quality inspection ii.pdf

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Quality Gear Inspection Part II ,Robert E. Smith ,R.E. Smith & Co., Inc. ,Rochester, NIY Diagnostics This section will deal with the use of gear inspection for diagnostic purposes rather than quality determination. The proper evaluation of various characteristics in the data Carl be useful for the solution of qual.ity problems, It is impor- rant to sort out whether the problem is corning from the machine, tooling and/or cutters, blanks, etc. An article by Robert Moderow in the May/June 1985 issue of Gear Technology is very useful for this purpose, Machine Preblems, The following tecb- niques and devices win indicate when prob- lems Originate with the machines. Lead and Involute Charts. Lead charts will often indicate problems with machine align- ment as well as blank mounting (see Fig, I), A comparison of Examples I and 3 in this figure is instructive. Whether the charts of both sides of the teeth are parallel is a good clue to the source of the problem. Accumulated Pitch vs. Runout Data. If a gear comes directly off a bobber (no subse- quent finishing operations) and has a very low runout, but a high accumulated pitch or total index variation, there is probably a problem with a worn-out table drive gear set. Hob Problems, Hob problems generally show up in involute charts. The e problems have several possible causes ..The hob could be made very accurately, but sharpened or mount- ed poorly; or it could be a hob that was made with excessive runout at the time of original manufacture, Even if it is sharpened and mounted correctly. it will not cut a good gear. This problem usually shows up as some form of waviness 01' slope error in. the involute charts (see Fig. 2). These characteristics of slope or waviness may very well fit within the allowable bandwidth of the appropriate AGMA Kchart. but the gear may still be unsuitable for many applications, especially when noise is an issue. One must study the characteristics in order to sort out the cause .. A good example is the dif- ference between Examples 4 arid 5 in Fig, 2. If the waviness is the same on all teeth, the prob- lem is probably within the hob or the accuracy of the mounting. [f the waviness is different. on every tooth, the cause is likely to be a machine looseness problem, Hob Feed Marks. When parts are bobbed at a high feed rate. uch as for pre-shaving, scal- lops in the surface characteristics are very evi- dent. Generally, these scallops are so deep that involute charts are quite useless. Usually the only valid data are lead charts. The slope of the charts can show if the lead is correct. and the Gea[ tlal Ia~( P,alJable c.",," Machi"" M,,,,lignmenl ~N t- t::; -' l- f::: f::: I-f-: t- f-: t:.:: , ;'- t::: ~ t- ~~ .r::: j::: ~ ...!l;2!L t: Gca! t::m~ Lead ~iuiallgD ProlJable c.\lSe Gear Blank Has Fate fiunaul-Wobbl. ""ti f::: .... ~ I- 3 . ~ ~ Numtiefi\== ::: sa 2 SEal t::Ia:iiLiliid Em:u: .PrDbaIJleC.USOlMac~"le r MiSilhgn""'n~ Setup Error- G.a's. Hob AI1lor End I'1<Iy ;i:: r-r-,- ~ F-: ~ :::- t; , , , ...:; I F::E= uaac- t-::t:-- 3 Probable C.use- Hob Br.a i"!l OUI of Cut. Hob ArIxlr or Watt Arbor FI •• ng K""" P'obable c.use- Machine MlOal'lllVllllnt. . Hob NOI Accu,ale Enough When O'.gonalllobtJ'"!I 4 Fig.1-lead and il1l1ollute chart. 5

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  • Quality GearInspection Part II

    ,Robert E. Smith,R.E. Smith & Co., Inc.

    ,Rochester, NIY

    DiagnosticsThis section will deal with the use of gear

    inspection for diagnostic purposes rather thanquality determination. The proper evaluation ofvarious characteristics in the data Carl be usefulfor the solution of qual.ity problems, It is impor-rant to sort out whether the problem is corningfrom the machine, tooling and/or cutters,blanks, etc. An article by Robert Moderow inthe May/June 1985 issue of Gear Technology isvery useful for this purpose,

    Machine Preblems, The following tecb-niques and devices win indicate when prob-lems Originate with the machines.

