View Full Version : Please help a feller out with some SFM training.

11-30-2010, 12:43 PM
I freely admit my lack of knowledge in this area and am seeking guidance from you guys.

Iíve been in the hobby for over 6 years now and have made a considerable pile of swarf without ever calculating feeds & speeds. I hate to admit this but have always guess-timated the rpm/d.o.c. and adjusted the feed on-the-fly. There, Iíve said it and I feel better already. Iíve only broken 1 endmill
(1/8Ē 2-flute ballnose HSS) and so far have always got the job done with decent surface finishes and my endmills last pretty well. Must not be too far off with the seat-of-the-pants method.

I now have my little Denford/Sherline CNC mill working pretty well and I have a job for it cutting 4, 1/8Ē slots in the steel ring spanner nut for the ER-32 aux. lathe spindle Iím building. Iím sure if I guess at it, Iíll either snap the endmill or burn it up.

Iíve looked at various online calculators but my brain turns into mush. Where does one start? There are a few known factors: (1) 1/8Ē HSS 2-flute (or 4-flute) endmill (2) RPM is easily variable between a few hundred and 2800 (3) No flood coolant, just manual drip feed of ReLiOn oil (4) Work piece is only 5/16Ē wide so itís not a long, arduous slot to be cut (5) The material is Scrapbiniumô that is what Iíd call medium hard but turned ĎOKí with HSS in the lathe (6) Feed rate is up to 10 i.p.m.

Iím confident I can do the job in my manual mill but want to move up the ladder a bit and let the CNC work on it. How do you guys determine where to start and how do you calculate it? Make it easy as possible for a mush-brain.:D

11-30-2010, 12:50 PM

I have a great chart that has RPM on the Y-axis, and cutter diameter on the X-axis. I keep it pinned to the side of the mill.

I'll see if I can find the PDF.


11-30-2010, 01:27 PM
To start with, use 2 and 3 flute end mills for aluminum and softer like wood, plastic, etc.. For all steels and cast iron use 4 or more flute end mills. The reason for this is the 2 and 3 flutes have more back clearance, thus a thinner cutting edge. The 4 or more flutes have a heavier duty cutting edge.
Keep the spindle speed low to start. Even with a 200 to 400 spindle, you can still have a decent feed rate. Too much spindle speed will wear out or burn up the end mill quickly. Too little spindle speed only burns up time, but the tool will last. Look for speeds and feeds on steel that keep the chips from turning blue without any oil or coolant. Heat is the worst enemy of cutting tools.
In your particular case, use a 4 flute end mill at 400 rpm and 2 in./min. feed.
You can go faster, but with one nut to do, you will be done before you have time to experiment.

11-30-2010, 01:28 PM
Let me see if i can help a bit.

Rule of thumb RPM:

(4 * CS) / diameter = RPM

For plain steel, Cutting speed (CS) is 100, tool steel 50, and aluminum 250. I use those numbers because they are easy to multiply in my head and are close enough to actual values. For your 1/8 end mill in scrapbinium, comes out to 3200 RPM. I always start conservative, especially for such exotic materials, so I would start at 2500 RPM.

Diameter depends on what your working on. On the lathe, it's workpiece diameter. On the mill or for a drill, it's end mill/drill diameter.

Feed is a bit more complicated. Feed per tooth is one place to start. .001" per flute is a good starting point for a 1/8" end mill.

The formula for feed is:

(chip per tooth * number of flutes) * RPM = feed

So for a 2 flute end mill (.001 * 2) * 2500 = 5 IPM.

CNC has the option of either feeding in IPM or in feed/revolution. I always go in IPM since I don't have spindle control on my CNC mill. If you choose to feed in per rev mode, just drop the RPm from the calculation (you'll get .002) in this example).

For your project, I would start this way:

2 flute end mill (2 flute better for slotting), 2500 RPM and 3 IPM.

See how it's cutting, the chips are clearing, etc and increase speed/feed from there.

11-30-2010, 01:47 PM
I was just going to post but baldysm beat me to it. Follow his numbers and you won't go far wrong, if the formula puzzles you just say speed times four over diameter. Peter

11-30-2010, 02:19 PM
Okie-dokie! I have something to try when I get home tonight. There's quite a bit of difference in ya'll's suggested speeds. I think I'll try the slower of them first and go from there. By the way, what about depth of cut? Is there a way of calculating that as well? My gut tells me to try about .010" d.o.c. on this little mill but I may be way off.

11-30-2010, 05:21 PM
Should be OK with about .030. I think 2500 rpm will burn up the end mill, but that is probably a "by the book" formula. Might start with one inch per minute feed and see what the chips look like. If the chips are dust or very tiny, increase the feed. If they look fairly thick, back off a little.

11-30-2010, 10:31 PM
Here is a link to a couple charts that may help one of the charts will help with the terminology so you will understand what the heck every body is talking about LOL Speeds & Feeds Charts (http://www.machinist-guide.com/speeds-feeds-charts.html)