Property of Fairfield Atlas Ltd. Not to be used outside FAL

PROBLEM CAUSE SOLUTION Pitchline Runout or Total Index Error

(a) Tooling not true. (b) Workpiece arbor not true. (c) Check change gears on

mechanical machines.

(a) Indicate top of gear to verify that the support plate and base are true. If runout exceeds 0.00005*PD, then remove support plate or base to ensure that there are no chips, burrs, etc. on the base or the support plate. If chips or burrs are present, then stone support plate or table to remove.

(b) Check workpiece arbor for runout. If runout exceeds 0.00005*PD, then re-true the arbor.

(c) Look and listen for runout or bad teeth in index, speed, feed and differential gears.

Involute Waviness See chart# 1 for maximum hobbed profile waviness.

(a) Hob runout caused by bad spacers. (b) Hob arbor bent.

(a) Indicate hob hub diameters (both front and back). If runout exceeds chart# 2, replace spacers (b) Indicate hob arbor and if runout exceeds 0.0001 (arbors 1.25” or less) or 0.0002 (arbors larger than 1.25”), replace hob arbor.

Minus or Plus Involute Hob produces minus or plus

involute. Sharpen hob to correct involute. See PC program: “Y:” “Shop programs” / “Tool Sharpening” / “Hob Sharpening”

Property of Fairfield Atlas Ltd. Not to be used outside FAL

Lead Error Lead Taper or Helix Error

(a) Support plate or base not true.

(b) Adjust or tram machine

centers. (c) Check change gears on mechanical machines.

(a) If runout exceeds .0003, remove support plate or base to check for chips or burrs, etc. If chips or burrs are present, stone support plate or base to remove.

(b) See PC program “Y:” “Shop programs” / “HP calculator” / “Hob tailstock lead corrections”

(c) Look and listen for runout or bad teeth in index, speed, feed and differential gears.

PROBLEM CAUSE SOLUTION Lead Variation (a) Support plate or base not

true. (b) Check change gears on mechanical machines.

(a) If runout exceeds .0003, remove support plate or base to check for chips or burrs, etc. If chips or burrs are present, stone support plate or base to remove.

(b) Look and listen for runout or bad teeth in index, speed, feed and differential gears.

Hob Holes – Tooth Chatter (a) Tooling not tight.

(b) Feeds and speeds not set correctly. (c) Machine set to Conventional Hob.

(a) Recheck setup and tighten all fasteners. (b) Recheck feeds and speeds and adjust if necessary. (c) Change to Climb Hob.

Property of Fairfield Atlas Ltd. Not to be used outside FAL

HOB TOLERANCES CHART 1 CHART 2 MAXIMUM PART PROFILE ERROR AS HOBBED MAXIMUM HUB RUNOUT In Tenths (1/10,000) inches BOTH ENDS CLASS CLASS AA A B C AA A B C N.D.P. 1.00 / 1.99 -- 35 51 70 -- 10 12 12 2.00 / 2.99 -- 23 33 50 -- 5 8 8 3.00 / 3.99 10 13 22 32 2 4 6 6 4.00 / 4.99 8 11 16 19 2 3 5 5 5.00 / 5.99 6 8 13 15 2 3 4 4 6.00 / 8.99 5 7 10 12 1 3 4 4 9.00 / 12.99 5 7 10 12 1 2 3 3 13.00 / up 4 6 9 11 1 2 2 2 IS THE INVOLUTE PROBLEM DUE TO THE HOB SHARPENING OR YOUR SETUP? a) After cutting a piece part, double check the hob’s hub runouts on both ends. Find the

maximum allowable hub runout from the chart above. b) If the hub runouts are o.k., then the problem could be found in the sharpening or in very rare

cases, the hob could be manufactured incorrectly. c) If the hub runouts are not o.k., check the hob arbor runout in a couple of places with the arbor

mounted in the machine. Also, check the spacer bushings for parallelism and nicks and bumps. The arbor runout and spacers must be near perfect for a good hobbed profile.

d) Do not over-torque the arbor nut when tightening up the hob, this can really introduce runout

in your hob if you are not careful. From ANSI B94.7-1980 W H Hayward 1/2000