EMF T V `
1.4.3 EMF Sequencing
EMF sequencing normally consists of two parts: (1) multiple setup sequencing, and (2) EMF sequencing within each setup. The issue of multi-setup sequencing is addressed implicitly when selecting locating directions (primary, secondary, etc.) for the EMF grouping, in terms of the generalised accuracy grade and critical datum reference. The true challenge of EMF sequencing is now shifted to how to sequence EMFs within each setup, when their machining sequence cannot be determined simply by the datum relationships and manufacturing constraints among the EMFs.
In DPP, an EMF-based geometric reasoning approach is proposed by tracking and
comparing the IMV against the AMV (or volume feature) of each EMF. By applying the following five reasoning rules sequentially, a machine-neutral sequence plan with multiple setups can be created. For example, in the case shown in Figure 1.7, the IMV of the Hole varies between its MMV (Figure 1.7(d)) and its AMV (Figure 1.7(f)) along the machining process. As a rule of thumb, if the IMV of an EMF equals the AMV of the EMF, it is the time to machine the EMF.
Rule 1: During sequencing, when the IMV of an EMF equals the AMV of the EMF, or IMV=AMV, this machining feature is ready for machining.
Applying Rule 1 to the case shown in Figure 1.7, it is easy to conclude a sequence of Step1 o Step2 o Hole for machining. Figure 1.8 shows 30 typical cases after applying Rule 1.
MAIN FEATURES
ASSOCIATED FEATURES Face
Step
Step2 Step1
Step Pocket Face Step
Face o Step
Face Pocket
Pocket2 Pocket1
Thru Slot Blind Slot 2-Side Pocket 3-Side Pocket 4-Side Pocket
StepThru Slot
Step1 o Step2
Step o Slot
Slot // Face Face o Slot Face o Pocket Face o Pocket Face o Pocket
Slot // Step Step // Slot Pocket o Step Pocket o Step Step // Pocket
Slot1 o Slot2 Slot2 o Slot1 Pocket o Slot Pocket o Slot Pocket o Slot
2-Side Pocket
Step o Pocket Slot o Pocket Pocket // Slot Pocket2 o Pocket1 Pocket2 o Pocket1 Pocket1 // Pocket2
Step o Pocket Slot o Pocket Slot o Pocket Pocket2 o Pocket1 Pocket2 o Pocket1 Pocket2 o Pocket1
3-Side Pocket
Face Slot
Slot Face Face Pocket Face Pocket
Slot Step Step Slot Step Pocket Step Pocket
Slot Step Slot1 Slot2 Slot1 Slot2 Slot Pocket Slot Pocket Slot Pocket
Slot Pocket Pocket Slot Pocket1Pocket2 Pocket1 Pocket2
Step
Pocket Slot Pocket Pocket Slot Pocket1Pocket2 Pocket1 Pocket2 Pocket1 Pocket2
1 2 3 4 5 6
7 8 9 10
Pocket Step
11 12
13 14 15 16 17 18
19 20 21 22 23 24
25 26 27 28 29 30
Figure 1.8. EMF sequencing results after applying Rule 1
This reasoning rule works effectively for EMF sequencing in the same setup and with feature interactions. However, after applying Rule 1, there still exist some cases that cannot be handled by this rule, in which the sequence of two machining features remains in parallel (shown as // in Figure 1.8), such as Case 8: Thru Slot + Step. In this case, if the Thru Slot is cut first, the Step will be divided into two smaller ones, which is contrary to the definition of a machining feature being a basic single machinable shape.
Rule 2: If the IMF of machining feature A is to be divided into more than one piece as a result of the machining operation of machining feature B, the machining feature A should be cut first.
In addition to the feature-splitting case encountered in Rule 2, there are cases where incorrect sequences may result in different types of machining features, e.g.
Case 2: Thru Slot + Face. In this case, if the Face is milled first, the Face feature in machining is actually changed to a Step. This is not allowed, as different EMF types require different machining data (tool type, tool-access direction, and tool-path pattern, etc.). Rule 3 is therefore established to prevent such ill cases.
Rule 3: If a machining feature is to be changed to another feature type as a result of its own machining operation, this machining feature is not ready and should be cut later.
The remaining parallel cases after applying Rule 1 to Rule 3 do not have feature interactions and their machining sequences are not critical. They are further handled by adopting the knowledge of best practice or know-how of operators. One rule commonly used by machinists is that the bigger volume is to be removed first, because removing a bigger volume generally produces more cutting force and cutting heat that may result in more deformation and poor surface quality, especially for large workpieces.
Rule 4: A bigger machining volume is to be cut first.
Figure 1.9 shows the EMF sequencing results after applying the reasoning rules 2–4 to those parallel cases remaining in Figure 1.8.
Sloto Face Stepo Slot Stepo Slot Pocket o Step Slot o Pocket Pocket2 o Pocket1 Slot Face Slot Step Step Slot Step Pocket Pocket Slot Pocket1 Pocket2
2 8 9 12 21 24
Figure 1.9. Sequenced results of the 6 parallel cases in Figure 1.8
Although Rules 1–4 are applied sequentially during EMF sequencing, Face and Side features are handled differently, except for Case 2. These two types of EMFs usually cover large surface areas and are frequently used as datum references. They are normally removed first in each setup. In addition, the tool-type information embedded in each EMF is used to group the sequenced EMFs into clusters (within each setup) to minimise the tool-change time.
Rule 5: In a setup, the machining features sharing the same tool types are grouped into clusters.
By applying the five rules, a machine-neutral sequence plan can be created.
These rules cover all critical EMF sequences of a prismatic part. The remaining parallel sequences, if any, are not critical and will be up to the controller-level operation planning (see Figure 1.2) to determine.