v1.0 - SEP Step 5 - Detailed Design - Mechanical System

Added by Jeremy Wright about 10 years ago

I have a question on the units (English vs Metric) for Shepard's CAD dimensions. Which system should we use? It seems like Metric would be the way to go for a world-wide audience, but this version of the stand was built with 2x10's. No matter what we decide for this version I would suggest going Metric for future versions.

J, I may have some more questions about the measurements of the stand as I go along. I'm not sure I can draw the whole stand based on what's in the wiki .


Replies (6)

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by J. Simmons about 10 years ago

Ah, the age old units discussion. Very early on Greg and I very vigorously said we wanted to use SI units for Mach 30 hardware projects. And, I still think this is the right call. SI units are the more modern units, and they are much more consistent than the English system. However, I realized something while putting together the Shepard structural prototype. All of my tools and materials (tape measures, power tools, drill bits, squares, screws, dimensional lumber, the concrete block, etc) are in English units.

This makes it very hard to work in metric, which is something we should consider when we are discussing open source hardware projects as a concept and how groups in maker spaces can participate. Having to switch out tooling and material suppliers can be a significant barrier to entry (not unlike having to purchase commercial software).

SO, I am torn about how to proceed. I overwhelmingly believe our design and analysis is better done in the metric system. However, the practical issues of manufacturing hardware in a country that is dominated by the English system need to be addressed or we will be left with the very unfortunate choice of either designing things that are difficult to build here, or having to make our choice about unit based solely on the available tooling in the US.

Perhaps it would be best to first discuss the tooling situation some, separate from the Shepard Test Stand, and see where that takes us. Thoughts?


And I will of course be happy to help fill in any details I missed on the wiki. What is up there now is just a first draft, and I am not surprised I missed a few things.

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by Jeremy Wright about 10 years ago

The last company that I did mechanical design for shipped automated machinery all over the world. They would do a machine in either English or SI depending on where it was going to be installed and what was requested. I would try to spend a little extra time with the machinists beforehand to see what Metric tooling they had or could easily get, but I'm sure I still managed to make some poor design choices tooling-wise.

As you said, when dealing with Makerspaces it will be difficult to force tooling on them that that's not readily available. What about doing the designs in Metric, and then specifying English substitutes on the drafts? This could make things a little messy, but it allows us to keep one design and the builder would just have to be responsible for keeping track of which units they're using. When we're doing the design we can try to pick Metric measurements which correspond as closely as possible to their English equivalents. Charts like the following would help.

https://www.dot.ny.gov/main/business-center/engineering/specifications/english-spec-repository/bolt_table.pdf

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by Ben Barnett about 10 years ago

My shop is english, and the probability that I will convert to metric is near zero. I sometimes work to metric drawings, but it is a pain to convert all the dimensions to english.

The real messy issue is threaded fasteners. British standard threads are different from American, though both are based on inches. Trying to chase metric threads on an American lathe is mostly not possible. Maybe converting to CNC machine tools will finally give us the opportunity to switch systems from a keyboard. Is there a "Maker Space" somewhere working on converting antique machine tools to CNC?

If I remember my "History of Technology" class correctly, the US Congress first passed a law converting our measurement standards to metric in 1830. I guess there were few lobbyists back then because the crap didn't hit the fan until after the law was passed. It was quickly repealed as too expensive.

Ben

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by Jeremy Wright about 10 years ago

This certainly presents a challenge, that's for sure. It's hard not to leave a large group of people out no matter what system of units you choose.

I did find something important out about FreeCAD a little bit ago. It looks like only Metric units are available at this time. I drew a few parts of Shepard two nights ago trying to get a feel for the user interface, and so wasn't worried about changing the units (and assumed you could). When I went to look for the setting tonight it wasn't there. After some research it looks like English units are on the to-do list for the FreeCAD development team but nobody's sure when it will happen.

We knew going in to this that the state of open source 3D CAD is behind the current commercial offerings, so the question becomes how much are we willing to deal with before switching to another system like 123Dapp (closed source but free for now)? My personal preference would be to struggle/suffer through with an open source CAD package like FreeCAD, and try to contribute code and/or funding for fixes that Mach 30 needs as our time and funding permits. If we want everyone around the world to be able to use and improve upon our designs, the tools are going to have to be freely available. We know that we (Mach 30) are going to have a heavy need for CAD and CAM in the future, so this is an investment in our future use as well. That's my preference, but I tend to be willing to suffer for open source more than most people.

I'll re-research the current state of open source 3D CAD, but I think FreeCAD is the leader right now.

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by Jeremy Wright about 10 years ago

As I think we've documented elsewhere, we've run into a problem with the exhaust from the motors eventually fouling the bearings of the drawer rail we're using. Extending the motor mount plate over the length of the drawer rail has been suggested. Another possible solution that came to me while I was doing the CAD for Shepard (yes, I know, the CAD should have been done before it was built) is turning the drawer rail over so that the slide mounts to the test stand. It looks like we could probably reuse the motor mount base plate without changing it, and the bearings would be better (although not perfectly) shielded from the motor exhaust. Since we're getting ready to move on to version 1.1 of Shepard, simply noting this change here might be the best course of action.

RE: v1.0 - SEP Step 5 - Detailed Design - Mechanical System - Added by Jeremy Wright almost 10 years ago

The CAD work on Shepard 1.0 is now finished.

https://opendesignengine.net/dmsf/shepard-ts?folder_id=10

If you download the "Shepard v1.0 CAD" zip file, there are PDFs in the "Drafts" directory. If you have the time and inclination, please take a look and let everyone know what you think (and if there are problems).

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