[JerryBurks]

I am quite satisfied with my moving gantry machine (see the Big Bamboo thread) but once a while I want to do something bigger than the 34"x31"x8" range allows for.

I surely can not set up a full size 8'x4' machine but was wondering if I could do a 8' or maybe 9' by 31" moving sled as a compromise. That would be a removable table, maybe on skate bearing slides that is clamped to the extrusions of the main table. While the gantry moves left (-x) the sled would move right (+x) and vice versa. I might even use my rotary A-axis stepper with harmonic drive reduction to actuate this sled (steel cable or belts). The Planet-CNC controller allows for slaving this 4th axis to "x" but I need to check if it can do that with different steps/inch. When I don't need the long travel I would just take the sled off and store it vertically in a corner of the shop.

Anyway, it looks like a wacko compromise but has that been tried before?

Thanks, JB

[PaulRowntree]

Jerry, I remember seeing a concept drawing of a belt driven system, with the gantry attached to the lower side of the X belt, and a sled attached to the top side of the X belt. It would double the effective X range, albeit at the expense of resolution. And it would take long belts to get your sizes ...
Cheers!

[RomanLini]

I think you main problem would be overhang?

A moving table machine already cuts to the edges of the table, which must be the edge of the bearing supported area.

So if your gantry moves past that you are cutting stock that is hanging over the edge of the table or at least over the edge of where the table bearing supports are?

[JerryBurks]

I think you main problem would be overhang?

A moving table machine already cuts to the edges of the table, which must be the edge of the bearing supported area.

So if your gantry moves past that you are cutting stock that is hanging over the edge of the table or at least over the edge of where the table bearing supports are?

Overhang is surely to be considered and the sled table would need to be pretty rigid to avoid sagging. However, the cutting point would never be in the overhang area because the the spindle can not move beyond the main table.

The 80/20 extrusions that make the main table are actually 48" long, quite a bit more than the usable x-range of 34" which should help. In full extension, the sled would hang out about 5 feet from the table. The linear guides obviously would have to deal with that. But at over 500 pounds I guess the machine is not going to tip over.

[harryn]

I have wondered about the same challenge for cutting full size 5 x 9 ft sheets. If you are not going to use the longer setup that often, then consider to make the table removable.

Example I have considered:

Main Frame
- Fixed gantry style frame roughly 3 ft high x 7 ft wide x 3 ft deep
- Double as a work table when not in use.
- Frame is made so the table is within 12 inches of the floor.
- Main frame is placed on a poured self leveled epoxy floor surface large enough for the entire length - so approx 20 ft. long x 6 ft wide.
- 3 - 4 IVT bearings mounted along width of frame to manage local flex near the cutting area. (along the Y direction)
- Y axis moving up and down for the Z motion (not mandatory, but possibly allows more depth with less flex)

Moving Table
- main table made from a large sheet of polymer honeycomb and reinforced edges with 80/20. Al honeycomb will "ring" a lot, so I suggest the polymer type used for sound deadening.

- Rail system pacific bearings IVT type rails mounted on table base for support
- Drive system using rack mounted on the moving table and pinions on main frame
- Some kind of rubber wheels (perhaps urethane skate wheels) mounted to the ends of the table to "approximately follow" the relatively flat epoxy floor surface.

The final precision of the table flatness will come from the area where the cutting occurs, not the epoxy surface, but this helps manage the table flex as it extends out.

It still requires that you can store a table roughly (desired cutting length + 3 ft) x 5 ft, so it isn't exactly small.

[dodger889]

Jerry the best you could do is setup an indexing system with 4 pins. This will allow you to move the work from one set of pin to the next set. And this is going to have to relied on your repeatability of your machine. This would be the how I would approach it setup and indexing program first then do the cutting program second. Index the materiel first with the pins using 4 pins and 4 holes per section to be cut. This should keep you cuts alined and smooth.

[ger21]

I think what you want is similar to the ShopBot Buddy with PowerStick.

http://www.shopbottools.com/videos/B...%20320x240.wmv

They also point out some limitations of this method.


Tempering Expectations for PowerSticks with a Little Realism

The Buddy with PowerStick is a whole new concept for flexible CNC. We have a number of applications in mind for this new flexibility but we believe that even more interesting applications will arise as ShopBotters begin to put these tools to use and develop creative new approaches and techniques in their use. That means we can give you some ideas about where to start and issues that may arise, but also realize that you are an early adopter of a new type of CNC tool having new and un-explored opportunities and possibilities -- and there’s a lot still to be worked out.
It is true that some pretty big material can now be cut on little tools with PowerSticks, and that a small tool can now be used for a big job. But a PowerStick will not replace a table and gantry tool for intense commercial applications such as cabinet production from full sheets. A PowerStick does not offer the same rigidity or perfect flatness afforded by a gantry CNC router for cutting full sheets of plywood. Additionally, it does not offer the space for installing an industrial vacuum hold-down system in the way that a gantry table offers. A PowerStick tool will require almost twice as much floor space during the cutting of a large project because the entire panel must move to each side of the Buddy for full cutting.
When deck material, sacrificial spoilboard, and project material are added to a PowerStick, the X-axis can become heavy. This will place a large load on the X-axis motor. Particularly with PRSstandard Buddies, be careful to avoid pushing the PowerStick beyond the motors’ capabilities and causing it to miss steps. Although this is less of a problem with the more powerful PRSalpha Buddies, be aware that when large loads are placed on the X-axis, mechanicals and speeds should be adjusted downward accordingly. In any case, please note that it is dangerous to move the tables too quickly as persons standing around the tool may not be expecting the table to move towards them at a high rate.

