Spindle Speeds for Mild Steel

[CJ5Dave]

I am trying to learn how to use my new to me milling machine. Doing some projects and practice on mild steel. Using 4 flute end mills, mostly 1/2 inch. I am running the mill at the slowest non-back gear speed (660 RPM). Seems to extrude more than cut the metal. These end mills came with the machine, and may be a little dull. I am doing manual feed on the Y axis and feed speed does not seem to change this. Is this spindle speed too slow?

 

[rabler]

RPM = FPM * 4 / diameter
roughly, actually (FPM * 12) / ( pi * diameter).
You want about around 100 FPM on HSS, or around 800 RPM for 1/2" endmill. Number of flutes effects feed rate, not rpm.
You're in the right ballpark. I take all of these calculations as starting guidelines, and try to build experience on how to adjust.

If by extrude, you mean the cutter is drawing up a significant burr, then yeah, probably dull. Is it hot rolled mild steel? The mill scale can dull endmills pretty quickly.

 

[Ischgl99]

Little Machine Shop has a nice speeds and feeds calculator. Looks like you are at the right speed range for your cutter.

Speeds & Feeds

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It sounds like the end mill might be dull, do you have any new ones you can try and see if they cut properly? If the end mills are good quality ones, it is worth getting them resharpened. I am very happy with Gruschow, turn around time is quick too.

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[CJ5Dave]

Thanks. Yes, there are new and old ones mixed together, some still in package, some not.

 

[Eddyde]

IMHO, You don't have to be too critical with feeds and speeds.Those calculations are for optimum rates but there is usually a pretty big window. I start off at the lower end of the range I think it should be, then push it up till its telling me it's too much (you'll know when you get there) then back off a bit. I almost never consult the charts or calculate F&S with HSS, but I do sometimes with carbide inserts as they tend to be more persnickety.

 

[benmychree]

I HAVE BETTER LUCK WITH 2 FLUTE ENDMILLS FOR HSS AND MILD STEEL, USING CUTTING OIL OR COOLANT FREELY.

 

[CJ5Dave]

The next speed step up is 1150 RPM. That’s a big step.

 

[benmychree]

The next speed step up is 1150 RPM. That’s a big step.

Not knowing all the details, it is difficult comment, but the next step up is usually a percentage of the prior RPM step. Relatively modern machines have speed steps in a geometric progression, that is, for example, the speeds may be in steps of 20% up or down between steps, it may look like a lot of RPMs between steps, but that is why machine too designers developed the method: It works and it turned out to be the efficient method, back in the old days, speed between steps were or could be quite random, either too much increase in speed between steps or too little for best efficiency.

 

[verbotenwhisky]

In mild steel with HSS 1/2" tool I run around 900+/-.