Adjustable bearing block (from 1/2" EMT to 1.25" plain tube)

[jamiek (Jamie)]

How important is it that the tubes ride in the center of the bearings?

Crazy idea: the attachment of the brackets to the plate could be adjustable if there were slots that accommodated adjustment. Let’s assume this can be made adjustable and still secure.

Then you could install one side, hook it onto one rail, and install the other side so that it touches the rails. Slide inward until it presses against the second rail.

This would support the plate on the rails but it would not prevent lifting. Before we get to that, note there is some leeway (actually quite a bit) in the size of the tube that can fit on these parts. Smaller rails will ride closer to the blue parts, riding on one side of the bearings, and larger rails will ride farther from the blue parts, on the opposite side of the bearings. I think this can accommodate almost a half inch range of tube sizes, e.g. 1" to 1.5". Each 1mm increment in the tube diameter moves the point where it rides on the bearing by 0.5 mm, so a 7mm bearing width would allow up to 14mm of range in tube diameter. I would be reluctant to assume the rails can ride the full edge to edge 7mm of width so restricting to somewhat less range would be safer.

Then to prevent the plate from lifting off the rails, additional pieces can be slid under the tubes and anchored to the plate.

These smaller parts on the underside will not be able to accommodate as wide a range of tube sizes, so there will need to be several parts, each of which accommodates a narrower range of tube sizes. But the adjustment/installation process does allow for some tolerance. The parts do not need to be precisely matched to the tube size and if someone screws up it’s relatively easy to print new ones for just those parts.

One disadvantage, which I think it a disadvantage for any adjustable system, is how to get a non-expert to get the “right” amount of tension. To “press lightly and tighten it down” can yield different results for different people. But any adjustment system will probably have this issue.

[jeffeb3]

Some part of the bearing would still be on the center until the width of the bearing was reached (8mmx2 would be about 16mm of slop).

You could also play with the inside parts being fixed. They would be the reference. The outside pieces would both need to be adjustable.

If the plate could be used to hold the tension, then it may be easier to have a “print to size” template that could be used to drill a second set of holes for the adjustable parts.

I do wonder if the lateral force on the screws through sheet material may not be able to hold the tension well enough.

Then there is the xz connection that would need to adjust for the varying location of the tubes.

[jamiek (Jamie)]

An additional assumption I didn’t realize I was making was I was thinking the XZ parts would clamp the tubes in such a way that the distance between them would vary too. So the outer pieces accommodating variable width is handling the tube size and also the tube spacing. But the concept is not tied to a particular sequence of certain dimensions being independent and other dimensions being dependent, so it’s whatever works.

Also the 608 bearings are 7mm wide with an 8mm inside diameter, but some bearings are slightly beveled on the edges so the usable width might be a slight bit less. I think we are in agreement on the concept of using the entire bearing width even though the numbers are not the same.

The lateral force of the screws holding tension through the plate is a valid concern. The plate is being asked to do more duty than before so it is a potential risk of discovering a new weakness.

Another possibility which is perhaps interesting is that the bearing supports do not necessarily need to be coplanar. I’m not sure if there is any advantage to offsetting them, except that it might help make room for beefier parts and more distance between anchor points.

[merrittgene (Gene Merritt)]

Instead of carrying the weight on the “top” bearing,

@jamiek

what about carrying the load on two bearings, with a common spacing for all tube diameters. There would only be 1.9mm difference between the high and low positions, and I assume a lower bearing holder could be adjustable, or swappable based on the tubing diameter:

[vicious1 (Ryan)]

Something looks off about that drawing but I am not seeing it…unless it is that center and tangent on the right bearing. I have made a part it just barely fit 23.5 to 25.4mm rails. When I go back into the office I will find my drawing. I decided not to go with it because the extremes are too close to an edge and any bad prints would not work.

I was trying that with the new design…I wonder if I made a mistake. It would be awesome if that is correct.

Try just tangents on the bearing 7mm line and the circle, no center lock. Meaning both sides should be moving up to keep that circle center point on the center line, what you have is rotating I think.

[vicious1 (Ryan)]

Wait it looks like you have it correct. Try adding a 0.2 ± on those tubes, somewhere that fails.

The bearings are 7mm wide but the edges have ~.5mm radius as well.

[merrittgene (Gene Merritt)]

@vicious1

The bearings are 7mm wide but the edges have ~.5mm radius as well.

I just threw the drawing together to see “how far off-center would the contact point be” to see if the idea was feasible.

If I understood you correctly, here are sketches with +.2, 0, -.2 variations of the 23.5 and 25.4 versions

[vicious1 (Ryan)]

23.3-25.6, but that looks good. You are right I double checked, Not sure why I didn’t think it worked. I have a printed part I know it fits, the rails are super close to the edges.

This shows that with 2 points fixed we only have about a 2-3mm diameter tolerance, and have to having a moving third…That could make for a more simple LR set of parts (with more assembly work) but doesn’t work on the MPCNC because of the center.

[jeffeb3]

It’s about time you give up on that MPCNC anyway.

#LowRider4Ever

[vicious1 (Ryan)]

Not gunna happen

[timonjkl (Tim W)]

Not till he finishes this one MPCNC. Then lowrider I have a room in the basement for a full sheet machine so please hurry @vicious1

[jamiek (Jamie)]

Just throwing this out there because I love algebra : the angle between the bearings determines how much the position on the bearing changes in response to the diameter of the tube. Larger angles produce a larger difference in position and smaller angles produce a smaller difference in position, for a given change in tube diameter.

Specifically for an angle α, and a change in diameter of Δd, the change in position Δp is
Δp = tan(α/2)*Δd/2

In particular when α is 90 degrees, Δp = tan(45)*Δd/2 = Δd/2
And when α is 120 degrees, Δp = tan(60)*Δd/2 = Δd * 0.866

So for Gene’s diagram, a range of 23.5 to 25.4 would shift the position by only 1.9 mm * 0.866 or about 1.64 mm.

If we allow a 6mm range of position on the bearings then the difference between the max and min diameter would be about 6 / 0.866 = 6.93 mm of range in diameter, which could cover about 23.5 to 30mm diameter at the extremes. All the other tolerances could eat some of that range but anyway that’s what the math predicts.

[timonjkl (Tim W)]

You math geeks make my brain numb or maybe it’s the Jack but that looks like it will work and does the edge bevel make a difference or will the they stay because you only have 7mm not8 to work in? Now I see the end and 6.93

[blainesgarage (Blaine)]

1” EMT. YAY. I’m in.

[RobinBennett]

Here’s a thought for making one adjustable bearing. It’s very similar to the standard design, except the hole for the axle is elongated into a short vertical slot, and two bolts threaded into the plastic push down on the axle to hold the bearing against the tube.

The gray blocks on the sides are supposed to be an M8 nut and bolt, the two smaller T-shaped blocks on the top are the heads of two vertical M4 bolts.

[kubark42 (Kenn Sebesta)]

Wow, this really took off! I love the ideas, and it’s definitely a huge upgrade on my original notion.

Boiling down the past few posts, I see two directions emerging. One is to do it as @jamiek suggests, where the bearing block is printed in two sections, with a sliding member. The other is as @RobinBennett suggests, with two adjustment screws.

I have some thoughts on which way I’d like to go, but I’m going to think a little longer before diving in.

One thing to consider with both strategies is that the Y-plate doesn’t lend itself to these kind of 1-D adjustments.

Notice that individual bearings don’t line up vertically. This means that we couldn’t fix two and leave a third mobile. Lining up bearings would require redesigning the plate and bearing holders. This isn’t necessarily a bad thing, but it could be if there’s a reason why the original design has contact with the tube every 60 degrees.

It’s also getting toward a larger redesign and it might make sense first to get input from @vicious1 about lessons learned during y-plate design and testing.

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@vicious1

The bearings are 7mm wide but the edges have ~.5mm radius as well.

Measuring with calipers on the bearing set from the kit, I get .75mm fillet radius, i.e. 5.5mm of usable flat width.

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@jamiek

If we allow a 6mm range of position on the bearings then the difference between the max and min diameter would be about 6 / 0.866 = 6.93 mm of range in diameter, which could cover about 23.5 to 30mm diameter at the extremes. All the other tolerances could eat some of that range but anyway that’s what the math predicts.

Great closed-form analysis. I think that we can grow the envelope a little by allowing the bearing to be positionable along the axis. Basically, make the notch wide enough that there’s a 3-4mm gap, and then use some washers to position the bearing so that the center of the bearing is somewhat close to the contact point with the tube.

This doesn’t need to be precise, and washers of this kind can be made out of any convenient material. For coarse work, some convenient thin plywood works. For fine work, I would just use a pair of scissors on some old plastic packaging.

The upshot is that we go from ~7mm of range in diameter, to ~10mm. That’s the difference between 3/4EMT (23.4mm ±.2) and 1.25" (31.75mm ±.2). Therefore, a 3mm gap-- quite small all things considered-- allows for the entire range of likely tubes, including some wiggle-room for tube and bearing tolerances.

Sliding along that axis is also not a control variable, so it’s not something we have to tune or which could get out of alignment with other parts in the system. Whew.

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@merrittgene

what about carrying the load on two bearings, with a common spacing for all tube diameters.

FWIW, I think that there’s an argument to be made for optimizing bearing loading directions. It’s unclear to me if the tube is driving the plate downward, or keeping it from falling. This depends a lot on the cutting loads. However, when I think of a drill, it’s definitely the case that I have to push down on it, its self load is insufficient to drive it into the material. However, if I’m using a hand-held router, even a palm one, there’s no need to push. So I’m kind of at a loss here.

[kubark42 (Kenn Sebesta)]

I gave a stab to the adjustable bearing approach. I realized that this block could be used for all 12 locations, which would simplify the build and prints.

Screw adjustable bearing block

This uses hollow brass tubes as axles (we couldn’t use a bolt as an axle because the bearing will wobble on the thread), and thus retains the zip-tie features. However, the zip ties no longer carry any load, they simply keep parts from falling out.

The two adjustment screw holes can be seen at the top, as well as slots for captive nuts. The holes are sized for M3 (SAE #4 will work if standardized hardware isn’t available).

Also note the 3mm extra width in the notch around the fixed-axis axles. This provides for the longitudinal sliding adjustment required to handle from 3/4" EMT to 1.25" plain tube.

Using these bearing blocks for the Y-plate looks something like this:

It does require splitting the wheel arm from the bearing block. I feel that this is only a problem if it introduces flex in the structure, but I believe any flex can be engineered away.

Of course, the lateral block location can be adjusted. And if additional plate mass is a problem, it can also be significantly lightened by carving out some holes. However, the plate raw material dimensions don’t change, as the enclosing rectangle is identical.

However, the distance of the tube from the plate grows by ~3mm and this can’t be reduced anymore without wall thicknesses getting very small. I don’t know if this increased distance is a problem or not.

Maybe worse, this moves parts of the bearing block past the rubber wheels. That means it cuts marginally into the working area.

Thoughts?

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CAD file

https://cad.onshape.com/documents/9bfd836abff9fc3b04fc7b23/w/8b821b524858853269344a9f/e/4130cd71536d5b5ee8f2260a

[kubark42 (Kenn Sebesta)]

@kubark42

However, the distance of the tube from the plate grows by ~3mm and this can’t be reduced anymore without wall thicknesses getting very small. I don’t know if this increased distance is a problem or not.

Maybe worse, this moves parts of the bearing block past the rubber wheels. That means it cuts marginally into the working area.

I take that back, if we cut notches out of the y-plate, the brackets can be mounted on the inverted side, bringing the z tubes much, much closer to the plate.

The downside is that we have now introduced new unique parts (unless we modify the spindle plate in the same way).

[kubark42 (Kenn Sebesta)]

Here’s the proposed piece. This moves the Z-tube as close as 1mm from the y-plate, but the mounting tabs can be shifted backwards so that the Z-tube will have more clearance. Is that desirable, though?

Here’s how the modified assembly looks:

[Paradox_Pete (Pete)]

I would recommend modifying that part to reference off the inside face of the plate rather than the outside.

All the current LR2 parts that attatch to the Y plate reference on the inside face. This eliminates any reliance on or limitation to the thickness of the Y plate.