Critique my flycutting results

chris.trotter said:
I read something somewhere (lol) that said HSS needs faster speeds (like 1500+rpm). During my geometry tests, I try low and high speeds.

If by diameter you mean, the circle the cutting point turns around, I think it's something like 3-4". Is this something I can look up in Machinery's Handbook? (I have a copy, but have no experience in using it as a tool)
Click to expand...

Yes, the diameter of the circle that the cutting point turns. The appropriate rpm is determined by the diameter of the cutter, the larger the cutter, the lower the rpm must be. In my previous post, I gave an approximate formula for determining the correct rpm based upon the cutting speed of the material (in surface feet per minute) and the diameter of the cutter. The formula is: Cutting speed (of material) x 4/diameter. This is for high speed steel cutters, carbide can be run 2-10 times as fast.

The cutting speed of C1018 steel is given as 80-100 sfm (surface feet per minute). 4x80=320/4 (diameter of cutter)=80 rpm.
You may find higher cutting speeds listed in different charts, I tend to be conservative to prolong cutter life.
This formula is used for any type of high speed cutter including lathe tools, drills etc. Memorize the formula. For quick reference, a 1 in. diameter drill should be run at 320 rpm, 1/2 in. at 640 and 1/4 in. at 1280. Remember these numbers and you will never be very far off.

I think your grind is fine, you're just running the cutter much too fast. As I suggested before, with a fly cutter of this diameter, you would be better served with a brazed carbide bit.
 
In Machinery's Handbook, look up the section on Speeds and Feeds. There are tables of cutting speeds for all types of materials.
 
That's excellent, thanks. Apologies for glossing over it earlier, and thanks for taking the time to respond.

I have a pile of brazed carbide lathe bits, will give it a try. In my head I keep hearing "only use carbide if your machine can handle it" (my understanding is that to effectively cut with carbide, you need horsepower and speed), which is mentally blocking me from thinking it'll work well.
 
Jeeeepers...hope the outlook is good for you. You're a champ for posting at all!

Yup, that confirms that the geometry is correct, is what my latest attempt looked like, for the most part. The top angle (clearance angle?) on mine was less aggressive, and the radius was much smaller, but the 'end' view looks almost exactly like it.

The radius I have read needs to be smaller (1/64, 1/32) for steel, and larger (1/16, 1/8) for aluminum - is that information in Machinery's Handbook, too?
 
that seems good. i use the larger 1/8 for aluminum. For steel I start off with a 1/64 and end up going larger as I need to sharpen the tool bit.
 
You guys are awesome. I re-ground the tool bit (attempted to give the face some more angle, too), stoned it, then gave this another shot using what I estimated to be ~80-100rpm. It works!! Still not a mirror finish or anything, but an order of magnitude better than my previous attempts.

I experimented with feed speeds, too. Well, as best as one can using a mini-mill and no power feed. :)

Speed did not appear to affect the visual quality, but it is semi-noticeably smoother when I was feeding it veerrrry slowly. At the fastest I dared to go, the appearance was the same, but you can barely feel with your fingernail some ridges. It did actually manage to bind up entirely for a second, but you could push it pretty fast, lots of "nice" chips coming off. This was also a pretty thin cut, fwiw. I'm still struggling with conversions, but ~2-3 thou probably?

So I guess the last question would be - is this an acceptable flycut appearance? Anything else I can do to improve?

apr2-flycutter2.png
 
That finish looks good. Different materials will give you a slightly different appearance also. Some will shine like a mirror and others may stay dull but look and feel good.
 
If I have time this week, I'll give the carbide a shot, too, just to see what happens. Thanks again everyone! I was so pleased after this, recharged me after the many failures. :) Was a great lesson in perseverance.
 
Looking much better. I'm guessing that the rigidity of your mill is the limiting factor for further improvement. Now try the carbide at about 500 rpm. Remember that a carbide will not hold up well if ground like HSS. I don't regrind brazed carbides, I use them as they come, but I've got a full-size BP copy and I'm lazy. YMMV

Thinking about this a little more, that's an awfully big fly cutter for a mini mill. I'd suggest getting one about half that size.
 
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