Introduction to Gear Design

5 How Should They Be Made?

Gears can be made by a number of machining and “near-net shape” processes. The “near-net shape” processes (plastic molding, powder-metal forging, and stamping ) require large “upfront” investments in tooling, and are usually restricted to very high volume (5000+ pieces) applications. It is very expensive to make changes to these tools, so it is advisable to make prototypes by less expensive methods, usually machining them from the same material as planned for the final product. Each “near net” process has its own unique requirements, and it is best to work closely with a couple of suppliers to make certain the part design is compatible with the process desired. The power capacity of plastic and powdered metal gears is not well “standardized”, and a thorough testing program is suggested for all new applications.

The machining methods used to make gear teeth can be divided into a number of subcategories, each of which must be well understood if the gear designer is to avoid manufacturing problems and high production costs. Careful selection of tooth size (Diametral Pitch or Module), for example, can avoid the need for special hobs and shave 8 to 12 weeks off the required lead time. Asking for a ground tooth when there is only clearance for a 3-inch diameter cutter might triple the cost of the part, double the lead time, and restrict you to a couple of suppliers. The following paragraphs will provide some insight into the most common machining methods, and help you select the best process for your gears.

5.1 Milling

Milling gear teeth with a cutter having the same profile as the tooth space is the oldest method still in current use. Milling is most commonly used to produce special course-pitch (less than 1 dp) gears or unusual non-involute forms that are difficult to generate with a hob. Milling cutters are available to cut a “range” of tooth numbers (see Table 5.1). If more accuracy is required, special cutters with the form of an exact tooth number can be made. Modern cutter manufacturing techniques can produce excellent results, but it is difficult to produce much better than an AGMA Class 7 gear by this method. It is advisable to make both mating parts of a gearset by milling to avoid meshing problems related to profile errors. Most of the machines used to mill gear teeth are unable to provide “double plunge” cutting cycles, so it is important to allow plenty of cutter clearance on one end of the part. Special small-diameter cutters can be made, but there are limitations, so make sure to consult your gear supplier or a tool manufacturer.

5.2 Hobbing

Hobbing is the most popular gear-manufacturing method, combining high accuracy with high production speed. A wide variety of cutting tools are available “off the shelf” (see Tables 5.2 and 5.3)

in quality levels to match part-quality requirements, making it easy to get the tooth form you need without having to wait for or pay for custom tooling. Hobbing is a “generating” process — the tooth profile is developed in a series of cuts as the hob ( a threaded worm with slots or gashes that act as cutting edges (see Figures 5.1 and 5.2) rotates and is fed at an angle to the workpiece. Due to this “swivel” angle the hob must have room to “approach” and “overrun” the needed face width and insure that “full depth” teeth are cut in that area. (This is one of the first things to investigate if load-distribution or noise problems appear.) Allowances must also be made so that the hob does not destroy other features on the part when entering or exiting the cut (see Figure 5.3).

Undercutting is another problem that occurs in the hobbing process. On parts with low numbers of teeth (the limiting number of teeth varies with the pressure and helix angles), the tip of the hob can remove or “undercut” the lower portion of the part tooth, destroying the involute profile and reducing the strength of the tooth. This problem is usually corrected by changing to a higher pressure angle, increasing the number of teeth (with a corresponding reduction in tooth size), or “enlarging” the pinion teeth and “contracting” the gear teeth (also called the “long and short addendum” system). If the number of teeth on your part is less than that shown on Table 5.4, you may want to read the information on undercutting found in the books listed in the reference section of this guide.

Hobbing is used to produce spur, helical, and double-helical gears. The helix angle of helical gears necessitates larger approach, overrun, and clearance allowances. For double-helical gears there is the additional complication of determining the “gap width” required to avoid the hob cutting one side of the gear from damaging the other side when it reaches the end of its cut (see Figure 5.4). The analysis of these situations is quite complex. Several different methods are detailed in the reference books, including graphical techniques that may be adaptable to cad systems. Where little or no gap width can be allowed, a shaped or assembled double-helical gear can be used. Use of “staggered” rather than “in-line” teeth slightly reduces the gap required for hobbed gears.

