A proven process and resources to prototype using as many purchaseable components as possible

edward_omagbemi

Hi everyone,

I'm new to Onshape and I'm probably my worst enemy in getting to work with it.
I studied mechanical engineering and learned "sort-of" how to design. "Sort-of" because I went into fluid and aerodynamics and spent more time on wind tunnels and programming than designing. Now I have enough knowledge to be ambitious but no experience in actually prototyping. A bad combination.

Fast forward 30 years and I'd like to build stuff using components I can buy and using Onshape or Shapr3D to design and build brackets, fixtures,... to make it all work.

So the "designing" a shaft from the ground-up approach of the beginner tutorials are not really what I'm looking for. That's basic stuff that is kind of self-explanatory. I'll go through them in order to learn Onshapes features and how to use them but this is not what I'm struggling with.

I'm struggling more with
A: Finding components like bearing assemblies, gearboxes, castor wheels,... that I can actually buy and use without having to buy the components and reverse-engineer a 3Dmodel. Everything you get in an engineering shop or the Lowes and Bunnings of this world is so standardised. And what good is a castor wheel in the public library that I don't know where to buy in physical form?
B: Defining the correct clearances for assembling those components. I'm thinking there must be "proven standards" by now for bearing seats, holes/shaft combinations,... that I'd like to use without having to start calculating everything. Is there a good source for knowledge like this around?
C: Placing said components in the 3D space so I can design what I need around the components. Imagine a sensor that needs to be at a specific location in the 3D space so I then know how the bracket has to look. It's this working in the 3Dspace without having 3 axis coordinates to work within Onshape that I struggle with.
Working with virtual constraints. Imagine designing an autonomous lawnmower where you want to achieve a certain ground clearance and want everything to fit into a given envelope but of course, those things do not exist in material form. They are a constraint on the project.
E: Taking a 3D scan and defining axis, layers,... that allow me to design brackets etc to fit those 3D scans.

So in a nutshell: Defining constraints around objects and boundaries that I then use to easily design and build what I can't buy.

Does this type of approach actually exist? All I find are tutorials on how to build components from the ground up.

What approach would you recommend I take?

Any suggestions are welcome and please do not hold back if I'm talking rubbish.

Regards,

Edward

S1mon

A. McMaster.com
B. Machinery's Handbook

tim_hess427

I agree with @S1mon on A and B. Mcmaster even has CAD models that you can download and import into onshape.

C and D - These two are hard to address because your questions are still pretty general and you're basically asking, "How do I design things?". If you have more concrete examples, we could provide more specific advice.  I would suggest starting with paper. Write down your design constraints using words and sketches. Then you start defining the shapes and dimensions that you want (again, sketching by hand is really a good way to iterate in this process). For the lawnmower example, you know your ground clearance and you can decide on a wheel diameter. Then, you know where the axles for the wheels need to go. Then, you can decide on width/length. Now, you have some basic dimensions that you can use to start designing things. 

E. There are some posts from @billy2 where he shows his workflow for this process. 

 

bruce_williams

@edward_omagbemi - Welcome to Onshape and the forum. You will find lots of helpful people here.

@S1mon & @tim_hess427 give good advice. The search in Forum tools should help you find the discussions on designing from digitized data.

To add a tip - for what you call constraints, using Layout Sketches is a powerful way to do design from Top Down.  You make sketches at correct orientation in the Part Studio and then use them in that studio, or derive to other part studio, or insert the sketch(es) into assembly to help position inserted  parts.

edward_petrillo

I mentor a high school robotics team that builds a new robot every year for the FIRST Robotics Competition.  A typical robot comprises a roughly equal number of commercially available off-the shelf (COTS) parts and parts that are custom fabricated- typically 200-300 parts in all.  The workflow you may be looking for resembles fairly closely the approach my students and I use.  We start by creating layout sketches on the three native planes in a "Master" parts studio to define the 3-dimensional shape of the robot and any critical geometry such as the location of  wheels and other components.  We insert the layout sketches into a top level assembly and fix them to the origin.  

Nearly all of our suppliers provide CAD models of needed components, and generic hardware items are available as already mentioned from McMaster-Carr.  The next phase of design involves inserting the components into the top level assembly and placing them in their approximate position. They can be left unconstrained, or they can be mated to mate connectors located at key points in the layout sketches.  Onshape allows you to place a mate connector anywhere in the assembly by using distance and angle offsets as needed.  We often carry this phase of design to the point where nearly all of the moving parts are present in the top level assembly.

At this point, one of Onshape's most powerful features comes into play.  Right-clicking on a sketch in the top-level assembly allows you to edit it in context.  You'll be returned to the Master parts studio to see the layout sketches overlaid with images of all of the COTS parts you've added. The "Use" tool allows you to capture geometry features from the parts, such as the positions of mounting holes, bearing diameters, and so forth.  This geometry becomes the starting point for designing the supporting elements that will tie the entire assembly together. 

