My plasma table design

[m_c]

Having just bought a plasma cutter, I am now looking at building a CNC table for it.

I have been looking at several sites and have started on the basic mechanics of the table.

My plans so far are (see attached sketches)-

Table frame will be contructed using 2"x 2" steel box section(or what size is available locally), and will be large enough to be able cut an 8' x 4' sheet. It will also be mounted on castors to allow easy moving.

For the X-axis, I was planning on using a similar design to the torchmate machine. This would involve having a T-Shaped carriage mounted on bearings, running on CRS (1/2" x 4"), welded on top of a double height of 2" x 2" that makes up the table frame. The carriage would have 8 bearings on the T-side, 4 at either with 2 at either end, running on top off the CRS to support the carrier, and 2 at either end, running along the side of the CRS to guide the carriage along the X-axis. There would also be 2 bearings located at the opposite side of the carriage running on CRS to support the carriage.
Power would be transmitted via a gear rack mounted on the inside of the double box section on each side of the table, with pinions mounted on each end of a drive shaft.

This is as far as I have got. I am open to any suggestions regarding my ideas.

My next problem is the Y-axis. I did consider using linear slides, however, now that I've priced them, I'm looking for other ideas.

What would be the best material to make the carriage from. I know that aluminium would be lighter, but I would prefer steel, due to cost + availabilty. Would using steel make much difference performance or cost of suitable motors?

Any suggestions would be gratefully received.

Moray.

[metlmunchr]

Moray, probably one of the most important design considerations in a full sheet plasma table is to have nothing projecting above the surface which the sheet lays on when the gantry is positioned at the end of the table for sheet loading. In other words, work toward building a TABLE to load the sheet onto, and not a BOX to try to stuff it into. This sometimes requires a bit more design creativity with respect to the frame and gantry, but that extra work is nothing compared to attempting to wrestle full sheets of material in between a pair of rails. Even a sheet of 16 ga, which weighs only 80 lb will feel like it weighs 300 when you find yourself in an awkward position in trying to place it on the machine. Over the long haul, the ergonomics of any machine will be the factor that makes you like to use it or hate to use it , much more so than how perfectly the machine itself may perform.

[Hobbiest]

Is it really necessary to cut a full sheet? What are your intended purposes (i.e. art, automotive, sheetmetal, etc.)? This can help greatly in deciding what size is actually needed, and not just what size is possible, or prefered. I think that pretty much everyone here will agree that hassle, cost, and frustration pile up when you get a table with a working area larger than 4' by 4'. I have been considering on that is 2 by 4, and since most supply yards will shear a sheet free or cheap if you buy it from them, that seems quite reasonable IMHO.

[traveler]

Moray, Metalmuncher has it right. It is a huge mistake to have anything above the level of the table except the bridge (gantry). One thing that needs to be addressed is the weight of the gantry carrying the y axis. any weight that is not absoulutly needed should be trimmed off. This is to reduce the momentum when cutting at anything more than very slow speeds. If the gantry is too heavy and you are using steppers without encoders it is easy to lose a step or two and that translates to inaccurate cuts. This is the reason aluminum should be considered for this part. I bought aluminum at a salvage yard that came from store front windows to build my gantry. The design you are patterning after may be "hell for stout" but is not thought through very well for real life use. I agree with your plan to have a table that will use full 4' X 8' sheets because I often cut pieces larger than 4" X 4' and to index would be a pain as well as potential mis-alignment. Think it through then go for it.
remember my motto "don't set a pace that you can't maintain")
Traveler

[m_c]

Thanks for the input guys. The main reason for making a full sheet machine is I occasionally use full sheets for automotive repairs, and as I've got the space to house a full sheet machine, I may as make it that size, rather than have to cut sheets up to fit them in. The machine will be used for a variety off purposes from cutting steel plates upto 10mm thick for machinery, to the odd bit off thin metal artwork.

Now that I've started the sketches, I'm thinking about altering the height that the metal would sit, so that the table could be loaded with a forklift if needed. I have been looking at the Torchmate 3 machine, and have noticed that they have dropped the height of the support rail that the gantry runs on. I was thinking about doing something similar, however this would mean that the gantry would only be driven from one side, and the slightest bit of play in the guide bearings would lead to inaccuracy with the x-axis (play would obviously become more of a problem the further away from the driven side you are).
The other idea I had, was to make one side like a letter box, whereby there is a big enough gap between the gantry support rail and the holder for the metal, to allow a full size sheet to be loaded.
I've not decided on any particular design yet, but I want to make sure I've thought about everything before I decide. I don't want to get the table built and realise it's not going to work very well.

I'll keep a look out for what aluminium sections are available, as this would probably save alot of hassle.

Thanks,
Moray.

[m_c]

I've just done a rough sketch for a raised gantry.
It's similar to my first idea, however the gantry is raised up, and the rack + pinions have been moved to the outside of the table. Drive would be via shaft running across the gantry, and then to the pinions via a belt at either end. This would allow the sheets to be raised above the x-axis tracks.

[metlmunchr]

Moray, take a look at this page. http://www.data-cut.com/router.html There are lots of detailed pics of Chris's router on the various pages. Although this is a router, the same ideas can be applied to a plasma cutter. You may even find it worthwhile to buy a set of plans from Chris because he's got a lot of the details already worked out and drawn up which can take 2 or 3 tries to get right sometimes on a first try at building a machine. If nothing else, his pictures will give you some ideas about how to build a flat-top machine and the gantry to go along with it. No association with Chris. I just know him from another CNC group and know him to be a knowledgable and helpful guy to anyone with a question he can answer.