5.3 Shaping

Shaping involves a reciprocating pinion-like cutter (see Figure 5.5) which is rotated and in-fed against the rotating blank to generate the tooth profile. Spur, helical, and double-helical gears can be produced by this method with either internal or external teeth. Cutting-tool availability is not as great as with hobbing, and machine capabilities are far more limited, especially with regard to helix angle. Special “guides” are needed for each helix angle to be cut — at a cost of several thousand dollars each — along with matching cutting tools. To minimize these expenses several standard helix angles have been adopted (23°, 30°, and 45° are most common). Special “herringbone” machines have been developed to cut both sides of a double-helical gear at the same time with little or no “gap” at all. Larger gears (6 dp and lower) can also be produced on “rack”-shaping machines which use a straight “rack” of cutter teeth to generate the tooth profile.

Except for herringbone gears, all shaped teeth require a chip-clearance groove beyond the tooth face (see Figure 5.6). The cutter must also have open access to the start of the cut. Shaped parts can be cut between centers, on an arbor, or in a locating fixture. Fixtures are also used when the gear teeth must be aligned with another feature on the part. It is usually easier to cut the teeth first and then produce the aligned feature, but where this cannot be done it is much simpler to shape the teeth rather than hob them. If alignment is required, remember to make a special note of the relationship to be maintained and a tolerance on that alignment. Normal operating procedure in the gear industry is to assume that no alignment requirement exists unless specifically noted on the drawing. Any alignment shown on the drawing but not noted is assumed to be a drafting convenience only. Tooth alignment of hobbed teeth is very difficult.

Another important design aspect of the shaping process is the need to relate cutter size to the number of teeth on internal gears to prevent “trimming” of the teeth by the exiting cutter tooth. Table 5.5 shows the minimum number of part teeth that can be cut by a given number of cutter teeth. This varies according to the tooth form, and can be adjusted slightly by enlarging the minor or inside diameter of the internal gear. This can reduce the contact ratio and must be analyzed very carefully. Methods for this are outlined in several of the reference books.

Table 5.5: Shaper-Cutter Teeth vs. Minimum Internal Gear Teeth to Avoid Interference

# teeth in cutter	# teeth in internal gear
	14.5° fd	20° fd	20° Stub	25° fd	30° ff	30° fr
3	15	11	11	10	9	9
4	18	13	13	12	12	10
5	20	15	15	14	13	11
6	22	17	16	15	14	12
7	24	19	17	17	16	13
8	26	21	18	19	17	14
9	28	23	20	21	18	15
10	30	24	22	22	19	16
11	32	26	23	23	20	17
12	34	27	24	24	21	18
13	36	29	25	25	22	19
14	38	31	26	26	23	20
15	39	33	27	27	24	21
16	41	34	28	28	25	22
18	44	36	30	30	27	24
20	47	38	32	32	29	26
21	49	39	33	33	30	27
24	54	42	36	36	33	30
25	55	43	37	37	34	31
27	58	45	39	39	36	33
28	59	46	40	40	37	34
30	62	48	42	42	39	36
over 30

Note: Small shank cutters do not produce a true involute form.

Small shaper cutters are typically made as “shank” cutters (one piece with the tool holder) which have very specific face-width limitations. Wide face widths can cause cutter life and rigidity problems. These parts may be more suitable for the “broaching” method described below. It is best to avoid designing parts that require shaper cutters with less than five teeth.

