Defining tangency with GD&T? 2

[meridian5850]

I have a solid cylindrical part with a diameter of .375" and a SR.275" at one end with a blend radius of SR.050" between the dia. and spherical radius. The SR.275 and SR.050 blend radius must be tangent, but we cannot figure out how to specify this with GD&T without having an unreasonable profile tolerance. The boss wants a profile tolerance of .0001, but the .275R and .050R can vary +/-.002 each. Any ideas?

 

[ctopher]

Can you show a pic to be more clear? From your description, I think I would call out the SR .275, but leave the .050 as a fillet or radius callout R .050. Then add a note on the dwg that all radii/fillets to be tangent and free of burrs. (or something like that).
Hard for me to tell without a pic.

Chris
Systems Analyst, I.S.
SolidWorks/PDMWorks 05
AutoCAD 06
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

 

[wgchere]

First, you can't have a SR to blend a Cylinder and a Spherical Dia. that is just a regular old fillet radius. Call the radius out to blend--in other words, don't define a center point for it, just the radius. A fillet radius by definition has no steps. It starts tangent to one surface and ends tangent to the other. To nail it down (if it is critical that there be no steps) do as Chris suggested and put in a note that there can be no steps, or that there must be a smooth transition.

 

[MechNorth]

Actually, a fillet radius isn't specifically identified in GD&T and is called out with a standard R symbol. As of Y14.5M-1994, a radius can have a step unless it is a Controlled Radius (CR). Y14.5M-1994, Section 2.15, Fig. 2-19 shows that the controlled radius cannot have any flats or reversals, and must be tangent at the ends of the radius. A radius marked with just an R symbol can have flats and reversals, and therefore does not need to be tangent at the ends (Fig. 2-18).

The difference is not typically a concern until finer toleraces are needed, then use a CR along with a suitable size tolerance. I love Surface Profile, but this isn't a good use for it as it sets up a symmetrical tolerance zone which conflicts with the CR tolerance zone.

This was not the case in '82, when CR did not exist and R had the meaning that CR now has. (Fig. D-2)

Hope that helps.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[ewh]

Good update, Jim!