[Josh*k]

I'm contemplating increasing the cutting envelope size up to 24" x 36". I would like to avoid driving both sides of the gantry, so am intending to use an integrated tension cable system such as discussed here: Make your Gantry rock solid!. It's always amusing to think one has an original idea, then discovers shortly afterwards that it's already been done...

Has anybody integrated the above mentioned cable system into a Momus gantry? I am intending to use a like-sized brass rectangular tube for the gantry, which should be more than adequate to maintain stiffness for the extra 9" of length added. I'm also intending to add an extra 4" of length to the gantry side bearing blocks, which will widen the bearing footprint and aid in reducing gantry racking.

Any thoughts...?

[the_tool_man]

Your link doesn't go where you say it does.

The packaging machinery company I work for makes a device that uses a cable system to keep both ends of an assembly aligned as it moves. It works quite well, and lasts literally millions of cycles at rates up to about 60 strokes per minute. So the concept is certainly valid. However, in our case, the accuracy requirement isn't nearly what a CNC router would need. I wonder what the extra mass of the cable and pulleys would do in a transient situation? I suspect it would work if you use good quality pulleys, like those used in aircraft control cable assemblies. Even so, there will be an initial break-in period where the cable will stretch a little. So you'll have to build in some tension adjustment and keep it adjusted with some frequency. Fix your link so we can see the system you're referring to.

I wouldn't use brass for the gantry tube. Depending upon alloy, you get 1.5-2 times the stiffness at 3 times the weight. You're better off increasing the size of the gantry tube by an inch. For example, if you're worried about vertical deflection, going from 1-1/2 x 3 to 2 x 3 will almost double the vertical bending stiffness, and increase torsional stiffness by about 40%. Such a change only adds minimally to the mass. Of course, this ignores the significant vertical stiffness contribution of the steel guide bar, which would also have to get larger and heavier.

Widening the bearing blocks would also help with the gantry alignment. But, if you're going to go the cable route, you probably don't need it. Keep in mind, your base and X rails will have to increase in length for the given work envelope, so it's definately diminishing returns at some point.

[Josh*k]

the_tool_man: thanks for your reply.

Weird. Here's the same link I posted above: http://www.cnczone.com/.../diy.../51...ock_solid.html

I opted for the brass tube for the gantry arm as it can be acquired in the same size and price as the spec'd AL piece. So no additional changes would be needed for the remaining assemblies. Also, the plans indicate that the modulus of elasticity for brass in 170% stiffer than AL, and it would only add around 4 lbs to the gantry. I figured it would be a simple improvement...

[jim_r53]

Your new link doesn't work either.

[ger21]

Try this.
http://www.cnczone.com/forums/diy_cn...ock_solid.html

[Mac.CNC]

Here: http://www.cnczone.com/forums/diy_cn...ock_solid.html

I've came across this a while ago too - by accident, but immediately thought it would be much, much simpler to just slave a 2nd motor in as Bob suggests, but that is just my opinion....
--
Mac

[Mac.CNC]

I was just thinking about this again and I'm beginning to wonder why the steel cable setup really needs to be present in something like this. The space to put it is a commodity too..

What about this?
We already have a reinforced belt in play that can be purchased in unlimited length. So why not use a system of skate bearing pulleys to bridge the existing x-axis belt over onto the other side of the Momus and then use an inverted belt plate to clamp the other side of the gantry onto on to that belt? Basically running the belt down one end of the machine, then going over the other side, using secondary idlers and running it all the way up the other side with another idler?
This would still run off a single stepper motor and negate the racking on the gantry, in theory.
That's an awfully long belt loop in the end, which might expose another problem, where it will force the two side walls to want to fold into each other.
Some special bracing would be required between the walls where the belt crosses over, apart from general concerns about the stretching properties of the belt...
Just some food for thought :-)
Mac

[the_tool_man]

Reverse-bending a timing belt shortens its life significantly, unless it is designed for that. I'd make the idler pulleys bigger to reduce the stress on the belt.

[the_tool_man]

I didn't read the entire linked thread. But I assume from what I saw that the cable system would have to traverse the gantry. This would attempt to pull the base walls toward each other, correct?

The arrangement I've seen is more like what I've pictured below. The ends of the gantry would attach to the cables at opposite sides, one high and one low. It still pulls the base walls toward one another where the cable crosses. But this crossover point is fixed at the back of the assembly, where the rear wall of the enclosure can resist it.

Attachment 197174

[Josh*k]

I don't think it would pull the base sides inward, as the fixed pulleys are mounted to the gantry, and rotates the cable tension around to run parallel with the side walls. This tension is then in shear load with the side walls.

This seems so much easier and less expensive than needing electronics for a slaved fourth axis.

[bearded]

If you use cut-to-lenght belt, couldn't you turn it like a mobius strip to eliminate reverse-bending forces?