# Pm-932m Feeds/speeds/doc/etc



## Bray D (Mar 25, 2015)

I wasn't sure where to post this, but I have a PM-932M so I figured I may as well post it in this forum. 

I'm new to machining and I see a lot of references to feeds and speeds. The tool speed makes perfect sense, however I'm struggling to understand feeds and appropriate depths of cut. 

Being a manual machine, I assume it will be difficult to quantify feed rates. That being  said, could you guys school me on machine feeds rates? What are the variables used, how do you quantify those variables, and what is the ultimate equation that's used to calculate said feed rate? 

That brings us to depth of cut. Is there a rule of thumb for finding the 'correct' depth of cut? 

I've just been simply listening/feeling/observing my machine and trying to obtain a feel through mechanical empathy. It would be nice to put some numbers to this stuff. Hopefully it increases productivity as well. I could very well be treating my machine 'too nicely'.


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## planeflyer21 (Mar 28, 2015)

Do you have a Machinists' Ready Reference?  Nifty palm-sized book, with literally hundreds of formulas machinists need.  There is an entire section on what speeds...roughing and finish...for what materials, with what type of tool (HSS or carbide).


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## wrmiller (Mar 28, 2015)

What a machinist uses on large automated machinery doesn't map well to what a hobby machinist does on a much smaller, less rigid, manual machine. Unless that hobby machinist is doing CNC of course, and even then unless you machine weighs several thousand pounds those formulas may not be apropos. 

You are already doing what most, including myself, do in that we 'listen' to the machine/cut. You can tell when it's happy, and when it's not (chatter/poor finish/broken bits). Each machine is different, and will respond to a particular operation differently. There are some general guidelines of course: slower/smaller cut for harder materials, while aluminum and other similar materials can go much bigger/faster.

As for treating your machine "too nice", well that just means it will last longer. You can of course attempt to always take the maximum cut you can get but that just means you will wear out your tooling and machine that much faster.


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## JimDawson (Mar 28, 2015)

+1 what Bill said.  I would say you are doing it correctly.


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## coolidge (Mar 28, 2015)

Now add the cutting tool variable, HSS vs carbide insert. Heck there's a maddening variety of just different carbide inserts to choose from that will perform quite a bit differently on the same machine. Carbide grade, shape, size, rake, did I mention it was maddening? You just have to figure out what works best on your particular machine.

This is no less true for 12,000 lb CNC machines, they also have to figure out what works best on their machines, how long the insert lasts is a factor, they do this largely through trial and error, recommendations from suppliers, other shops on what is working for them on the particular machine. Typically once they find something that works they stick with it, but they will still kick the tires on the latest new age insert from time to time. In production work if you can shorten cycle times 10%, 20% that's a pretty big deal.

As for being too easy on your machine, unfortunately we don't have a spindle load meter to help guide is. Even on industrial CNC machines its not generally a best practice to run the machine at the limit, sometimes it is but machines do wear out. If a machine is dedicated to churning out a part that pays for the machine in 3 months say, and it makes financial sense to run the machine into the ground at its max limit for 4 years then throw it away that's what you do. That's why I'm leery of purchasing a used CNC machine frankly.

My brother never ran his Haas VF2SS at more than 50% rapids for example. That thing will rapid at 1200 ipm which is insane. Spindle speed is 12,000 rpm but again he throttled it to between 6,000 and 8,000. After 5 years the machine still looked brand new and performed like new.


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## planeflyer21 (Mar 28, 2015)

I found all the recommended speeds/feeds in Machinists' Ready Reference to work very well on my Smithy Granite, as they did for the big machines in school and at work.  The only widely vastly different variable was depth of cut.

I'd agree that just because your machine _*can*_ hog off .100" cuts doesn't mean you need to operate there all the time.  Treat it well and make it last.


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## wrmiller (Mar 28, 2015)

Now the OP has a 932 which is very similar to my 12Z, and in HIS case, a .1 DOC is nothing. I've done .75" (not a typo) DOC on mine and while it was a little hard on the cutter as it was a dry cut, the mill handled it well. I just wouldn't recommend a steady diet of that type of cut if you want the machine to last very long. That is what I was thinking when I typed the above comments.


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## Bray D (Mar 31, 2015)

Thanks for all of the replies, guys. Sounds like I haven't been doing anything too out of line so far. I have access to a Machiney's Handbook, though I haven't spend a whole lot of time studying it as I assume it was created for far more rigid commercial machines. I may have to pick up a Ready Reference for future reference though. 

I haven't broken any bits yet so that's a plus. My heaviest cut thus far was a full cutter width .100 DOC with a 1/2" 4 flute HSS end mill in steel. It seemed to handle it pretty well. I ran 6 parts under those same conditions. .190 DOC with a 1/4" cutter in cast iron on another project. Definitely no issues with the 1/4" end mill. 

I'd like to start experimenting with different cutters, though that will get expensive quickly. I just ordered a set of 4 flute cobalt roughers, though. I'll keep 'playing it by ear' and see how it goes.


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## wrmiller (Mar 31, 2015)

I used a 1/2" cobalt rougher for that 3/4" deep cut. I'd not recommend that if you want the cutter to last very long though.


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## JimDawson (Mar 31, 2015)

wrmiller19 said:


> I used a 1/2" cobalt rougher for that 3/4" deep cut. I'd not recommend that if you want the cutter to last very long though.



.875 deep. 1/2 cobalt rougher, 1018 steel, WD-40 mist coolant.


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## wrmiller (Mar 31, 2015)

Ok Jim, now you're just bragging. 

That big honker of a mill you have there is a lot more rigid that our (mine and the OPs) bench mills. I DO like the mister or fogbuster idea though...been giving more serious consideration to something like that since getting the 12Z and having the ability to use larger cutters/more aggressive cuts.


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## JimDawson (Apr 1, 2015)




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## tmarks11 (Apr 1, 2015)

Find out what the sfm recommended for the material is.

Steel is 60-200 sfm (depending on hardness).  Aluminum is 150-300 sfm. These numbers are for HSS, about 3x that for carbide.

rpm = sfm * 4 / dia

so for 80 sfm steel with a 0.5" endmill : rpm = 80 * 4 / 0.5 = 720 rpm

Now that is for cutting with coolant, so drop by about 1/3 to keep temperatures good if cutting dry (and then adjust to get straw colored chips).

When starting out, feed rate tends to be the hardest thing to judge (hence the beauty of a DRO that tells you how fast you are feeding), and a lot of novices just wing it, which is not the secret to getting a good surface finish.  Adequate chip load is important as the metal the endmill removes helps keep it cool.  Too slow a feed rate results in excessive heating of the end mill, to fast a feed rate results in a poor finish.

ipm = rpm * chip load * flutes = 720 * 0.002 * 4 = 5.8 ipm

For endmills, generally use a 4 flute for steel and ALWAYS use a 2 (or 3) flute for aluminum. 

Chip load depends on material, and diameter of endmill (YMMV)
Steel: 0.o01"/flute for 0.25", 0.002"/flute for 0.5".
Aluminum: 0.004"/flute for 0.25:, 0.006"/flute for 0.5"

DOC choice is generally based upon spindle power.  There is a calculation for it, but I generally don't bother and cut based on sound.  A little bit of experience gets you comfortable with how deep to make your starting cuts.

Another way to this is to use a free online calculator (like the link below) or the more advanced offline GWizard of HSMAdvisor (all of which give you spindle power calculations):
http://zero-divide.net/?page=fswizard


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