5.4 Broaching

The broaching process is used to produce internal spur gears and splines. A broach having the same shape as the required tooth spaces is pushed or pulled through a pilot hole with each row of teeth removing a little more metal. Some parts may require more than one “pass“ with a series of broaches to reach the final size. The process is very fast and accurate but requires expensive tools and careful blank preparation. Some machines are susceptible to “broach drift” and require final machining after broaching. Part size is limited by the “tonnage” or power capacity of the broaching machine and the length of the broach that can be pulled. Fixtures can be made to align part features and the teeth, or to broach more than one part at a time. Broaching is usually limited to parts under 45 hrc, and tooling design can be very tricky if allowances must be made for heat-treat distortion on parts broached before heat treatment. Broaches are very expensive to make so it is wise to check if your gear manufacturer has an existing broach before finalizing your design. It is important to tell them the length of cut involved as there must be at least two broach pitches in the part at all times to avoid trouble. Give the maximum amount of tooth-thickness tolerance possible, as broaches cannot be adjusted to vary the depth of cut like a shaper.

5.5 Lapping

Lapping — the oldest gear “finishing” method — involves running a set of gears with an abrasive fluid in place of the lubricant. This process was developed to adjust for cutting inaccuracies, increase backlash, and improve surface finish. Lapping is no longer widely used, as sophisticated gear inspection techniques have revealed that excessive lapping can destroy the involute form cut into the teeth. Modern gear-cutting equipment can usually produce parts that do not require lapping, and lapping requirements noted on older drawings are frequently ignored. The most common use at lapping today is as a “last resort” in solving field problems or in making very fine adjustments in backlash for pump or instrument gears. In some cases gears are lapped with “dummies” (made of cast iron or another soft material) to replicate the mating part. This reduces the tendency to damage the involute form.

5.6 Shaving

Shaving is a gear “finishing” method that is used to improve surface finish and gear geometry (lead and involute). A serrated gear-like cutter (see Figure 5.7)

is rotated and led axially while in mesh with the part. The part teeth must be cut with a special “pre-shave” hob or shaper cutter that finishes the root area at the part but leaves “stock” on the sides of the teeth. The cutter axis is at an angle to the part axis which creates a shaving or planing action as the cutter moves axially across the gear face. Spur, helical, and double-helical (with gap) gears can be shaved. Both internal and external teeth can be accommodated if proper tooling can be designed. Tool clearance is an important consideration, as shaving cutters are very fragile and expensive. Internal gears are subject to the same cutter-size limitations discussed in the “shaping” section of this manual. Shaving cutters are often designed for a specific application, and “off the shelf” tooling availability may be limited.

On hobbed parts, care must be exercised to avoid hitting the hob runout area. A technique known as “hob dipping” (feeding the hob past finish depth at the ends of the cut) is sometimes used to minimize this problem (see Figure 5.8). This is similar in principle to crown hobbing. Shaving is often used to provide “crowned” or “tapered” teeth for special applications where shaft misalignment or adjusting backlash is desired. Shaving is most successful on parts less than 50 hrc.

5.7 Honing

The honing process is similar to shaving except that the cutters are coated with an abrasive material. As shaving machines can also be adapted to honing most of the limitations discussed above still apply. Honing has been used to produce very finely polished surface finishes (as low as 6 aa) and to finish surface-hardened gears that are not suitable for gear grinding. This is a very “hot” area of gear research and is best studied by reading technical papers and manufacturer’s literature.

5.8 Gear Grinding

Gear grinding is the “Cadillac” of gear-finishing processes. High accuracy, excellent surface finish, and special features (see Figure 5.9) like crowning, tooth taper, and profile modification (tip relief) are possible with most grinding methods. While there are many different brands of grinding machines (see Figure 5.10), each with a slightly different operating principle, they can be divided into two basic types — form grinders and generating grinders.

Form Grinders

Form grinders employ a thin wheel that has been “dressed” with the profile of the desired tooth space. This method is very versatile and can be used to grind internal and external teeth of almost any type, including non-involute forms. Form grinding is also used to produce racks and gear segments that cannot be done by generating methods. Wheels as small as 1.5 inches in diameter can be fitted on some machines, allowing one piece “cluster” gears and small internal gears to be processed. Dressing the proper form into the wheel requires a high degree of operator skill or an expensive cnc control. Wheels must be re-dressed frequently, adding time to an already slow process. Newer form grinding machines are able to use longer-lasting abrasive-coated steel wheels which may make form grinding more cost competitive. Form grinding should be avoided in the design of new parts if at all possible.

