General profile and draft angle 1

[Kedu]

A feature is defined based on its general profile tolerance (lets say a note such as: profile within 0.30 to A, B and C) which is the best way to combine its “basic” size defined in the model with the draft angle specified in the general note?

Is it allowed to have a basic dimension with a draft angle?

I was trying to find a good academic example of the above issue within Y14.5 and also Y14.8 with no success.
Any “do this” example/ template I can follow you can recommend would be greatly appreciated.

 

[CheckerHater]

If I understood you correctly, your 3D model is "basic" but draft is only specified on the "paper" drawing.

I would say it's a bad idea, but I don't know how complicated your part is.

I would include draft into model, but once again, it is not something one can decide completely blindly.

It depends on several factors, size/complexity of the part, what kind of documentation you prepare, what level of understanding you have with your supplier.

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

 

[chez311]

Kedu,

Do you have an example of what you are describing? From what it sounds like you would be setting yourself up for providing an ambiguous drawing that could be interpreted a myriad of different ways. Specifying a draft angle with a basic dimension on one of the drawing views as well as reflecting this in the 3D geometry would avoid this.

 

[Kedu]

If I understood you correctly, your 3D model is "basic" but draft is only specified on the "paper" drawing.

Correct.


Our company's standard business practice is to not include the draft angle because in the early stage of design (when the model is built) the process is not yet definitive and firmly confirmed and might be adjusted/ changed.

I would say the parts are fairly complex (highest complexity could be rates rated/identified as body air compressor but there are multiple vales as a standalone products that can be considered complex too)

https://www.northerntool.com/shop/t...MIgvzb1ZP-3AIVQ77ACh1lBQcnEAYYAiABEgJdI_D_BwE

So, if the draft is to be included in the model, how will interact with the basic dimensions shown in the model? Will the draft angle be considered basic too?
Let’s say a hole has only a size dimension shown (Ø 4 DFT “+“) and a position within Ø0.01. If the dimension applies at the smaller end of the feature, what would be the “correct” dimension at the other end? Draft angle 2°±1° (paper drawing note) or 2° basic angle draft and default profile shown in the general note (profile within 0.30 to A, B and C)

 

[SwinnyGG]

Not including draft in the model is a very bad idea.

You will be leaving fine detail of how surfaces interact up to the moldmaker, which is not something you should do.

Draft the model and dimension accordingly.

 

[Kedu]

Not including draft in the model is a very bad idea.

Probably here I agree with you. The question is how to do it correctly with no conflict to the general profile shown.

Specifying a draft angle with a basic dimension on one of the drawing views as well as reflecting this in the 3D geometry would avoid this.

Could you, please, expand a little bit?

That is exactly my main goal: how to do it in an unambiguous way to avoid multiple interpretations.
Any good/ academic examples that can be shown online?

 

[drawoh]

Kedu,

Are you trying to control your draft angle, or just your profile?

If you specify a sufficiently sloppy profile, a functional draft angle will be contained within it. You need to distinguish between an angle that is part of the functionality of your part, and an angle required for ease of manufacturing. I have designed lots of mirror mounts where I have specified an angle and a much sloppier profile on my feature control frame. The angle is critical. The location is much less so. If you are showing a draft angle, you can call up 1[°]MAX. Perhaps on a perpendicularly modelled side, you can call up 90+1/-0[°]. This way, your 3D[ ]model is at MMC/MMB, and you eliminate some nasty surprises at assembly modelling time.

--
JHG

 

[SwinnyGG]

The question is how to do it correctly with no conflict to the general profile shown.

I guess I don't understand what you're saying here. The drawing needs to be adapted to the model with the draft included, not the other way around.

If you have machined mating surfaces, dimension them as normal. If you don't, dimension the drafted body. Either way, it's the same end result.

 

[Kedu]

So, IF a basic dimension shown on the drawing (or on the model) is subject to the draft then how the true profile is defined? Still not clear.
I assume (due to the draft) the profile (when a feature is governed by the general profile note) is allowed to be outside of the profile tolerance zone. Am I correct?

I am making an analogy with the ± and DFT (+) or DFT (-) where a feature is allowed to exceed the shown tolerance in the direction shown by the draft. (ex: a hole shown with DFT (+) can be bigger than the size tolerance.
Am I missing something?

 

[SwinnyGG]

The profile of a surface applies to that surface, not to the 'extruded' shape behind it.

When you show a surface in your drawing and dimension it, you are dimensioning that surface only, unless otherwise specified.

If you have, for example, holes with drafted walls, the position and shape of the hole on the dimensioned surface does not change.

The walls of the cylinder created by an undrafted hole become the walls of a cone when the hole is drafted. A cone can have position, perpendicularity, concentricity, etc applied to it just like a cylinder.

