Not sure how to drive it …I don’t see any way a ball & screw will work….some Kind Of rack & pinion…is weight is enough, will I need any thing—can just run it on a rail on the floor.don’t know..How are the big machines made… the plates I am cutting are not more then 1/4 inch thick…. Is there any kind of special cam soft ware am I going to need to handle that size of table.

There is one manufacturer (Esab) that uses a floor mounted rail and rack and pinion drive, it has a fairly huge gantry, the operator actually sits at the console that is mounted on, and carried with the gantry.
Most commercial machines of this size use Servo motors with a master and slave servo either side of the X axis. And use a rack and pinion on the X.
There is one older table that was made by Linde that had a gantry that weighs around 350lbs and is driven only on one side by rack & pinion, the method used to keep both sides of the gantry tracking is the use of small gauge railway type rail both sides with a axle from one side to the other with two 8 inch dia. wheels riding on the track, this works suprisingly well due in part to the large mass of the gantry.
In the past a couple of the major US players, MG sytems and Koike-Aaronson used the Acroloop PC based card with the DOS version of Acrocut, this makes for a great retrofit as the software was designed specifically for gas and plasma cutting.
If you want to set up a table of this size, it might be worth looking out for an older Linde to retrofit, they can be picked up cheap as originally they were Gas Trace cutting machines. The last one I retrofitted was 10ft x 60ft.
Al

Do you know the price of the Acrocut software and the motion card?

Thanks,
Alex

The ACR2000 which has an on board PLC and 24vdc opto isolated i/o is around CAN$2000.00 it will run the Acrocut DOS s/w http://www.compumotor.com/manuals/Acroloop/AcroMILL_Cut_DOS_OEM_Supplement.pdf on a 386 if need be.
If you have a large amount of i/o you would need the expanded i/o board.
Unfortunately, Acroloop has been taken over by Parker-Hannefin and most of the guys that knew the software have seem to gone by the wayside.
I have been involved with Acroloop products for quite a few years now and were sorry to see the drop in support from the older software. They were also in the process of developing the AcroMill s/w, both in DOS and WinNT, they now have a new company Axiom, designing the software called MotionMAX, at the moment it is just for CNC mills under win2000 & XT.
I have used the AcroMill DOS software and apart from a couple of bugs it has in the G41/42 and feed overide, it was showing great promise as a retrofit s/w.
I am not sure wether you can still get the AcroCut for DOS as although they still show alot of the products on their web site, I believe they are pushing the new Win s/w.
The DOS software was a one-time buy so it is pretty cost effective.
If there is anyone that uses the Acroloop software, I would gladly offer any help in setting it up.
Al

I would sugjest looking into OneCNC Profile version of Cam Software
Dont spend 2000.00 for dos based software
not shure what the cost is but dont think its more than 2000.00
as for a cheap way of driving the table for plasma cutting
how about using belt or steel wire loop to drive the 30' axis
with V grove rolers
that is how the older plotters are driven
rack and pinion will cost a fortune.

I would suggest looking into OneCNC Profile version of Cam Software
Dont spend 2000.00 for dos based software
rack and pinion will cost a fortune.
The $2000.00 is for a motion control card which offers many more advantages over a serial port software driven system, the card operates the CNC control independantly from the PC or operating system, the PC is just used for an operator interface.
The cost of the rack and pinion to do the 60ft table was a little over US$1000.00 which I would not call a fortune, for the table mentioned in the first post that for a travel of 30ft, I assume is going to be used for commercial purposes.
Al

How do high tension timing belts compare to rack and pinion for an 8 to 9 ft table?

I was thinking of driving with them servos from either side. I read that using two half power servos either side of a gantry notably increases it's acceloration over one big servo.

Best wishes,
John

The $2000.00 is for a motion control card which offers many more advantages over a serial port software driven system, the card operates the CNC control independantly from the PC or operating system, the PC is just used for an operator interface.
The cost of the rack and pinion to do the 60ft table was a little over US$1000.00 which I would not call a fortune, for the table mentioned in the first post that for a travel of 30ft, I assume is going to be used for commercial purposes.
Al

Where did you get the rack and pinion you mentioned, the places I have found, Only have rack in 2’-6’ long ….

Can any Drive software or g-code conversion software be told to make
enough steps to go the entire length of the table without having to sacrifice detail.

What I am doing is building boat…. not for commercial purposes…. I already Have the steel, some there on the order of 19-20 sheets of 8’x30’ something like 28,000 lbs of steel, cutting all 300 pieces by hand seems like a near impossible task.. I sure it could be done…. all of my pieces are nested on one AutoCAD drawing with all the sheets separate not sure how to separate them. Cross that bridge When I come to it…

Was thinking i could use the geckodrive..or the rutex stuff ??? it really dont have to be user friendly i will be the only using it..

