Machinist

[Lawrence London <lfljvenaura@gmail.com>]

ProTag LastCut Tools & Machinery

Feeds And Speeds

It is impossible to develop strict speed and feed rules for any machining operation because machining conditions vary from job to job.

In this post we will examine how to establish the correct cutting speeds and feed rates. I will try to explain in layman's terms how to calculate the correct speed and feed rate for any drill bit.

If you're a machinist or an engineer, I'm sure you already know what I'm talking about. If you're not, speed rate is the speed a cutting tool rotates, and feed rate is the amount of material the tool cuts per revolution.

How do we arrive at the correct speed to run any size cutting tool? The cutting speed is calculated in surface feet per minute (SFM). Why are we measuring the speed of our cutting tools in surface feet per minute when machines measure speeds in revolutions per minute (RPMs)?

It doesn't seem quite right, does it? Read on to see why it does make sense.

When we refer to feet per minute, we're talking about how far a set point on a rotary cutting tool will travel when rotated for one minute.

We can calculate the distance a set point will travel for any size cutting tool by multiplying its diameter by Pi (3.14159). A one inch diameter drill will travel 3.14159 inches in one revolution.

Another way to think about this is to imagine placing a 1 inch diameter cutting tool on a flat surface and rolling it one revolution. The distance it will travel is 3.14159 inches.

Knowing this, we can now calculate the distance a set point, for any size cutting tool will travel.

Because we know a one inch diameter cutter will travel 3.14159 in one revolution, we can calculate the distance a set point will travel for any cutting tool by multiplying its diameter by Pi (3.14159).

Charts are available from cutting tool manufactures with a range of SFM for each material to be machined. For example, the recommended speed for mild steel is 80 to 120 SFM. Aluminum is 200 to 300 SFM. Cast iron is 75 to 125 SFM. I always start with the slowest speed. For example, in the case of mild steel, I start at 80 SFM.

Now we have to convert SFM into RPMs. If we are drilling mild steel, the chart recommends 80 SFM, which is equal to 960 surface inches per minute (as there are 12 inches to a foot). When we divide 960 by Pi, it equals 305 RPMs.

Feed rate is the depth of cut of the drill bit per revolution and depends upon the machinability of the material being drilled. The softer the material, the heavier the feed rate. For example, if I'm drilling a 1 inch hole in mild steel, I set my feed rate at .015” per revolution. For further reference, feed rate charts are also available from cutting tool manufacturers.

Drills have two cutting edges which reduces the feed load by half. To avoid damaging the cutting tool, be sure to let the cutter do the work and don’t overload it with too heavy a feed.

I enjoy sharing these articles on Facebook and hope you found this information helpful. I have been in the tool business all my life and continue to provide great products backed by years of experience t the marketplace.

Pete The Drill Guy.

Please click on the link below and visit our eBay store to take a look at all the great tools we have stocked there.

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Lawrence F. London, Jr.
lfljvenaura@gmail.com

https://sites.google.com/site/avantgeared