RDOC, ADOC, IPM, IPT, oh my... Help!

[wildo]

Can we talk speeds & feeds for MANUAL milling for a second? I'm trying to do things "right" and utilize my tools appropriately. There are online calculators, so it's not hard to figure these things out according to the spec sheet from the end mill manufacturer. But here's what I don't understand. Say I have a 1/2" end mill with these statistics and we want to cut aluminum:




You can see that the recommended SFM is 800.
800 x 3.82 / .5 = 6112 RPM

Ok, well my manual mill doesn't reach those kinds of spindle speeds. I max out at 2660 RPM.
2660 x 3.82 x .5 = 348 SFM.

**348 SFM is only 43.5% of the recommended speed.

We have our speed, but now we move on to the feed. The chart recommends .005 IPT for the 3 flute 1/2" cutter.
6112 x .005 x 3 = 91.7 IPM @ 6112 RPM
2660 x .005 x 3 = 39.9 IPM @ 2660 RPM

**39.9 IPM is still 43.5% of the recommended feed, so it's directly proportional with the reduced SFM


We have our speed & feed, but now we need our depth of cut. The general rule of thumb that I've seen is that profiling cuts should be 1D (one times the diameter of the cutter) and slotting cuts should be .5D. I can't seem to find any formula that relates spindle speed and work feed to the actual depth of cut. So here's my question: Since the feed rate is directly proportional to the spindle speed as proved above, does my ADOC or RDOC need to be modified as the speeds & feeds are reduced? If I can't possibly reach 800 SFM, or if I can't possibly feed 91 IPM, does that mean that I should not be taking a 1D profile cut or a .5D slotting cut? I can't find any formula that relates speeds & feeds with DOC, but it seems reasonable that if my speed & feed are only 43.5% of the recommended values, then maybe my DOC should only be 43.5% of the generally accepted values. This is the confirmation I'm looking for.

 

[benmychree]

I think that the depth of cut would be the same regardless of surface speed and feed.

 

[mikey]

I'll take a shot at this. I'm sure the other guys will be along to correct me.

First thing that comes to mind is that you have a small mill with 1 HP and a max speed of 2600 rpm. You have no power feed and no X-axis DRO or any kind of computer control of that axis. The numbers in the charts are intended for industrial CNC machines that crank out part after part and where MMR actually has some significance. Would that you had Haas's new 60HP, high precision 10K spindle but you don't.

Second thing is that you have what you have. If you can only go 2660, then you can only go 2660. If you're feeding by hand then you need to learn how to feed by hand. Not being a dick; I feed by hand, too, but I know how to do it. I cannot nail the feed in IPM but I can get the job done accurately, usually the first time and with a good finish.

Your DOC determines your speed, and hence, your feed. You do not have to take a maximum ADOC or RDOC. In fact, most of the time you will NOT be taking these depths of cut. The shallower your depth of cut, the faster your speed can be. However, in the case of aluminum, you cannot go fast enough anyway so max your speed and make your cut. I posted those charts from Niagara Cutter; use those to determine speeds for the various depths of cut you need to take.

It is always faster to bandsaw stock to size than to cut it to size with an end mill. Most of the time, you will get your stock as close to size as you can with rough methods like this. Then you need to square it up so you can mill it. We see all the videos where a chunk of material is put in a vise and the end mill does it all. In a hobby shop, that is not reality. Well, at least not in my hobby shop.

I actually don't care about MMR, IPM, chip load and that sort of thing. It isn't that I ignore it or that it isn't important. It's more that I'm a manual machinist in a non-CNC shop and I like it like that. I can still make accurate parts; I just can't do it as fast and I can't churn them out. On the other hand, if something goes wrong ... I am not paralyzed. I have pushed end mills until they snap, just to see what it took to do that. I'm over it now. Now I am content to make parts that are accurate, albeit much slower than a CNC machine can.

Bottom line: you know the DOC limits of your end mills; you do not need to work at those limits. Your machine can only go so fast so learn to machine at those speeds. You can certainly go to max depth of cut and run at slower feeds and speeds. Most of us do this all the time and the parts actually don't know what it took to cut them.

 

[wildo]

You have no power feed

I do, it's just broken at the moment.

If you're feeding by hand then you need to learn how to feed by hand. Not being a dick; I feed by hand, too, but I know how to do it.

Didn't think you were being a dick, and I'm not either when I ask: ...so how do you do it then? I mean, ultimately that's the real question here. How do you know if you're cutting "appropriately" or not? If the end mill is capable of making those crazy fast HEM type cuts, then does that mean that I can definitely push it to the point of stalling the motor without worrying about breaking it? I don't think it takes a rocket scientist to figure out "the feel" as the difference between the two end mills I tested was night and day in the feel. But feel alone doesn't mean that I'm working the right way; it just means that I'm working an acceptable way. Does that make sense? I'm here to learn, so that's why I asked the question.

