Development of Impeller with CAM

In the past, complicated shapes like an impeller of a centrifugal compressor were produced from investment casting. An impeller is a rotating component which transfers energy from the motor by accelerating the fluid outwards from the center of rotation.

Gone were the days of NC using punch-card and later paper tape looping methods. As well as the multiaxis equipment in the days when all the axes where driven mechanically by cam plates and elaborate lever systems. 3 axis CNC milling is similar to conventional milling in that all movements are in 3 axis only (X, Y, Z, left-right, forward-backwards, up-down). Typically the angle of the head is fixed or cannot be moved in anything but the up and down(Z axis) direction automatically. All conventional milling processes such as end milling, slot milling, face milling, drilling, tapping, trepanning, chamfering and others are possible. Also possible is 3D contour milling and this feature is commonly used on a 3 axis milling machine for milling out mould and die cavities as undercuts are undesirable.

In most instances when the CNC milling job requires undercuts and milling on multiple sides of the job, a 5 axis CNC milling machine is required. In both vertical and horizontal milling, two additional axes are added to the existing X, Y and Z axes. These two axes are typically a rotary axis that rotates around the Z axis and a tilting axis that rotates around the Y or Z axes.

There are currently three main ways of achieving simultaneous five-axis milling:
(1) a dedicated five-axis machining center;
(2) a tilting/rotary or trunnion table; and,
(3) a spindle head attachment.

The development of milling machines was essential in this transformation. When milling impellers, all translational and rotational axes are used simultaneously in order to be able to follow surfaces shaped according to the demands of fluid mechanics. This simultaneous milling process requires permanent acceleration, deceleration, or both of all axes involved, which brings translational dynamics to the fore.

Various movement of a 5-axis milling machine



A typical 5-axis milling machine workstation



Our main focus: 5-axis machining – Impeller.

Various Designs of Impeller

There are different types of impeller blade; straight, curved and split. 

Straight blade impeller produced from investment casting





Curved Blade Impeller
(Note: the aerofoil cross section of the blade)



(a) Split Blade Impeller Configuration          
(Note: the split blades between each full blade)

The above 3 samples are Open-type impellers, where the below two sample shows the Closed-type impellers.



When milling closed impellers, the milling machine works
along the channel from its intake/outlet opening inwards.

 Below is a short clip on the manufacturing process of a open impeller.
 

Modern Methods

Impellers can be manufactured with channels that are open or closed by a cover disk. Casting in expendable moulds with subsequent polishing is a classic manufacturing method for closed impellers. Combined methods are milling of the open channels and welding or soldering on a cover disk. The latest method of manufacturing closed impellers is milling the blade channels from a blank. Modern CAD/CAM systems allow you to control all aspects of 5-axis machining: the cut pattern, the tool axis direction as the tool is following that cut pattern and the exact contact point of the tool tip as it touches the drive surfaces. This method leads to highest strength of the workpieces and reduced quality costs. Case studies have shown that roughing cycle times can be reduced by more than 60 percent compared to traditional roughing techniques.

Maximum Precision

In the last few years, milling of vane channels out of full material has increased in importance. The milling process has many advantages: 

• Shorter delivery time through higher processing speeds and, in some cases, shorter material delivery time.
• Lower costs than forging/welding and, in small numbers, also compared to casting.
• Lower costs for quality assurance and fewer processing steps.
• Maximum manufacturing precision.

An example(below) from Sulzer Innotec where a stainless-steel blank, diameter of 929mm, mass of around 1200kg was machined into a impeller. Experience showed that the 5-axis simultaneous process attained considerable machine performance. With the low-vibration proces and small infeed depth, it was possible to use a one-step finish, without separate roughing and finishing passes. Three different milling bits with varying lengths were used, which made it possible to keep the cost of tool wear very low, obtaining maximum precision.
 

The effective machining time for this impeller machined
from a solid was just approx. 200 hours without running in. 


Collison Avoidance
The advantages of integral production through milling, however, come with the disadvantages of complicated programming and geometrical limitations. Above this, spatial constraints hinder avoidance of collisions between milling shaft and the component. Length and shape of the cutting head influence the rigidity of the tool. If the tool is used with cutting parameters and settings that fail to take these factors into account, vibrations, unsatisfactory surface qualities, and reduced tool life will result.

The individual blades of an impeller are often thin, warped, short on one end, tall on the other and close together. It is desirable to keep the cut pattern parallel to the hub surface. It is also preferable to cut the whole blade in one continuous motion to avoid leaving undesired tool marks on the workpiece. The challenging part of this operation is dynamically controlling the tool axis, as the tool gradually spirals down the blade. The CAM system must be capable of continuously monitoring the tool tip, flute, arbor and toolholder and also take action if a user predefined near-miss is imminent.

However, with modern cutting tools, the flow of the cutting forces is induced primarily axially instead of crosswise throught the milling tool. Tool deflection is minimal because cutting forces are aligned with the cutter’s center axis, dramatically extending tool life. A complete CAD/CAM package also allows users to watch a virtual machine simulate the entire cutting process while developing it, rather than using an expensive 5-axis machine as a verification tool.

Split Impeller machining demostration created using wax.
Watch out for the real part in Aluminum towards the end of the video!

Watch this video to see Mastercam in action:



With five-axis machining, parts can be machined in a single setup which reduces cycle times. Improved tool accessibility allows the use of shorter tools that provide more accurate machining. The main focus of using five-axis machining in industry is to reduce cycle times, dimensional and surface errors in its nature. However, these desired points cannot be achieved satisfactorily without modeling of five-axis milling mechanics.

Five-axis machining is a growing, challenging and exciting field.

Should engineers give way to the limitations of machining process and sacrifice aerodynamics efficiency using complex curved aerofoil geometry?
In the future, engineers will love the free rein they have in designing impellers that can withstand higher temperatures and pressure with better efficienvy without worrying that no machining process can materialize their dream designs!