pic: Introducing BarBox: My First Attempt at Gearbox Design

[Trent B]

Another benefit of designing for an endmill radius, is it will get rid of some of the stress concentrators in your design. The round rings and cross member meeting spots appear to be a pretty sharp angle which could be prone to failure.

[Adrian Clark]

Originally Posted by bardd

The output shaft sits on two different bearing sizes (0.875" in the back and 1.125" in the front)

Why is this necessary?

And a note on standoffs: It's very hard to find .375 OD tube with #10 clearance ID. Rather than buying .375 rod and drilling it out I prefer to buy .0625 wall with .375 OD and put .0625 deep counterbores in the plates to capture the standoff. Makes things much lighter and simpler to make.

[R.C.]

Originally Posted by Adrian Clark

Why is this necessary?

And a note on standoffs: It's very hard to find .375 OD tube with #10 clearance ID. Rather than buying .375 rod and drilling it out I prefer to buy .0625 wall with .375 OD and put .0625 deep counterbores in the plates to capture the standoff. Makes things much lighter and simpler to make.

Ditto,

If your turning down the back guy currently to .375", you might as well save yourself time and just lathe it down to .500".

I know it might be lighter to go that smaller bearing but honestly, its much easier programming/making 2 of the same part then it is 2 different parts.

Adrian,

We do about the same thing, buy .375" OD Tube with a .235" ID. 1658T41 - McMaster. We go parting crazy and you can make a lot of spacers in no time, especially on a bar feed . We use this material for the gearboxes we make. Do you really need the cbores? We've noticed its incredibly harder to assemble gearboxes that way.

-RC

[MichaelBick]

Originally Posted by R.C.

Ditto,

If your turning down the back guy currently to .375", you might as well save yourself time and just lathe it down to .500".

I know it might be lighter to go that smaller bearing but honestly, its much easier programming/making 2 of the same part then it is 2 different parts.

Adrian,

We do about the same thing, buy .375" OD Tube with a .235" ID. 1658T41 - McMaster. We go parting crazy and you can make a lot of spacers in no time, especially on a bar feed . We use this material for the gearboxes we make. Do you really need the cbores? We've noticed its incredibly harder to assemble gearboxes that way.

-RC

I agree that counterbores are harder to assemble with and unnecessary. What we've been considering for next year is buying aluminum 6063 .375" OD tube with .145" ID and still machining it out. While we will have an extra step, the ID will be closer, the hole will be centered throughout, and our drills will not gum up because we are taking out quite a bit less material. However I'm worried about the 6063 aluminum. I've heard it is a bit less strong than 6061 and more gummy. I'm assuming it will be fine but if anybody else does this I would love to know.

[R.C.]

Originally Posted by MICHAELABICK

I agree that counterbores are harder to assemble with and unnecessary. What we've been considering for next year is buying aluminum 6063 .375" OD tube with .145" ID and still machining it out. While we will have an extra step, the ID will be closer, the hole will be centered throughout, and our drills will not gum up because we are taking out quite a bit less material. However I'm worried about the 6063 aluminum. I've heard it is a bit less strong than 6061 and more gummy. I'm assuming it will be fine but if anybody else does this I would love to know.

That's not a bad idea. You shouldn't have issues with 6063, it is weaker:

6061: http://asm.matweb.com/search/Specifi...ssnum=MA6061t6

6063: http://asm.matweb.com/search/Specifi...ssnum=MA6063T6

But I can't see you having an issue with loading them as "spacers". As we've been using 6063 for all our spacers since its so darn cheap/easy. We also use a handful of plastic spacers on gearboxes as well. No issues with either or.

6063 can gum up pretty badly when machining but your taking off so little that it shouldn't be an issue. We sometimes drill out the current spacers for 1/4" bolts.

