I’ve been creating technical drawings to machine parts for a summer project, but all my drawings seem to never result in easy communication to the machinist. I figured I could start learning GD&T to use it on my technical drawings and it seems pretty complex and difficult to understand. We also do not have access to CNC machining that does not require technical drawings.
Basically:
Do other teams use GD&T?
Should I start using GD&T? If so, how can I learn it easiest?
I can’t speak for other teams, but we typically use Onshape drawings as its integrated. That makes it easy for whoever is CADing to make drawings.
GD&T is not separate software for drawings. It is a system for communicating engineering tolerances between design and manufacturing. Onshape drawings have the geometric tolerances and datums needed for GD&T, but the designer needs to know how to use those tools, which is a topic with some complexity.
I don’t know how many teams employ the rigor of GD&T. I suppose if you are using a sponsor for machining and that sponsor is expecting GD&T tolerancing, it would be good for that team to learn and use GD&T.
GD&T will not result in easier communication.
It sounds like you need to spend more effort understanding what your machinist needs from your drawings to make the parts. Bring open ears and your honest questions. Explain that you’re doing your first project and that you need their help to make a better drawing set. They know what’s missing for their process better than some strangers on the Internet.
GD&T is a very specific form of tolerancing for high precision components and process control measures, eg communication of engineering data. Unless your machinist explicitly prefers it (unlikely), adopting it will not help communicate your design more easily.
Communication of your design is a hard skill on its own. You need another human to understand everything you need and will check on this component, and you need to describe to another human how to make the component. .
Most modern machine shops accept CAD files now, which they’ll use as a guide for the majority of the part, and then only reference the paper drawing for critical quality control dimensions. So, in that context you’d only need a drawing for the interfaces, the things you will check with a caliper or another tool.
Typically point to point dimensions for critical references are used here, specific bore callouts, etc.
If you’re not going to check anything you don’t need a paper drawing.
Making a paper drawing set for manual manufacturing processes is a completely different skill, where you will need to communicate all the dimensions they need to actually produce the part by hand on the paper.
Typically an ordinate dimension scheme is used here, so that the machinist can take one reference and then do all their machining from that one point.
If you don’t know how to make the component, it will be difficult to set up your dimensions to describe how. You may need to sit down with your machinist and listen to them for a while to better understand the problem
If I understand correctly, you’re having parts made by your team in your school workspace, versus parts sent to an outside machine shop.
If so, I would NOT suggest you use GD&T. There are not enough hours in the day for you AND the manufacturing kids to learn it. You could consider including the common symbols in your drawings, and teach their meanings as time and energy permits, but I wouldn’t expect much. The students are often having enough difficulty keeping things in (simple) tolerance and adding further constraints to the dimensions are not going to make it easier.
At the FRC level and possibly a young learner of creating technical drawings, I would suggest you first become very proficient at dimensioning drawing. Make the dimensions complete, not redundant, not ambiguous, minimizes the maker doing a lot of math, toleranced appropriately, use notes, etc.
I used to be proficient in the use of GD&T, but I have’t actively used it in 25 years. -I’ve forgotten a lot. Having said that, as the internal machinist in several companies, I found it much more useful to walk over to the engineer/designer and have a conversation about what was important (and not) on a drawing they gave me. I could then best provide them what they wanted.
Also, do you have the ability to inspect and check the parts as drawn with GD&T? If not, then how do you know you’re getting what your “paid” for? Adding GD&T, tight tolerances, over tolerance, etc. cost money. -“money” could be currency, or the extra time/energy/frustration that is spent trying to make your part.
CNC machinery is not a requirement to use GD&T.
You can use it on parts made with a hacksaw and file 
If engineering is in your future goals, I would say yes, learn it. But take your time as you likely won’t need it during your time in FRC.
Coincidentally, I receive an email this week that led to this link:
link to GD&T Introduction I am NOT endorsing this paper, I just happened to see it this week. There are many other sources.
If you want to use GD&T, keep it simple and use just the basics: True Position, concentricity, parallelism, perpendicularity, circularity, straightness, flatness and angularity. That should cover most(?) things that you would likely design. But as I already mentioned, first learn to dimension your drawing well.
If I had to pick one to start with, true position (especially on holes) is the one I can see as being most useful to an FRC team.
When I learned GD&T, the class had a large emphasis on creating the drawings to communicate how to inspect the part after it is made and not using it to communicate how it should be made.
But in the case of FRC, sometimes we want to communicate how it should be made, depending on how experienced the team is in manufacturing.
Usually when drawings don’t communicate what you want to the machinist, it’s usually because the dimensions are hard to understand. Sometimes the dimensions is hard to measure and layout, or are dimensions are shown on the incorrect face, etc.
Drawings, yes.
GD&T, nope.
First of all, thank you for your great explanation.
Now, sadly my team is pretty inexperienced at the moment so this isn’t my first project, and I’ve made many drawings with simple dimensioning rules for two build seasons now, and I still can’t perfect the art of getting the machinist to understand what I want to be made. Not even because they end up making crappy parts, it’s mostly because we have very few and very short meeting times and I am usually actively doing other important things as the machinist is making parts, so I don’t want to keep having to babysit them.
