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gootee 12th November 2007 11:49 PM

Knob: Shaft-length Problem
 
I'm sorry that this post is so long.

I have a 1P12T rotary switch that has a 0.236-inch (6 mm) diameter "D"-shaped shaft, which only protrudes through the front panel by about 3/16-inch (4.76mm), which is at least 1/16-inch (1.6mm) too little in length, for set-screw-type knobs.

I need to find a solution that will enable me to use a knob on the shaft, while still retaining the capability of removing the knob, when necessary (i.e. no glue). The knob can be up to 3/4-inch (19mm) diameter, max, although something closer to 1/2-inch (12.7mm) is preferred.

Possibilities that I have thought of include: 1) finding a knob that will work, as-is, 2) drilling a larger hole in the panel so that the knob can extend slightly behind/inside the panel face, to enable it to use more of the shaft, 3) making or buying a shaft-extender, 4) using a different switch, i.e. with a longer shaft, or, 5) completely replacing the existing shaft with a longer one.

All of the knobs that I have seen, that use "set screws", have the set screw >= 3/16-inch from the base of the knob skirt. The ones that are exactly at 3/16-inch are just barely too far from the panel to work with my 3/16-inch of shaft that protrudes, since only 1/2 of the set screw lies over the end of the shaft, in that case.

If I could find a "push-on" type of knob, made for a D-shaped shaft, which required NO (or an extremely small) gap between the panel and the internal "D" portion that slides onto the shaft, it "might" work, well-enough. That seems like the best solution, to me, IF it would work OK. Does anyone know of such a knob? And a knob that had an "extended" D-shaped hole, even extending below the knob base, might work even better.

A "collet"-type knob might work, too. But the only ones I've found either don't have the special tightening tool available, or are the wrong size. And I don't know if they would work well-enough on a D-shaped shaft, anyway.

One alternative: I "could" make the hole in the panel larger, so that the knob could slide partially "into" the panel (and farther onto the shaft). But I want to try to avoid that method, if at all possible.

Another alternative: I found some "shaft extender" parts. But they are too long to fit between the panel and the PCB, and are 1/2-inch in diameter. IF I could find an extender that had a D-shape on one end, and slid onto a D-shaped shaft at the other end, made of thin material, with a length that was short-enough, or could be cut down, THAT would probably work. Does anyone know if something like that is available? Or is there a better way to extend the shaft? I really only want about a 1/8-inch (3.175mm) increase in the shaft length.

SOME DIY SHAFT-EXTENDER IDEAS THAT I JUST THOUGHT OF:

The shaft has a round (non-D-shaped) portion, near the switch body, that is slightly over 1/4-inch (6.35mm) long, before the D-shaped part starts. I could, possibly, cut the shaft off, leaving about 1/8-inch (3.175mm) of the round portion before the D-shaped portion on the piece that I cut off, and then find a piece of round metal tubing of the proper inner diameter and length, and apply some good glue to the round parts of both the remaining shaft and the piece I cut off (or maybe, instead, apply the glue to the inside of both ends of the metal tube), and then stick the tube onto the remaining shaft, and stick the cut-off piece of shaft into the other end of the tube, resulting in a longer shaft. But I'm wondering if 1/8-inch is enough "overlap", to be reliable-enough, when glued. Maybe it would be, if I used the right type of glue, and had metal tubing with the right inner diameter. (Or, maybe I could even, or also, solder it.) I only really need about a 1/8-inch (3.175mm) shaft-length increase. But, the only type of metal tubing that I've seen that is readily available is copper tubing, at the local hardware store. I'll have to check to see if it comes with 1/4-inch inner diameter, which is probably as close as I'd be able to get to 6mm, here in the USA. But, is copper tubing strong-enough, and durable-enough, for long-term reliability? Any opinions? Is there any other type of tubing available, that I might be able to use? (I could also glue a 1/8-inch (3.175mm) length of same-diameter metal shaft inside the tubing, and between the cut ends of the original shaft, to try to strengthen the whole assembly.)

Along those same lines (i.e. cutting the shaft off and splicing-in a DIY extension): How else could I do that? I wonder if plastic tubing would work well. And what about just soldering or glueing several pieces of flat metal between the cut-off part and the original part of the shaft, overlapping the outer surfaces of the two parts? And, I suppose that if I had to, I could even drill some tiny (radial) holes into the sides of the two pieces, near the cut ends, and through the "splint" pieces, and use metal pins or screws to help hold any splints in place against torquing.