Generating Grinders

One type of generating grinder employs a large diameter (8 to 14 inches is most common) threaded wheel that acts much like an abrasive hob. A relatively simple dressing mechanism is used to maintain a very accurate straight-sided rack with the proper pressure angle and depth in the wheel. The large diameter provides a long life between dressings, but creates clearance problems on many parts (see Figure 5.11). Small diameter “solid-on-shaft” pinions and the pinion on one-piece “cluster” gears are often impossible to grind by this method.

Other generating grinders use either one or two narrow grinding wheels, which simulate a rack. These wheels are very easy to dress but require more frequent attention than the threaded type discussed above. Cycle times are slightly slower also, although they are much faster than form grinding. The same wheel-interference problems are also encountered. For new designs it is best to consult several gear suppliers and read as much as you can on the method to be used before final drawings are prepared.

There are other problems that crop up frequently with all ground gears. Improper feeds, speeds, wheel materials, and coolants can cause grinding cracks and re-tempering (also called soft spots or grinding burns). Nital etching is used as an in-process check on grinding quality, and gear designers are wise to note a nital etch requirement on the drawing. It is also difficult to maintain tight controls on tooth thickness and alignments with other part features because heat-treat distortion may not occur uniformly, and the amount of stock removed from each side of the space may not be equal. These tolerances should be discussed with the supplier before finalizing a design.

5.9 Bevel Gears

Bevel-gear-cutting methods fall into two general categories — the reciprocating-blade method for straight bevels, and rotating-cutter methods for straight, spiral, and Hypoid bevels. Bevel gears are probably the most difficult to manufacture. The theoretical “summary” (a computer calculated set of machine adjustments) is just the starting point for developing the proper contact pattern between the gear and pinion. During a production run this contact pattern must be constantly monitored and adjustments made for cutter wear and diameter changes due to re-sharpening. It is very important that the same mating part or master gear be used for these checks if the parts are to be interchangeable. Many field problems are caused by attempts to use non-interchangeable parts as a gearset. Subtle differences in cutting methods can often result in wide variations in contact pattern. The best policy is to order bevel gears as a set and to replace them as a set. If that is not possible, the use of a master gear and a specified cutting method, complete with a proven summary, is the next-best choice.

Cutter clearance is an important consideration in the design of bevel-gear blanks, especially on parts with a through shaft or hub on the “small end” of the gear (see Figure 5.12). Consult your gear supplier early in the design process, and review all parts that might have this problem. Minor changes in hub or shaft diameters can often result in considerable cost reductions.

5.10 Worm Gears

Wormgear sets consist of a threaded worm and a mating gear. The worms can be produced by rolling, milling, or grinding, and usually present no significant manufacturing problems. Wormgears, on the other hand, can be almost as tricky to make as bevel gears. Successful development of the contact pattern and tooth thickness depends on proper initial tool design and accurate adjustment of the hobbing machine to account for cutter re-sharpening. Fly tools, commonly used on low-volume jobs due to lower tool cost, are particularly prone to this problem due to the short life of the single cutting point per lead. This method requires low tool loads and relatively long cycle times to produce an accurate gear. Wormgear hobs are custom-designed to replicate the mating worm and have very distinct limits on the number of times they can be re-sharpened without compromising the tooth geometry. Many field problems are caused by trying to use a hob which has been sharpened below the acceptable limits of outside diameter.

Wormgears are usually checked against a master worm at both the mounted center distance (for backlash) and a “tight mesh” center distance (for total composite error). If no master worm is available, it is acceptable to use a representative mating worm. Cutter clearance is not a problem with wormgear hobbing but part tolerancing should be watched carefully. Excessive runout can cause the contact pattern to “wander” across the face width. Parts cut with a topping hob will have fairly wide variations in “throat diameter” (see Figure 5.13) depending upon how many times the hob has been sharpened. As long as the contact pattern is good and the backlash is within limits, this should not cause a functional problem. Throat diameters should usually be “reference” dimensions.