Daniel's Work Thread: Toward a Hackerspace Scanner

[daniel_reetz]

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Also I am 90% through drawing up jck57's cradle. just dying to test that thing.

If you are making the sliding tables according to revised design with pressed-in axles, don't forget to cut the rabbets to 7/32" depth instead of 3/8" shown in my drawing.

Ah! I had missed that. Thanks.

[jck57]

This is arrangement I'm talking about. The arm is machined from one side and the bearing is on the "inside." This is the way I set up my scanner and it works fine. I can't see any advantage to flipping the arm around and using a 2" bolt with a spacer as opposed to a 1.5" bolt with a flat washer. In both cases, you are depending on the press-fit to keep the bearing where it belongs. And yes, I am an asshole when I think I'm right.

[dpc]

Heh, in the end (for this scanner application), it' probably won't make a bit of difference where the bearing is so whatever reduces the parts count and makes it easier to machine is probably the design to use.

Q: From that sketch you've posted, the only thing keeping the arms from coming apart is the press-fit of the bearing on the left arm. I had thought at one time you used a retaining washer and wood screws on the inside of the arm that captured the bearing into the counter bore to prevent this?

Just throwing out another idea - use a socket head cap screw and run it through the arms in the opposite direction. The counter bore in the left arm then could be much smaller as you won't need clearance for a socket to tighten the nut.

One of the main reasons that I chose to put the bearings on the outside of the arms is that the bolt itself is what keeps the arms from pulling apart. Even if the press fit becomes loose over time, those arms can't come apart unless you remove the bolt (or the bolt fails). But as I mentioned earlier, there's likely never going to be a problem due to the way the arms operate with this scanner, so any advantage in keeping the arms and bearing captured with the through-bolt would never be realized.

[jck57]

Heh, in the end (for this scanner application), it' probably won't make a bit of difference where the bearing is so whatever reduces the parts count and makes it easier to machine is probably the design to use.

Q: From that sketch you've posted, the only thing keeping the arms from coming apart is the press-fit of the bearing on the left arm. I had thought at one time you used a retaining washer and wood screws on the inside of the arm that captured the bearing into the counter bore to prevent this?

Yeah, but If you can get a good press-fit the retaining screws are over-kill IMO.

Just throwing out another idea - use a socket head cap screw and run it through the arms in the opposite direction. The counter bore in the left arm then could be much smaller as you won't need clearance for a socket to tighten the nut.

Uh-huh. Good one. They spec a button head cap screw anyway.

[daniel_reetz]

At the moment I'm using Grade 2 hex cap bolts because they're less than half the price of the expensive button head cap screws. Could change my mind on that, though.

[rob]

Personally I can't trust a press-fit that's wood on metal. I'd much rather flip the arm around and have the bearing on the outside. That way, no large force in any direction on the arm has the potential of working the bearing out. But that's just me.

On the other hand, if you glue the bearing in, perhaps it will work fine.

[dpc]

Just to be clear, the only thing that holds the bearing into the arm is the press fit, regardless of whether the bearing is on the outside or the inside of the arm.

The arm can still slide off of the bearing when it's located on the outside of the arm, but the arm will slide inward toward the other arm and begin to rub against it.

There's typically not enough gap between the two arms to allow the arm to move enough to where a bearing on the outside of the arm would be completely out of its counter bore though, so maybe that's a plus.

[rob]

The arm can still slide off of the bearing when it's located on the outside of the arm, but the arm will slide inward toward the other arm and begin to rub against it.

It won't actually slide off the bearing, because you've cleverly arranged nuts and washers to prevent this

[jck57]

From a machining perspective, it's best to have all machining operations on just one side of the part. A quirk of the current design is that the bearing on the lifter arm would be better on the "outside" as drawn by DPC.

I'm going to try pressing and/or pressing with epoxy or cyano glue. I guess I wouldn't mind pressing the arms if I did 50 or 100 at once and didn't have to think about it for a long time.

Just a reminder that Fab52 devised an ingenious two bearing system that can be machined from one side:

[dpc]

It won't actually slide off the bearing, because you've cleverly arranged nuts and washers to prevent this

I'm a bit slow today. I still don't see what keeps that arm from moving toward the piece with the stud coming out of it and rubbing wood-on-wood. Could you draw a cross-sectional sketch and post it here? I'm lost on where those flat washers go.

That wood pulley is pretty sexy. Did you turn that on a lathe or come up with some other way to do it? I'm going to make a number of those an exact size and I'm planning to use my router circle jig to cut the OD, then build a fixture on my router table with a 1/4" cove bit to cut the groove.