Machining (9/26/21)

Machining, Lathes and Rockets

Machining is the primary form of subtractive manufacturing and in most cases the terms are synonymous. Subtracting manufacturing means manufacturing that subtracts material from a blank or stock of material. This has been in use since the industrial revolution in gargantuan scale in every industry and the advent of CNC (Computerized Numerical Control) has simply built upon the basics of machining but done it more effectively and economically. There’s many kinds of machining operations including drilling, grinding, facing, boring, spinning and my absolute favorite: Explosive molding. Explosive molding works by putting an explosive charge inside a hexagonally welded tank to turn it into a spherical tank of extreme strength, nearing the tensile limits of the material.

Until the 1960s, all machining was done manually, meaning a machinist would turn the knobs on the machines and not a computer. As computers began to advance and the ability to program them became standard CNC machines started popping up left and right and slowly from the 1980s to the present day, became the de facto mass production method for many industries, including aerospace. We got involved in machining in late November of last year, Pricing on machining begins with a high cost for single digit parts as they require hours of cnc programming, which is all manually done on the computer, before the machine is loaded with the material as well as fine tuning of a few test pieces before the production cycle is loaded and the material is made. Once the initial work is done, the price drops off a cliff and stays there. So it would go from $4000 to $50 per piece. This is because every hour of labor costs the machine shops a lot of money. They are extremely high overhead businesses and finding a well equipped one to donate time and services is extremely rare. Once Ben and I found a machine shop willing to help we sent the most complex piece, the injector and had Ben’s dad, a mechanical engineer at Raytheon, appraise it at around $10,000. This piece had nearly 300 precision drilled holes at various angles impinging on each other. It is specifically an uneven triplet impinging injector with film cooling on the outside, requiring several operations, one of which is radial holes going all the way through followed by the drilling of the angled inlet and outlet holes and followed up by a surfacing sweep which left is with a very fine surface finish of 8 (best way to describe it is very smooth) which in hindsight was unnecessary.

Learning about machining over the course of the project was really more of a trial by fire than anything else. We would get feedback from a few engineers before sending them to the shop and sometimes they’d ask us about some small change we’d need to make. A lot more pieces than I care to admit were redesigned for machining and logistical/price considerations.

Ismail

Sources:

Walker, J. C. (1981). Fundamentals of Machining. Springer International Publishing.