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HighTechDIY • View topic - Bearing / Linear Motion Design?

Structural Design

Bearing / Linear Motion Design?

Design of structural and load bearing framework.

Bearing / Linear Motion Design?

Postby Awesomeness » Mon Dec 06, 2010 1:55 am

One of the key objectives of the Spartan Project is to make a design that has specific dimensions. Many people have built machines, including myself, that used Patrick Hood-Daniel's bearing trucks or "Bearing Rail Assemblies (BRA)". These were a great step forward because of their simple design and use of inexpensive bearings. They had the disadvantage of not being easily adjustable, which caused the builder to have to cut the actual MDF parts of the machine to an arbitrary size to cause the proper fit. They also had the side effect of requiring the use of a 45-degree chamfer bit, which I found to be uncommon and not easy to borrow, which adds cost to the build.

Here are some ideas of linear motion systems that could be used, along with some of my opinions on pro's and con's. I'd like to discuss what options we have, which would be best to use (and why), and begin to come to a decision on which will be incorporated into the final design.

"Bearing Rail Assemblies":
Pro's: Simple to construct, use inexpensive rollerskate bearings
Con's: Limit adjustability, require purchase/use of special 45-degree chamfer bit, strength is limited by small number of threads holding through thin aluminum angle

V-groove:
Pro's: Very simple to construct, adjustable, requires no special tools, allows for a design with specific dimensions
Con's: Expensive (adds $200-300 to the build)

Square-tubing BRA's: As seen on Peter Chick's site... http://www.chickfamily.net/blog/
Pro's: Use inexpensive rollerskate bearings, may provide necessary adjustability
Con's: Require careful construction and possibly the use of a jig or drill press
PeterChickSquareBRA.jpg


Commercial linear slide bearings:
Pro's: Easy to construct
Con's: Very expensive
Image
Image
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Re: Bearing / Linear Motion Design?

Postby airnocker » Mon Dec 06, 2010 4:57 am

Nice job Awesomeness on getting the site up quickly.

One this topic, here is a link to VXB http://www.vxb.com/page/bearings/PROD/Kit8407 and their 3/8" V-Bearing page. Today only $7.77 US each and I've received email specials from them on occasion for just over $4 each.
I don't know if you are in UK or US, but for US folk that comes out to just under $128 for 16 bearings. This is were I got both my skate bearing and recently V-bearings.

Cheers,

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Re: Bearing / Linear Motion Design?

Postby Awesomeness » Mon Dec 06, 2010 5:23 am

I'm in the US, too.

$8 is definitely a good price, and $4 would be better. It's still a long way off of $4 for a 10 pack, like the roller skate bearings. I fear that tacking on an extra $100-200 for v-groove bearings may be just too much for newcomers (in the context of a $800-1500 build). Does that seem like a concern to you too?

Obviously the v-groove bearings make design and assembly VERY easy. The only thing better and easier are the commercial track systems and linear bearings (but they get even more expensive).

One of big things I noticed is that even though some machines claim to be built for "$800", to a new CNC'er by the time they build the machine, buy the router to go in it, buy some bits and a spoil board and a ShopVac, etc., they're actually up to 1.5-2x that claimed cost. So I think keeping costs honest, and as low as possible, is important.

I've been thinking a lot lately if there is a clever and cheap way to use 3 skate bearings around a piece of square tubing, similar to the way BuildYourIdea.com made the linear bearings. I think that would be the holy grail for a project like this. Each bearing truck would have 6 skate bearings, which costs like $2. It would be sort of a combination of Peter's use of square tubing above, and these...
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Re: Bearing / Linear Motion Design?

Postby beermkr » Mon Dec 06, 2010 3:51 pm

I actually am working on the square tube idea and will try to get some pictures drawings up in the next day or so with my concept. It uses 3/4 square tubing in place of the 3/4 angle but with the bearings still coming off the sides vs. the corners. The edge of the router table will have a piece of 1x1x0.125 architectural channel on it for the bearings to ride on.

R/
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Re: Bearing / Linear Motion Design?

Postby Awesomeness » Mon Dec 06, 2010 7:25 pm

Here is an idea I had. I whipped up a quick CAD drawing to illustrate. Unfortunately, I used aluminum angle that was too big (or bearings that are too small) and the bearings wouldn't reach, but I think the idea is clear. This uses 6 skate bearings that can ride on 3 sides of an edge of a piece of flat "strap" steel (maybe 1.5" wide, and 1/4" thick?). You would bolt that flat stock to the table, and it would stick over the edge, and could be adjusted slightly.

LinearBearingIdea1.png

LinearBearingIdea1a.png


Some alternative uses of this same idea...
- Space the aluminum angle further apart, so that it runs on square tubing instead of flat stock.
- Use square tubing instead of aluminum angle, so that you bolt the tube to the face of the wood, instead of having to cut a hole. (The hole would be tricky to make with hand tools.)
- Flip the aluminum angle, so that it points out, instead of in through a hole in the board.

It's not a perfect idea, but maybe someone can add to it...
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Re: Bearing / Linear Motion Design?

Postby beermkr » Mon Dec 06, 2010 8:49 pm

Here is a quick picture of what I was thinking. Replace the angle with square tubing.

IMG00277.jpg


I think separate pieces will allow for more adjustment options. Here is my plan for adjustment.

IMG00280.jpg


Obviously my gantry side design is not as easy to cut out but I have some ideas on that too...
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Re: Bearing / Linear Motion Design?

Postby Awesomeness » Mon Dec 06, 2010 8:59 pm

Looks like we were thinking nearly the same thing. :) And it probably took you less time to demonstrate with a real picture, than it did for me to draft it in CAD, haha.

We're certainly up against a tough challenge... to use as inexpensive bearings/parts as possible, but minimize the build difficulty (eliminate fancy slots and keep the number of parts/cuts/holes small). I like where this is going already though!

Slots may not be too big an issue. Since all they do is provide up/down adjustment for the bearings, they don't need to be "precision" slots. They could probably be made by drilling a bunch of holes and connecting them with a chisel or file. That's time consuming though, so I'm sure it would be more encouraging to new builders if they were eliminated. What ideas do you have on that?
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Re: Bearing / Linear Motion Design?

Postby beermkr » Mon Dec 06, 2010 9:42 pm

I actually think I went overboard on my slots and that the could simply be two 1/4 inch holes right next to each other and then connected.
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Re: Bearing / Linear Motion Design?

Postby Awesomeness » Mon Dec 06, 2010 9:52 pm

One of the trickiest parts of these bearing trucks is the bearing that rides the edge of the table (as opposed to the ones that ride on the top and bottom of the table).

I saw some surface rollers, and wonder if they could be used. All they would require is drilling a hole in the side of the board for the surface roller(s), and then smaller holes for the skate bearings that ride the top and bottom of the table.

Image
http://www.vxb.com/page/bearings/PROD/C ... gs/Kit7766

They cost $4, which is far worse than $4/10 like the skate bearings, but far better than $8-10 each for v-groove bearings.
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Re: Bearing / Linear Motion Design?

Postby beermkr » Mon Dec 06, 2010 10:00 pm

That might work for the side bearings. I think that their height and cost are the biggest issues.
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