BIZOMAT MAKEOS · Gears
Gear Hobbing Plan (Time, Hob Life and Cost per Gear)
How long does each gear take to hob, what share of hob sharpening and hob cost falls on each gear, and what does the batch cost?
Plan of a gear hobbing job: hob and work speeds, approach and total hob travel, cutting and floor-to-floor time per gear with stacked blanks (as the hobbing time tool), machine-time cost with set-up spread over the batch, sharpening cost per gear from the gears cut between sharpenings, hob cost per gear over the hob’s sharpening life, the cost per gear and for the batch, and the hob speed against the machine maximum.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Gear teeth | Number of teeth on the gear. | |
| Hob starts | Number of starts on the hob. | |
| Hob diameter | mm | Outside diameter of the hob. |
| Cutting speed | m/min | Cutting speed for the hob and material. |
| Axial feed per work revolution | mm/rev | Axial feed of the hob per revolution of the work. |
| Face width | mm | Face width of the gear. |
| Tooth depth | mm | Whole depth of the tooth. |
| Helix angle | ° | Helix angle of the gear. |
| Overrun | mm | Extra hob travel past the face. |
| Blanks per stack | Blanks cut together in one stack. | |
| Load and unload time per stack | min | Time to load and unload a stack. |
| Machine and tooling | ||
| Largest hob spindle speed | rpm | Highest hob spindle speed of the machine. |
| Machine and labour rate | $/h | Machine plus operator rate. |
| Set-up time | min | Set-up time for the batch. |
| Gears in the batch | Gears in the batch. | |
| Gears cut between sharpenings | Gears the hob cuts before it needs sharpening (your value). | |
| Cost of one sharpening | $ | Cost of sharpening the hob once. |
| Sharpenings in the hob’s life | Number of sharpenings the hob can take before it is scrapped. | |
| Hob price | $ | Price of a new hob. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Gear teeth
- What it is
- The number of teeth on the gear being hobbed. With the hob starts it sets how fast the work turns relative to the hob.
- Units and format
- Enter a plain number with no unit. The form accepts 5 to 1000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More teeth slow the work speed, so cutting time rises in proportion.
What this means: Hob starts
- What it is
- The number of thread starts on the hob, marked on the hob or its certificate. Each hob revolution advances the work by this many teeth.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 6. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More starts speed up the work in proportion and cut the cutting time.
What this means: Hob diameter
- What it is
- The outside diameter of the hob, which sets the hob spindle speed for the cutting speed and the approach length.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure the current diameter: it reduces each time the hob is sharpened.
What this means: Cutting speed
- What it is
- The surface speed at the hob teeth, from the hob maker’s data for your hob material and gear material.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 600 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Hob and work speed rise in proportion, so cutting time is inversely proportional to it.
What this means: Axial feed per work revolution
- What it is
- How far the hob moves along the gear axis for each revolution of the gear blank. It sets the cutting time and the feed marks on the flanks.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 20 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting time is inversely proportional to the feed.
- Get it right
- Per revolution of the work, not of the hob.
What this means: Face width
- What it is
- The axial width of the gear teeth to be cut.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Tooth depth
- What it is
- The whole depth of the gear tooth, from tip to root, as cut by the hob. It sets the approach length while the hob enters to full depth.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5.625 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use whole depth, not the addendum or working depth.
What this means: Helix angle
- What it is
- The helix angle of the gear teeth. A helical gear needs a longer hob travel across the face than a spur gear.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 45°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 for a spur gear.
What this means: Overrun
- What it is
- The extra hob travel beyond the gear face at the end of the cut, so the last teeth are fully formed.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Blanks per stack
- What it is
- The number of gear blanks clamped together and hobbed in one pass. Time per gear is the stack time divided by this number.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 50. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The hob travel is worked out from one face width and is not lengthened for the stack height, so check stacked cutting times.
What this means: Load and unload time per stack
- What it is
- The time to load a stack of blanks onto the machine, start it, and unload the finished gears.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 600 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Per stack, not per gear; the tool divides it among the gears in the stack.
Machine and tooling
What this means: Largest hob spindle speed
- What it is
- The highest hob spindle speed the machine allows. The plan is flagged if the cutting speed needs more.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 10000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Machine and labour rate
- What it is
- The cost of one hour of the hobbing machine with its operator, from your costing or finance team.
- Units and format
- Enter a number in $/h: money per hour. The form accepts $0/h to $10000/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $90/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It is applied to floor-to-floor time and set-up time.
What this means: Set-up time
- What it is
- The time to set up the machine, hob and fixture for this batch before the first good gear.
- Units and format
- Enter a number in min: minutes. The form accepts 0 min to 10000 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter minutes for the whole batch; the tool shares it across the gears.
What this means: Gears in the batch
- What it is
- The number of gears made from one set-up. Set-up cost is shared across them.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10000000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger batch lowers the set-up share of the cost per gear.
What this means: Gears cut between sharpenings
- What it is
- How many gears the hob cuts before it is blunt and must be resharpened, from your wear records or trial gears.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1000000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sharpening and hob cost per gear are inversely proportional to it.
What this means: Cost of one sharpening
- What it is
- What one resharpening of the hob costs, from your tool room or sharpening supplier, including any freight.
- Units and format
- Enter a number in $: an amount of money; use one currency for every money input. The form accepts $0 to $100000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $80 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Sharpenings in the hob’s life
- What it is
- How many times the hob can be resharpened before it is too small and is scrapped, from the hob maker or your records.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 200. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Count resharpenings only; the tool adds the first use as new.
What this means: Hob price
- What it is
- The purchase price of a new hob, spread over all the gears it cuts in its life.
- Units and format
- Enter a number in $: an amount of money; use one currency for every money input. The form accepts $0 to $1000000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $1200 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Hob speed n = 1000 V_c ÷ (π D_h); work speed = n × starts ÷ z; approach = √[h (D_h − h)]; travel = b ÷ cos β + approach + overrun (as the hobbing time tool).
- Cutting time per gear = travel ÷ (f × work speed) ÷ stack; floor-to-floor adds the load time ÷ stack.
- Cost per gear = rate × (floor-to-floor + set-up ÷ batch) + sharpening ÷ gears per sharpening + hob price ÷ [gears per sharpening × (sharpenings + 1)].
- Example: each gear takes 6.9709 min floor to floor and costs about 11.582; the batch costs 5,790.9.
Limitations
- Single pass at constant feed; roughing and finishing passes need their own lines.
- Hob life must match the speeds, feeds and material.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Quote a hobbed gear
- Compare stacking blanks for a batch
- See what share of cost the hob takes
Who it is for
- Gear engineers
- Estimators
- Process planners
What you get
- Hob and work speeds, travel and times
- Machine, sharpening and hob cost per gear
- Batch cost
Interpreting the result
- Stacking blanks spreads the approach and loading over several gears; it pays most on narrow gears.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3347–3349. Topic reference; no source tables or recommended settings are reproduced.
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Open the Gear Hobbing Plan (Time, Hob Life and Cost per Gear) calculator