# Thread: Short beam/Long beam? - Basic design

1. ## Short beam/Long beam? - Basic design

Conventional design of moving gantry type m/c s is generally type A in my sketch below - short beam - nice and rigid and within reason - the long axis can be quite long...

I like to push things or at least try!

so type B in my sketch below has the long axis in the other plane and means a longer and bit bigger section beam is used... I'm wondering about going down this route perhaps myself - I'll make clear that I intend to use two Acme screws each end of the long beam.

what are the pro's and cons of these basic designs?

A long beam means you can hang more routing heads on it? more accessible ?

2. Good question, two things come to mind;

As the beam increases in length so does its weight which might be a factor in motor size, acceleration etc.

As the beam increases in length the further apart the bearings on the base axis need to be as the moment arm increases.

Andrew

http://www.cncrouterstore.com/detail_info.php?num=105

4. Pat

I've done exactly what you are suggesting.

My y is 1.5M; my x is 1.1 giving a cutting envelope of 1200*800.

Why? because I can easily convert my machine into a 1200 * 2400 by bolting together three frames 800 + 800 + 800 = 2400. All I have to do is build the frames and longer timing belt for both sides of the x axis. The gantry and head assembly remain unaltered.

Having a 1.5M Y will cause sag in the middle as I have currently built it, but I have designed it so that I can introduce an array of measures that will strengthen it. (multiple torsion boxes).

Because torsion boxes ideally require a CNC to cut all the pieces, I will use my machine to do that. As the machine will sag in the middle, I will cut the torsion box parts away from the middle in order to reduce/remove inaccuracies

My current y axis is two parallel 40*40*3 mild steel square beams with the head assembly between them. On the beams is stainless steel tubing with the trucks assembled using aluminium section. The tubing is held onto the beams using angle iron and flat plate.

Downstream, I can put torsion boxes under the beams and I can strengthen the insides of the beams through designed inserts.

If after that, I still have a problem, I can introduce a more comprehensive solution.

The challenge is the fun.

My wife tells me that after I have built my machine I probably won't use it but I will have enjoyed getting it to work.

I hope that helps.

Andy

5. right, I think I'm with you Andy, you've got that shape so you can simply lengthen the router in the future, thanks, worth bearing in mind.

6. Pat

That's right.

Andy