 

[Kedu]

Thank you for your answer.

So, is it okay (good design practice) to use basic dimension (let’s say a curved surface dimensioned with basic radius dimension) with DFT(+) or DFT(-) ? As far as I understand basic dimensions they are theoretically exact and locate/orient the tolerance zones. Now, how in one hand a basic dimension is theoretically exact (no tolerance whatsoever), but DFT(+) or DFT(-) are applied to them? Sorry, not sure I understand the concept.

Any figures from Y14.5 or Y14.8 (or any other standards or books or that matter) that you can reference for proper visualization of the idea (show-and-tell)

 

[Kedu]

In other words, I do not see any difference between having the above radius basic only and having the radius basic and DFT(+) or DFT(-) applied. The actual feature still has to reside within the general profile note regardless of the draft. Am I correct?

 

[SwinnyGG]

No, I would not apply generic draft angles to a basic dimension.

You need to establish reference planes and allow your molder to determine the parting lines as needed- it is counter productive to do this yourself.

Measure the part on the surfaces, not through the depth.

Remember that the purpose of GD&T is to describe the importance of features and their relationships- avoid creating GD&T callouts that are difficult/impossible to inspect.

 

[Kedu]

Imagine this is an ASME drawing Y14.5 and Y14.8
Also general profile note shown in my original initial post.

 

[Kedu]

I forgot the question:

See all those R5 basic, R30, R4, etc. The surfaces dimensioned with these basic dimensions are drafted (drafted angle being specified). Are the applicable surfaces allowed to go beyond the general profile surface due to the draft angle? If no, why not?
And how the depict that the outside contour is drafted, if R basic is not recomaded to be shown with basic angle AND DFT(+) or DFT(-)?

 

[Kedu]

Again, I have read Y14.8 -- 3.6.1.
"(c) +DFT (“+Draft”) is the symbolic means of indicating
that the dimension may increase due to draft in addition
to the increase allowed by any applicable tolerance
applied to the considered feature (see Fig. 3-6).
(d) -DFT (“–Draft”) is the symbolic means of indicating
that the dimension may decrease due to draft in addition
to the decrease allowed by any applicable tolerance
applied to the considered feature (see Fig. 3-7)."

but still not sure on how to apply it with the general profile note (profile within 0.30 to A, B and C)

That's why I came here for guidance.

 

[CheckerHater]

Doesn't look like ASME drawing to me.

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

 

[Kedu]

As I said, pretend is ASME. Replace shown position with profile and see if the described issue is solved. If not, could you explain why not and how to solve it. Thanks.

 

[SwinnyGG]

It's solved by dimensioning a real drawing that shows the draft.

 

[chez311]

Kedu,

I am not very familiar with Y14.8 and its methods, however I will take a stab at answering your questions after reviewing the applicable sections.

1) Just to get it out of the way, I can see some of the advantages of specifying a part using the methods shown in Y14.8, mainly to provide flexibility to the tooling designer and casting/molding/forging supplier, especially during the initial development/prototyping phase. That being said, utilizing these methods just adds additional ambiguity to your product definition (as you have already seen - it is not nearly as thorough and vetted as Y14.5, and even that is open to interpretation in certain sections) and I would highly recommend that a 3D model and associated drawing be developed ASAP at some point prior to production with the EXACT geometry of the product per Y14.5, and ditch most of the conventions suggested by Y14.8 (at lest those with little to no support in Y14.5 such as +/-DFT) otherwise you are leaving yourself open to a myriad of issues down the line, especially if disputes arise over issues with the product at your assembly plant or in the field resulting from geometrical variation/print specifications.

2) Your +DFT/-DFT looks to be some sort of pseudo profile tolerance, and I do not think it should be combined with a traditional profile callout. In the section you mentioned 3.6.1c and 3.6.1d they both include the verbiage "in addition to the increase allowed by any applicable tolerance applied to the considered feature" and reference the figures 3-6/3-7 both of which ONLY consider directly +/- toleranced features. I would not combine this with basic dimensions.

3) Located on several of the figures and at the end of section 3.6 is the below cautionary statement. This leads me to believe that +/-DFT and traditional profile tolerance are mutually exclusive and cannot or should not be combined as I read this statement to mean "use profile instead of +/-DFT for critical features" not to combine them. That being said, after doing some searching I've seen the two combined as you described and interpreted several different ways. Since this is not supported by Y14.5 and even in Y14.8 is not clearly defined, you will never get a clear answer on this and everyone will have their own interpretations. Feel free to develop your own internal standard to define this for your use if you really feel you need it but I would personally avoid this at all costs.