GD&T Datums for idler wheel forging

[ofted42]

Morning Everyone,

Been doing GD&T for a while but it's been a while since I've tackled castings/forgings. I'm looking for some advice on GD&T setup for an idler wheel I'm working on. The part is a forging that is then 100% machined down to the part as used on the machine. The bore is pinned through and a belt rides on the outer diameter. Outer two faces are thrust faces.

Currently the forging and machining are all +/- toleranced, but I'd like to put together GD&t that would allow for better control of the part and better inspection. My initial thought is to use one of the two large diameter side faces as the primary forging datum and the outer diameter as the secondary. From there you'd machine the pin bore and one of the thrust faces. Then you could use those two datums to control the orientation as you machine the other portions, controlling the flat faces and diameters through runout and position. However, that seems overly cumbersome to me. I would think you could control the face dimensions by using overall width dimensions and possibly using positional to control their center planes relative to each other.

Not sure if that made sense, the picture shows the forging and the finished machining. Any tips on how to lay this out would be much appreciated.

Thanks

 

[drawoh]

ofted42,

If you are starting from some inaccurate process like casting for forging, consider adding datum target features. Call up datum targets, such that your forging and your machining all work off the same datum features.

--
JHG

 

[ofted42]

Right, that's what I would normally do and have done in the past. However, this part gets 100% machined, which is where I'm running into a bit of an issue. Any datum targets that are on the forging would be machined off, meaning I need to establish some machined datums based on the forging before the rest is machined.

 

[drawoh]

ofted42,

If there is no way to leave an as-forged feature on your part, then you are stuck.

You could specify an initial machining process that locates the forging, and that shows a face and a perpendicular diameter that would be your datums in the subsequent processes. Your manufacturing process would be three steps...
[ol]
[li]Forge the blank[/li]
[li]Machine the datum features, located to features on the forging[/li]
[li]Machine everything else[/li]
[/ol]

--
JHG

 

[ofted42]

Yeah, that's the three steps I'm coming up with as well. I think where I'm getting stuck right now is the best way to lay out those initial few machined datums to properly represent the function of the part. The initial drawing that was done used the forged bore and center plane of the wheel as the two initial datums, but the D datum that came later for machining used the same face to determine the center plane and so was referring itself back to a face that was gone. Hard to check that. Keep going back and forth on what to use for the machined datums, assuming that the forged bore is A and one of the side flat surfaces is B. Might end up using the outer diameter of the forging as A and the sides of the larger rim as B. From there I can machine the bore for the pin as C reltive to A and B, the two thrust faces to establish the center plane D by using B and C, then use those datums to machine the rest.

 

[CheckerHater]

You may benefit from the idea of temporary datums.

There was a discussion on this forum recently: thread1103-451313

Could be helpful for whatever it's worth

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Dean Watts]

ofted42,

I think what Checkerhater brings up about temporary datum features is necessary in this case. I also think your plan to use the outer diameter as the primary temporary datum feature is probably best. What I'd like to add is that the datum feature should be referenced at least material boundary (L). I think the machinists who will handle the part are used to basing their "balancing" of the forging (or casting) on the low points of the features. That's how they ensure material is present so the features all clean up during machining (as you may already be well aware of). Referencing the as-forged outer diameter as the primary, at (L) will just mean that the temporary primary datum axis will need to be based on a maximum inscribed cylinder, so driven by the low points on the outer diameter, just like a good machinist will do if no GD&T is specified.

This all assumes the outer diameter has enough length to orient the datum axis in a repeatable enough way.

Since the two outer faces are functional thrust surfaces on the finished part, your plan to use the sides of the larger rim as the secondary temporary datum feature also sounds good. I think it would be best to apply the datum feature label to a width dimension between them, so the center plane in the datum, and reference this datum feature at (L) also. The datum would then be the center plane that is perpendicular to the temporary primary datum, and otherwise based on two "maximum inscribed" parallel planes that are based on the low points on those two faces.

From that temporary datum reference frame, the center bore and the outer thrust faces can be machined, which should create a nice set permanent datum features (the center bore as primary and the width between the two outer thrust surfaces as secondary) from which the other features on the part will have the best chance possible of cleaning up when they are machined.

Dean

http://www.validate-3d.com