timing gears

[HPS]

I am wanting to replace a set of timing gears on a small lathe.
Id like to be able to use a single cutter od 1/4" HSS toolbit held in my mill collet, and cut the gear blank while using an indexer for the correct tooth positioning

Im a little unclear as to determining how far I must cut into the blank in order to have correct belt engagement between gear and belt

I know that The pitch diameter of a timing gear coincides with the reinforcing cord centerline of the belt. Also, that the pitch diameter is derived from knowing the tooth pitch x number of teeth /pi.


When I model the gear in Solidworks using the calculated pitch diameter..placing the bottom of the tooth profile tangent with the PD....and create the appropriate number of teeth for the PD...the measurement of the pitch(tooth spacing) is off by about 0.02" or more.Seems to me that this would create a problem with correct meshing between the belt and gear.

I have several stock catalogs from WMBERG,SDP-SI, ect. and am using thier profile specs for the belt I need to use...and it expains it all pretty well..with exception to where the profile of the belt grooves are in relation to PD of the gear and how to determine how far the cutter must be engaged into the gear blank.

What am I doing wrong?
Where should the profile be placed in relation to the pitch diameter in order to cut the gear correctly?
How do I determine what the OD of the blank should be and where/how far into I should cut the grooves?

I hope Ive explained what I am seeking help with well enough.

Please help as Im beating my head against the wall, and would prefer to not make any unecessary scraps in the process of figuring this out, dueto the lengthy setup time in using a single cutter/indexer on a mill.

Thanks

[Oso]

Do you mean change gears?

While you *can* do the job with a timing belt, I don't think I would choose to.

What you seem to be referring to I would call a pulley, since you are meshing with a tooth belt.

But, your issue is to have the belt lay on the pulley OD and the teeth go into the toothspaces. I don't think you need to worry about anything other than that fit.

If that fit is right, then the tooth ratio works (neglecting belt stretch etc).

If you get it off, then the belt length and tooth spacing won't match the pulley, and it will want to climb the pulley and skip teeth.

Seems that the belt lays on the surface AFAIK and the teeth just need to not get in the way, and not be so loose in depth that they let the belt wear on the tooth edges.

[HPS]

I am going to replace the belt drive assembly from the motor to the head....it uses an XL timing pulley type setup.

I understand that the only real issue is getting a good fit between the the pulley and belt teeth....

In the SPD-SI catalog for instance......it lists XL timing belts as having the reinforcing cord centerline(or pitch diameter on a timing pulley/gear) as being 0.010" from the bottom of the tooth profile(listed as "U", ... ie. distance from pitch line to belt tooth bottom).....
but when I model it in CAD the pitch measurement from tooth to tooth is off, even though Ive copied there specs exactly on a few of there listed stock pulleys.

Im thinking that there is something thats not being listed in the specs or possibly that the specs are intentionally listed wrong...

I dont know.

All I know is that I cant cut a pulley blank OD right until I determine how to position the tooth profile in relation to the pitch diameter and how far to actually offset the pitch diameter to determine the pulley OD to begin with.

I know that timing pullies are cheap , but Id like to know how to machine them myself.......
That was the whole point in learning to be a machinist.......to make my own parts.

Thanks for the replies

[Evan]

Model the belt as if it didn't have teeth. The belt acts like a flat belt but the teeth keep it from slipping. The proper diameter is the top of the teeth on the pulley or the bottom of the teeth on the gilmer drive belt as wrapped aound the pulley.

Imagine each belt tooth center as a line extending on a radial to the axis of rotation. Then imagine the tooth form that is required.

These are not gears.

[This message has been edited by Evan (edited 10-12-2003).]

[LES A W HARRIS]

THE XL SERIES PULLEYS HAVE THE PITCH DIA .010" RADIALY ABOVE THE OD. HOBS & SHAPER CUTTERS ACTUALLY FINISH CUT THE OD TO SIZE, THE DEPTH IS WORKED IN. AS YOU ARE CUTTING A TOOTH AT A TIME YOU WILL NEED A SINGLE TOOTH CUTTER OR CUTTERS DEPENDING ON THE NUMBER OF TEETH YOU WISH TO CUT. AS YOU DID NOT SAY WHAT NUMBER OF TEETH YOUR ARE CUTTING I CAN NOT GIVE YOU YOUR SIZES. FOR EXAMPLE A 2O TOOTH PULLEY COULD BE CUT WITH A STRAIGHT SIDED CUTTER OF 50deg:36min:23.6094secs AND A POINT WIDTH OF .050" GIVING A DEPTH OF .055" ABOUT.

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[Evan]

Caps lock off please.

[chipeater]

Try this link:

http://bostongear.com/pdf/product_se...pp_142_156.pdf

Page #11 has the specs for the 5mm pitch belt.

If there's not enough info here, you might try Machinery's Handbook.

On second thought...
www.sdp-si.com has 3d CAD models for the timing pulleys that they sell. Can't you just download the CAD model for a particular pulley and get the dimension from there?

Steve