    Lead and Involute Charts. Lead charts willoften indicate problems with machine align-ment as well as blank mounting (see Fig, I), Acomparison of Examples I and 3 in this figureis instructive. Whether the charts of both sidesof the teeth are parallel is a good clue to thesource of the problem.

    Accumulated Pitch vs. Runout Data. If agear comes directly off a bobber (no subse-quent finishing operations) and has a very lowrunout, but a high accumulated pitch or totalindex variation, there is probably a problemwith a worn-out table drive gear set.

    Hob Problems, Hob problems generallyshow up in involute charts. The e problemshave several possible causes ..The hob could bemade very accurately, but sharpened or mount-ed poorly; or it could be a hob that was madewith excessive runout at the time of originalmanufacture, Even if it is sharpened andmounted correctly. it will not cut a good gear.This problem usually shows up as some form ofwaviness 01' slope error in. the involute charts(see Fig. 2). These characteristics of slope orwaviness may very well fit within the allowablebandwidth of the appropriate AGMA Kchart.

    but the gear may still be unsuitable for manyapplications, especially when noise is an issue.One must study the characteristics in order tosort out the cause ..A good example is the dif-ference between Examples 4 arid 5 in Fig, 2. Ifthe waviness is the same on all teeth, the prob-lem is probably within the hob or the accuracyof the mounting. [f the waviness is different. onevery tooth, the cause is likely to be a machinelooseness problem,

    Hob Feed Marks. When parts are bobbed ata high feed rate. uch as for pre-shaving, scal-lops in the surface characteristics are very evi-dent. Generally, these scallops are so deep thatinvolute charts are quite useless. Usually theonly valid data are lead charts. The slope of thecharts can show if the lead is correct. and the

    Gea[ tlal Ia~(P,alJable c.",,"Machi"" M,,,,lignmenl~Nt-t::; -'l- f:::f:::

    I-f-:t- f-:

    t:.:: ,;'- t:::~t-~~ .r:::j:::~ ...!l;2!L t:

    Gca! t::m~Lead ~iuiallgDProlJable c.\lSe Gear BlankHas Fate fiunaul-Wobbl.

    ""tif:::....

    ~I- 3 .

    ~ ~ Numtiefi\== :::sa2

    SEal t::Ia:iiLiliid Em:u:.PrDbaIJleC.USOlMac~"lerMiSilhgn""'n~ Setup Error-G.a's. Hob AI1lor End I'1

  • fig. 2 - Lead and involute chart.

    Probable Cause ~Hob Sharpened WithPositl'''' ~.k.

    very useful for diagnosing gear noise andtransmission error problems. Other technique,such as separate determination of "mean invo-lute slope" and waviness in involute charts,should be used for these evaluations ..AGMAQ7 or Q8 gears can be made that are noisy, andQ]3 or Q14 gears can be made that are quiet.Fig. 4 shows involute traces from a gear that isabout a Q7 in terms of runout, The effect ofrunout shows up in the traces as a variation upro .00066" in slope. However, the mean invo-lute slope is only .00015", and it matches itsmate properly, (See the middle example in Fig.4.) Originally, the mean involute slopes for thegears were like the upper example in Fig. 5.These gears were noisy. Experimentally, gearswere also made with the mean involute slopecharacteristics of the bottom example, andthese also were quiet. In all cases, the runoutwould make them a Q7 set of gears, and theywould have slope variation ..This points out theinadequacy of the traditional methods of quali-ty determination as far as noise is concerned.

    ISO Approach. The International StandardsOrganization (ISO) uses a different approachto the evaluation of involute and lead charts. Inthe main body of the standard, they use a rec-tangular tolerance zone, much as AGMA usesthe K chart. However, in an Annex, they havealternate methods of evaluation that are moreappropriate for diagnostic evaluation and thecontrol of noise and transmission error.

