Breaking small end mills!

[Arnak]

Hi,

I could do with some advice please.

My 1/8" milling cutters keep breaking when used on steel, I use a slow rate of feed and only take small cuts but still they break.

Also plenty of cutting oil.

What am I doing wrong?

Arnak

[AndrewP]

Are you running it fast enough? The theoretical speed for a small cutter like that is usually 'off the scale' on our mills. I broke several small mills before I plucked up courage to run them at 2000 rpm which is the maximum for my X1. No more breakages but lots more noise!

Cheers, Andy

[chris vine]

Hi Arnak,

I wonder if you are running them fast enough? I don't have the table to hand but I expect the correct speed with cutting fluid is at least 2000 rpm if not more.

The other thought is if they are running true? if a few thou out, then they will only be cutting on one tip.

Are they long series cutters? These are specially designed to break!

Are they blunt?? The difference between blunt and new cutters is amazing. If needed, I find that cutters from J & L supply are excellent value.

Chris.

[chris vine]

Andrew and I replied at the same moment!!

C

[alanstepney]

Ideally you need (roughly) 100ft/min speed with (again roughly) a cut of 6 thou per tooth.
At 1/8" dia, that gives you a speed of around 10,000 rpm.
Wouldnt that be nice!!

As both Andrew and Chris have said, we run our machines far too slow.
Assuming you have 1,000 rpm available, a a very light cut, you should manage OK without the cutters breaking.
The other possible cause is lack of rigidity. Anything moves whilst cutting, and goodbye cutter.!

(Errum above figures done quickly in head so dont take them as gospel!)

[havoc]

Indeed, speed is the point. I just learned that running the lathe at 1000rpm instead of 250rpm and a cutting depth of 1mm instead of 0.2mm like I used to do not only works faster but also gives a better finish.

Where did the myth of "slowly" came from.

[AndrewP]

OK, I found the table, Tubal Cain's Model Engineer's Handbook - where else?

He lists for endmills (4 flute) in bright drawn mild steel an rpm of 1920 and a feed rate of 1 in/min.
For a 2 flute slot drill these figures become 3200rpm and 1 5/8 in/min.

Usual caveats I suppose of endmill width of cut less than 1/4 cutter diameter and depth less than actual cutter dia, for slot drills the test is depth of cut less than 1/2 cutter diameter.

Simultaneous agreeing responses eh Chris? so is that 'great minds.......' or 'fools seldom.....' ;D

[Lurkio]

Arnak,
Further to the good advice given above, make sure the swarf is getting away from the cutter. Wash it away with a powerful flow of coolant, and gently clear it away from the area with a small brush.

Lurkio.

[Arnak]

Hi Folks,

Thanks very much for the replies, first to answer the questions. )

Brand new end mills, not long series and yes plenty of lubricant, like Niagara Falls....

Speed... ah well,I bet that is the answer to the problem.

I tend not to change the speed as it is a fairly large mill and awkward to do and I've always erred on the side of caution with regards to spindle speed.

I bet it's because I tend to think of them as drills and try not to run too fast to avoid burning them.

So right it's time to shift the belts to the max speed for these small sizes and see if that makes a difference.

Thanks again for the advice.

I'm going to the bristol exhibition so I can buy some more there I expect.

Arnak

[Jo]

The other question is are you actually in need of "plunge cutting"? So often my natural laziness encourages me to use a slot or end mill when actually I am cutting a slot and a slitting saw would do the job much easier....

Talking about thin slots: Anyone got any ideas on how I can cut the 20 thou wide slots for my steam siren? They are 70 thou long: I am assuming that I will take the brass down to 40 thou thick after I have cut the slots.

Jo

[Tel]

jo said:

Talking about thin slots: Anyone got any ideas on how I can cut the 20 thou wide slots for my steam siren? They are 70 thou long: I am assuming that I will take the brass down to 40 thou thick after I have cut the slots.

Jo

Cut-off wheel in the Dremel, set up on the toolpost?

[Jo]

That was my initial thoughts but with the 70 thou length and 40 thou depth the cut off wheel could only be 80 thou in diameter and then it would only just touch the bottom of the slot.

Jo

[Arnak]

Hi Jo,

Slitting saw, Bangs head in frustration.. why didn't I think of that. 8-((

To right, that would have been a lot easier, thanks very much Jo.

Arnak

[ausdan]

on the subject of feed rates, having to be self taught on my mill....How do you actually work out how fast to feed to obtain the feed rate speed needed?

Do you just work out in your nogen (head), something like 1 foot a minute, takes 3 turns of the feed to move a foot, so about 20 sec per 1 revolution of feed wheel, would do it?

[Myford Matt]

There's a nice explanation of how to break (and avoid breaking!) cutters in GHT's Model Engineer's Workshop - reason no.786 for owning his great book.

Available here: www.hemingwaykits.com/acatalog/Miscellaneous_Items___Books.html

MM

[Jo]

I would not calculate them rather look them up, from a table such in TC's handbook:

 

Cutter Steel                  Brass Or Iron       Aluminium             Cutter 
 Dia     RPM      Feed       RPM    Feed          RPM   Feed            Dia
                  mm/in            mm/min               mm/min
1/16”   2000     5           3800   12              *          *               1.5
1/8”    1000     10          1920   25             8100        *	    3.0
3/16”   700      16          1280   35             5250        *	    4.5
¼”      500      22          960    45             4000        200             6
3/8”    350      28          640    60             2700        *               8
½”      250      50          480    75             2000        *              13
5/8”    200      35          385    80             1600        *              16
¾”      150      35          320    85             1350        *              20
1”      130      37          240    72             1000        *              25

Feed rates marked * are even higher!!

As TC says in the book this is a starting point... slower means longer tool life and longer machining times, which is not normally so critical for us. Slot Drills are different again, faster.

Jo

[chris vine]

Hi Andrew,

We must be Telepathic.....

Arnak, Maybe see you at Bristol on the Friday. I am taking Bongo down, so will problably be lurking not too far away.

Maybe see some others as well, I hope so anyway....

Chris.

[Myford Matt]

Chris, I'm a big fan of your book and would really like to see Bongo in the flesh. Unfortunately, I won't make Bristol - do you have plans for any other shows this year?

Cheers

MM

[chameleonrob]

ausdan said:

on the subject of feed rates, having to be self taught on my mill....How do you actually work out how fast to feed to obtain the feed rate speed needed?

Do you just work out in your nogen (head), something like 1 foot a minute, takes 3 turns of the feed to move a foot, so about 20 sec per 1 revolution of feed wheel, would do it?

I'm partially self taught as well, (I learned the theory on big machines with power feed and the like, different entirely to my X1) you can tell allot about the feed rate by the metal chips coming off, too thick and the there is too much heat even if you don't break the cutter, not enough and you won't cut much metal in the life of the cutter, practice with larger cutters!

on a separate point I tend to ignore the advice of Tubal Cain on depth of cut, I normally set the cut up to use about one diameter in depth but only about quarter of a millimeter per pass, just enough so that any one tooth is cutter all the time (less chatter too), this means that you are using most of the teeth of the cutter rather than just the last quarter diameter of it.

I always try for the largest feed rate I can, even if it means slowing the spindle speed to stop the cutter getting to hot, I find that this substantially increases the amount of metal that can be removed in the life of the cutter.

rob

[Arnak]

Hi,

Thanks foor the chart Jo, I'll check what speed I am running at.

Chris, Bongo? a model I assume, can't be the dog or the wife? 9-))

I'll look out for you tomorrow.

Arnak