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Why use a tapered pin?
Dick Z
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Hi
can somebody point me to a link or post where this is discussed?
My plan:
Clamp the two parts together, drill the smallest diameter and then ream the taper?
After that, press fit the taper into the stationary part and hand-ream the
removable part until it fits to my liking?
I think that is a good plan, but I am not sure how the professionals do it.
Thanks!!!
Why use a tapered pin?
Dick Z
DZASTR
A taper pin is usually driven out, by the small end when disassembling. The whole pin is removed. If you make one half lose the pin is not aligning the two parts correctly.Originally Posted by eblaschka
I can't think of any situation in 50 years of work where a tapered pin was used to allign or locate two parts on a flat plane surface.......collars on shafts for radial location yes, but linear as in flat plane surfaces....nope, and a lot of taper pin situations are now "tied togehter" with roll pins.
Usually when you have to have two parts located together so that they can be dissassembeled and then reassembled in the exact position, you drill and ream on first assembly for a straight hardened dowel pin.....the dowel is tight in the one half and a close but sliding fit in the mating piece....and the dowel is parallel with a radiused end face.....they can also be flat on the end.
Dowels, when they are driven in should have the radiused end just above the surface of the job at the shoulder of the dowel, and if'n they are driven in to be flush, the edge of the hole will get burred and make dowel extraction very difficult......if'n the dowels are required to be flush with the surface, the ends are machined flat.
I fit dowels by drilling the two halves of the job 10% undersize and reaming with a hand reamer to size.
I use the hand reamer in the mill or drill, whatever, because it has a tapered lead in for 1/4 of it's length, and if'n you ream to a predetermined depth and leave the bottom of the hole with the tapered size at the bottom for the last 1/4 of it's depth you will get a tight fit when you drive the dowel in, but the top half of the job will be to the reamed size and so the dowel can be held in position without falling out or working out from vibration.
Dowels are NEVER tapered, and NEVER fitted in a blind bore....you, as far as possible and by preplanning, make sure the dowel can be driven out with a punch from the back before dissassembly.
On the occasion when a dowel has to be fitted where the bore is blind, IE not drilled through, then the dowel has a threaded hole in it's end to allow a drop hammer or slide hammer, whatever you want to call it, to pull the dowel out.
When you disassemble a part that is dowelled you must first drive the dowels out or else you'll be hitting the job all over trying to get it to move apart, and screw drivers don't make good wedges.
Bolts or screws hold the parts together, but dowels locate them.
For a quick and easy location you can just drill the hole to a nominal size and drive in a roll pin, which is a dowel type pin made from rolled up spring steel and it is oversize to the hole you are going to drill..IE a 5mm diam hole will have a roll pin of 5.3mm diam and the rolled spring steel body will close up as it is driven into the hole......no genuine toolmaker will use a roll pin by choice, but they do have their uses.
Where dowels are to be used for location, the two parts are finished machined AFTER doweling....NEVER before....there are exceptions, such as spigoted bearing housing covers etc.
Ian.
Dowels will not give you the precise registration that taper pins will give you. The use case is a bit different though. With taper pins, you do all of your alignment, then you drill and ream for your pins. This way if you ever have to take things apart, you can re-establish the original orientation. You can't do that with dowels since dowels either require clearance or will cause wear upon removal. Taper pins are zero clearance. If there is any wear when they are removed, they can be put back in a bit deeper and they will snug up. They are also easy to work with if things shift a bit after an overhaul. Simply ream clean again. They weren't uncommon in older military fire control equipment.
Tormach uses them in the head and column. See the sections "Head Removal and Moving" and "Column Removal and Moving" on this page:
Tormach Disassembly
Clearly they can be fitted to blind holes.
bob
Nobody in their right mind uses taper pins today unless they really have to.....the exception as I said before is for fastening collars to shafts, but roll pins are now the norm.
The statement that says, "If things shift a bit on overhaul they can be reamed again" is ludicrous....the whole point of a dowel is to ensure that the parts go back together EXACTLY as before disassembly, anything otherwise and you don't need a dowelled location.
Machine parts are alligned at manufacture to be precisely located, and if'n they go out of position and the taper pins are re-reamed the parts are out of allignment....period.
A hardened dowel pin does not cause wear on removal....the dowel is tight in one half of the assembly and a relatively loose but still tight fit in the other half......You cannot detect any movement with dowelled parts......if'n you can you have not fitted the dowells properly.
Some people drive dowels tight into both parts as if the dowel is there to fasten them together.....they have a lot to learn.
I would like to see you get a taper pin out of a blind hole without damaging it.....taper pins are soft.
Ian.
ref: post #1. Ream hole to my liking? With a taper pin?
Dick Z
DZASTR
Hi
thank you for your exhaustive replies.
I am a rookie and somewhere I read about tapered dowel pins that they
make assembly and disassembly easier.
I am planning to make a jig where I will open and close the "lid" often but
also want to have a nice fit when closed.
So it seamed a good idea to use tapered dowel pins, but then I started to think about how to ream them properly.
I also thought about the mentioned below possibility the seat the pinto the depth where the fit will be "to my liking".
You definitely can by the reamers and pins so they mus have some use.
Thanks a lot again.
Hi, before you start to cut metal I'd suggest you have a talk with a toolmaker for a design that satisfies your needs.
No matter what the design is, if'n you don't have the skill to work with heat treatable steels and also the combination of materials and fits, any form of dowelling will be lost in the making.
Just for starters, you WOULDN'T use taper pins in a jig for a manufacturing environment, unless you were a Masochist.
Ian.
Perhaps if we abandon the use of the word "tapered". Are you thinking more of a coned projection on one part of a jig that mates with a female cone on the other part - thus giving you perfect registration every time?
Please define "perfect registration".
http://www.kirkcon.com/
I'm with Ian on this one...
You are entering the world of jigs and fixtures. There are a few good books on the subject. Go to google books and search for "jig and fixture design". One of the first hits should be to a book by Hoffman. The preview for that book is pretty good and has enoug information to give you a good base to work from. The book is worth the price if you choose to buy it.
If you go to Carr Lane Mfg. Co., tooling components and toggle clamps and look at their online catalog you will get a good idea of what people use to make jigs and fixtures. Of course you don't have to use bought parts but it is good to know what kinds of things are typically used. They also have a book on jigs and fixtures: Carr Lane's Jig and Fixture and Modular Fixturing Handbooks, Trig Book
Workholding is fascinating and will forever tax your creativity.
bob
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