That's why you're not structural engineer... Look back at my post #64. The structure is actually cut back so you CAN see the huge gap. :facepalmLooks like those parts are extruded with a radius at the edges, in any photo it would look like it has a gap from the shadows that they create at the seam. Only a feeler gauge would tell the truth.
:wave: :wave: Oh, oh, oh.... I know the answer to this one.... ask me! ask me! :wave: :wave:The gaps in the pre-"grind" pics certainly looked to be a lot more than .005" as well...
I'm curious - does anyone know the engineered bondline thickness for these joints? I guess since Lotus says "do not repair" the factory documents are unavailable...
Well…the good news is that two crashed chassis are still crashed chassis (no more chassis are written off)…this chassis (SCCPC11195HL3478) and the donor chassis. The bad news is that a potentially salvageable chassis is now scrap metal! You guys had the right plan and the right approach but executed it in exactly the wrong way! I had high hopes for this project.
Your friend that is helping you with this project…I think you’ve called him an aircraft mechanic…he isn’t working as an aircraft mechanic, is he? He has been fired for doing what he has guided you to do on this chassis. That is cause for termination in the aerospace industry!
You have not followed my previously advice/recommendations/instructions/guidance, but please follow it now. Scrap that chassis. Someone is going to die because of your “work” on this chassis. At best someone is going to get seriously injured.
Because you “ground” on the aluminum, you’ve taken metal off. Two things happened because of that; the metal got thinner and the scratches put stress concentration points (where cracks will start) into the face f the aluminum chassis. The other bad thing you have done to the metal, you have heated it. Aluminum starts to anneal at 250F and full on heat treating/annealing temperature is about 800F. Heat guns put out 1000F to 1200F. Since you have no control over the heat to the aluminum by a handheld heater, that whole front end is junk. These are just the damaging things you have done to the material properties of the aluminum…not even considering the corrosion protection that was mechanically removed, because this chassis will not last long enough to corrode!
Now for the structural SNAFUs…because you ground off the face of material, I also have to assume that ground on the joint surfaces. Even if you only took off 0.001” out of the joint surfaces, there are 4 of them on each side that stack up because that beam is sandwiched in between an upper and lower “clip”. But you are not that good to only take off 0.001” when grinding off all that adhesive. My guess is that at least 0.005” have been removed. Now you’ve got a varying gap of at least 0.020”, probably closer to 0.050”. For one, the adhesive will not survive that kind of a gap…there will be no strength in it. But the biggest problem is that if you use the correct structural rivets, they will draw the metal to each other and put significant stresses into that metal. Now that the metal has been weekend by the thinning, scratching and heating, that front clip will crack off! If you don’t use the correct structural rivets and use only pop-rivets, that front end will break off even sooner.
If you can stick a 0.005" feeler gauge in any one of the the 6 part lines, your chassis is junk... I really don't know that you ground on the parting surfaces (I am making that assumption from the work I see.)... If you did, and you can stick a 0.005" feeler gauge in, it is junk!
I'm sure we could fill volumes (at least a few cubic centimeters) with your knowledge, and several stadiums with your B.S. You are just totally wrong here.
First off, a primary reason that structural adhesives are used is for their gap filling properties. The adhesive manufacturers have several versions to fill various gaps, but they usually specify a MINIMUM GAP. An adhesive REQUIRES A GAP for it to function properly. If you squish out all the adhesive, then it will not be able to hold the parts together.
"Though only light pressure is needed, it should be applied as evenly as possible over the whole bond area. Excessive pressure leaves the joint starved of adhesive."
The Huntsman Araldite 2015 structural adhesive (use in the construction of the Elise) TDS states that it is "gap filling" and "non sagging up to 10mm thickness"
The TDS does say "a layer of adhesive between 0.05mm and 0.10mm will impart the greatest lap shear strength to the joint". However thicker gaps will work.
Huntsman gives a example graph of the relationship to failure stress to bond-line thickness. And the statement "In thicknesses greater than 1.0 mm shear strength is approximately constant". www.freemansupply.com/datasheets/adhesivesguide.pdf
As you do not know the tolerances that Lotus allows for gap in their bond-line areas... you really can't say whether he is out of tolerance. You could say that his bond might be sub-optimal if it greater than the recommended gap of .05mm to .10mm for greatest lap shear strength... and that is it.
In a structural adhesives guide from 3M (for example) , they say this "For the best adhesive bond, there should be at least a .003"-.005" gap between the parts for shear and 0.015" - 0.020" for peel."
The Huntsman Araldite TDS for 2015 also says that mechanical abrading is required for a good bond. "The strongest and most durable joints are obtained by either mechanically abrading or chemically etching ( pickling ) the degreased surfaces. Abrading should be followed by a second degreasing treatment." www.intertronics.co.uk/data/ara2015.pdf
"Grinding" is actually a good method of mechanically abrading a surface, as it DOES NOT over heat or anneal the metal. That is ridiculous. Redoing the heat treatment is not required after machining, drilling, turning, or grinding.
In order to anneal 6063-T6 alloy, it requires an annealing cycle consisting of holding at temperature of 775 °F for 2 to 3 hr; cool at 50°F per hour from 775 to 500°F. A hot air gun will not be able to anneal a thick wall section 6063 aluminum tube... won't happen, the aluminum will conduct away so much heat, the spot under the gun will not get to 775F, much less for 2-3 hours.
Great to hear the update!! Look forward to the photos!!Moving forward. Bonding was compleated a few weeks ago. All parts are here exept the crash structure and front clam. Wishbones, shocks, etc should go in next week. Hope to have the car aligned before end of month.
Have plenty of pictures to show you.
Need some time to write it properly for it t be usefull.
Thanks for the read. That was great info. Definetly super usefull.Daniel,
Glad to see you are still working on it and making progress.
Not sure if you have seen this. It is a 2013 PDF from Lotus about the adhesives and design considerations they used.
Take a look at these pics.;3557265 said: