Ok, Rookie Here-- What Am I Doing Wrong?

[JimDawson]

Your calculated spindle speed and formula are correct. BUT, those numbers are for a new, sharp end mill, cutting under optimal conditions in a production environment for maximum metal removal rate. The home shop rarely has optimal conditions, so we make allowances.

Normally half of those speeds is a good starting point. The worst enemy of an end mill is heat. Slower speeds = less heat.

A air nozzle or a mist coolant system would be very helpful. I use one or the other depending on what I am doing.

 

[T Bredehoft]

Surface feet is diameter x pi (3.14) for any cutter. .5 x 3.14 is 1.57 inches. That is 13% of a foot so 7.64 revolutions is one foot. to go 90 feet it must go 90 times 7.64 or 678 ROM.. My 180 is off by a factor of 3.82 or nearly 4.

Your chart is right, but here's the rub. 90 is for the mildest steel. I work in 4140 because I want a tough steel to hold my lathe tools where I want them. It's cutting speed is nearer 26, you may be working in something more difficult to work than 1018, common cold rolled steel.

 

[randyc]

Randy, is that "ball and socket" thing with the black base the magnet? Looks like a great idea. I have air 5 feet from my mill. Do you remember where you got that magnet thing? I recognize most of the other pieces of the system-- but any parts, specs would be appreciated.

Yes, you've correctly identified the little magnetic air valve gizmo. I think that it came from Harbor Freight but if it's no longer available, you can rig up something with fittings from the hardware store ...

 

[randyc]

Basic rule of thumb, High Speed Steel shouldn't be run faster than 90 surface feet a minute. Sure you can get away with 120, if you're cutting Ledloy, or brass, but for steel, 90 surface (of the cutter) feet per minute is good. A half inch tool then is turning 180 RPM.

Tom, I get 688 RPM from the following: (SFPM x 12) / (Pi x cutter diameter). Are you sure that you're not multiplying by the cutter diameter instead of dividing ? Just a thought

Edited: Never mind, I didn't look down far enough to where you corrected it, Sorry Tom

 

[randyc]

.....BUT, those numbers are for a new, sharp end mill, cutting under optimal conditions in a production environment for maximum metal removal rate...

Just to add one more qualification to Jim's comments: cutting speeds are generally based on some nominal time between sharpenings. IIRC, this figure is generally around 2 hours.

So as an example, starting with a new cutter, you can probably run at 90 SFPM in steel for about 2 hours before the tool dulls to the point that it needs either replacement or sharpening.

Using half the RPM from the tables will likely increase tool life by at least a factor of three, all other things being equal

 

[Bob V]

Randy, Jim, and everyone else on this thread:
-Thought I would give you a brief update after all your great advise-

Yesterday: I ran the .5 inch cutter in the same steel (Tom, it may well be cold rolled stock!) at 180 RPM and a manual feed rate (don't know exactly what this was but "slow"), and squirted the cutter with cutting oil whenever it looked "dry". I had cleaned out the collet with brake cleaner (and the mill shank), and installed it almost to the flutes.
--It went great! I ended up cutting the side of the piece about .25 inch deep with about half the cutter diameter. - No hard feeding, no cutter spinning in collet, minimal "smoke".

Without your advise I would still be scratching my head about all this.

--Much appreciated!
Bob

 

[T Bredehoft]

That's what we'ere here for, Bob, to keep the world's wheels turning.

 

[Bob V]

Yes, you've correctly identified the little magnetic air valve gizmo. I think that it came from Harbor Freight but if it's no longer available, you can rig up something with fittings from the hardware store ...

Randy, I will rig up an air cooler/chip clearer. It will look pretty much like yours-- why reinvent the wheel with generous folks like you out there!

 

[benmychree]

Thanks, Randy. I did notice piles of chips developing around the mill. I had to stop it to brush them away.. My machine has a sump for coolant and motor to pump it, then it drains thru a hose back into a sump in the base--maybe I should hook this up--will have to wire it to the phase converter-- another project!

From what I have read so far in this thread, I think your biggest problem is the lack of coolant, and yes. by all means hook it up and use it; steel should never be milled dry, especially for removing a significant amount of stock. With lighter work, cutting oil and a brush can be used, but with care not to overspeed or feed or depth of cut; in hand feeding, your hand on the crank and your hearing will tell you when you are pushing things too hard. The speed of 650 rpm should have been fine with coolant being used, as it is only about 85 fpm cutting speed; up to about 100 fpm can be used with steel with coolant. As others have suggested, sharpness is also a big issue, indeed, it should be sharp enough to cut your finger; if the cutter is dull, high heat is generated, further dulling it until failure is the result. A suggestion for guidelines on feed and speed can be derived from handbooks, or more simply from a slide rule speed and feed calculator that used to be given away by tool manufacturers, made of cardboard. They integrate all the factors relating to cutting speeds, feeds, tool material and material hardness, available horsepower, etc., but there is one scale devoted to tool diameter, cutting speed and rpm only that would likely serve your needs. Mine was made by Kennametal. You could likely find one on E Bay.

 

[janvanruth]

nothing wrong with your arithmatic
tables were made for industry, tend to give numbers for maximum toollife in respect to maximum amount of chips, a matter of making the most money out of a cutter, and will presume flood coulant and auto feed.
taking it easy on the feeds and speeds with no coulant and hand-crancking will not make as much money out of a cutter but will make the cutter hold on for much longer