Specifically, there is a mentor who asks a lot of questions about my drawing to help the machinist make the part. (It’s not that this annoys me, my mentor is definitely right, and my drawings are, I guess “not good enough.”)
And another part of having less time is not having time to have students like me be taught how to make effective drawings.
And since that’s out of the way, my overall question is:
How should I take the step from my beginner level to a better understanding of how to effectively communicate my intent of the part with a drawing without wasting my time explaining the entire part function face to face?
(I use OnShape)
Face to face communication is NOT a waste of time. Let’s just make that clear.
That said, I’ve marked up and sent back drawings on my team, for missing dimensions.
You ask what you should do to communicate the intent of the part in a drawing. I think you’re approaching the question wrong. Clear communication is important… but it gets easier when both sides have common ground. This isn’t a CAD problem.
IMO, your first step is to learn how to machine, with the tools used. Make parts from drawings. Maybe even from drawings you’ve made, but mark them up as you go with missing dimensions and/or notes.
THEN you’ll understand better what’s missing in the drawings you’ve been sending for machining. Add that in, and you’ll have a much easier time.
I agree 100% with the comments from @EricH . He makes some good suggestions.
Like most situations in life, good communication (which includes active listening) is paramount.
Another thing to consider: Your machinist at school are probably as inexperienced as you. They too are in a learning environment. You might give them a perfect drawing that any professional machine shop could give you exactly what you ask for, and your peers would still struggle.
FWIW, in my 30 years of machining professionally, I doubt there was a week that went by when I didn’t have questions about a dimension or feature on a part. …these drawing were coming from name recognizable, multi billion dollar, medical, tech, defense, aerospace… companies as well as little start ups and “mom and pop” businesses.
Don’t be too hard on yourself. You recognized there are shortcomings to your work and you want to learn to fix them. Step 1
e.g. “Please make sure the hole in this block lines up with the other hole because a shaft needs to slide smoothly between them.” (bonus point if you hand them the shaft.)
The machinist doesn’t necessarily need to know the parts are for the lateral intake on the bot."
If you have the means, the best way to understand how to communicate how you want a part to look is to learn how to make the part. Work with the machinist as they make the parts, and make some parts yourself. One of the most useful classes I had in highschool (almost a decade and a half ago…) was my intro to machine tools class. I only received a C on the final project, but that class taught the basics of what dimensions and details are needed to make a part.
Lot of good advice here. Here’s what i would check first on a drawing:
On a related note… Use reasonable standard tolerances. There’s a good chance there is already a standard tolerance in your title block if you’re using one, and if there isn’t any standard tolerance on the drawing, make sure you and the machinist have the same expectation on tolerance. But for the love of everything holey, if you have a standard tolerance, make sure it makes sense mathematically, and for the material and manufacturing process in question. One of my former employers used the same tolerance on all drawings: X.XXX ± 0.003, X.XX ± 0.010, X.X ± .015. This meant that we had stuff like 32.0±0.015 on molded plywood that suppliers couldn’t get better than ± 0.25 on, and if you wanted 5/8" but didn’t need it to be very precise rounding to a single decimal place would result in perfect accuracy to the CAD model not being in spec. All our suppliers ignored our tolerances and when parts came in to get inspected I would have to fill out a sheet that documented exactly how much they failed to meet 75% of the dimensions by and then wrote approved at the top.
Agreed with others’ comments.
On 254 we do drawings with dimensions and just default tolerance block, (typically 0.XXX = +/- 0.005").
To the question of “do I need to know this for my future career as a Mechanical Engineer”:
I work for J&J on surgical robots and we use GD&T extensively. It better communicates design intent to the machine shop and ensures the parts / tolerance stackups actually fit as intended.
Interns at J&J (perhaps in general in the industry) are not expected to have learned GD&T, and nearly all haven’t fully. It is something that be taught on the job with guidance and reviewing from peers.
Seconded on the “interns are not expected to have learned GD&T”. Everything I’ve learned about it (at that’s still just scratching the surface) has been on job, and coming from engineers who also had to learn it on the job.
I was actually supposed to have been exposed to it a little bit in one of my classes but that class was taught by one of the worst human beings I’ve ever encountered, so that didn’t happen.
I don’t know that I would even agree these are simple or the basics.
If you’re going to be a ME, eventually you’ll want a strong working knowledge of GD&T. If you exit college knowing what all the symbols are used for and what they control, you’re way ahead of the curve, even if you don’t know exactly how to apply them or what tolerances/datum reference frames would be appropriate.
GD&T is of zero use in FRC though.
@Cory Thanks for the update! When I learned GD&T at a several days long Mitutoyo class long ago, True Postion (changed to Position in 2009) and Concentricity were taught. Best practices are always being updated and I have not kept up.
Out of curiosity, which GD&T symbols would you consider simple and/or basic that a student should begin with?