One consideration with the letterbox design in cutting heavy plate is the ease of UNLOADING when the parts are cut. 10mm plate weighs about 16#/sq ft, so a 4x8 is going to be around 480 lbs. Often if you've cut a bunch of heavy plate parts, its easiest to first pick up the remaining skeleton and move it off the table, then start removing the parts. Here you'll find the ability to pick the skeleton straight up without worrying about bumping into part of the machine structure can be even more important than when loading. I'd agree it wouldn't be wise to drive only one side of the gantry on a 4 ft capacity machine, but there are ways around this, either via using a cross shaft or by using two motors and slaving one to the other. Instead of focusing on the details of commercially built machines aimed at the hobby customer, it might be good to look at the general arrangement of a Lockformer Vulcan or a Komatsu Rasor machine, or any one of many other true commercial quality machines. You'll be amazed at how similar they all look. That's because it's an operator-convenient design. The structure of most commercially built hobby machines has the look of being designed with one criteria.......cheap.....in mind. Building a machine with maximum utility requires little additional money, but a bit more forethought. The final product will return your investment of additional time many times over.

[m_c]

metalmuncher, thanks for that link and the other info. I've done some more sketches that'll I'll upload once I can get them scanned in.
Here's a quick description of them for just now.
I've come up with a design where the gantry is supported and runs on the rack. The rack will be tack welded to some CRS (1-2" wide by 1/2" thick). Each side will have 2 pinions (only 1 on each side will be driven), to stop the gantry rocking. On one side there will be 2 bearings running on the sides of the CRS so as to locate the gantry on rails, and to stop any sideway movement. The reason for just 2 bearings is they're only to stop side ways movement, and not to hold the gantry square.
All this will be mounted below the cutting table on the inside off the gantry legs, so the gantry legs can be relatively close to the edge of the cutting table.

Also, each side will have it's own motor, mounted on the outside of each gantry leg, with drive transmitted to the driven pinion via a belt. I think this would be a far simpler (mechanical) design than trying to transmit drive to both sides using just one motor.

Thanks
Moray

[InventIt]

I wouldn't worry too much about the weight of the gantry. Most cutting will be done less then 100IPM and a good 400Oz stepper will move 70+Lbs fast enough (300IPM+)

What are you thinking of using for drives? Geckos? Rutex?

Steppers or Servo?

Is this a dedicated Plasma Only machine? Or will you be tempted to mount a router to your gantry also?

[metlmunchr]

I tend to agree with Traveler's comments about keeping the gantry weight to a minimum. The ability of the motors to accelerate or drive the gantry is but one part of the equation. The heavier any object is, the more resistance it puts forth to any change in the direction in which it's moving. The dynamic response of any high speed cutting machine is an important element of design. The design should aim to minimize weight while maximizing stiffness. This results in a gantry with the minimum of deflection inducing inertia, while at the same time maximizing its inherent resistance to deflection. A side benefit comes with the ability to use smaller (read that cheaper) guideway and drive components. A person who attempts to build a machine with a maximized design should look at "equivalent sections" a bit. It's surprising to learn how one section, via its geometry, can weigh half as much as another, yet be twice as stiff.

[m_c]

Here's the scanned drawing of what I described a few posts ago.

I've been thinking about the gantry, and have decided it would be best to make it from aluminium. I know the basics about using different shapes of materials to maximise strength/rigidity (ie. box section is stronger than using a solid bar of the same cross sectional area). I'll have to see what different aluminium extrusions I can source locally, before I decide on the final design.

As for the drives, I've still not decided on anything. I have been trying to locate UK sources for steppers/servos/drives to find out what is available and approximate prices. This will be the next problem I face as all the power/control side is completely new to me.

The machine will be for plasma cutting only.

[m_c]

Having listened to the comments made about my previous designs, I have taken them on board and have come up with the attached design.

All grey shaded parts will be aluminium (at least 10mm thick, will probably use 12mm though). The guide plate will be approx 12mm x 75 or 100mm CRS. The CRS will be mounted on top off the 50mm box section used for the table frame.

The plate that the motor + pinion are mounted on, will be spring loaded, to keep the rack+pinion in constant mesh and also to allow for slight variations in the rack. I'm not sure how this will be achieved, however was thinking about machining groves in the gantry and the motor mount plate, to allow the motor mount plate to run on some ball bearings, and then somehow have spring tension to keep the motor mount plate held tight to the gantry.
The pinion and motor will be connected via a toothed belt, to allow a suitable drive ratio to be selected. The motor will be mounted on the outside off the gantry to allow for easier maintainance, and aslo to allow the gantry to be nearer the table.

The support rollers will be mounted in the gantry base plate, and the guide rollers will be mounted in the gantry side plate. The rollers on the underside of the guide rail will only be there to prevent the gantry from being knocked over, as the weight off the gantry should be enough to hold it upright.

There will be guards placed over the mechanicals, to help prevent any plasma dust from falling in. I was also thinking about mounting wipers onto the ends of the gantry to keep the guide rails clean.

The attached drawing is only for one side of the gantry, as the other side will only need support rollers, and the motor + drive components.

I hope everyone can understand this, if not just ask and I'll try my best to explain. As always, I'd be very grateful for any comments.