Your DOC determines your speed, and hence, your feed. You do not have to take a maximum ADOC or RDOC. In fact, most of the time you will NOT be taking these depths of cut. The shallower your depth of cut, the faster your speed can be.

Right- this makes complete, and obvious, sense to me. DOC determines speed. And yet... I can't find a single formula that actually quantifies this. That's why I posed the question.

It's more that I'm a manual machinist in a non-CNC shop and I like it like that.

ME TOO!!! Believe me, you won't be seeing a CNC machine in my hobby shop any time soon. They are neat, but I'm interested in machining. I already write code for a living...

 

[mikey]

I do, it's just broken at the moment.

Unbreak it quick! I hate cranking ...

Didn't think you were being a dick, and I'm not either when I ask: ...so how do you do it then? I mean, ultimately that's the real question here. How do you know if you're cutting "appropriately" or not? If the end mill is capable of making those crazy fast HEM type cuts, then does that mean that I can definitely push it to the point of stalling the motor without worrying about breaking it? I don't think it takes a rocket scientist to figure out "the feel" as the difference between the two end mills I tested was night and day in the feel. But feel alone doesn't mean that I'm working the right way; it just means that I'm working an acceptable way. Does that make sense? I'm here to learn, so that's why I asked the question.

I don't know about you but the way I answered the question as to how hard I can push an end mill and my machine was to actually push the machine and break some end mills. Honest, that's what I did. I learned that when I perceive the machine pushing back, I am about to bend that end mill. Shortly after that, it will break. To learn how that feels, I had to break some end mills. And I broke good ones, too. Nowadays, if I feel the need to feed faster, I lighten up on the depth of cut and make myself happy. Sorry, Willy, but I don't know how to describe what I feel, any more than I can tell you how long a piece of string is.

Right- this makes complete, and obvious, sense to me. DOC determines speed. And yet... I can't find a single formula that actually quantifies this. That's why I posed the question.

Don't know of any formulas, although I'm sure someone does.

ME TOO!!! Believe me, you won't be seeing a CNC machine in my hobby shop any time soon. They are neat, but I'm interested in machining. I already write code for a living...

Then you're exactly the kind of guy that CNC was made for!

 

[Bob Korves]

Chip thickness is what you really want to compute and realize. Too little and you unduly dull the cutter and don't get much done, too thick and you bog down the machine and/or break something. I usually try to get real chips, not skinny sharp needles -- at least when I am roughing.

 

[wildo]

I don't know about you but the way I answered the question as to how hard I can push an end mill and my machine was to actually push the machine and break some end mills. Honest, that's what I did. I learned that when I perceive the machine pushing back, I am about to bend that end mill. Shortly after that, it will break. To learn how that feels, I had to break some end mills. And I broke good ones, too. Nowadays, if I feel the need to feed faster, I lighten up on the depth of cut and make myself happy. Sorry, Willy, but I don't know how to describe what I feel, any more than I can tell you how long a piece of string is.

OH!! OK, to be honest that's not how I took your initial message. But now I see what you mean about pushing a few to breakage in order to develop that feel for the threshold. That actually makes a lot of sense... Thanks! So maybe I bit off topic, but what happens when a carbide end mill breaks? Does it fragment like a grenade? Do I need a blast shield? haha! (No really, I don't know what happens)

 

[mikey]

I snapped a 3/8" carbide end mill, 4 flute, on entrance to a slot. Seems I was going a bit fast on feed and it didn't like it. All I heard was "snap" and half the end mill dropped off. I haven't 'sploded anything yet.

 

[JimDawson]

But feel alone doesn't mean that I'm working the right way; it just means that I'm working an acceptable way. Does that make sense? I'm here to learn, so that's why I asked the question.

Feel is all you really have to go by when operating manually (or in many cases CNC for that matter). There really is no right or wrong, but rather what works for your machine and job. If your machine is happy, then all is good.

 

[MarkM]

No matter what machine your working on learn what the chips are telling you. Personally I feel a deeper cut helps keep things rigid. Not talking about full slot here. I tend to have a 10-20 percent radial cut of the cutter diameter.
Say a half inch four flute endmill. Take a depth of .100" at .050" and take a cut. Now do the same and take a 1 inch. depth at the same radial cut of .050". The deeper cut has less flex in the tool and will be more rigid.
Each machine has it own capabilities. Take recommended speeds and feeds as an indicator. Maybe stick with hss to keep the rpms more in line. Carbide will do fine at the lower speeds but your paying for something your not using.
I tend to climb cut whenever I can. Tighten all gibs or you ll ruin your cutter and maybe the work.