I may have a cnc mill in my garage, but I still want a good condition Bridgeport conventional mill too to teach my son on so he knows what it means to cut alloys and materials manually and so he get a machinist's mind on how to setup unusual or difficult to hold or clamp parts. Not everything starts in a mill vise or from a block of material.
Many times you have to machine tooling first to make parts or modify existing parts you need to machine on. I've made literally millions of parts in my machining days. Most were scratch made cnc production part, or one off prototype parts, Or even production manually machined parts or manually machined one off prototype parts, but many many were parts that had already seen and been "In Flight Use" used parts that just needed repair. You can legally repair many parts under FAA guidlines. So knowing how to make precision tooling and hold an exsisting part correctly is VERY important.
I'll walk you through just one single part I used to machine all the time.
Restrained conditions greatly affect a part and weather you can hold tolerance or not. I've had regular weekly parts where I had up to 12, .0005 high precision indicator's on a part the size of a TV tray, that could ONLY be held by the split line of the housing as that was the datum the dimensions were called out from, and the only flat surface with any sort of area of flatness to even hold the part from in a precision manor, and come off a precision surface to holf tolerance to.
So my tooling had to be tall precision blocks with precision tooling plates to create a big overhang on each side of the part, but yet be rigid, solid, and dead precision to .001 MAX flatness runout. Once I had my tooling stoned flat and checked for dings and dents, and the parts we're already varifyed flat within .005and still in tolerance in spicific locations, before repair by inspection, I would locate the housing half generally in the necessary location on the very clean mill table and loosely clamp the large tooling angle blocks in place.
The tooling over hang plates mounted on top of the angle blocks had numerous holes in them that were for all the different housing part numbers we would repair. So I'd pickup the correct holes for this particular housing on the plate and bolt through the housing and tooling plate and snug up. Before tightening the tooling blocks down tight, you have to KNOW they are precision flat AFTER being tightened down. So with the part located where it will bolt up to the blocks up against the bottom of the precision surface and be flat, I could then tighten down the tooling blocks.
Then I'd remove the part by removeing about a dozen or more fasteners to remove it. Then with a height gage and .0005 indicator, I would sweep the bottom of the overhang plates and check for flatness. If flat, all good, if not, fix it right and make it DEAD FLAT. SO lets say it was flat as it usually 99.999% of the time was. I always take GOOD care of precision tooling. So then I'd bolt the part back up to the bottom of the plates and mount up to about 12 if necessary .0005 tenth indicators all over the split line of the housing.
The area I had to mill was gown inside the housing. All dims we always plus minum .0005 to split line so when I say I would indicate the split line in to .001 max. to have the tolerance help me I'd indicate it into to as close to .0000 as possible on every part. THATS how you hold TRUE precision and allow the tolerance can actually help you IF you make a mistake and cut a little deep, and by deep in this case I mean .0003 deep. YES I mean .0003 TEN THOUSANDS DEEP, is to be DEADLY presision period. ONLY if it's warrented of course.
Anyways, after varifying it's correct, each and every parts gets bolted up, swept with in indicator to check for flatness, adjust if necessary with jack screws under the part, how ever necessary, which was how these parts could only be machined. These housing had seen so many hours of service there was NO WAY they were in shape as new parts. They had beed heated with use and cooled down over hundreds of hours of usage. So originally they had a flatness of .001. They had to be within .005 to even be repairable, but HAD to be held to the original tolernaces. So I HAD to restrain and jack screw then into shape to even machine them.
But thats what it took to run these parts as they came in. Ran them weekly about two to ten a week. Was only a few hour job to setup and run a few parts. A dozen was a good days worth of machining. Very simple parts to an experienced machinist. Some guy with not so much experience would kill his self over a difficult part like this.
Not every part needs to be held to tenths though, and MOST parts CAN NOT be restrained for good reason. Some jobs are quick and dirty jobs too. Saw cut finish good enough type thing. (I HATE that type of quality though) I like as close to dead nuts as possible. Perfection is beautiful. I've rambled enough. Later
