More Z Please

[Design Dude]

Last weekend I moved my 48x96 PRS out of the cramped basement and into the detached shop that i have been working on for way too long. With the machine taken apart I got to thinking about modifications that would be really convenient to make while the bot was in pieces. I have been doing a lot more 3D work in foam lately and figured 3 more inches of Z would really open up some capabilities. So I purchased two aluminum T-slot extrusions and went to town.

I raised the original 6" rails up two slots on the legs and slid the 3" new rails under them and bolted up to the legs. So i have 4 bolts in the lower two slots of the 6" rail and four bolts in the two slots of the 3". I feel like I need to add a few splice plates that have two holes for each slot in them (12) but it feels just a rigid as before.

I am aware from reading everything i could find on the forum about extended Z, that i am going to have more bit deflection when cutting sheet goods and that a torsion box is in order to raise up the cutting surface. I was thinking this would be made out of 1/2" Baltic birch with ribs 8" on center with an overall height of 4". Do you think i need a full sheet skin on both the top and bottom. I am concerned about the weight of this thing and moving by myself.

Anyone using this type of setup? How about those of you with vacuum hold down systems. I am thinking ahead about how to incorporate a vacuum hold down with this type system. Seems like Brady Vac might be a contender. I could put pass through ports in the torsion box and use the same vac fixtures with or without the torsion box.

That's what I got. Any advise welcome. You guys have been super helpful in the two years I have been lurking this forum. Thanks in advance.

[myxpykalix]

As far as the design and construction of the torsion box, i think you need to have a sheet of plywood on top and bottom. Others may disagree.

I'm unclear on how you would distribute the vacumn with the torsion box. How I would see it is that the top sheet of plywood on the torsion box would be where you cut the design of your plenum and your mdf would attach to that.

Maybe to make it movable you attach the plenum and the mdf as one piece that then removes from the torsion box itself.:confused:

[steve_g]

Chris

Without the 2nd sheet of BB on the bottom, it's not a torsion box... just an insert. If I understand your Idea correctly... you'll also need the second sheet to seal the unit and make what will become a giant plenum for vacuum. My next concern is if 8" on center ribs is adequate to prevent deflection of the top and bottom between ribs while under vacuum forces...

A few 1" holes in the center of each rib should be more that adequate to distribute the vacuum throughout the box, but you will still need a sacrificial top sheet, adding still more weight.

I have a 12" Z and have been thinking about something similar, I'll be watching to see if anyone has tried it yet!

Steve

[myxpykalix]

Steve,
If i understand you correctly you mean that the box itself would become a big suction box? I can't see how you would get much of any good vacumn if you have to encompass this big box?

My thought was that you would go thru the bottom layer of ply to the underside of the top layer where you would have fittings that attached to pvc pipes that attached to the plenum on top and ran thru to the bottom where they would be hooked to the vacumn system.:confused:

[steve_g]

fittings that attached to pvc pipes that attached to the plenum on top and ran thru to the bottom where they would be hooked to the vacumn system


Don't forget, the purpose of the whole thing is to quickly and easily remove it and use the extended Z feature!


I can't see how you would get much of any good vacumn

It would work like a huge reservoir or buffer.

My thought was just have a fitting on a side and be able to tap the top anywhere as a source... No matter, it's still going to weigh more than I want to deal with.

Steve

[wowhuh.mike]

I also have a 12" z and have been grappling with how to solve all the issues addressed above. One additional potential solution I have considered for cutting sheet goods is to bolt brackets into extrusions and basically recreate the table surface and cross supports closer to the gantry. Biggest concern would be how to switch out quickly and if surface would need to be decked each time. I wonder if you could make the raised table surface slide out of the way when using full Z travel? Similar to gantry style machines where table moves in x axis.

I use lighthouse motors bolted directly to bottom of current table. Was thinking I could use hose to swap between setups??

[Design Dude]

The way i was thinking about it was inspired by this (http://www.shopbotblog.com/index.php/2006/11/a-removable-vacuum-plenum-that-maximizes-hold-down-potential/) article from Brady. If i understand correctly all he is doing here is plumbing the vac source to a hole in base sheet of his table. The vac grid is then cut into one side of ultra light mdf and placed grid side down over the vac hole. Then the top side is surfaced and serves as the spoil board. Single piece vac plenum and spoil board. Pretty slick. So when you add a torsion box to the mix you would only need to provide a pass through pipes in the box that align with the vac holes in your base sheet effectively just making the vac pipe 4" longer. All this would need some clever gasketing and such, but you think the concept is sound?

Steve, given that the vac forces are only developed between the top sheet of the torsion box and the vac plenum sitting on top of it, you think the 8" ribs are still good, or should i try for even less say 12" o.c. to lighten up the weight?

The box if made from a 1/2" sheet of ply top and bottom with 8" oc ribs (also 1/2") it will weigh at least as much as a sheet of 1" plus the weight of the grid. Not too intimidating as long as you are sliding on and off of a cart.

[steve_g]

Chris

I wouldn't go with rib spacing more than 8", I imagine that at some combination of cut speed and chip load you're going to get a harmonic resonance, effecting both cut quality and noise level. My thinking was that by pulling a vacuum on the whole spacer you would stiffen it some, the spoil board would still be done the same way.

If you have a room for a cart to slide the whole thing on and off that should work well. I can't imaging not needing to get the thing vertical for storage though...

Steve

[danhamm]

When I modified my prt to the prs design, I made it so it would range from 6 inches to 12 inches.It works very well.
It allows me to do Thick fireplace mantels and use long bits for thick work,then in minutes go back to normal usage..

[Design Dude]

I agree with your thoughts on the 8" spacing. I will go with my gut on this one. Hopefully i'll have a few inches of space under the bot to store this thing. If not it can go vertical with the rest of the sheet goods in the shop.

Dan, do you use a 6" thick torsion box to do this?

[danhamm]

No my gantry has 6inches of movement.

[wowhuh.mike]

so you unbolt and move your beam? how long does that take? sounds like a tricky job for one guy.

[danhamm]

Not at all, just use a small bottle jack and it takes less than 5 minutes, just have to pull the motor cable through and reinstall it.

[Design Dude]

That's actually kind of awesome. Have you had issues with keeping the spindle square to the table surface? Do you have the bolt holes indexed some how to maintain square?

[danhamm]

I havent had any issues with the spindle being out of square, but have only changed it 2 times once down to cut a alu sheet, the other time up to 3/d cut 3 mantels...