One of the most frequent questions I get asked is how to work out speeds and feeds.
Now, although our website is a brilliant place to start looking for this information, it’s not always as black and white as “use this RPM with that feed per tooth”. Some cutting data gives just surface speed in m/min when your machine needs RPM and some cutting data gives RPM (when you need surface speed in metres per minute). Once you know how to work out speeds and feeds, it is relatively simple.
What is the difference between RPM and Surface Speed?
In simple terms, RPM is the number of revolutions per minute i.e. how many times something rotates per minute. RPM differs based upon the diameter of the tool: smaller tools rotate faster; bigger tools rotate slower.
Surface speed is given in meters per minute. This is the speed (or velocity) that the cutting edge is travelling at as the tool rotates. Surface speed is generally constant regardless of the diameter.
How are feed rates measured or expressed?
Feed rate is the velocity at which the cutter is fed against the workpiece. It is expressed in units of distance per revolution. It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically millimetres per minute), with considerations of how many teeth (or flutes) the cutter has then determined what that means for each tooth.
Feed rate is dependent on the:
- Type of tool (for example a high feed tool like HRM Double may feed at 1.6mm per tooth, whereas a traditional 90 degree mill like ALPHA MILL may have a feed rate of just 0.25mm per tooth).
- Depth of Cut (for example a high feed tool like HRM Double may have a small depth of cut at just 1mm, whereas a traditional 90 degree mill like ALPHA MILL may have a large depth of cut of 8mm).
- Surface finish desired.
- Available power to the spindle (to prevent the tool or workpiece stalling).
- Ability of the tooling setup to mitigate vibrations.
- Material characteristics & hardness: chip flow depends on material type and feed rate. Ideally, chips should be small and are removed early, reducing the heat levels of the tool and workpiece.
- Cut Width: Any time the width of cut is less than half the diameter, a geometric phenomenon called Chip Thinning reduces the actual chip load. Feed rates need to be increased to offset the effects of chip thinning, both for productivity and to avoid rubbing which reduces tool life.
Calculating feed rates for single-point cutting tools is fairly simple, largely due to the work being concentrated at one point (or tooth). With a milling machine where multi-tipped/multi-fluted cutting tools are involved, then the desired feed rate becomes dependent on the number of teeth on the cutter.
When you have a larger number of available cutting edges, the feed rates you can use the tool at also increases. However, more teeth and higher feed rates can cause vibration, so a balance should be aimed for.
What is meant by Depth of Cut?
Depth of cut (or DOC) is reference to the amount of material that can be removed from a work piece per pass. These depths vary with every cutter, with some cutters taking small cuts at high speed & others taking full cutter flute length/diameter cuts. The width of cut is taken when slotting (unless trochoidal milling a larger slot with a smaller cutter). The manufacturer recommended values for both depth and width of cut can be found under the “Technical Information” tab on our website, or in the resources section.
How do I work out my Width & Depth of Cut?
Once we have the recommended DOC, the equation is quite simple
In the picture to the right, we can see that the recommended width of the slot would be 1 x D, or 1 time’s diameter & the recommended depth is 0.5 x D, or half times diameter of the selected cutter.
So, if we imagine the selected cutter to be 10mm, the equation would be.
10 x 0.5 = 5 ∴ the Depth of cut would equal 5mm.
In the picture to the right, we can see that the recommended depth would be 1 x D, or 1 time’s diameter & the recommended side cut (or stepover) is 0.5 x D, or half times diameter of the selected cutter.
So, if we imagine the selected cutter to be 10mm again, the equation would be.
10 x 0.5 = 5 ∴ the Depth of cut on the side cut would equal 5mm, with a depth of 10mm.
How do I work out RPM from M/Min (surface speed in metres per minute)?
To find the RPM from surface speed, we need to follow the equation example below:
Step by step…
1. Multiply surface speed (metres per minute) by 1000 (e.g. 100 x 1000 = 100,000)
2. Work out pi (3.142) x diameter (e.g. 10 x 3.142 = 31.42)
3. Divide step 1 by step 2 (e.g. 100,000 / 31.42 = 3,182 RPM)
How do I calculate feed rate from feed per tooth?
Once we have established the RPM, we can use this to work out the recommended feed rate. If we use the RPM from the above example (3,180) & establish the recommended feed per tooth (Fz) from the manufacturer, we can work out the velocity at which the cutter is fed against the workpiece in millimetres per minute. For this example, let us use a feed per tooth of 0.10mm and imagine the cutter to have 2 teeth (or flutes).
The equation we need is below, along with an example calculation:
How do I calculate feed rate from mm per rev?
To find the Surface Speed from RPM, we need to follow the equation example below.
1.Multiply pi (3.142) by Workpiece Diameter by spindle speed (e.g. π x 50 x 2000 = 314159 (rounded))
2.Divide answer by 1000 e.g. 314159(rounded) ÷ 1000 = 314(rounded)
3.∴ VC = 314
I hope this has cleared up any questions you may have had on the subject, but if things are still a little unclear, please feel free to call our Technical Team on 01924 869 615, one of our team will answer any questions you may have.
Our Product Training & Technical Manager Liam has also summarized the key points in the video below. Click below to watch now:
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