
Posts: 951
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Re: Choosing an end mill for the job

Post by Dave_C » Mon Oct 22, 2018 10:03 am

SFM which is stated as a constant for both end mills.
Correct, the SFM would be the same for both end mills! However, the chip load can be larger for the bigger end mill as the flute depth is deeper.

I do most of my work with my CNC mill and I use the largest cutter I can to remove chips as fast as I can. When I have to use a small 1/8" end mill, the chip load per tooth goes way down and so I have to slow the feed rate and thus the material removal rate goes down as well.

I think I'm lost in this thread as well as I don't see the point. Even though your SFM is constant for a given type of tool and material and the RPM may double however you are still up against feed per tooth for material removal. No way a small mill can remove as much material as a larger end mill.

Unless of course we are setting limits and making both end mills cut at the same chip load per tooth. That seems a bit silly to me! :roll:

Dave C.
I learn something new every day! Problem is I forget two.

User avatar
tornitore45
Posts: 1568
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Choosing an end mill for the job

Post by tornitore45 » Tue Oct 23, 2018 6:50 am

Thank you Dave_C for adding value to the thread. Your explanation make sense and explains instead of sterile arguing.
I like that attitude.
Mauro Gaetano
in Austin TX

Post Reply