ANOTHER IDEA (MAYBE BETTER): I could probably drill somewhat-larger (maybe about 3mm diam) AXIAL holes, into the ENDS of the two cut-off 6mm-diameter round shaft portions, and glue (or solder) a metal rod into the holes, to hold the two shaft pieces together and extend the length. OR, I could possibly drill two smaller holes into each shaft piece's end, instead, i.e. off-center, and glue in two smaller rods, so the torque load would be on the rods instead of the glue joints. In either case, I'd be a little worried about having enough hole-depth, while not getting too close to the start of the D-shaped part, i.e. in the 1/8-inch (3.175mm) long, 6mm-diameter round portion of the shaft that was cut off.

It seems like using tubing would be easier to get right than drilling holes in the ends of the shaft pieces.

Does anyone have any better ways to make a DIY shaft-extender, or have any opinions or gut-feelings about the robustness and long-term durability of the various DIY shaft-extension schemes (such as those I just mentioned)?

Any other ideas would be greatly appreciated.

Regards,

Tom Gootee

P.S.

SOME DETAILS OF THE SWITCH, MOUNTING, AND DIMENSIONS:

The switch is mounted on a PCB that is parallel to the front panel. There is exactly 1/2-inch (12.7mm) between the front surface of the panel and the rear surface of the PCB, and exactly 3/8-inch (9.525mm) between the rear of the panel and the front of the PCB. I cannot change the distance between the panel and the PCB. And I cannot change the thickness of the panel or the PCB.

The "backward-facing" switch's shaft extends in the same direction as its PCB mounting pins. So the switch is mounted on the rear-facing side of the PCB and there is a hole in the PCB for the shaft, which goes through the PCB and protrudes through another hole in the front panel. (I couldn't find a flat-enough switch to allow me to place the switch BETWEEN the PCB and the panel, or I probably wouldn't be having this problem with the shaft-length.)

The pointer on the knob needs to point in the direction that is opposite of the flat side of the shaft.

The switch that I use is part number 105-SR2513F-112NS at http://www.mouser.com . It is a Taiwan Alpha part number SR2513-0112-19F0A-T-N . The datasheet is here: http://www.mouser.com/catalog/specsheets/TW-700086.pdf .
I do not want to change the switch unless I can find one with the same PCB footprint, unless I could make a small daughterboard to hold it. The switch needs to be able to handle waveforms of up to 15 volts 0-to-peak, into 10 Ohms, i.e. 1.5 Amps 0-to-peak. Waveforms can be sine, triangle, and sawtooth.

Conrad Hoffman 13th November 2007 12:32 AM

A couple thoughts- there are/were collet type knobs where you put it on the shaft, then tightened a screw in the center front. This drew up a split collet (like on a lathe) to grip the shaft. A center cap then snapped over the front. No idea where to get them, but a search might turn them up. Maybe start with Rogan. You could also get hold of the guy who posted here recently about his new metal lathe he bought for making knobs. Perhaps he could be persuaded to make you a custom, with the setscrew in just the right place to hit the short shaft. The idea of making a sleeve to lengthen the knob and bring the diameter to 1/4" for standard US knobs is appealing. If it were a good fit and installed with something like Loctite 271 or stud lock, I'd expect it to be reasonably permanent.

gootee 13th November 2007 02:08 AM

Thanks, Conrad. That gave me some more ideas:

If I could find some combination of metal tubing, and possibly some inner sleeves if necessary, that could make an "adapter", with one end that would fit snugly around the outside of the original 0.236-inch (6mm) round shaft stub, while the other end had a 1/4-inch outside diameter, and could find a solid rod with diameter about equal to the 1/4-inch tubing's inside diameter (or also use sleeving, there, if necessary), then I could cut off and discard the D-shaped part of the shaft, slide the tubing/adapter over the remaining round shaft (with glue), slide the rod into the tubing (with glue), and have a round 1/4-inch shaft. Maybe I could also file a small flat into it, for the set screw.

i.e. If I can't fit any 1/4-inch O.D. tubing over the 0.236-inch (6mm) diameter round shaft stub (which I probably can't), maybe I could find slightly-larger-diameter tubing that WOULD fit over it (even if an inner sleeve of some sort were needed, to get a snug fit), into which the 1/4-inch tubing would fit snugly (maybe even requiring another inner sleeve). Then, with a solid rod that fit snugly inside the 1/4-inch tubing (perhaps requiring another sleeve), and the right glue everywhere, it seems like it might work well.