The rack I used was from Boston gear http://www.bostongear.com/literature/index.asp
L2012-6 6ft rack and YD28 spur gear.
It sounds like you just need a table for the one project? It might be worth while looking at getting the local steel supplier to cut them for you, the time and effort to build the table will probabally equal the cutting time by hand.
Al

Can any Drive software or g-code conversion software be told to make
enough steps to go the entire length of the table without having to sacrifice detail.

.
I have retro-fitted a table as long as 80ft and if you keep the rack and pinion meshed with a slight pressure, and calibrate the system accurately, there should be no problem with accuracy.
Al

I have retro-fitted a table as long as 80ft and if you keep the rack and pinion meshed with a slight pressure, and calibrate the system accurately, there should be no problem with accuracy.
Al

Hi Al,
I'm planning a 6ftx20ft table, I need some advice, Im planning to build a heavy steel table, and a light wieght gantry, to drive a plasma torch. My idea is to drvie the x,y with rack and prinion (boston gear), I don't know what to use if Vee Bearings ridding over steel channel or Cam Folloers over a rectified steel flat on both sides od the x of course. Next to choose the vee bearings or cam followers, and next what kind of motors (and torque needed) and drive bards...I don't know to go for servos and geckos or rutex or to go for steppers...Could you help me figure out this doubts? And what do you mean with slight pressure in the rack and pinion, do you mean like a spring mechanism appling pressure to the rack from the pinion? as in the shopbot design?
Thanks....
Hector

The motor size etc depends on the projected weight of your gantry and the acceleration and feed speed you can/want to achieve.
For meshing, I used a motor tensioning product by Rosta It can be seen in the photo's as the pivot point for the motor assy. It will be most probable that you are going to need some kind of reduction on the drive, I used NOS Bayside planetary gearboxes off eBay.
Pic's 1 & 2 show two different machine X axis.
Al

The air cyclinders seen in the photo's are because the user wanted to retain an original feature, because of the long travel, when the cyclinder is operated it disengages the rack drive and the gantry can be pushed alon the rail by by the operator.
This is the Y axis.
Al

The motor size etc depends on the projected weight of your gantry and the acceleration and feed speed you can/want to achieve.
For meshing, I used a motor tensioning product by Rosta It can be seen in the photo's as the pivot point for the motor assy. It will be most probable that you are going to need some kind of reduction on the drive, I used NOS Bayside planetary gearboxes off eBay.
Pic's 1 & 2 show two different machine X axis.
Al

Well, the maximum speed that the plasma can cut is 380 IPM thats the maximum speed that the table can go in cutting, I dont have a clue what will be a good positioning speed, could you give me feedback on this? that way I can pick my reduction ratio for the servos, I want to use timing belts on that matter. that Rosta product, by any chance do you have the part number, I got to Rosta web site, but I can not see which component you are talking about...
About calculating the motors torque do, you have a formula to calculate the dynamic force applied upon the gantry?
Regards...
Hector
Regards...
Hector

Well, the maximum speed that the plasma can cut is 380 IPM thats the maximum speed that the table can go in cutting, I dont have a clue what will be a good positioning speed, could you give me feedback on this? that way I can pick my reduction ratio for the servos, I want to use timing belts on that matter. that Rosta product, by any chance do you have the part number, I got to Rosta web site, but I can not see which component you are talking about...
About calculating the motors torque do, you have a formula to calculate the dynamic force applied upon the gantry?

The Rosta parts are DR-A27X100 BR27 & WS18-27
If your cutting at 380 ipm then I would aim for a max of at least 500 or 600ipm.
Commercial plasma machines run around 1000ipm max.
There is an industry standard that it is considered normal to keep the inertia ratio of the motor to final load to under 10:1 the deciding factors are motor, all gear/speed reduction components, the weight of the final load and the acceleration required.
It is a good idea to obtain one of the free motor sizing software's available from many of the motion/servo motor manufacturers and play around with it, plugging in various numbers. One of those is Kolmorgen http://www.electromate.com/technicalsupport/?c=kollmorgansoftware
There are others, this allows you to get a feel for what you are looking at for final sizing.
Increasing your reduction ratio greatly improves your inertia factor, but if you go too high, you will need a very high motor rpm to get the max feedrate you want. To grasp the effect of inertia, Imagine cutting a square at 380ipm with a 200lb gantry, the Y axis has completed it cut, now the 200lb gantry has to immediatly travel at 380ipm in the X with, ideally, hardly any visible sign of acceleration.
Al

