DP of gears, whats it all mean?

[irving2008]

Anyone throw any light on how to ensure gears will mate correctly. The changewheels on the SuperRelm are different to those on the Relmac, although they both have the same 3/4 boss and width, so how do i determine whether a gear on eBay has a chance of meshing?

[John S]

DP is shorthand for Diametrical Pitch and is the number of teeth that will fit on 1" of the circumference of the PCD or Pitch Circle Diameter.

Two main diameters on a gear that the user needs to know, many others but you can forget them.

OD, Outside diameter is obvious and the PCD is a theoretical diameter inside the PCD that if the gear had no teeth and transmitted drive by friction [ 2 rubber rollers ] would be the PCD.

Basically a point on the PCD that contact the other gear.

DP is found by adding 2 teeth to the number of teeth and dividing it by the OD.

So a gear 1.100" OD with 20 teeth = 20 +2 =22 / 1.1 =20 DP

The PCD is number of teeth divided by the DP so 20 / 20 = 1.00"

If you have two gears identical to above the centres will be 1/2 PCD + 1/2PCD so 1.00" in this case.

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[irving2008]

ok, found that formula... so both sets of gears are DP16, so whats the chances of them meshing?

[John S]

Every chance.

One other detail required on gears is to know the pressure angle, PA, the involute [ shape of gear tooth ] is generated at.

Modern gears are cut at 20 degrees, older gears at 14.5.

Bad news is it's really hard to measure a PA in the home shop without specialist equipment.

Good news is most machine tool maker use 14.5 and stick to it because of legacy issues.

Early Colchesters also had 16DP gears at 14.5 PA.

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[irving2008]

Every chance.

One other detail required on gears is to know the pressure angle, PA, the involute [ shape of gear tooth ] is generated at.

Modern gears are cut at 20 degrees, older gears at 14.5.

Bad news is it's really hard to measure a PA in the home shop without specialist equipment.

Good news is most machine tool maker use 14.5 and stick to it because of legacy issues.

Early Colchesters also had 16DP gears at 14.5 PA.

.

Both DP16, but his are 3/8 wide and mine 5/8. Also I have 55 and 65 while his set includes 56 and 64 so maybe intended for a different pitch leadscrew (mines 8tpi)

[John S]

Width and pitch of leadscrew have nothing to do with whether the gears will fit.
That's all down to the DP and whether they have the numbers of teeth you require.

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[irving2008]

No I realise that John, but it does have a bearing on whether the gear set is useful for cutting standard threads... if i can't make meaningful trains with the pitch of leadscrew then there's no point in buying them.