I guess trying to find tubing and solid rod sizes that would eliminate most of the sleeving requirements might be the tricky part.

mpmarino 13th November 2007 02:45 AM

Rather than all that wouldn't it be easier to come up with a custom knob somehow?

gootee 13th November 2007 02:59 AM

It looks like "hobby" suppliers carry brass tubing that has 0.014-inch wall-thickness, in a variety of diameters.

There is 9/32-inch O.D. brass tube available, which would have a nominal I.D. of 0.25325 inch, which seems like it should provide a snug-enough fit for a 0.25-inch O.D. tube, inside. Would the .001625 nominal clearance be too loose (or too tight)?

To fit the 9/32" O.D. brass tube over the 0.236" (6mm) shaft stub, a sleeve of a little less than (0.25325" - 0.236")/2 = 0.008625 would be needed. And the hobby suppliers also show some 0.008"-thickness tin sheet metal available. Would .000625 nominal clearance be too tight (or too loose)? They also have .005"-thickness brass sheets, if more clearance would be better.

The 0.250-inch O.D. brass tube would have an I.D. of 0.222 inch.

A 3/16-inch solid brass rod would have a diameter of 0.1875 inch.

To fit snugly inside the 0.222" I.D. of the 1/4" O.D. tubing, the 3/16" rod would need a sleeve of slightly less than (0.222" - 0.1875")/2 = 0.01725" thickness. The hobby suppliers show some 0.016"-thickness brass sheet metal available. Would .00125" nominal clearance be too loose (or too tight)?

So, it looks like it might be do-able!

Re-capping: I could use a 1/2" length of the 9/32" O.D. brass tube, over the 1/4" long 0.236" diameter shaft stub (with .008" tin sleeve), and an appropriate length of the 0.250" O.D. brass tubing slipped inside the protruding 1/4" of the 9/32" O.D. tubing, and an equal length of the 3/16" diameter brass rod (with a .016" brass sleeve) inside the 0.250" tubing.

Assuming that the clearances would be OK, and something like Loctite everywhere, Voila!

gootee 13th November 2007 03:10 AM

Quote:

Originally posted by mpmarino
Rather than all that wouldn't it be easier to come up with a custom knob somehow?
I don't know. It might be easier, for some people, to make a custom knob, or have one made. But, for me, right now, it seems easier to be able to do it with cheap "off the shelf" parts and simple tools.

Magura 13th November 2007 03:11 AM

Quote:

Originally posted by mpmarino
Rather than all that wouldn't it be easier to come up with a custom knob somehow?

If you take a closer look at the BAF-07 knobs, I think you should be able to see the solution.


http://www.briangt.com/gallery/album64


Magura :)

mpmarino 13th November 2007 03:17 AM

Quote:

Originally posted by Magura



If you take a closer look at the BAF-07 knobs, I think you should be able to see the solution.


http://www.briangt.com/gallery/album64


Magura :)

As ugly as they are(:)), they would probably do the job.

Tom, if your not in a hurry, maybe I can help.

Conrad Hoffman 13th November 2007 03:30 AM

I wonder if you could buy the right size shaft collar- the kind with a split and a clamp screw that goes across the split. Just clamp the thing on the shaft, and apply a foil disk to the front. The split could be positioned so as not to show too much, or could be used as an index. If they don't have the right size, just buy a smaller one and drill it. Even cooler than that, do the same thing, but have somebody machine you up a cup that snaps completely over it.

gootee 13th November 2007 03:38 AM

Quote:

Originally posted by Magura



If you take a closer look at the BAF-07 knobs, I think you should be able to see the solution.


http://www.briangt.com/gallery/album64


Magura :)

I guess "should" was the operative word, there. :-) I can't see the solution.

It's causing me pain.
So would you please explain,
to my sleep-deprived brain,
a little more-fully, what's germane?

- Tom


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