[louieatienza]

I don't see how this frees up shop space, as you'll need at least double the space to allow the table to move. Which is a primary reason why we make moving gantry routers.

But for one-off stuff, it might be cool to have a moving table systen that bolts to the machine, needing just a swap of the long axis motor cable.

[harryn]

I don't see how this frees up shop space, as you'll need at least double the space to allow the table to move. Which is a primary reason why we make moving gantry routers.

But for one-off stuff, it might be cool to have a moving table systen that bolts to the machine, needing just a swap of the long axis motor cable.

For a pro shop, it probably does not make sense. For my own DIY hobby goals, it makes perfect sense. My "shop" is the amount of space open in the garage at any given time. This changes a lot, depending on if a car is parked in there or not.

I already have to move the car outside to cut a piece of plywood with a circular saw while kneeling on the concrete and goofing around with 2x4s to space it off the ground, so if the self leveled pad is where the car usually is, it all could work. (in theory)

Another option of course, is to consider making one axis of the table vertical and see if that would help with the footprint problem.

[JerryBurks]

I don't see how this frees up shop space, as you'll need at least double the space to allow the table to move. Which is a primary reason why we make moving gantry routers.

But for one-off stuff, it might be cool to have a moving table systen that bolts to the machine, needing just a swap of the long axis motor cable.

That is true, but the idea is to roll the machine out of the shop (a.k.a. garage) for such bigger jobs. I have all the space in the world in my driveway.

I think what you want is similar to the ShopBot Buddy with PowerStick.

Interesting, have not seen that but it comes actually close. But that machine has a moving table and a fixed gantry anyway and they just replace the short x-drive with a longer one. For my concept I would take advantage of the 34" movement of the gantry, so that the table only has to move another 62" for a full 8 foot machining range. To allow for a reasonable load on the linear bearings I could e.g space them also 30-40" apart and get away with an 8-9 foot long sled table. I should probably draw that sometimes.

The cost would be reasonable. I was thinking of a 30" x 80" composite slab door as lightweight (and cheap) stiff table, and maybe 8.5 foot long linear bearings from Glacern ($300 a set).

Obviously that is still too narrow to slice up a full 4x8' sheet but at least I could do a lot of cabinet and other furniture parts. I know that can also be done with tiling and dowels but it needs a lot of additional planning, measuring and fixtures and can lead to expensive mistakes.

[louieatienza]

That is true, but the idea is to roll the machine out of the shop (a.k.a. garage) for such bigger jobs. I have all the space in the world in my driveway.





Interesting, have not seen that but it comes actually close. But that machine has a moving table and a fixed gantry anyway and they just replace the short x-drive with a longer one. For my concept I would take advantage of the 34" movement of the gantry, so that the table only has to move another 62" for a full 8 foot machining range. To allow for a reasonable load on the linear bearings I could e.g space them also 30-40" apart and get away with an 8-9 foot long sled table. I should probably draw that sometimes.

The cost would be reasonable. I was thinking of a 30" x 80" composite slab door as lightweight (and cheap) stiff table, and maybe 8.5 foot long linear bearings from Glacern ($300 a set).

Obviously that is still too narrow to slice up a full 4x8' sheet but at least I could do a lot of cabinet and other furniture parts. I know that can also be done with tiling and dowels but it needs a lot of additional planning, measuring and fixtures and can lead to expensive mistakes.

If you have another 1515 screw, you could just convert your gantry to single stepper running both screws, with 2:1 reduction and keep the slave axis. You'd then have to decrease the steps/in on that axis (probably about 2/3 what it is now.)

[harryn]

There is another possible approach to having a large cnc router with limited garage space - build it inside of an enclosed trailer. 20 + foot versions are pretty common.

There are some reasonably priced, 2 axle trailers used for moving cars and horses on the market. It might be feasible to reinforce one of these sufficiently to hold a full size router. If your driveway is large enough, you could store it there, otherwise, for around $ 100 / month, it could be kept in a storage yard until needed.

In theory, the trailer could also act as the dust and noise enclosure as well. With some planning, it might all fit and free up some garage space as well. It might need a "park position" for use when moving the trailer around to prevent fretting of precision parts.

[JerryBurks]

.............build it inside of an enclosed trailer. 20 + foot versions are pretty common...........

Excellent idea but I suspect this would be way overkill for my occasional one-off jobs for large parts. Actually my budget for such a large table is rather like $200-300.

As for Louie's proposal I would not need to disconnect the second x-stepper because they are mechanically synchronized and not electrically slaved. I still have the 4th axis output available that I normally use for a rotational axis but that can be configured as a slave. If I make an 18" sheave and install it in the rotary axis chuck, it could drive the table sled with a steel cable at twice the speed of the gantry (in opposite direction) effectively tripling the x-range. But I am not sure if the small 180in-oz stepper and 1:50 harmonic drive for this rotary axis would be strong enough to haul the large table. I would probably have to reduce the acceleration quite a bit.

[JerryBurks]

Thought about it again and I believe I can not use my rotary axis drive because of backlash and gearbox elasticity. It is just not made for this and would lead to lousy positioning.

I will probable even simplify this and drive the table sled with a leveraged pulley/cable system directly from the gantry. This will surely be much less accurate than the bare machine but for such large parts probably good enough. I am going to set up an experiment after the holidays and if it works I will post in the Big Bamboo build log thread.