Hardware store design CNC router #1

[phillby]

Hi,

This end of the torsion box design may be unnecessarily complicated as Spalms design relies on the pipes being stationary, oK, (parallell and Level to the cutting surface} but so is the torsion box hopefully. All the adjustment is on the Bearings which in the case of the Gantry are moved in relation to the Gantry Legs (sides) the Torsion Box cut-outs must support the rail solidly to prevent the slight (but to some unacceptible) flex in the rails. These adjusters of course will do this but so will grouted ( for want of a better term ) with epoxy ends and the rails pulled into place. The end of the torsion box could be like shown with a screw pulling the rail into the opening.

However without a cantelivered end in the torsion box or leaving the bottom open it will be hard to adjust these bolts and reduce the cutting surface for a given size gantry. If the bolts are needed then it is possibly better to use countersunk screws from the outside. Must stay away from the bearing tracks though.

Spalm now believes he has cured the problem by pulling the rails together on the ends with allthread.

[spalm]

Hey guys,

I spent about a month trying to come up with an easy way to build these torsion boxes. The half lap dados are tempting, but just try to get them all in 3 dimension parallel (alignment) before the glue dries. I tried twice, and gave up. That is why is used the individual spreaders. Gerry’s boxes were a lot larger than mine and he used screws to hold them together. If you are trying to build thinner boxes, I found that the parts floated around a lot.

I used the base ply or MDF as the ‘base’. Double check this for square. If it was ¼ inch ply, I used double sticky tape to make it lay flat. Then I glued the first stretcher and checked for perpendicular while using my finger to make sure that it touched the 3 edges of the base. I then spread glue on the first set of spacers and squished them into place (placement is not important). I then spread glue on the second stretcher and squished it into place, again checking for perpendicular and using my finger to make sure that it touched the 2 edges of the base. Repeat this along the entire base. The last set of spacers never came out to the right length, so I custom cut them at this point to make sure that the last stretcher was just touching the far edges of the base. I then let this dry overnight before adding the second skin by applying glue and a weight to hold it in place until it dried.

I did not use a nail gun or other fasteners. Only glue holds the box together.

Just my $.02
Steve

[mikeschn]

The basic construction of a torsion box can be seen here...
http://www.diynet.com/diy/ww_materia...278181,00.html

Chron,

Your cad drawing is a good start. The board is 3" x .75 thick. The upper and lower bolts need to be above and below the board.

I'll start modeling the torsion box this morning.

The other approach is to let the pipe lay on the bottom of the circular cutout, and just use the other two bolts to snug it up. The only reason you'd need the bottom bolt is if the cutouts are inaccurate.

When I built my sample rib, I found it very easy, because precision wasn't critical. And all 3 holes were very easy to drill and tap. Oh, and the drill and tap was available at the hardware store too, for $4.39. Throw it in your drill motor and away you go. I would harden the threads with some CA though.

Mike...

P.S. I threw a few lines on paper, to get a feel for where I am headed. The bolt size changed from #10-32 to 1/4"-20 (not changed on the drawing yet)!

[mikeschn]

Here's the lower torsion box and the skate bearings... I don't want to go any further until you guys tell me that's what you had in mind, and that we haven't created any weak links...

Mike...

P.S. Just added the centering bolts...

[ger21]

Mike, if you'd like, you can come by and take a look at mine.

[mikeschn]

Originally Posted by ger21 Mike, if you'd like, you can come by and take a look at mine.

I'd like that! PM me your address and phone!

Thanks,

Mike...

[Jimmy Southern]

spalm,

I was just reading your post and had a thought. If the pieces are cut like ger's, it should be a simple matter of putting together the outer shell of the box first. If this is square and true when the glue dries the rest should just follow it's lead. Also the assembly will only be as good as the master pattern. The more time spent making the masters perfect the easier the assembly will be. Just my opinion though.

Mike,

I did notice one thing I would change. On the bearing bolts I would go with a flush, or countersunk, headed bolt. This would give a bit more room for the supports to clear. It will also center the bearing on the bolt and take out any play in the bearing itself.

So far I like it!

Also did anyone find the eccentric bolt idea that I was looking for this might be handy for the cad guys if we can get them a picture.

See ya,
Jimmy Southern

[mikeschn]

About the best we could do is to go with a low profile socket head cap screw.

I'd like to see a picture of that eccentric bolt too!

Here's a major question for you guys...

do you want some kind of built in adjustment plate like spalm has, or do you want all your adjustment in the pipe adjustment blocks?

Mike...

[triticale]

I just dug out the pair of drawer slides I knew I'd scavenged some time back. Turns out that they are Accuride precision units rated 150 lbs the pair, with a retracted length of 28 inches. I'm going for a drawer slide design for my first machine.

One question for discussion. I am considering angling the mounts so that the slides are 30 degrees off vertical. This way I can assemble with preload without needing thousandths precision in construction. I'm also thinking that it will deal with sideloads better. Any comments?

[spalm]

Jimmy, I was just pointing out what I had learned. Note that Mike’s link on torsion boxes also uses individual spacers. Both ways work. Just watch out during assembly time. A couple of things that I was worrying about when building these were:

Most people at this level do not have a pneumatic nail gun. I do, but I was trying to build to their level of tools. I was using 1/2” MDF and screws at the cross joints did not work to well. Remember if you use Baltic birch ply, it is metric, so you may have to get metric router bits (or adjust cuts) for a good dado fit.

My most critical alignment was to make sure that the pipe seats were all in line and the box was square. This is a little different than normal box construction as the struts stick out.

Clamps just seemed to get in the way and can add twist to the box that you don’t realize until it’s dry.

I like where this is going,
Steve

[ger21]

Originally Posted by spalm My most critical alignment was to make sure that the pipe seats were all in line and the box was square. This is a little different than normal box construction as the struts stick out.

The top and bottom of my torsion box have a rabit along the edge with the tubes. The cross members are notched to allow the rabbit to fit in. By carefully adjusting the rabit, it can only go together perfectly straight. An alternative would be to cut a small slot in the cross members, and also in the top and bottom. Then use a spline during assembly to line everything up relative to the top and bottom skins. You could do this with a table saw or router table.

[spalm]

Gerry, my point exactly. All I wanted to point out is that these boxes are a little different than normal, and probably require some careful thought on how to assemble them.

Steve