    This process involves doing a least-squares-best-fit line (usually a straight line) tothe involute or lead trace (see Fig .. 6). Thisline is laid on the trace and the various para-meters, such as slope, average slope, wavi-ness and total error, are read and quantifiedindependently. This is much more useful formany applications and diagnostics.

    In addition, the ISO standards give recom-mended tolerances for accumulated pitch errorand transmission error. These are characteris-ticsthat are much more direct and functional.

    Good Measuring Equipmentdepth of the waves win tell if there is stock for Good measuring equipment is essential forcleanup in the shaving operation. However, the achieving quality gear measurements. There

    Probable Gause ~Hob Mounting E~orsHob Sharpening ErrDISIFlul. Spacing Off)

    ~HOb Not Accurate Eno~h

    ~, r

  • Mec.lumical Machines. The mechanicalmachines an example of which is shown in Fig ..7. use means such as base disksv sine bars,levers, formers, etc., to generate the theoreticalmotion such as involute or helix (lead). A probeon the tooth surface then measures the variationbetween the actual tooth surface and the theoret-ical shape being generated. Usually 3. recorderof some type makes the resulting charts.

    CNC or CMM Machines. The newer CNCmachines come in two types. On some. theCNC i used to generate the theoretical motion.much as the older mechanical machines did(see Fig. 8). On the other hand. CMMs mea-sure the tooth forms by moving the probe to aseries of X, Y, Z, and e locations. High resolu-tion and accuracy is necessary.

    Cal.ibration. Whether the machines aremechanical or CNC, alignment and calibrationare necessary in order to have confidence inthe results .. Any machine should be calibratedon a regular basis. A new standard, AGMA2010, Part I, is in the final stage of prepara-tion, Part 1 deals with involute, but a standardfor other parameters will follow, and the sameprinciples apply.

    The first step is to check the alignment ofthe machine for squareness, parallelism, runoutof centers and spindle, etc. Then mount a certi-fied artifact such as an involute master. Manymeasurements are then made and plotted on Xand MR bar charts for a statistical evaluationof capability. This establishes bia: and vari-ability (accuracy and confidence levels) for themachine. Remember that resolution is not thesame as accuracy.

    Certified Masters. Unfortunately, theNational Institute of Standards and Technology(N[ST) has not calibrated and certified mastersfor about ten years. The recent work by the cal-ibration committee of AGMA ha stimulatedintere t by NlST ill resuming it effort in arti-fact calibration. A project is now under way toset up a calibration service again.

    Cost of Inspection ..Many people seem tofind it hard to justify the cost of good in spec-tionequipment ..After all, the inspectionmachines don't make chips. However, the bestcutting or grinding machine in the worlddoesn't automatically make good gears. Thereare still a lot of other variables. such a tile cut-ting tool and its mounting, the gear blank and

    its mounting, work-holding equipment, opera-tor, etc. If you don't have good inspectionequipment and practices, had product canbecome very expen ive in terms of lost time,poor quality and bad customer relations.

    One should look hard at the true cost of nothaving good equipment compared to. thecostof buying it.

    A Look At The CompetitionA good example of the value of good

    inspection equipment and practices was foundat the 1991 AGMA Hear Expo, A Japaneseplastics gear manufacturer was displayinggears made to AGMA quality levels of QW to.Qll. Our American plastics and sintered gearmanufacturers typically claim the ability tomake only Q6 to Q7 gears. What is the differ-ence? Good diagnostic measuring equipmentand practices. Even though this company ismaking fine-pitch gears (20 DP and finer), itregularly uses CNC elemental measuringmachines and even single-flank. testers to mea-

    5.24"

    40

    Results - , /1 0,000

    Left Flank Right Flank

    1 2 3 44.4 -5.5 0.8 3

    -1.5

    4 3 2 1lHa 1.9 16 3 -6.6fHam (MIS) 1

    Fig. 4 -Involute traces with runoul.

