How do I know a cutter is still sharp?

[chris.trotter]

So I've started experiencing a lot of chatter/jumping around with my hogging cutter, and I'm pretty sure it's the cutter going dull?... maybe? But I have no experience with dulling a cutter, so don't know what 'dull' symptoms to look out for.

I tried moving the table into the most rigid position, locking the axis not in use, ensuring the workpiece is really clamped down. This is happening on both large and small workpieces, both clamped direct to the table and in the vise. The exact same setups a month ago were cutting fine. Even a few days ago things were cutting just fine, although by the end of the second-to-last session I could see the cutter was doing weird stuff.

I generally only ever use the bottom 10-15 thou of the cutter. Perhaps this is also a case of wrong tool for the job? (this is a Sieg SX2P/LMS 3960, so can't get super big cutters on it I think) . The cutter itself is only a few months old, maybe in use for ~8-10 actual machining hours at most?

The job in question is squaring up raw stock (mild steel), and I have been using a 3/8" hogging 4-flute cutter, almost always just making passes across the surface until flat, then flycutting it smooth (last 1-2 thou).

Ideas? Are these classic 'dull cutter' symptoms?

 

[extropic]

When you mention "chatter/jumping around" it makes me think your machine isn't rigid enough. Your cutter may still be sharp enough to do work in a more rigid set-up but not in your current set-up. As the cutter gets less sharp, the cutting forces go up so maybe you've reached the tipping point.

Is the cutter (end mill?) in question an economy model or top quality brand?

 

[chris.trotter]

It's definitely economy, but has cobalt. This is the exact cutter

I am pretty sure this is not a rigidity issue, but I suppose I could be reading things wrong.

Is it normal for cutters to wear out this fast? Is there a guide to 'extending cutter life' somewhere? Or a list of 'things that contribute to cutter wear'? I use brushed-on motor oil when cutting steel, and have a different cutter for aluminum.

 

[mikey]

Chris, have you looked at the tips of the flutes under magnification? Your symptoms do sound like the cutter is dull or damaged and a quick inspection will tell you if this is so.

As an aside, I am going to guess that the vast majority of us do not use a roughing end mill to flatten the surface of a work piece before fly cutting it. We just start with the flycutter.

 

[chris.trotter]

Not under magnification, but it does kinda look like they are damaged. Just inexperienced, so don't know how long a cutter should last, or what a bad cutter is like to operate.

I haven't been using the flycutter to flatten material because the mill can't deal with more than ~3-5 thou per pass - it feels very smooth and comfortable taking 1-2 off, but more than that and it gets a bit sketchy.

Would something like this work better for hogging off (relatively) mass amounts of material on a mini-mill? this

 

[mikey]

I'm wondering why your fly cutter can only take such light passes. My little Sherline mill routinely takes 0.020" deep fly cutting passes in steel and my RF-31 can do far more than that.

Which fly cutter are you using?

A roughing end mill can last a long time when used properly. I have roughers that are near 10 years old and still cut well. One thing that will prematurely wear an end mill is to take baby cuts with just the end of the tool. All the wear is concentrated there so the tips of the flutes wear fast.

 

[chris.trotter]

Yeaaaah that's exactly what I've been doing. Probably a number of things going wrong here.

I'll give the flycutter another shot, probably needs some regrinding/sharpening. How often do you have to/should you true up/sharpen a flycutter toolbit?

 

[mikey]

I use carbide but back when I used HSS, I would hone the tool after every use.

 

[extropic]

Big cutters on mini mills exacerbate the problem.
I think your weak link is the economy cutter.
I recommend you buy yourself one or more top quality end mills and see what that experience teaches you.
You need to keep the cutting forces low so your low power/not rigid machine can handle the work. Use correct speeds and feeds. Don't let the cutter overheat.

 

[benmychree]

Motor oil is not a good substitute for proper cutting oil, in fact, I'd call it a very poor substitute; I use TapMagic for most all cutting applications in my shop, including threading, drilling and tapping and milling on my vertical mill, on the lathe and horizontal mill, I use flood coolant. Lubricating oil does not promote free cutting as does proper cutting oils.