-RC

[bardd]

Originally Posted by sentientfungus

Small nitpicky thing on your latest version-- the edges are looking rather sharp. Whether you're planning on manufacturing it or not, it's best practice to design for manufacture, and nearly any manufacturing method that you would use in competitive robotics is going to need to have some sort of radius where you're cutting. I personally design most of my practice stuff to be manufactured with a 1/2" endmill because our primary machining sponsor uses CNC mills.

I took this into account. the edges do look sharp, but that's because of the render's angle. In reality I planned radii for the corners, so the whole thing can be milled with a 0.2" endmill.

Originally Posted by Adrian Clark

Why is this necessary?

And a note on standoffs: It's very hard to find .375 OD tube with #10 clearance ID. Rather than buying .375 rod and drilling it out I prefer to buy .0625 wall with .375 OD and put .0625 deep counterbores in the plates to capture the standoff. Makes things much lighter and simpler to make.

The different sizes are necessary because of the shaft's shape: the back end is 0.375" and round, the front is 0.5" hex. The fitting bearings for each side are different in size. I could change the shaft to allow using the same bearing, but then it'll have to be manufactured instead of just buying it from andymark.
As to the standoffs, I didn't actually consider how to make them... I'll look into it.

Thanks for the comments everyone!

[AdamHeard]

Originally Posted by R.C.

But I can't see you having an issue with loading them as "spacers". As we've been using 6063 for all our spacers since its so darn cheap/easy. We also use a handful of plastic spacers on gearboxes as well. No issues with either or.

6063 can gum up pretty badly when machining but your taking off so little that it shouldn't be an issue. We sometimes drill out the current spacers for 1/4" bolts.

-RC

We do everything mentioned in this thread in various spots; counterbored, the specific 3/8" tubing above, and plastic spacers.

We choose them in different spots depending on the alignment required. Very few of our gearboxes have their alignment and spacing entirely set by the standoffs. Those that do (like our drive boxes) we generally will counterbore, but not always (as it is a more time expensive process).

The aluminum tubing we use when we just need to attach two plates together for stiffness, but aren't counting on it for any alignment. I'm a big fan of clamping frame members with gearboxes, and the frame member handles all alignment. This specific tubing size is something 254/968 found on mcmaster a while ago and have shared with others as it's convenient. There is no concern of failing this tube, even w/ it being 6063. We use it in high loaded situations no problem. The time saved over drilling is really nice.

We use the plastic standoffs for the same reason as above, but when the load is lower. This is an example; 94639A134. This ENTIRE series on mcmaster/fastenal is just awesome. I'm proud that we introduced it into FRC as it has saved us so much time. They come in standard thickness from 1/8" to inches, in varying OD's per bolt size. We use them all over the robots and save massive time. They come as an ugly nylon, but we RIT dye them black and they're beautiful.

In summary, all the ideas posted about standoffs are valid so far, but they apply to different cases. It's not a good idea to assume that because you want things to work well at the system level, all parts MUST be super precise.

The different sizes are necessary because of the shaft's shape: the back end is 0.375" and round, the front is 0.5" hex. The fitting bearings for each side are different in size. I could change the shaft to allow using the same bearing, but then it'll have to be manufactured instead of just buying it from andymark.
As to the standoffs, I didn't actually consider how to make them... I'll look into it.

Using an off the shelf is a great idea if you have limited access to fabrication. It can even be a great idea when you do have access if the shaft meets all your needs. We've got a full, capable shop for FRC and we LOVE not having to make parts!

[Adrian Clark]

Originally Posted by bardd

The different sizes are necessary because of the shaft's shape: the back end is 0.375" and round, the front is 0.5" hex. The fitting bearings for each side are different in size.

Like Adam said, using an off the shelf shaft is a great idea. But I would recommend using a .375 ID 1.125 OD bearing from andymark so you can make the plates identical.

[bardd]

Originally Posted by Adrian Clark

Like Adam said, using an off the shelf shaft is a great idea. But I would recommend using a .375 ID 1.125 OD bearing from andymark so you can make the plates identical.

I haven't thought of that... Thanks for the tip. I'll try and incorporate this in the design.