Why Tolerances are Important

As rocket projects become more complicated and with the advent of cheap and readily available machinery, and machinery services (3D hubs for example) a lot of people are starting to push the limits with what is being made in the garage.

When it comes to rockets you are never going to get it right first time and you will soon find yourself in the iteration process as you improve on your designs. If you are having to remake parts from rough or non-existant drawings you may find yourself in a dilemma with parts not fitting and potentially botching a few.

From a mechanical standpoint, there are a few things you can do to make your life easier, a simple tolerance is one of them.

We must first understand what a tolerance is, in engineering, a tolerance is the limit or variation of a physical dimension. This can be set by yourself on how accurate you want your part or it is sometimes set by the machine used, a bad operator can also play a part but for this write up I will not consider this.
As a general rule the higher the tolerance you put on your part the more it will cost, if I wanted a shaft with a diameter of 20 mm ±0.1 mm (19.9 mm to 20.1 mm) this would be easily achievable on a lathe with no extra tooling. If I was to make this ±0.01 mm (19.99 mm to 20.01 mm) then things start getting harder, the shaft would now require a grinding process to achieve this, meaning more time and man-hours and thus a more expensive part.

Not only is cost a factor, but also the fit of the part, which is what we probably really care about matters. If my 20 mm diameter shaft had to fit inside a hole, a bushing for example, and if there were no tolerances involved then how would I know it would fit every time? It could be oversized, undersized or it could be ok.

Luckily for shafts and holes (or anything concentric like a rocket tube and bulkhead), there is a simple ISO tolerance letter/number designation system to make life easy, shown below.

Fits (Credit: Machinery Handbook 29th Edition)

To go along with this, there is a handy Limits, Fits and Tolerance calculator from Amesweb which makes this ISO system easy to understand.

Let’s look at out 20 mm diameter shaft and the bushing it must go into. From the above chart I’d like a sliding fit, with a basis on the hole (hole limits are maintained but shaft limits can vary), therefore I want a H7/g6 tolerance on the shaft and hole.
Plugging this into the above-mentioned calculator yields the following,

As can be seen, I have a nice tolerance dimension that will always enable a sliding fit, but what are these dimensions?

My bush dimension becomes 20 mm -0 mm on the lower end and 20 mm +0.021 mm on the upper end, while my shaft diameter becomes 20 mm -0.020 mm on the lower end and 20 mm -0.007 mm on the upper end.
This gives me a range that I can make each part too, and as long as each part is within that range I will always have a sliding fit, no matter who or where it is made.

This is a very basic introduction, more specifically relating to cylindrical components and fits. In a future post, I’ll go into a bit more detail into the next steps you can take to ensure your parts are concentric and cylindrical using the Geometric Dimensioning, and Tolerance (GD&T) language as well as covering the three basic types of tolerances you may see on a drawing.

RPA Standard Edition v2.3.0 Release

RPA (Rocket Propulsion Analysis) is the tool to own for liquid fuelled rocket engine design.
Although, having been around for a while, downloading the latest update I thought it may make a good post to talk about for those who have never come across it.

RPA Screenshot

Basically, RPA allows you to design your liquid rocket engine from the ground up, carrying out performance analysis, chamber sizing, chamber cooling analysis to name a few of the features.
It is very easy to use and allows quick design and iteration of your rocket engine and can also export a .dxf file of the inner engine geometry for further CAD design.

It can be downloaded as a trial in the lite edition but I would highly recommend buying a full version, it will be an investment you would not regret.

Click here to download and give it a go.

Team Ursa

I was recently made aware of Team Ursa, who are building some very cool rockets and hardware and have been doing so for a few years now.

Ursa 2.1 launch operations (Credit: Team Ursa)
Ursa 2.1 launch operations (Credit: Team Ursa)

Team Ursa’s mission as stated on their website,

Team Ursa and its partner, Mavericks Civilian Space Foundation, find space exploration to be a potent motivator for students and adults alike. By using the sub-orbital aerospace platform, Team Ursa works with Mavericks to inspire students and communities to invest in STEM through the development of open-source reference designs. These reference designs are intended for the educational and research community’s use to further younger generations’ involvement in STEM, and aid in making sub-orbital space a more accessible laboratory for students.

The team started out as 6 University of Maine senior capstone students who got together to build their first rocket, Ursa 1.0. Ursa 1.0 was a 2 stage solid propelled rocket designed to achieve 100,000ft in altitude, as shown below.

rocket_assembly_000
Ursa 1.0 (Credit: Team Ursa)

Continue reading “Team Ursa”

89mm Sugar Shot to Space ‘M’ impulse sugar motor test two at FAR

Video Caption: The second static test of an 89mm ‘M’ impulse KNSB sugar motor at the FAR site in the Mojave Desert of California for the Sugar Shot to Space project. (Friends of Amateur Rocketry dot org and on facebook)
sugar-shot.org

3D Printed Servo Valve

2016-07-06 09.47.14
Servo Valve

As I progress on my own rocket projects, I thought I would let everyone know what I have been up to lately. Juggling study and life can make things a bit slow but things are coming together for my test stand.

As part of this stand, I wanted some servo actuated valves for the pressurant and or vents etc. I was inspired by Rocket Moonlighting and his 3D printed valve holders so set about making my own.

Continue reading “3D Printed Servo Valve”