Gear Hobbing
Gears used to be milled one tooth at a time: index, cut, index again. In hobbing there is no indexing at all. Tool and workpiece rotate in a fixed ratio, and the tooth form appears as the envelope of many cuts — the tool does not carry the tooth shape within it.
US 640,728, “Apparatus for Cutting Worm-Wheels”, Robert Hermann Pfauter, Zschopauerstrasse 147, Chemnitz, Kingdom of Saxony. Filed 10 August 1899, granted 2 January 1900. Pfauter first filed the process in Germany in 1897; that German patent number cannot be verified to this day, whereas the US document can — and it shows the same invention, drawing included.
Staying honest. Pfauter was not first to the idea: Christian Schiele described the generating principle as early as 1856. Pfauter built the first machine that could do it reliably and universally — which is the difference between an idea and an industry.
说明
Write the claim down first
Write the claim down first
Note the core: tool and workpiece run coupled, and the tooth form appears as an envelope. No dividing head, no tooth shape in the cutter.
Cut a rack from cardboard
Cut a rack from cardboard
Cut a straight rack from cardboard: 20 mm pitch, 20° flank angle, 12 mm high. That is your tool.
此步骤所需材料：
纸板样板1 张所需工具：
钢直尺
量角器
手工刀Cut out a gear blank
Cut out a gear blank
Cut a 127 mm diameter disc from cardboard. Circumference = 20 mm × 20 teeth.
此步骤所需材料：
纸板样板1 张所需工具：
剪刀钢直尺Mount the disc so it turns
Mount the disc so it turns
Pin the disc to the board through its centre with a machine screw so it rotates freely.
此步骤所需材料：
平木板1 个
#6-32 机螺钉1 个Set the rolling condition
Set the rolling condition
Draw a line tangent to the disc. The rack must travel exactly one pitch while the disc turns by one tooth.
此步骤所需材料：
坐标纸1 张所需工具：
油性记号笔钢直尺Roll and trace, step by step
Roll and trace, step by step
Advance the rack 5 mm, turn the disc by the matching amount, and trace the tool outline onto the disc each time.
此步骤所需材料：
纸板样板1 张所需工具：
油性记号笔Trace twenty positions
Trace twenty positions
Repeat until the disc has gone once around. Between the lines the involute form appears — it was never drawn, it was generated.
此步骤所需材料：
纸板样板1 张所需工具：
油性记号笔Cut out the envelope
Cut out the envelope
Cut the disc along the inner envelope curve. You are holding a generated gear.
此步骤所需材料：
纸板样板1 张所需工具：
剪刀Measure the pitch
Measure the pitch
Measure tooth centre to tooth centre with the caliper at five places. Record the largest deviation.
此步骤所需材料：
坐标纸1 张所需工具：
游标卡尺Mesh it with a partner gear
Mesh it with a partner gear
Make a second gear of 10 teeth from the same rack. The two must mesh with no rework — that is the real proof.
此步骤所需材料：
纸板样板1 张所需工具：
剪刀Check the ratio
Check the ratio
Mark one tooth on each and turn. 20 teeth to 10 gives exactly two turns to one. Count them.
Show the difference from indexing
Show the difference from indexing
Draw a gear made with a dividing head: 20 separate indexings. Every indexing error stays in the gear. Generating has no indexing step to get wrong.
此步骤所需材料：
坐标纸1 张所需工具：
量角器Look at a real cutter
Look at a real cutter
Examine an HSS end mill under the magnifier. Its edge has nothing to do with a tooth form — which is precisely the point.
所需工具：
高速钢立铣刀套装
放大镜History & Context
History & Context
US 640,728, “Apparatus for Cutting Worm-Wheels”, Robert Hermann Pfauter, Chemnitz. Filed 10 August 1899, granted 2 January 1900. The German application of 1897 is regarded as the original; its DRP number could not be verified and is therefore not asserted here. Where a number is missing, the function is described — not guessed.
What came before. Christian Schiele described the generating principle in 1856, and Ramsden was working along similar lines on dividing engines as early as 1768. Pfauter's contribution is the ready-to-use, universal machine from 1897 in Chemnitz; the Pfauter company led gear hobbing for over a century and is today part of Gleason.
Why the process won. A single hob of a given module cuts any number of teeth. With indexing you need the matching form cutter for each tooth count, and every indexing step contributes its own error. Generating is faster, more accurate and cheaper all at once — which is rare.
The link to mass production. Without accurate, cheap gears there is no gearbox, and without gearboxes no automobile in quantity. Hobbing stands at the head of that chain. The Fellows gear shaper (1897, USA) solves the same problem by shaping rather than milling — both processes are still in use side by side today.
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