"Aerotech" 38mm Seal Disc Compatibility Issues

I have a couple of different brands of Aerotech designed and licensed motor casings. I have RouseTech, Dr. Rocket, and Aerotech and until recently I thought that all of the parts were completely interchangeable. However at a recent launch I found out the hard way that I could not fit my Dr. Rocket 38mm forward seal disc into my Aerotech 38/600 casing - the Dr. Rocket seal disc was just barely too tight. It would go into the area with the threads but would not go past the threaded section into the main tube body. Luckily the case was purchased with an original Aerotech seal disc as well.


Has anyone else experienced this?

It sounds like machining tolerances Brad.

Lets take an H999 Reload.

In some cases it's tight a very tight fit, and in some it fits just fine.

The differance in my example is not really brand, but the production runs of the cases.

Machining always has tolerances, and sometimes the tolerances on one end add up with the tolerances on another part that causes an excess in tolerances.

Art Upton said:

It sounds like machining tolerances Brad.

Lets take an H999 Reload.

In some cases it's tight a very tight fit, and in some it fits just fine.

The differance in my example is not really brand, but the production runs of the cases.

Machining always has tolerances, and sometimes the tolerances on one end add up with the tolerances on another part that causes an excess in tolerances.

Click to expand...

Exactly. I assembled 3 H100's for my fatboy, 2 went together like butter! The third I had to peel the liner down 2 layers!! I decided to jam it in good instead of keep peeling. It worked fine but it was tight!

Ben

Nit picking a little....but Art you can take it

Tolerances are limits on the variability of finished part dimensions.

The example in this case is the variability of the dimensions due to the machining process.

Tolerances should be chosen so that the parts fit 99.9% of the time (depending on the sigma level that is targeted).

If this happens frequently then either the machining variation exceeded the tolerance on the drawing which is a DEFECT...OR the tolerances were incorrectly specified which indicates a design MISTAKE.

My bet is on the latter.

jderimig said:

If this happens frequently then either the machining variation exceeded the tolerance on the drawing which is a DEFECT...OR the tolerances were incorrectly specified which indicates a design MISTAKE.

My bet is on the latter.

Click to expand...

Well, why do you bet it is a design mistake?

If I am not mistaken, the first condition you mention ("it happens frequently") is not met, so it is more likely that it was a simple machinist error, isn't it?


Juerg

Art Upton said:

It sounds like machining tolerances Brad.

Lets take an H999 Reload.

In some cases it's tight a very tight fit, and in some it fits just fine.

The differance in my example is not really brand, but the production runs of the cases.

Machining always has tolerances, and sometimes the tolerances on one end add up with the tolerances on another part that causes an excess in tolerances.

Click to expand...

Good guess - but I left out critical info. Another flyer in my club who obtained his 38/600 at a completely different time had the exact same problem with HIS seal discs. He too has multiple "brands" of Aerotech compatible hardware.

So I'm betting on an error or a change. Both of us have older 38/600's and newer Dr. Rocket / Rouse stuff. The difference is maybe 3-5 years with the Rouse / Dr. Rocket stuff being newer than the Aerotech branded stuff.

Ok, now this is new information, not an insulated single event but at least a second one.
Can you provide the exact diameter of your seal discs?
I'll measure mine tonight...

Juerg

Juerg said:

Well, why do you bet it is a design mistake?

If I am not mistaken, the first condition you mention ("it happens frequently") is not met, so it is more likely that it was a simple machinist error, isn't it?


Juerg

Click to expand...

Just from my experience machinists are pretty good at making parts from drawings and tend know how to check their parts.

Designers are generally pretty poor at specifying tolerances. Often they are an afterthought without any real analysis. Statistical tolerancing is not something you can figure out yourself without some training or decent math skills.

I have seen much more fit problems from parts that were made to the drawing than from non-conforming parts.

I bought a Dr. Rocket 38/240-480 set 2 years ago. Some months ago I played around with the casings and some cardboard tubing. It seems like this tubing has the perfect outer diameter for using it as a liner for the casings when used with the 240Ns and 480Ns casing. However, it required significantly more force to insert it into the 360Ns casing. Till now I never had a problem with these casings.

Its surprising that the tolerances between the seal disc and the casing are so tight. This contact surface doesn't contribute to the seal, so it shouldn't be a problem to increase the gap a little bit.

Reinhard

Art Upton said:

It sounds like machining tolerances Brad.

Lets take an H999 Reload.

In some cases it's tight a very tight fit, and in some it fits just fine.

The differance in my example is not really brand, but the production runs of the cases.

Machining always has tolerances, and sometimes the tolerances on one end add up with the tolerances on another part that causes an excess in tolerances.

Click to expand...

Boy did you hit the nail on the head!
I've got one or two case that are new from Dr Rocket and their not right, one is a 38/720 case that no reload will fit ( way to tight diameter, off by about 10 thousands)

sad but true