    Tip - .0001"

    GearMIT Gear MIT PinionMn

    PO

    -

    --. Tip +.0006"Tip +.000'"

    Tip + .0006" - Root

    Gear MITRoot

    IPinionl Mil

    , Root

    Fig. 5 - Mean involute traces (MIT).

    NDVEMSE.RrOECEMBER 1994 33

  • Prufile Diagr~mA = Tooth Tip or Start of ChamferE = Start of Active ProfileF = Start of Usable Prali Ie1 = Design Profile2 = Actual Profile3 = Mean Profile

    La = Profile Evaluation RangeFa = Tota I Profile Deviation'Ha = Profile Slope Deviationffa = Profile Form Deviationfa = Pressure Angle Deviationfwa= Profile Undulationft.a = Profile Undulation Wavelength

    Fig. 6-ISO tolerance system.

    3

    B

    1 - Measuring Slide2 - Rolling Sliae3 - Vertical Slide4 - Control Slide5 - Helix Guide

    7

    6 - Compensation Guide7, 9, 12 - Compensation Mechanism

    10 - Work Adaptor11 & 13 - Optional Base Disk and

    Straight Edge

    Fig. 7 - Mechanical generative involute and lead tester.

    Tesler Witll 7CNC-cornroll'ed Axesz

    ~ Z Axis - Vertical Slide Movement.. I Linear Scale Aesolution: 0.5 J.Im

    (20 rnicroinchesl,

    TAxis - Radial Slide

    /Movement.

    T linear Sea la Resolution:O.5[.lm

    ~. Axis - For AngularPositioning of ---I-"+--tlll-fl-+Probe, 90"

    X Axis - Tangential Slide.Linear Sea IeResol ution: 0.5J.1m

    --- Y Axis - Carriage"""'_:c-'_ Positioning.Linear ScaleResol ution: 0.5 urn

    II' Axis - WorkRotation. AngularResolution: 0.0001" i

    fig. 8 - Generative machine involute and lead tester.

    34 GEAR TECl-mO~OGY

    sure gear quality. The company also has agood symmetrical gear blank design that pro-motes uniformity of teeth around the gear.

    db American gear manufacturers in this fieldonly use double-flanktesters. They can't reaUytell what tooth form they are making or evenknow what is wrong when the product doesn'tmeet the specifications. Double-flank testersare not good measuring devices for diagnosingproblems: thereforevenglneers don't do theiterations necessary to develop good gears.One of the major reasons that American plasticgear quality is as low as Q6 or Q7 is that thegears have too much runout caused by not cen-tering the hole in the center of the gear. Thishould be an easy problem to solve with propermold and cavity design.

    Better measuring technique would soonpoint manufacturers III the right direction formeeting the competition and opening up newpotential markets for their products. Theirproducts are very uniform from piece to piece,so it is just as easy to make all pails good as tomake all parts bad.

    ConclusionsProcess Control. More use should be made

    of elemental inspections rather than double-flank composite methods. This also applies togears that are finer than 20 DP. This kind ofinformation fosters better diagnosis of prob-lems as wen as better process control.

    Standards. Our AGMA standards needchanging. Elemental tolerances and inspectionneed to be extended to finer pitches (even asfar as 80-100 DP). We should be using accu-mulated pitch variation rather than runout. Thestandard should also allow the optional use ofsingle-flank inspection as well as double-flankand elemental methods.

    One way of doing this is to make use of thenew ISO standards currently being developedand published. In fact, AGMA is now veryactive in the development of these standards,and perhaps someday that is what our gearindu try will be using .RefereneeseI. Moderow. Robert H. "Gear Inspection and Chart Interpreta-tion," Gear Technology. Vol. 2, 0.3, May/June 1985, p. 30.Acknowledgement: First presented or lite AGMA 21st AnnualGear Mamifacmring Symposium. October /0-12, 1993. Reprint.ed with permission.

    Tell Us Wha~You Think ... lf you found this article ofinterest and/or useful, please circle Reader ServiceNo. A-79.