Gears...

[Ken I]

Charles,
Two different problems

1) Non-Standard pitch circle - in the case of the 7T 64 DP gear it had to go over a 2mm shaft so an oversize pitch circle was used.

The modified PC approach is used when circumstances dictate you cannot use the correct theoretical value (shaft size, axle pitching etc.).

2) Interference when using small tooth count pinnions - Can be solved by addendum modification or as you pointed out by my slightly different regeneration method - there are probably a number of tooth profile modification methods.

Either way, the moment you deviate from theoretical you introduce an element of "sliding" - relative motion of one tooth face to the other at point of contact - as opposed to the correct "rolling" motion of one tooth against the other with no relative motion at the point of contact.

Which of course leads to greater friction losses and wear problems.

I was not advocating pitch circle enlargement as a means of getting away from interference.

If you have a pair of ruined gears - all you need is the number of teeth and the axle pitches and everything else follows - some people are amazed by this resurection of manufacturing data from apparently little to go on - however when a non-standard approach has been used, all bets are off - hence my dislike of non-standard gearing.

Ken

 

[precisionmetal]

As an FYI -- the point where undercut happens on an external gear is...

2 / (sine of PA)^2 ≤ = number of teeth where undercut happens

(Two, divided by the sine of the pressure angle squared)

17 teeth on 20°
32 teeth on 14.5°

PM

 

[Todd...]

this might be a dumb question, but i just got a dividing head to try and make a new gear for my lathe. when counting the teeth of the gear, what is being counted the tooth itself or the void in between the teeth?

also i see the angles of the teeth mentioned a lot in gear making, i am going to be using my shaper and grinding a form cutter by using the old gear. are the angles as important when reproducing a gear like this? or just when making one from scratch

are there any online articles or literature explaining some of these things more? i have the machinerys handbook 28, and get the ideas and theory of it but am looking for something more applied rather then theory

Thanks
Todd

 

[Tin Falcon]

this might be a dumb question, but i just got a dividing head to try and make a new gear for my lathe. when counting the teeth of the gear, what is being counted the tooth itself or the void in between the teeth?

well I always count teeth but do not think it matter unless I am miss remembering the # of teeth and spaces are equal.

if you are trying to copy an existing gear just coy it as close as you can and do not worry about numbers . be careful not to copy wear. I have a couple of acquaintances that were handed parts and told make like this. and they reproduced them to every detail. This included the wear that made the original part useless and in need of a new replacement.
so an understanding of the original specification can be valuable info.
Tin

 

[precisionmetal]

Todd,

Hopefully every gear you check has the same number of teeth as it has spaces!

Are you referring to "pressure angle" when you ask about angles?

PM

 

[Todd...]

pressure angle i guess, as well as DP, how can i measure an existing gear to find out what DP it is? i have read through machinery handbook and understand the theory of the different measurements of the gear, but how do i obtain them?

 

[Tin Falcon]

The proper method of measuring gear teeth is a gear tooth caliper.
again if you are making a copy of the existing gear just make it match the old one.
Tin

 

[Ken I]

Measure the OD of the gear and the root diameter (arkward if an odd number) subtract one from the other and divide by 2 to get total tooth height (including clearance) divide that by 2.139 (the .139 being the clearance) and that will give you the Module of the gear - if that turns up a metric value like 1mm then you have a metric gear.

If not divide it (in inches) into 1 to get the DP. If neither gell try CP - same as module but in inches like 0.1" etc.

Once you have established the basics of the gear - ie the number of teeth and its module / DP and this tallies with the diameters - you've almost got it.

Next check out the pressure angle - that is the angle that would exist for a rack or a gear with an infinite number of teeth - most modern gears are 20° and older being 14½° - generally the a 14½° looks a lot "skinnier".

However a wide range of angles is possible (but unlikely).

There are ways to determine this using rods as measuring wires (use drill shanks etc.) - consult your handbooks.

Regards,
Ken

 

[precisionmetal]

A quick and dirty way to get very close is to simply measure the base pitch.

Pick 2 "numbers of teeth" where it's clear you can get the jaws of a vernier onto the *profiles* of the teeth (don't touch the tip or the root). e.g. 3 teeth and 4 teeth, or 6 teeth and 7 teeth, etc.

Span one "set" of teeth, and zero the calipers, then check the other "set" (whether one tooth up or down). Read the caliper. That's the base pitch.

While the base pitch will not give you the DP or the Pressure Angle, it will point you to the combination. In other words, there is a given DP/PA combo that will yield a certain base pitch. If you have a good idea that a gear is 20°PA by looking at it, then the base pitch will show you what DP combines with 20° PA.

Base pitch is the magic number around which all gear design is based... even if it's a term that's not often used.

The great thing about using base pitch as a measurement is that it doesn't matter if the gear was cut on an oversize blank, if the OD was reduced for some reason, if some non-standard cutter was used, etc. Base pitch still shows the DP/PA combo that did (or could) produce that gear.

... and more interesting yet is, you can use a different combo DP/PA to arrive at the same gear. e.g. Say you have a gear that is 16DP 30°PA. That has a base pitch of .1700". An 18DP 14.5° gear has a base pitch of .1691" (.0009" off). Pretty darn close. So if you happened to need an 18DP 14.5° gear, and you only had a 16DP 30° cutter (I'm speaking about hobs or shaper cutters now, that "generate" the tooth form), you could probably get by.



PM

 

[MachineTom]

Measure the OD of the gear, the more teeth the better, even teeth is also better. count the # of teeth, add 2 to the number of teeth, then divide the number of teeth +2 by the diameter, the result will be the DP of the gear. This for 20° PA gears, which is a large % of all gears

example OD of gear 1.042, number of teeth 48. t+2=50, 50/1.042=47.98~48dp

 

[popnrattle]

The video demonstrates how I draw the gears I need to make from metal or import sketches into 3-D software to prototype them with an ABS plastic modeler.

[ame]http://www.youtube.com/watch?v=xjQ9R7-vEvs[/ame]

 

[viumau]

Wonderful work

https://www.youtube.com/watch?v=1rckGCN1kwY