precise orientation fastener ideas

How about a V-block setting on a cylinder? Two linear points of contact, and impossible to misalign unless you screw up very creatively. Controls both rotational and vertical relationship of the two parts. If you need precise 3-dimensional placement, make two of 'em at right angles to each other, and you're locked in.

If that doesn't make sense, please describe the parts for a better suggestion or description, depending.

Dave Hinz

Yes that makes a lot of sense, simple too! Can this easily be made to be accurate and not sloppy, I mean, practically speaking, how tight can I get those V's made on a cylinder? Is there a material of choice for this application?

Steel works great, but v-blocks could be made with a router, a straight guide, and a chunk of wood if you want. What's your application?

The classical solution to eliminate all 6 degrees of freedom (3 linear, 3 rotational), without redundant constraints, is one each of a sphere/plane mate, sphere/cone mate and cylinder/slot mate. Google "kinematic mount" for more details.

If you already have two surfaces clamped together, and only need to minimize relative rotation, two pins in a slot is easy.

If you need accurate linear location as well, think about two pins in cylindrical holes, or one hole and one slot. You can loosen up the accuracy required in making the parts by the judicious use of "diamond pins." For example...

Here's a high-tech solution...

Ned Simmons

I'd simply suggest that the existing screw holding the parts together be replaced by a tapered pin. Readily removable, yet provides precise, repeatable positioning.

Or use the existing screw to lock the parts in alignment, and drill a new hole for the tapered pin, then ream that hole. If the existing screw holes don't quite line up, then this way is better.

Doug

"Shoulder Screws" have a precisely ground cylindrical shank between the head and the threads that is very good for locating parts.

Jim Wilkins

But they don't always have the cylindrical part truly concentric with the threads -- at least based on my experience using two of them to allow a stack of parts to slide together.

Enjoy, DoN.

I can see the tapered pin providing alignment, but I would also need to add some type of fastener to hold the parts together, correct? Or is this some type of locking pin.

No, they are *not* good for locating parts. You're at the mercy of the threads being concentric with the ground body, and that isn't guaranteed.

This job would be a classic example of how tools are built-----with the use of dowel pins, either straight, or diamond, as Ned alluded. If exact orientation is desired, pins can be installed such that the parts will come together in only one way----and held together by screws of one's choosing. Tapered pins would be a PITA in this instance, for they don't always release without a struggle. Dowel pins, properly applied, do, and have the potential to locate within tenths of a thousandth.

Need more info? Feel free to ask. I worked for years as a tool maker.

Harold

"steve" wrote in news:1119301033.308925.119370 @g47g2000cwa.googlegroups.com:

Something similar to a carriage bolt, but with ground flats would work also. If you want only one orientation, make it D shaped.

A taper pin is self-locking. That means the taper is shallow enough so that when you bang it in with a hammer or press it in with an arbor press (or a drill press, for small pins), it won't come out.

Small Parts

and MSC sell them. You need a tapered reamer to make the mating hole. A "taper pin reamer", not a "repairman's tapered reamer".

Doug

Two precision pins (cheap) separated by 1" on flat mating surfaces with a screw about half way between them. Drill the two holes for the pins in one operation and slightly undersize. Ream on set to press fit size and the other set to snug sliding fit. The objects slide together and the screw merely holds them. It is quite easy to get considerably better than .5 degree this way.

Ted

The ones I used were close enough that they didn't bind badly in holed drilled and tapped for the thread and bored and reamed above that for the shank. The arrangement would have been better if the shoulder screws threaded into a floating nut plate but it was a blind hole in the top of a dovetail slide.

I made a successful jig to drill precisely spaced dowel pin holes in the top jaws for a Microcentric lathe chuck by turning down 3 disks on a freshly turned stub mandrel that fit snugly in the disk's reamed center holes. The disks were all shaved down until any one would just fit between the other two which were resting on the chuck's locating pins.

To drill the top jaws I located the first dowel pin hole to the mill's accuracy, put a pin and disk in it, chucked another dowel pin with the second disk on it, then moved the table until the third disk would slide between the other two. Lock the table, recheck fit, drill and ream. All 3 top jaws fit onto the chuck with finger pressure.

Jim Wilkins

As others have suggested, dowel pins are specifically made for this appliction.

However, you need to consider who will be assembling/disassembling the product in question, and whether they're capable of taking the appropriate care required to not bugger the dowel/hole fit.

While a pair of dowels will allow repeatability within a couple ten-thousandths of an inch, there is a significant risk that an untrained operator will incorrectly assemble the joint and (typically) use the dowel to "shear" its mating hole due to the dowel entering on an angle. Once this has occured, the hole will require rework to function correctly.

There are ways to minimize the risk. Having the appropirate hole size is important (precision slide fit as opposed to a light press fit), using a relieved dowel pin, using one standard pin and one diamond pin, using hardened bushings instead of soft reamed holes, etc. You pretty much need to have lubrication every time the dowel is assembled into the hole as the fit is virtually completely intolerant of corrosion or excessive friction.

The bottom line is that dowels are used extensivly in the tooling industry and would probably do an excellent job for very little cost in your application.

HTH.

Regards,

Robin

Holy cow. Does anybody still use diamond pins? That's what they used decades ago to assemble master jigs and master watch plates, but I thought those things disappeared.

Did they teach you about watch plates in your tooling classes? That was once the ultimate expression of the toolmaker's art.

-- Ed Huntress

It's been a few years (ok, more than 20) since I last built tooling, but I was still using them at that point in time. I can't imagine not using them when the situation calls for them. They're the ultimate for fitting more than one item to the same set of pins.

Harold

The object is a small (2 ounce) electronic orientation sensor and it needs to know the orientation of it is relative to the object its attached to.

Robin, Yes the removal/replacement will be done by the end user, so I can't use the dowel suggestion (although a cheap and excellent way to do it). I'm now thinking more of the keyed male/female set (maybe just a simple square shaped key and hole), slide it in with some type of lock-down mechanism that doesn't effect the alignment. .5 degrees isn't extremely precise so maybe this will work. The object is a small (2 ounce) electronic orientation sensor and it needs to know the orientation of it is relative to the object its attached to.

steve

(I replied to this message but it didn't show up)

Yes you are right, the dowel method is excellent but I need a method that's more user friendly, I'm thinking a keyed slot with some type of locking mechanism, accuracy requirement is not excessive so maybe it will work. The application is an orientation sensor that needs to be aligned to the object its measuring the orientation of.