Involute Knurling, No base diameter supplied??? 2

[jagodragon]

I am trying to make an exact model of the knurling below. The problem I'm having is that the Base diameter required to define the involute curve is not supplied. Is there a standard will supply the base diameter from the data below?

Tooth Profile = Involute
Module = .44
Number of teeth = 34
Pressure angle = 45°
Major Dia = 15.400mm
Minor Dia = 14.520mm
Pitch Dia = 14.960mm
Over pin Dia = 16.751mm
Pin Dia = 1.000mm

Any help on this would be greatly appreciated.

 

[Occupant]

The base diameter is always BD=PD*cos(PA). But what interest me about this is: why do you need an involute? The deviation between a straight line and an involute in this case is <0.005mm.

 

[jagodragon]

Occupant,

Thank you. I don't think it is, strictly speeking, essential in this particular case. But, there are more splines defined in the same fasion that have a larger tooth height. These are the reason that I needed the formula.

Aside from that, I was informed the other day that the CAD models containing involute spline shapes could not be trusted. mainly because the method for making the involute was pure math and had no geometry to show as proof. It left the engineers aprehensive. unfortunatly, in my work i found that they were right. the models did not take the pressure angle in to account properly. I am fixing this problem.

 

[TheTick]

I feel your pain. I've seen all kinds of hacksmanship in gear tooth modelling. Trust no one.

 

[Occupant]

I agree that you can generally not trust CAD-Programs to come up with an accurate profile, therefore I've uploaded a *.dwg file for this. I know it is accurate, I did it myself . If you can use it - fine.

 

[tbuelna]

...I was informed the other day that the CAD models containing involute spline shapes could not be trusted. mainly because the method for making the involute was pure math and had no geometry to show as proof....

A CAD surface or solid representation of a spline tooth flank can be extremely precise or it can be a rough approximation. It all depends on how the person creating it chose to model it. Often it is not necessary to have super high-fidelity surface definition on features like spline teeth since the CAD model is not typically used to produce the spline teeth. In order to simplify the digital model, the spline tooth flank profiles might be approximated by a series of arcs.

With modern CAD softwares and specialized design applications for modelling things like splines or gears, it is very easy to produce a CAD model of a spline or gear tooth flank that is extremely precise. If you are not being supplied a CAD model that meets your standards for accuracy, it's the modeller's fault and not the software.

 

[drawoh]

jagodragon,

Is this a knurl, intended to create a rough, grippable face, or is it a gear that must act on another gear? When I want a knurl, I call up a "MEDIUM DIAMOND KNURL", and I get what I want. I could specify pitch and depth, but I do not care enough, I do not want to call up anything that requires the fabricator to do anything other than select his favourite knurling tool.

If this thing is a gear, how are you going to fabricate it? Machinists select an off the self cutting tool to cut the gears. Your model has no effect on this. I model most gear teeth with straight sides, which are close enough to reality. For small numbers of teeth, my 1941 copy of French's Engineering Drawing has instructions on how to use a radius to fake the involute face. Is your rapid prototyping process accurate enough to do functional teeth?

How large and complex will the final assembly be? I have two assemblies up on my SolidWorks right now. One of them has 2390[&nbsp;]components. The other has[&nbsp;]2797. If you had modelled my knurled components, I would hate you with a passion.

All of this will affect how I model everything.

--
JHG

 

[TheTick]

Most CAD programs are capable of generating decent gear geometry. Most CAD users, not so much.

 

[gearguru]

The tooth thickness in this case has to be calculated too. To "model" the tooth thickness from the size over pins and pin dia is an option too, but I always prefer math.
To accurately model/draw an involute is not that hard and it can be done in any CAD. Just some trigonometry is necessary.
Long time ago we discussed it in these forums. Try to search, it should be still available.

 

[tbuelna]

There are existing standards for straight knurling (ANSI B94.6). There are also widely available standard tooling and machines for roll forming the knurls. If all that is needed is knurled surface for ease of gripping, then it would be far less trouble and would be far easier to use a standard straight knurl rather than an involute spline profile. Both in terms of digital modelling and manufacturing the part.