A Comparative Study of Chain Clamping Fixture with Other Clamping Methods for Gate Valve Body: Cycle Time and Rigidity Study

“Fixture rigidity influences the production rates”. In this paper, a comparison and feasibility study is demonstrated to improve the productivity and existing fixture rigidity for machining on a gate valve body by comparing the existing technique i.e. top clamping fixture with other possible clamping methods including newly developed chain clamping fixture. The CAD models of the fixtures are prepared in CATIA V5. The existing fixture concept, clamping methods, cutting parameters and cycle times have been studied and compared with the values of chain clamping fixture obtained by trail out results. Along with the increased production rates, other parameters like tool life, noise and vibration levels and surface roughness values are also improved slightly.


Introduction
Gate valve body shown in Fig. 1 are machine elements which are used to control of flow of volatile, often toxic gases and liquids and keep them away from being emitted into atmosphere [1]. A gate valve body of cast steel has to undergo various machining processes and to accommodate rigidity while machining, fixtures are required. In this paper, traditional top clamping method to locate the gate valve is compared with other fixtures along with newly developed chain clamping fixture.  The majors input to fixture design consists of product design data including CAD geometric model, the machining operation sequence, the quantity to be produced and the machines on which the various operations are to be performed [2]. The machining operations are done in a horizontal machining center (HMC). The B-axis machine tool combines the turning capabilities of a horizontal/vertical turning machine with milling and machining capabilities of a five-axis machining center. A B-axis machine can do the work of two because it supports the entire range of milling and turning operations possible, with the advantage of doing it in one setup [3]. OP-10 (Operation number 10 of process sheet of the component as per the production drawing)  Fig. 2 and Fig. 3 in which the machining have to be performed. The machining operations to be performed in operation number 10 (OP -10) along with the tools to be used are shown in Table I 1.1 OP-10 Operational details

OP 20 Operational details
In B0 side of OP-20, the operations are 2 x 1/2"NPT Tap and spot facing whereas in B180 side face milling is the only operation. Fig. 4 shows a top clamping fixture, this method employs concentrated clamping i.e clamping force will act on very small area on the component. Major drawbacks of top clamping technique are high tendency of component deformation, less rigidity than distributed load type and handling difficulties. This is popularly and widely used method but due to above mentioned demerits; its applications are limited to small components only where clamping force is significantly less. To avoid these limitations, alternative concepts were developed and evaluated.

Bottom Clamping
Advantages of bottom clamping concept shown in Fig

Chain Clamping with orientation by studs and additional clamping
In this concept, the component is clamped with two chain, one additional clamp and one suport pin as shown in Fig.  7. The orientation is easy, accessable and quick, also it is provided with top clamping for additional support as fall back option which is practically unnecessary as it will increase cycle time.

Chain Clamping Fixture Final Concept
In OP-10, the component is located on v-blocks with resting and butting pads. It is orientated by two M12 Studs and clamping by two Chain Clamping Sets against resting. The clamping and orientation is shown in Fig. 8. In OP-20, the component is oriented and located by two pins (Diamond pin type) diagonally which shown by color green and blue respectively. One central pin with top clamp is used for clamping the component. The fixture concept with component and without component is shown in Fig. 9. The operation wise parameters are taken from Kennametal catalogue [4] and total time taken in OP-10 and OP-20 is found out which is shown in Table 2 and Table 3.  The total time of both OP-10 and OP-20 operation is 130.3 min.

Cutting force calculations
For calculating the cutting forces, the operation in which max cutting forces will be induced was selected. The operations to be performed on the component are, 1. Face Milling of flanges (roughing and finishing), 2. Boring operation for ø53mm holes, 3. Drilling operation for φ25.4 mm holes.
Among the above the operations, major cutting forces will be induced during face milling (roughing) and then boring.

For side face rough milling operation (B-90 or 270)
Material of valve body is cast steel, cutting parameters and other details considered for cutting force calculations in this work are as follows: Considering maximum values of above preferred parameters, Spindle speed, feed per min, metal removal rate, clamping force, cutting force and cutting Power are calculated, formulae given above in brackets at the parameter [5]. n = 1061.5 RPM i.e., Taken as 1062 rpm F m = 850 mm Q = 1700 mm 3 /sec P c = 5.9 kW F c = 171.94 Kgf in Tangential direction. = 1.686 kN F =1.5 × 1.686 = 2.54 kN

Elements of Chain clamping fixture
For Valve body machining as provide in the component operational geometrical requirements, two setups were selected (OP-10 & OP-20) with two different fixtures with single part loadings.

OP-20 fixture concept details
The location and orientation is done by cylindrical locating pin and diamond pin respectively. The clamping is done by top clamp with stud and washer as shown in figure 9.

Conclusions
The distribution of pressure induced by clamping is uniform and noise reduction while machining is reduced significantly. By using chain clamping over the component the cycle time was reduced by 20% (from 145 min to 115 min)for Gate valve body Component. The experimental results shown increase in Production rate by 530 numbers (26%)components per year, from 1983 Numbers to 2513 Number per Year. Other parameters like vibrations and noise levels were observed decreasing due to increased fixture rigidity.