How to find GD&T Tutor / Drawing Consultant 1

[Ramblin_Man]

I'm very green and have found myself as the only Mechanical Engineer at a small startup.

Some good work has been done to design a sheet metal assembly (basically a complicated box made of folded parts bolted together). But no thought has been put into tolerances / manufacture. Even the holes were of informational size instead of realistic (3/16 hole for a 3/16 rivet is no good etc.). So a very small chance the parts would actually work as delivered. I need to figure out how to control the drawings I will be sending to a contract manufacturer.

I have a decent idea of how to do this, but given the scale of me %^&*ing up (tens of thousands of dollars and months of lead time) I would really like to have someone experienced look over my drawings. I also have been teaching myself GD&T over the past week to use for this process (might as well learn it) and I'm sure that however I've applied it to my drawings, it's not correct. I'm really just applying position tolerances to holes, but still.

Are there any consultant businesses / services that would look them over (probably only a few hrs time). Or could I hire a GD&T tutor and double dip by having them teach me on the drawings I need to make? I really just need to ensure that this first prototype production run doesn't come back as a disaster because I'm quite inexperienced at tolerance stack-up analysis and controlling manufacturing drawings. Or any resources on bent sheet metal parts, i.e. what are realistic punch, bend line, or angle tolerances.

 

[Compositepro]

The best consultants for this are the vendors who may be fabricating your assemblies, depending on how busy they are. Developing good relationships with vendors is a key part of a successful business. This is also a good way to learn which vendors you want to work with, and which ones you do not want to work with.

 

[Ramblin_Man]

Thank you that is good advice. They were originally handed the CAD and asked to make the drawings, and whoever made them put zero effort into it. That is understandable, it's not their job, but made me hesitant to go to them. It feels like the contract manufacturer just wants a CAD and perfect drawings. But perhaps I will be able to push them with the right questions to get help.

 

[CWB1]

Most companies today either hire draftsmen or outsource their prints to a drafting house bc engineers are generally terrible at drafting. I would recommend doing the same. Most job shops and even large contract production manufacturers aren’t going to have a draftsman on staff, so I’d save your breath with suppliers. There’s a ton of standard best practice with drafting that only years of experience can provide, and not following those quickly adds up to a lot of dollars wasted.

 

[drawoh]

Ramblin_Man,

Buy a copy of the standard. It's very readable, and it is officially correct.

[dt]ASME Y14.5-2018[/dt]
[dd]The latest standard. Is anybody working to this?[/dd]
[dt]ASME Y14.5-2009[/dt]
[dd]The previous standard [—] ASME Y14.5-2018 has removed concentricity and symmetry, so don't use them.[/dd]
[dt]ASME Y14.5M-1994[/dt]
[dd]The older, but still popular, standard[/dd]

--
JHG

 

[moon161]

Really? Concentricity a no-no? What do you use instead?

 

[3DDave]

It's Newspeak - now Position is used for both.

 

[drawoh]

moon161,

Concentricity is a messy and easily misinterpreted concept, and the positional tolerances work.

--
JHG

 

[3DDave]

Of course concentricity is messy - it is to control angular/rotational radial symmetry. Changing the symbol doesn't change the complexity, it just removes it from the lexicon. Wasn't concentricity in GM's internal - never use this - document? Does this mean that subs to GM can now use the replacement interpretation and force GM to inspect parts per a requirement they specifically did not want to inspect?

 

[drawoh]

3DDave,

Concentricity is meaningless without a well defined cylindrical feature to use as a datum. For position, you can use the cylinder, or two diametral features, or a diameter and a perpendicular face.

An untrained person will specify a diameter as a datum feature, then use concentricity to control another diameter. This is not sufficient. Concentricity requires an axis with a position, and some directional control.

Run-out is the other way to control "concentricity".

Note how I am distinguishing between a diameter, and a cylinder.

--
JHG

 

[3DDave]

Concentricity applies to shapes like propellers. Position doesn't cover that. The fact that the degenerate case, as applied to a nominally perfect cylinder, is similar to position, doesn't negate the cases position is ill suited to handle.