Saturday, 27 October 2018

Scratch built 29mm rocket anchor point (Or why do things the easy way!)

So I am working on a design for a minimum diameter 29mm rocket air-frame which means that the motor directly fits into the complete internal diameter of the rocket with no space around it. Common practice with larger rockets is to use an internal motor mount tube mounted in some centring rings, the fore centring ring usually is where the builder will place an eyelet or lifting bolt that will be the anchor for the recovery system/parachutes etc.

Obviously without this centring ring you need to come up with a different solution. I want to have the option of using the motor ejection charge so needed something that gases can expand through but also it needs to be strong. I recently picked up 2nd hand an aluminium anchor point for a larger 38mm air-frame off another rocket builder via Facebook and a discussion followed about how with a smooth metal outer finish it was difficult to get them to adhere/epoxy to the inside of an air-frame. So my idea was to make a similar anchor point to receive a lifting eye but to make it slightly under size and then knurl it to be a better fit and to maximise the keying surface for when it is epoxied into the air frame tube. The knurling is pretty badly done and definitely not decorative but it is definitely going to allow a small amount of epoxy to create the strong bond needed.

So why the "Why do things the easy way" bit... Well... after a search of the material/scrap pile I didn't have a piece of aluminium of large enough diameter to make this component. So I have done it the hard way! I started of making a very crude casting from some melted aluminium! With my very dinky little kiln!

I then poured the melt into a crude mould made from some of my diy casting sand (silica play sand and bentonite clay sourced from cat litter (unused... I a NOT that much of a cheapskate!)

The crude casting was then chucked up in the lathe (the cheap 2nd hand mini lathe I bought a while back) and turned to round.

I then realised none of the suitable boring tools I had actually fit in the toolpost of the mini lathe so I parted off the job and transferred it to my old faithful perfecto lathe and bored out the inside. Its somehow more pleasing when you really have made something from scratch!


Andy from Workshopshed said...

Nice job, I've found that if you leave the casting open to the air like that the top few mm is of poor quality. So mostly used closed casts now. You can also reduce the machining and amount of metal needed with a core. Something I've not tried out yet, will likely need to wait for better weather now so perhaps in the spring?

concretedog said...

Andy, yes.. I agree, this would have been much better if I had done it via a closed cast or using lost pla casting.. it was a bit of a spur of the moment get the job done type deal! Lost pla would have taken a damn site less alu as well... it seemed wasteful to bore all that lot out again after melting it all in the first place!