Great questions!
As an old tool&die maker building maintaining stamping dies, I have been working with dowel pins for many years.
So when should dowel pins be used?
Dowel pins are used in conjunction with the bolts to maintain precise alignment of the parts of the tooling. There is always some lateral movement possible with bolts, if they fit tight in the bore, you couldn't get the bolt in. Dowel pins also allow you to remove the tooling from the base (die set) for sharpening and repairs and be able to put the tooling back exactly where it was without having to go through the initial build alignment procedure all over again. When dies are built, the tooling is bolted into place, then aligned properly, then as about the last operation, the dowel pin holes are drilled and reamed for a tight fit. The dowel pins prevent any lateral movement, while the bolts secure the tooling to the die set. Dowel pins are normally installed with a tight fit, requiring a hammer & punch or a press to install and remove. Normally installed in through drilled holes so they can be punched through to remove.
So bottom line, if you don't want any lateral movement in any assembly, use dowel pins to prevent that.
Taper pins do exactly the same thing as dowel pins, maybe with even better alignment than dowel pins. But are harder to install properly because of the taper and require the use of tapered reamers. Taper pins are good for installation in a blind hole, and are many times threaded internally so you can extract them with a puller. You normally find taper pins aligning parts of machine tools, especially the older iron.
A lot of the modern equipment uses spring pins for alignment when ''precise'' alignment needs to be maintained. Much easier to install, just drill a hole and pound the pin into place. Reduces manufacturing costs, but not as good as dowel or taper pins.
Sticky Stuff
We all pretty much know the uses and grades of LockTite and similar products. If you don't want that bolt to come loose, use red LockTite, and it's pretty much there for ever. Red is also used for securing SpeedySleaves to shafts. SpeedySleaves are used to repair worn seal surfaces on shafts, including main seal surfaces on automotive crankshafts. The blue stuff doesn't set quite as hard, and disassembly is easier. LockTite also has a bearing mount product that is said to fix some problems with bearing bores and shafts that have had a bearing spun on them and caused wear, I think the limit is about 0.010 wear. I have never used the stuff.
I glanced at an article the other day about gluing car frames together, I haven't read through it yet. I guess it's in the experimental stages.
I am also a real fan of 3M VHB tape products, this stuff is great for sticking stuff together. It's pretty good in shear and tension, but not so good in peel. Good chemical resistance for most petroleum products, and water based solvents. It doesn't like MEK, acetone, and other stuff like that. I believe Grumman Aircraft was using the VHB products to attach the skins on their non-pressurized aircraft.
The most common metalized epoxy available for normal shop use is JB Weld. It can be used in a lot of places for lightly stressed metal repairs. It's machinable, drills & taps with reasonable holding power. With proper preparation, will seal up a cracked engine block in many instances. Could be used to repair a broken casting, but again only a lightly stressed part. I've never found anything metal that it won't stick to. It can also be used with some plastics with varying degrees of success.
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