The Rosta parts are DR-A27X100 BR27 & WS18-27
If your cutting at 380 ipm then I would aim for a max of at least 500 or 600ipm.
Commercial plasma machines run around 1000ipm max.
There is an industry standard that it is considered normal to keep the inertia ratio of the motor to final load to under 10:1 the deciding factors are motor, all gear/speed reduction components, the weight of the final load and the acceleration required.
It is a good idea to obtain one of the free motor sizing software's available from many of the motion/servo motor manufacturers and play around with it, plugging in various numbers. One of those is Kolmorgen http://www.electromate.com/technicalsupport/?c=kollmorgansoftware
There are others, this allows you to get a feel for what you are looking at for final sizing.
Increasing your reduction ratio greatly improves your inertia factor, but if you go too high, you will need a very high motor rpm to get the max feedrate you want. To grasp the effect of inertia, Imagine cutting a square at 380ipm with a 200lb gantry, the Y axis has completed it cut, now the 200lb gantry has to immediatly travel at 380ipm in the X with, ideally, hardly any visible sign of acceleration.
Al

I see you know what you are talking about :) Is nice to have guys like you around :cheers: helping guys like me :p
Thanks so much for your help...
Hector

Al,

You're replies are always helpful reading!

I am thinking of making an 8 by 4 plasma table. I was looking around on the net and have found a guy selling 600 oz/in servos. That's about 400 newtons per centimetre.

If I have a pulley with a radius of 30mm, I have about 100 newtons of torque available.

Force require = the mass times the acceloration

So if I have a force of 100 newtons peak available, and a gantry mass off five kilos say, I should have an acceloration of twenty metres per second. The 1250 Hypertherm has a cutting speed on thin metal somewhere in this region of 15 metres per minute.

I'm just wondering if you think I'm heading along the right tracks with this size of motor.

The 600 oz/in is their peak rating. But since the load of the gantry is interial, they'll only be running at their peak as the gantry accelorates. The rest of the time they'll only be running against the very low friction of the guides.

Best wishes,
John

Need a bit more info. What type of drive? Are you looking at driving by belt direct off of the motor? Any reduction?
Al

I'm not sure at the moment. I keep bouncing between rack and pinion and belts.

Is either inherently better in terms of performance?

Best wishes,
John

I have used timing belt up to 8ft with good results, You need to support it to prevent having to tension too tightly. I used the reinforced non stretch type from BrecoFlex.
If cost is not an issue I would tend to go with the rack, over 6ft.
Al

Thanks Al,

Looks like it's rack and pinion! :)

I'm also unsure about servo reduction ratios.

If a servo's torque is linear across it's rpm range, and falls off as the motor starts to spin really quickly, could I not just run the servo at a lower RPM, voltage, and do away with any reduction gearing?

Also, do you have any thoughts on guide mechanisms. I've seen 2" tube, angle iron and CRS slab being used as guides. The smoothest angle I've seen is alloy, a lot of the steels seem to be quite rough.

Best wishes,
John

Ok, this has been very inspiring and informational …I have my rack pinion----L2012-6 6ft rack and YD28 spur gear…as suggested by AL…Rack was only $78-6ft and spur gear $19…to my disbelieve there was a disturber in my area (50miles) that had it in stock…(usually can never find any thing you need where I live)…bough enough to run it on each side of
The 32’ table (just finished weld up tonight… 32’x10’ )…I decided to use gecko drives $99 each (Hope they come Friday I want to see this thing move!!) Because I all ready have 80v brush dc motors with reduction gears on them out of my junk Collection (dont remember what they come off of) sure I can make them work if I do have to use a different size spur gear?..Order optical encoders from US Digital E2… Trying to order a R953 Plasma THC - control PCB from rutex….do not have a plasma cutter yet , have a buddy that is a air gas dealer trying to get miller Spectrum 3080 for a song…will see ????

All of this pretty amassing…. Saturday I had known idea how I was going to do any of this….still do not know how the drive software knows how to get the scaling write… I would like to thank every one here…it is rare to find so many people so Passionate about the same thing and willing to share their ideas ….

Thank you,

Boy, you don't hang around do you! You can work out ahead of time the maximum speed you would get, calculate or obtain the linear travel of one revolution of the spur gear, if you know the reduction of your gearbox and assume a maximum of 2000rpm (typical for DC servos). divide 2000 by the GB ratio and that will equal revolutions/min of the spur gear x Lin travel of 1rev.
Al

I should have mentioned before about aligning the rack when you install it, as the L2012 does not have machined ends, in other words they are cut anywhere on the pitch, so it is sometimes necessary to trim the end, the best way to join pieces is to either use a spare length or cut a piece around 3ft and overlap (mesh) the piece with the two adjoining lengths before you bolt or weld them in place.
Al

Al,

In retrospect, would you recommend the exact same rack and pinion you used or would you go with a finer, coarse pitch, lighter rack and so on if you did it again?