Here is a link to a simple example- not a whole robot, but an assembly that is mostly COTS parts: https://cad.onshape.com/documents/767427fce062561957a110e8/w/8de83dde68b08ca9769bf0fb/e/0c6c7bf1bcd164d0be31eabd

nick_papageorge073

For sure get super familiar with McMaster Carr. They have the best mechanical catalog. Better than MSC, Grainger, etc. Their site is also the easiest to navigate, and the search on their site works way better than their competitors. They have accurate CAD for almost every single item they sell. They won't sell your information to salesmen when you download cad, and that will avoid you calls from CAD salesmen.

For things like bearing fits, yes the "Bible" (machinist's handbook) has everything. But its huge. There are generally 3 types of "fits". Clearance, slip, and interference. Clearance is something like .005" - .020" clearance. Typical for a machine screw is the clearance hole for the screw is .015" larger than the OD of the screw threads. Slip is usually about .001-.002" of clearance. Interference is about .0005-.001" of interference. The interference amount is dependent on the diameter and the material hardness. With the difference between a slip fit and an interference fit so small, the tolerance on the parts has to be super tight. Bearings are often made to be press fit into their housings, and/or onto their shafts. For prototype work though, its often easier to use slip fits on both, this way you can easily put together and take apart, as is often necessary during the prototype phase, without heating items or using presses/pullers.

For shaft work, you will want to look at the standard sizes in industry. These are tied to fractional inch sizes 1/4", 3/8", etc, Letter sizes, and Number sizes. That is because drill bits are made in these 3 types of sizes, and you can find super, super, cheap, every single size made for drill bits. Same with mm sizes, they are usually available in all the .5mm increments. These are called "drill rod".

Besides drill rod, another source for super accurate and cheap shaft material is dowel pins. They come in nominal size, undersize, and oversize. Another source is molding machine "heading pins". The last source is gauge pins. They come in any size imaginable, but they are the most expensive by far. McMaster has all of these.

For shaft work, the bearing will drive everything else. You buy the bearing first, then find a shaft from the above types that fit the way you want. Bearings you will want to use a 608 bearing if you can, because it is the commodity skateboard bearing, and is the cheapest.

For whatever reason, similar sized hardware is cheaper in inches than in mm at McMaster. Stuff like screws, shafts, etc. So when I'm doing prototypes, which I design in mm typically, I'll still use inch sized hardware to save cost.

A huge consideration for prototype is what is the volume (qty) of your final product? Is it going to be a mass produced injection molded product? Or is is something you want to do for fun at home in your garage and never sell it? In either case, you have to think of the manufacturing process, and the tools you have available. The manufacturing process will drive your design.

For stuff for home, a 3D FDM printer is usually pretty good. For mass produced molded items, FDM printers aren't good enough. You will want to use polyjet printers, or have the prototypes CNC machined.

Make an account at sendcutsend.com. They are a laser cutting service. Once you see how cheap they are, you will use them a bunch, and you will rethink your design direction to use the lasercutting manufacturing process. Even if you are ordering qty 1, their prices are almost too good to be true.

That's all for now, good luck.

edward_omagbemi

Happy New Year to everyone and thank you so much for your answers.

I was expecting to get an email telling me that someone has answered my questions so I expected my question to go unanswered.

Today, just out of curiosity, I sign in to the forum to see all these valuable answers :-) Very Happy.

edward_omagbemi

edward_petrillo said:

...Onshape allows you to place a mate connector anywhere in the assembly by using distance and angle offsets as needed...

So that's how you define placements in the assembly without yet having fully designed the parts. I'll have a closer look at mate connectors then. I thought they were only applicable to mate parts together.

Thanks for all the other useful details. That's what I was asking for.

I love the idea of designing shafts around standard parts and skateboard bearings. That will definitely do the job for any of my prototypes

I also love the idea of using slip fits for prototyping. I don't mind glueing things together on a prototype once everything is at the right place.

I design in metric as I'm in Australia so I'll see what I can get here.

Regards,

Edward

S1mon

I don't know if McMaster ships to Australia, but they do have decent amount of metric hardware, and the CAD models are pretty useful.

dirk_van_der_vaart

For bearings go to https://www.skf.com/au
For all other parts https://www.traceparts.com/en/
And this is something to have a look at
https://learn.onshape.com/courses/tips-and-tricks-for-working-with-imported-models-in-onshape

edward_petrillo

At least one of the main suppliers for educational robotics has set up to serve Australia.  https://www.andymark.com/categories/australia-wheels-gears-hubs

Looks like you might enjoy shopping in this arena based on the projects you're describing. 

And if you'd like to jump in with both feet, there are more than 100 FIRST robotics teams on your continent- maybe you'll find one close by! https://www.firstinspires.org/team-event-search#type=teams&sort=name&programs=FLLJR,FLL,FTC,FRC&year=2021&country=Australia

edward_omagbemi

edward_petrillo said:

... there are more than 100 FIRST robotics teams on your continent- maybe you'll find one close by! https://www.firstinspires.org/team-event-search#type=teams&sort=name&programs=FLLJR,FLL,FTC,FRC&year=2021&country=Australia

This is were I got my inspiration to start doing something again with my knowledge. The company I work for sponsors a Not-For-Profit robotic group that goes into schools. Their biggest event is the annual Roborave https://roboraveaustralia.com/.