Gears Pt2 Flashcards by Albert Falgui

2 ways of manufacturing gears

Form cutting

Generating

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__ uses a form cutter to cut away the metal between gear teeth. The shape of the tooth depends entirely on the shape of the cutter

Form cutting

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Form cutting is best suited for one off and repair work, t or f

True

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__ relies on the relative motion of the gear blank and cutter to shape the tooth. It is a more accurate way to cut a gear but requires specialized and expensive machinery

Generating

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Gears are milled with __ gear cutters. These are arbor mounted form cutters shaped to cut between gear teeth

Involute

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How many involute cutters are usually in 1 set?

8 with specific DP

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The __ of the table is thr distance the table travels in 40 turns of the crank. Assuming a 1:1 gear ratio

Lead of the table

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The __ of the workpiece, a gear in this case is how far the gear must travel to rotate once. Calculate the lead by dividing the circumference of the workpiece by the tangent of the helix

Lead of the workpiece

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The milling table must be swivelled to match the helix angle if the gear, otherwise, the profile of the gear is wrong. T or f?

True

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Swivel the table CW when cutting a LH helix and CCW when cutting a RH helix, t or f

True

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Milling a rack presents 2 challenges. It is

axis of cutter must be aligned with the axis table
table must be advanced by an equal amount to the linear pitch

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A __ milling attachment re orients the axis of the cutter by 90deg. This allows the machine to cut a rack that is as long as the table travel

Rack milling attachment

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Thr easiest way to advance the table is with a __. This allows you to input a repeated interval. When milling a rack, the interval is the linear pitch

Digital readout (DRO)

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The most accurate way to advance the table is with a __ attachment. This device uses change gears to move the lead screw by exactly the right amount. This attachment is extremely rare

Rack indexing attachment

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It is possible to mill a straight bevel gear, but a milled bevel gear is inaccurate and noisy. Bevel gears with curved teeth cannot be milled at all milling a bevel gear on a mill should be a last resort, t or f

True

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When generating gears, the 2 main generating processes are

Hobbing

Shaping

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__ is a fast and accurate way to cut a spur,helical and worm gear. The tooth form is generated by the relative motion of the cutter and the blank rather than the shape of the cutter

Hobbing

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When hobbing, the cutter is called a __ and resembles a worm thread with gasses in it for teeth.

Hob

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What machine do you need when hobbing

Hobbing machine

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A gear hob cannot cut internal gears or gears sitting next to a shoulder, t or f

True

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A __ uses reciprocating motion to cut a gear. The cutting tool is shaped like a pinion with the desired tooth size and pressure angle

Gear shaper

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Shaping is slower than hobbing but it allows the cutting of internal gears, herringbone and gears sitting next to a shoulder, t or f

True

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Mass produced spur and helical gears are often __. Typically, bar stock is fed between hardened rollers to make a length of gear stock and then individual gears are parted off.

Rolled

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Rolling is a cold forming process. Teeth are generally stronger as a result and the surfaces of teeth become work hardened. Usually needs less finishing and hardening compared to other gears, t or f

True

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Finishing takes place after a gear has been cut close to size by the other proceses. 3 ways to do it

Grinding Shaving Honing

High performance gears are usually hardened and then finished to a precise shape. Grinding is particularly useful for gears harder than 50rc. The 2 basic grinding process are

Form grinding Generating

__ a gear requires a wheel that has been dressed to the shape of the tooth space. This is easily done on modern grinding machines by a single point diamond dresses guided by a cnc controller

Form grinding

One advantage of form grinding is that it allows precise control over the shape of the root. T or f

True

__ a gear tooth requires that the grinding wheel (or wheel) mimic the action of a mating rack.

Generating

The purpose of __ is to improve the shape and surface finish of gear teeth. Results are as good as grinding but is limited to gears no harder than 30rc

Shaving

Honed gears last longer and run more quietly than any other type of gear. A gear hone is an abrasive cutting tool in the form of a helical gear. T or f

True

A single hone can finish thousands of gears before it reaches the end of life. T or f

True

3 common ways to measure gears are

Using a gear tooth Bernier Measuring over wires Measuring across several teeth with a caliper

A __ caliper allows you to measure tooth thickness directly

Gear tooth caliper

Most gear sets require a clearance in the form of __. It is the amount by which the tooth exceeds the thickness of the tooth at the pitch circle

Backlash

The amount of backlash varied depending on many factors including

-manufacture tolerances -variations in center distances -run out -oil film and lube requirements -changes in temp -speed and reversibility

3 ways of measuring backlash are with

-feeler gauge -dial indicators -soft wire

A _ drive transmits motion between non intersecting shafts at 90deg

Worm drive

A worm drive consists of 2 things

Worm thread (aka worm) Worm gear or worm wheel

Worm drives have the following advantages over other types of gearing, they are

-high torque -high speed ratios in small spaces -self locking when lead angle is less than 5deg -accurate rotary positioning (indexing) - quiet

Worm gearing is widely used for winches, jacks etc. It is also used whenever precise control of motion is required. T or f

True

3 types of worms in common use are

-fine pitch -coarse pitch -Browne and sharpe

__ worm threads are intended for light duty applications in which the accurate transmission of motion is the most consideration. Found in instrumentation and mass produced consumer goods. It has a 20deg pressure angle

Fine pitch worm thread

__ worm threads are intended for heavy duty industrial applications in which the transmission of torque requires a strong gear set. Typically has a PA of 20deg. Although 25deg is used when greater strength is needed. Size of thread is expressed in DP

Coarse pitch worm thread

__ worm threads have 14.5 pressure angle. They look like acme threads but they cut a little deeper

Browne and sharpe

Brown and sharpe threads are obsolete and are only found in older machinery, t or f

True

3 types of contact for worm gears

-plain -single enveloping - double enveloping

__ worm gearing is the simplest set up and consists of nothing more than a worm in mesh with a helical gear. Easy to make but lacks strength compared to other types

Plain worm gearing

A better form or worm gearing consists of a worm in mesh with a throated worm gear or __. It has teeth wrapped around or envelops the worm so contact is spread across the width of the tooth. Strong than plain worm gearing

Single enveloping

The best form of worm gearing consists of a throated worm in mesh with a throated worm gear. This is called

Double enveloping worm gear

Double enveloping worm sets are unique in both thread and gear have straight sided teeth. Gear tooth is a little thicker to compensate being made of a softer material such as bronze or cast iron, t or f

True

Rule of thumb. The number of starts is chosen to provide a lead angle of between 25-45deg. More starts = higher angle lead. T or f

True

Best way to make a one off replacement worm is to turn it on a lathe, although with proper set up and tooling you can mill a worm on a universal milling machine. T or f

True

4 methods of mass producing worms are

-milling -generating -rolling -grinding

For worms, the worm must always be harder than the gear to avoid falling, t or f

True

The strongest and most wear resistant combo for a worm is a hardened steel worm with a bronze worm gear. T or f

True

Linear pitch,also called __ , is the distance from one point on one thread to the next thread. It must be the same as the circular pitch of the mating worm gear

Axial pitch

Lead is the linear pitch multiplied by the number of starts . T ir f

True

Advantages of a multiple start thread

- larger minor diameter -efficient transmission of motion -larger load bearing area

4 ways of spacing starts when cutting multi start thread on a lathe

- move compound rest - indexing back gears - indexing drive plate - using thread chasing dial (best way)

Quick change gearbox is always in DP. Remember this. And for metric it is always Lead then Pitch when writing it down

True

Gears Pt2 Flashcards

(63 cards)