# PM932 - Milling depth and speed for A36 Steel



## tlmartin84 (Feb 4, 2020)

First off, I am using A36 because scraps of it are free to me.

This piece had been cut with a torch and I squared it up with a grinder and cutoff disc, so there is a chance I was still milling in the HAZ area......

I was just attempting to face it smooth, I've had the mill for a while now and besides aluminum this was the first thing I really need to mill that was "heavy".

I chucked up a 2 flute (probably should've been using a 4 flute) 1/2" HSS endmill and tried to go at it.  I started out trying to take .010" running at 1180 rpms, I had Excessive chatter so I slowed the feed rate down to a crawl, and it didn't help.  I backed off to .005" but even at that I had a lot of chatter, enough that the down feed would back (shake) itself off while going across the 2" long cut, losing a couple thousandths. by the end.  Through the cut, it had areas where it seemed to chatter more than others....

What do you think?  Should I be able to take a .010" pass?  Was it because I was using a 2 flute mill?  Was it the HAZ?  Wrong RPM's?  Or am I trying to do too much?


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## davidpbest (Feb 4, 2020)

Here's the feeds/speeds I get for your A36, 1/2" HSS 2-Flute end mill.   You might want to check that the head is locked down to the column.


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## tlmartin84 (Feb 4, 2020)

I definitely had the head stock locked down....check it multiple times to make sure the chatter wasn't loosening it...

Am I reading that right? Depth of cut is .875"??? Width is .1475"? @ 3.74 in/min?

I was making 4 passes across it. cutting with nearly the full edge of the endmill, where as I should have only been cutting with a portion of it?


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## TomS (Feb 4, 2020)

For what it's worth I have a PM-932 CNC mill and recently ran a A36 job using a 3/8" 4 flute un-coated carbide end mill at 1/2" DOC, .040" WOC at 3360 RPM and 28 IPR (.002" IPT).  Probably could have increased the WOC a bit.  I had no chatter.  You might have excessive gib clearance on your X and/or Y axis.


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## pstemari (Feb 4, 2020)

The other thing to consider is that chatter is usually solved by reducing the surface speed and increasing the feed rate. Very light feeds have a hard time starting the cut and the edge of the end mill kind of skips on the material. This is especially true if you're conventionally milling and not climb cutting.

With a conventional cut you're starting out sliding across the material and it gradually ramps into the cutting edge. With a climb cut the edges is going straight into the uncut surface of the material instead of traveling along the cut side and attempting to dig in. The downside, of course, Is that climb cutting will yank all the backlash out of the feed screw, and if there's too backlash much it'll snap the cutter doing it.

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## davidpbest (Feb 4, 2020)

tlmartin84 said:


> I definitely had the head stock locked down....check it multiple times to make sure the chatter wasn't loosening it...
> 
> Am I reading that right? Depth of cut is .875"??? Width is .1475"? @ 3.74 in/min?
> 
> I was making 4 passes across it. cutting with nearly the full edge of the endmill, where as I should have only been cutting with a portion of it?


Corre


tlmartin84 said:


> I definitely had the head stock locked down....check it multiple times to make sure the chatter wasn't loosening it...
> 
> Am I reading that right? Depth of cut is .875"??? Width is .1475"? @ 3.74 in/min?
> 
> I was making 4 passes across it. cutting with nearly the full edge of the endmill, where as I should have only been cutting with a portion of it?


Best practice is width of cut is maximum 60-70% of the diameter of the end mill.  Max depth of cut is a function of the rigidity, horsepower, and feed rate, but you should be able to hog off ⅛” at the proper RPM and feed rate.   Here is a revised calc based on .125” DOC, and .350 width of cut.  The feed rate is 4.4 IPM at 745 RPM, requiring 0.11 horsepower.   I too suspect your gib adjustments are too loose.


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