Comparing the influence of geometric variations in physical hob size on tool life; specifically, the overall length of the hob.

Manufacturers consider many factors when shopping for a gear hob solution: What material is being cut? What is the pitch of the part? Will the hob be delivered on time, and what will it cost? Unless the job is hot and lead time is critical, the cost of the tool tends to be the deciding factor. A less expensive hob reduces the tooling cost of the project and increases the profitability of each gear or spline cut. However, tool life cannot be overlooked when evaluating the overall cost of a hob or process, and the physical size of the hob partly determines the life of the hob.

There are any number of external influences that affect the life of a hob: quality of the hob material, quality of the coating, feeds and speeds used during the hobbing process, efficiency of the coolant, and so on. However, the actual dimensions of the hob affect tool life despite these external factors, independent of the hob manufacturer. It may seem intuitive, but it pays to compare the benefits of long hobs.

The purpose of this article is to compare the influence of geometric variations in physical hob size on tool life; specifically, the overall length of the hob. Four different lengths are used to determine the number of parts produced, compare the tooling cost per part for each over the life of the hob, and to compare the tooling cost for a hypothetical job run. All four hobs are dimensionally the same except for the overall length. For the purpose of this article, the life of the hob is estimated to be 15 sharpenings. After this, the hob tooth length would not be enough to produce acceptable parts.

The hobs being compared have a 60mm (2.362”) outside diameter and one right-hand thread. They are solid carbide tools, with 15 straight flutes, and Balinit® Altensa coating. Each hob has a lead angle of 1°15’58”. The overall lengths being compared are 60mm (2.362”), 80mm (3.150”), 100mm (3.937”), and 120mm (4.724”); hobs 1, 2, 3, and 4, respectively. The 60mm x 60mm hob will be referred to as the “square hob.” For this comparison, the length of the hubs on both sides of each hob is 2.5mm (0.098”). This brings the cutting width of each hob to 55mm (2.165”), 75mm (2.953”), 95mm (3.740”), and 115mm (4.528”).

The number of usable teeth on each hob is dependent on the overall size of the hob, the number of flutes, and the pitch of the teeth. A coarse pitch hob will have less teeth than a fine pitch hob at the same size. Similarly, more flutes on the hob will increase the number of usable teeth. This helps increase the number of parts produced per sharpening of the hob; however, it reduces the total number of sharpenings. Since the hob teeth need to be thinner to accommodate more flutes, the total number of sharpenings decreases as the number of flutes increases. There is simply less material for the teeth and more empty space for the flutes. This is shown visually in Figures 1 and 2, comparing two 28mm OD (outside diameter) hobs with 12 and 15 flutes, respectively.

Figure 1: A hob with 12 straight flutes.
Figure 2: A hob with the same diameter as Figure 1 with 15 flutes.

The number of usable teeth for each hob compared is approximated using Equation 1. This formula also uses the axial pitch of the hob, which is defined as π divided by the diametral pitch, shown in Equation 2.

Each tooth on the hob has a lineal cutting dimension available to it before sharpening is required. This value is higher for carbide hobs with quality coating versus an uncoated high-speed steel hob, for example. Essentially, this is a quantitative expression of the ideal tool life. This is also an idealized factor for production estimates. Poor selection of cutting feeds and speeds, low quality coating, and poor machine/fixturing rigidity can lower the actual value of this factor. For this article, each hob tooth has 300” available, and the available teeth for each hob is shown in Table 1. The lineal inches per tooth value is derived from industry experience and is a useful starting point for these types of estimates.

Table 1

From the lineal inches per sharpening comparison alone, a hob twice as long is more than twice as productive. Shank type hobs offer even longer cutting widths, greatly increasing the number of usable teeth on the hob. To determine the total linear inches available for each hob per sharpening, these values are simply multiplied together, shown in Equation 3. Finally, the number of parts per sharpen is estimated by dividing this value by the product of the number of teeth in the part and the face width, shown in Equation 4.

Hob costs can range widely depending on the hob material, the manufacturer, the quality class, the desired lead time, etc. When evaluating the tool costs over the life of the hob, the sharpening costs should be considered as well, including the cost to strip and recoat the hob. Table 1 shows the tool life and cost comparisons over the length of a high-volume job for a hypothetical gear. Equation 5 is first used to determine the number of sharpenings each of the hob lengths would require to cut the entire job. Then, the number of sharpenings required is divided by the estimated sharpenings per hob to determine how many hobs are required to complete the job. This is shown in Equation 6. These values are rounded up to the nearest whole value since a hob would not be sharpened a fraction of an amount.

Finally, the total tooling cost for the job can be calculated using the hob and sharpening service prices in Table 1. This can also be used to determine the tooling cost per part for the given job by dividing the total cost by the number of parts in the job. These are shown in Equations 7 and 8, respectively. The percent cost savings over the entire job is defined in Equation 9. Note there are additional factors not considered in this savings calculation. Fewer hob changeovers reduces machine idle time, scrapped parts during setup, and shipping costs to and from the sharpening facility.

The concepts behind these comparisons should be used when deciding on a tooling solution. They should also be considered when looking at machine solutions. If machine A offers a 100mm shifting range, while machine B offers 160mm shifting range, then machine B will be able to produce more parts per hob. Again, this makes sense intuitively, and when jobs come down to dollars and cents, it pays to compare the costs and benefits of a long hob. If you have high volume runs, please make sure you inform your cutting tool supplier so they can help you determine the best solution. It could help reduce your tooling costs by more than 10 percent. 

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Michael Weas has worked at Helios Gear Products, formerly Koepfer America, since January 2018. He graduated from Northern Illinois University that same year with a Bachelor of Science degree in mechanical engineering. Weas works as an applications engineer and oversees the cutting tool sales at Helios.