Mechanical Engineering

How can you assure the quality of the manufactured parts if you don't describe the part geometry by using GD&T? ;-)

Nowadays CNC machines dont need dimensions added to the sketch because you just input the correct file and voila, out comes the part. But imagine that you you order 1000 shafts. The CNC operator gets the file and he prints 1. He checks the part and because the drawing doesn't have any dimensions for him to check against he can only visually confirm that its a match with the file. Now imagine that the shaft needs to be 1000mm long, but because of some human error it's only 100mm. Well now you have 1000, 100mm shafts.

Of cource CNC machines need dimensional tolerances, too!

You can cut a surface slow, fast and superfast. Therefore you get very low tolerances with fine roughness, low tolerances with normal roughness or rough tolerances with harsh surface. So the CNC program needs to know what the engineer wants to produce.

Yes, but the CNC doesn't need the dimensions visible on a 2D plane, like the operator does. The 3D file has the dimensions that the CAD engineer used.

Dimensioning is always necessary. Most of the time the engineer who design a product doesn't know the exact technology used for part production-not every production phase use the 3d model in order to get the correct dimension.

Another aspect is dimension tolerances. Most of the time I avoid adding tolerances on CAD file.

And last but probably most importat is the quality department which always need a print version in order to create reports.

Have fun!

It's necessary. it gives accuracy in manufacturing.

its more than necessary, imagine don't having control on what your machines does, cnc for example can achieve really nice accuracy but a lots environmental/operator can affect on final product, lets say, your part need to achieve some flatness that is critical for the assembly, but the operator clamp the raw material at certain angle and naked it eye looks great but on flatness could be out of tolerances, so fundamental GD&T works to have control and convey to the manufacture place what you really care about of the geometry functionality of the part, there is many aspects why your part could be wrong and even sometimes involves the machines, cnc are no perfects, so you can have control on what you really care and for quality control as well.

Sundstrand, my employer for 20 yrs. had a CNC line for machining in the 70's when I started and it wasn't new. Magnesium is a flammable metal. The gear box housings for their generators sometimes were magnesium castings. The housings had GD&T drawings and those parts were inspected to determine if they met the tolerences. Note that CNC machining, G code, creates a curve by breaking it into many lines. To just the number of lines effects dimensions. The CNC machining process cuts many features that do not have to be inspected against GD&T dimensions. And today CNC is used to produce the complex surface of dies that produce all the plastic parts. The majority of the machining produces surface that form the outside and inside surfaced between mounting points. Visual inspection is normally fine for the outside of metal castings and plastic parts.

GD&T stands for Geometric Dimensioning and Tolerancing.

Most we use is the dimensioning and tolerancing like Tony mentioned 1000mm vs 100mm. Geometric Tolerancing takes place once assemble work happens. If you only deal with one part and no assemble, like 3D printed figure, you may not need Geometric one. Even you cut a copy of key, the key's profile (geometry) needs to be controlled, then Geo. one has be applied.

So, depends on what you and your company dealing with.

In heavy industries you should define your tolerances accurately. we do not use GD&T just for CNC but we use it in oil&Gas for huge components such as towers, flanges, marine equipment, etc. It shows to manufacturing department how to assemble and produce the components accurately.

GD&T is actually more permissive (from an inspection stand point) than standard tolerancing. Not many people realize this. There is an interesting bit of text about the origin of GD&T here: https://quality-one.com/gdt/

GD&T is also very useful for designers of complex parts and assemblies as it actually "means" something in 3D unlike normal basic +- tolerances that don't really correspond to what you really want to part to look like. For example, nearly all of the aerospace sector uses GD&T nowadays. It is a bit more complicated and expensive, but the savings generated down the line can be huge.

The case for using GD&T in your company really depends on what your company makes/needs. If your just welding some parts together to make a diner table or chairs, than you absolutely don't need GD&T. If you are make aircraft wing profiles, than you really should be using GD&T. Chances are you are somewhere in the middle!

Then there is also the matter of using GD&T in a company that has never used it (for good or bad reasons). I would not recommend using GD&T in such a case as barely anyone will understand the tolerances that you put on the drawings. Just keep using the good old basic +- tolerances. I've personally not used GD&T that much in the last 20 years, some places I worked at should have used it but most people did not know how it worked so they never changed. The few time I did use it, was for very specific needs or in aerospace.

Keep in mind that GD&T did not even exist during WW2 when they designed so many new aircraft. GD&T would sure have helped simplify drawings and specifications though!

GD&T is necessary because in industry to manufacture some parts it's give dimensions and tolarances .

Jimbo has an excellent point. GD&T in many cases actually provides a wider envelope for a good part to fit into than standard ± tolerances can provide.

As with any new system, there is a certain amount of education that needs to occur to understand how to dimension parts (as the engineer) to provide the fit that you need to accomplish but there is also some education required for the operators and QA personnel to properly apply the tolerances during inspection. Once it becomes the norm for your employees you will wonder why it wasn't implemented long ago because it has the potential to greatly reduce manufacturing errors as well as inspection errors.

A part scrapped on the shop floor isn't great but can be a necessary evil when fine tuning your process. But a good part incorrectly scrapped during final inspection that should be "good" can be even worse because of the added cost of finishing operations and inspection.

GD&T helps the engineer convey the intent of the design as well as the physical requirements that somebody else has to interpret from a drawing. Depending how big the company is there could be many people involved in producing parts or assemblies or it could be as few as one person wearing many hats.

it is vitally important. find someone who designs transmissions. if you have interconnected parts that move together, GD&T is vital to make sure that they match. perpendicularity and circular runout can make things go downhill in a rapid, expensive manner if critical relationships aren't held.

It is necessary, but your employers need to know how to use it or will be a pain in the ass