Also, amazingly, Boston Gear have two outlets within my reach... in England! I have been looking at the motion products from Gudel here in England, but they look too precise, read costly, for a plasma table - more like hardend and ground linear ways for a machine.

Best wishes,
John

John, Most of the tables I get involved in are larger than 4ftx8ft, but for smaller tables you could go finer pitch, I have also used nice machined hardened rack by STI products, I don't know wether you can get them in the UK. But there is Misumi Corporation that has a huge catalogue they send for free for all kinds of machine part goodies, evidently they have a office in Staines, Middlesex that you may be able to access. http://www.misumi-europe.com/
Al

Would there be any need to go with a finer pitch, or would the rack you mentioned be okay do you think?

If I go with this rack and a T-slot alloy gantry running on bearings along a piece of angle iron, what sort of peak torque ratings should I be looking at to achieve 1000ipm with a reasonable acceloration?

Like I say, I have seen a guy offering 600oz/in peak servos for roughly $169. I am hesitant to buy servos from the manufacturer, as they seem about five times more expensive than they do in surplus sales.

I would expect the servos to have 2500 - 3000 rpm spindles so I'd need to put them through a gearbox.

Many thanks!
John

If the weight of the gantry is not that heavy you could probablly go with half the diametral pitch of the rack I used which was 12 diametral, you could go down to 24 diametral for lighter systems.
What I would aim for in reduction is to calculate the maximum ratio you can go with, this means obtaining the maximum rpm of the motors, decide on a rack and spur gear size and measure the distance traveled for 1 rpm of the spur gear and then calculate the final rpm of the spur and size of gearbox.
e.g. say the spur gear travels a distance of 5"/rev and you want to achieve 1000ipm then the spur will have to rotate at 1000/5 = 200rpm so if you use a gear box of 10:1 then your motor will be rotating at 2000rpm.
As I mentioned before, Your torque size of the motors depends on many factors, Final (or estimated) Gantry weight, gearbox ratio and acceleration required.
Al

Hi Al,

Thanks again for your help. I had a look round Boston's site and found something with a finer pitch, a 24 tpi rack, and a pinion to match. The pinion has a pitch diameter of 1" (25.4 mm). If you could check over my logic here, to see that I haven't made an error, that'd be excellent!

Pinion gear = 25.4 mm in diameter

One rotation = 25.4 * pi (3.141) = 79.78 mm linear travel per rotation

Target rapid transverse = 30 m/m, 0.5 m/s; 30,000 mm/m, 500mm/s

RPM of pinion to achieve rapid transverse = 30,000 mm / 79.78 = 376 rpm

Estimate servo RPM = 3000 rpm

Required reduction ratio = 3000 / 376 = 8:1

Available peak servo torque = 600 oz/in

Conversion to metric = 423.69 N/cm

Torque output of reduction gearbox = 423.69 * 8 = 3,389 N/cm

Torque at radius of pinion gear = 3,389 / (7.978 / 2)
.........................................= 3,389 / 3.989 = 849 N

Acceleration = F = m * a

Available peak force at pinion radius = 849 N

Estimated mass of gantry = ~5 kg

849 = 5 * ?
= 849 / 5
= 169.8 m/s

Target rapid transverse = 0.5 m/s
Time to achieve rapid transverse = 0.5 / 169.8 = 0.0029s

Something seems to have gone wrong, and it's not jumping out at me. That's around 17.3G of acceleration.

Have I made a mistake here, or is that actually correct!? :)

Kind regards,
John

John, Your calcs look ok, but I would consider the fact that you will probablly not be cutting at 30m/m, on a plasma m/c this is usually the rapid move rate, the max cutting feed rate is more like 6m/m, so although you may want the higher rate for positioning etc, the acceleration for this is not so critical, so I would calculate for the max cutting feed speed.
So to confuse things.
Al

John, Most of the tables I get involved in are larger than 4ftx8ft, but for smaller tables you could go finer pitch, I have also used nice machined hardened rack by STI products, I don't know wether you can get them in the UK. But there is Misumi Corporation that has a huge catalogue they send for free for all kinds of machine part goodies, evidently they have a office in Staines, Middlesex that you may be able to access. http://www.misumi-europe.com/
Al

Do you have a link for that STI rack?
Regards,
Hector

John, are you sure that the gantry will weight only 5kg?