Milling feeds and depth of cut?

[raferguson]

I looked some on the internet and on the forums, but was not sure if the cnc info was applicable to a manual mill-drill. I also assumed that the info related to carbide tools was not applicable to HSS. Bottom line, I was not sure that any of the info that I found applied to my situation.

I knocked off for the day after breaking my third end mill. :-( Presumably I am exceeding the limits in terms of feed rate and/or depth of cut. Think in terms of HSS end mills, 3/8" or 1/2" in diameter. What is reasonable in terms of depth of cut? Does it depend on whether the depth of cut is down or sideways? How about feed speed on the X or Y axis? I was working on creating a shoulder on a piece of 1 inch square bar, starting with a length of bar under 1 inch.

I also noted that the part tended to walk in the vise, presumably also a result of excessive feed and/or depth of cut. I suppose the small size of the piece did not help. It made me think that I might have been better off starting with a larger piece, and cut off the excess after the other milling is done. Maybe parts walking contributed to breaking things.

If someone could point me to the right website and/or video, that would be great.

Richard

 

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[jwmelvin]

The part moving in the vise is a bad sign and seems like a good way to break end mills. You need to sort that out. Perhaps you need a small piece of deformable material like aluminum between the workpiece and the moving jaw. Or a small ball to create point contact there.

You need to decide on the surface speed of your cutting tool based on tool and workpiece material. For high-speed steel tools, say 100 fpm in mild steel; so 750 rpm for a 1/2” diameter end mill. Feed should be easy enough to feel (you shouldn’t have to turn very hard).

One approach is to run full depth of cut (length of the end mill), with no more than say 15-20% of the cutter diameter engaged per pass. If your machine doesn’t like that try half the depth and 10% of the cutter diameter.

 

[ttabbal]

I generally run about 1/2 cutter diameter as max depth of cut. Particularly for a slot. DOC is about cutter engagement. Side or end is similar, but deflection is more likely with side loads.

Most of the charts and such are for carbide and are really a maximum that will only apply to very big rigid machines. HSS is generally 1/3 speed from carbide. Note that different materials matter. Aluminum speeds tend to be much higher than steels. For a manual machine, feed speed is less of an issue. RPM is going to be the big thing.

If the part is moving in the vise, something is wrong. That will definitely kill tools. Did you square the stock? Not square material is more likely to slip.

If you are trying to remove lots of material, consider roughing end mills.

 

[Weldingrod1]

I'm guessing you have a power feed?
I don't think I've ever broken a HSS end mill! And certainly not a 1/2" one. Yeah, moving in the vise is a sign of waaaay excessive feed vs, really, anything

Carbide feeds and speeds are radically different from HSS. Lile 100x different.

What are you cutting, anyway? Pictures of the carnage?


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[raferguson]

Thanks to all for their comments.

We all agree that having the part move in the vise is a good way to break end mills. If I do something like this again, I will put more material in the vise, and cut off the excess at the end of the process. It would have been quite easy to do that if I used that approach from the start, but I apparently needed to learn this the hard way.

I understand surface speed per minute. I will double check, but I think that I am in the ballpark.

I will try to attach a photo of the carnage, for your amusement. I was trying to make a T-slot nut.

But what about depth of cut? Presumably it would be unwise to try to mill the shoulder in one pass. Would I be well advised to try to cut the full width (about 3/16") with a 3/8 or 1/2 inch HSS end mill, but only take off 10 thou per pass? What is the correct sequence? Maybe someone could make a sketch showing how I should do this...... I was trying to lower the end mill about 0.015" per pass, towards the end.

No, I do not have a power feed. I have a stopwatch to help me guesstimate my feed rate, if I knew what feed rate makes sense.

Or is this all really based on feel? Presumably feel is the result of the experience that I do not have.

Richard

 

[JimDawson]

Here is a video you should watch, the narrative addresses some of the problems that you are experiencing. This video is of course showing high end CNC machines and carbide tooling in action, but the same principles apply to manual machining. One thing to note is the amount of material that the vice is hanging onto relative to the cuts being made. It takes a lot less grab than you might think, but does require a decent vice.

To make T nuts, I would grab onto about 1/8'' of the T and then just machine the sides at full depth with about 15% stepover on your machine. Conventional cutting. Your machine won't climb cut at all. Also lock the axes that you are not moving, on a light machine you have to get set up as rigid as possible. On a mill-drill, I would not use anything larger that a 3/8'' endmill unless you are making light facing cuts.

I think you need to take a serious look at your vice and find out why the material is walking.

 

[ericc]

Did you square the stock before holding it in the vise. Sometimes it is a bit off square and will move around.

By the way, your picture looks like that metal is pretty chewed up. Reminds me of the time I was in TechShop and the only endmill available had a broken off end. So, I tried to sharpen it on their big bench grinder. It did not have enough clearance to center.

 

[raferguson]

I went back and checked the RPM, 820 RPM. With a 3/8" end mill, that is about 80 SFPM, if I calculated it correctly. (107 SFPM for 1/2" mill). One reference suggested 125 SPFM for HSS, so the RPM is not the issue. As I was researching, I found references to feed per tooth, a concept that I had not previously been exposed to. I think that the tools I broke were 4 flute. 820 RPM would be 3280 teeth per minute. If I was aiming for .001" per tooth, I could make a one inch pass in 20 seconds. At 0.005" per tooth, I could make a one inch pass in 4 seconds; I cannot turn the knobs that fast.

The suggestion that my mill is not appropriate for climb cutting may be the key. Even though I understood what a climb cut is, I had not understood that as something to be avoided. (Kind of stupid, as I knew to avoid climb cuts with a wood router). I was feeding in both directions, a no-no. The climb cuts may be the root cause of my issues with part holding and end mill breaking. Presumably I can travel in the "climb cut" directions if I don't feed before I do that. The correct sequence, then, would be to cut in the right direction, reverse the direction of travel, feed the desired amount, and then make a second pass.

I did not think about the depth of cut as a percentage of the tool diameter. What do you do when you are forced to cut the full tool diameter, as when machining a slot? Take very light cuts? I have been drilling before cutting a slot, as using the end mill as a drill did not seem to work very well.

I like the suggestion of locking any axes that I am not using. I had been locking the Y axis. It is probably not realistic to lock and unlock every 20 seconds, however.

Thanks again for all the comments. Further comments would be appreciated.

 

[compact8]

My mill is just hobby-grade ( EMCO FB2 ) and I judge whether the RPM / feed speed are right by feeling only. If the sound is right and the resistance of the hand wheel doing the feed is right then the settings are about right. Very unscientific I know but I just found the guidelines found on line unusable. For example, to cut aluminium with a 10mm carbide end mill, a guideline I found says that an RPM of at least 5800 is needed but my machine can only do 2000 max. However, my experience is that 2000 works just fine so I never looked at those guidelines anymore but just build my only experience.

The breakage of your tool is likely caused by the movement of the workpiece. It appears that you were clamping it at the sides so movement can happen especially when you are using end mills on the big side. I will try to fix the workpiece in other ways :

1) If several nuts are to be made, you can make a T bar first and cut them out into nuts afterwards.

2) If only one is to be made, I will clamp it at the top and bottom of the nuts instead of the sides when cutting the shoulders. The increased surface area coming into contact with the vice will be bigger hence it will be more secured.