7.466 DP which cutter to get; 7DP or 8DP ?

[iMisspell]

Need to buy a gear cutter (have little experience cutting gears) to make a new pinion for an arbor press.

The pinion has 12 teeth with an OD of 1.875

I used the following formula:
# of teeth +2 divided by OD
(14 / 1.875) = 7.4666666667

Not sure what to get, a 7 or 8 DP cutter ???
And if it don't matter because its too close, and having little experience cutting gears, which would be a more common cutter to get (for later use) if it does not matter.

Tried making my own cutter out of D2 but i have no luck hardening (have tried a few times (MAP gas and magnet check) with different projects and never get the results i want) its too soft and wore on a test cut in 1018.

The new pinion will be made out of pre-hard 4140.

Thanks for any help

_

[Forrest Addy]

That's very close to 10mm linear pitch. Is it an import press?

[iMisspell]

Greenerd 3-1/2 Ratchet

http://bbs.homeshopmachinist.net/threads/65441-_-The-resurrection-of-a-Greenerd-3-1-2-Ratchet-Arbor-Press-begins

Dont know the manufactured date, but i know for sure its 30+ years old.

Never crossed my mind it might be metric.

_

[Forrest Addy]

It might have what's called an enlarged addendum, a trick whereby gears having low tooth counts are strengthened. The teeth are cut to depth on an oversized blank with the object of avoiding undercut roots. It works very well but the calculations look funny to whoever has to make replacement parts. You might be looking at 8 diametral pitch 12 teeth cut on a larger blank.

Try this. Count the number of complete teeth on the rack in Pi inches. The number will be the diametral pitch of the rack itself. The gear has to be the same pitch I bet it works out to a common diametral pitch rack.

[oldtiffie]

I have not taken the OP's maths further but it may be that the odd DP is in fact a metric module number.

Further, the OP does not say what the pressure angle is either - it may be 17 1/2 degrees if the machine is relatively old or perhaps20 degrees if the machine is relatively new.

[dp]

If it is possible, pull the pinion out and roll it in a flat layer of modeling clay. This will product a rack profile. Slice it with a razor to get a clean edge. Stand that on edge in a flat bed scanner and you will have an accurate digital image from which you can determine the pitch and pressure angle of the original gear.

[EVguru]

If it is possible, pull the pinion out and roll it in a flat layer of modeling clay. This will product a rack profile. Slice it with a razor to get a clean edge. Stand that on edge in a flat bed scanner and you will have an accurate digital image from which you can determine the pitch and pressure angle of the original gear.

If this is for an Arbor press, then there is already a rack to measure!

[iMisspell]

Thanks all... posting from phone now so...
The pressure angle is another thing. Found something with COS (PA) x magic# and / by DP... something like that(have it at home). Gonna try the clay thing with the rack. The pinion is in bad shape.


If Pi inches are 3.1416 then there are 8 teeth in that distance.

[Mark Rand]

Unfortunately, you have to measure start-of-tooth to start-of-tooth or end-of-tooth to end-of-tooth etc. in order to get a whole number of teeth, so we're not looking at a whole DP pitch. Try measuring as many whole teeth as you can get on the calliper travel and then seeing what that computes to as DP, CP or mod. Then measure the pressure angle.

Note that with a 12tooth pinion, it's not going to be an un-modified 14.5° or 20° PA profile anyway, because that would have undercut teeth, but if you can find or make a B&S type cutter for it, that won't be a problem.

[dp]

If this is for an Arbor press, then there is already a rack to measure!

Mine's locked in place with a blind rivet so is hard to get to. Probably helps keep them assembled on the boat ride over from China.

[Rich Carlstedt]

Just "eyeballing" the photo, tells me it is a 8.25 DP
You have 8 full teeth and a crest. Looking at the crests and the linear scale, they occupy 1/4 of the distance of a "pitch" and the gullet and tooth sides are 3/4 of the distance.

Got to be a modular pitch near 3 Module I believe

Pressure Angle is simple...measure the tooth angle on the Rack !

Rich

[SVS]

Isn't that seven full teeth plus a crest? (7.25 dp)

I freely admit I might be looking at this wrong, but same method as counting threads per inch, right?

[iMisspell]

Gonna look into this farther over the weekend.

Like said before, i dont have alot of experience with gears or gear making so most of the basic stuff is new to me. The thought of using the ram as a 'guage' never entered my mind. A quick check with a protractor and it looks like a 20deg PA, gonna do alittle more research and measuring over the weekend.

Thanks again for the direction.

_

[Spin Doctor]

Measuer the depth of the teeth on the rack. I suspect what you have here may be one of two things. First is a propriatary tooth form. The second is a Stub Tooth Form. Either a Fellows or American Standard. A Fellows is described as for example 8/10 DP. The 8 represents the Pitch Diameter and Circular Tooth Thickness. The 10 is used to determine Addendum and Deddendum. Pressure Angle 20 degrees. American Standard Stub Tooth I'm really not up on. Also could this be a 7.5DP tooth form. That calculates out to 1.8666666 OD (14/7.5)(#T+2/7.5)

[Juergenwt]

Most likely metric pitch (module). Take a quick reading from center tooth to next center tooth in mm (gear rack) and divide by 3.14.
On a gear take the distance from one tooth to the next tooth in mm measured on the pitch dia and divide by 3.14.
That number is your module. Most likely they are even numbers like 0.5 - 0.8 - 1 -1.25 - 1.5 - 2 - 2.5 etc.
See here: http://www.micro-machine-shop.com/module_gear_data.pdf