New table build - looking for pre-build tips...

[davek0974]

I am embarking on my second plasma table build, the first one was an educational project that ended up being used for production, it was built from alu section and i've killed it It was belt drive and suffers from flex in the gantry causing wobbly cuts.

This one will be a little bigger at 1m x 1m and the normal steel frame construction.

I want to re-use my steppers/drivers/psu hopefully, to lower the cost. They are 3.1Nm Nema23's at 36v/4.2A

Now the design queries...

First-
I would like to hit a speed range of 100 to 9,000mm/min (4 to 355ipm) to get the full usage out of my Hypertherm 45 cutter.
To get good cuts I also need decent acceleration ( I have read that 0.1G is a good minimum)
I think rack and pinion, direct drive is the only way to achieve this - two motors slaved on the Y and one motor on the X.
I can't find much help in figuring if these motors will do the job or not, any ideas?
Gantry will probably weigh around 20kg I think, at a guess.

Second-
I see builds like TorchMate and Precision Plasma use flat steel plate and skates for the motion, is this a good model to follow or do I need to look at linear guides??

Any other tips feel free to throw them in

Thanks

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[davek0974]

Looking like 3:1 reduction on the drives is a good idea.

A 20t pinion on Mod1 rack will give me plenty of speed with the motor around 400-450rpm

still not decided on the linear rails or plate and skates though - any views?

[davek0974]

Ok, a specific question, looking at a cross-section of the gantry beam, what style will give me the best stability A or B??

Both have two linear rails, I was guessing to put two carriages on the lower rail and one on the top rail for type A or two on the front and one on the back for type B

Or the third design which also has the benefit of keeping the Z axis mount very close to the beam face??

I am looking at high speeds and want zero wobble induced.

[davek0974]

Looking at the main side rails now, likely 100x50x3mm box section, HiWin 15 only has M4 mounting holes so not good in side load support.

My idea is outlined in the picture - fit a strip of 3mm x something to the box, this could be drilled and plug welded or something, then bolt the Hiwin above using the plate as a support as recommended in tech specs, plus i can then fit the rack against the lower surface of the plate giving me a (hopefully) parallel relationship of linear rail and drive rack ???

I have no idea how parallel the sides of a strip of 3mm x maybe 30,40 or 50mm cold-rolled is, any ideas if it's good enough here??

Looks like I can fit a bigger build in my shop, hoping for a 4'x4' or 1200x1200mm cut area so looking at the racks and rails being maybe 1500mm long??

[davek0974]

ok, looking like one wide linear guide on the X axis with one carriage holding the Z axis

20mm lead ballscrews all round

frame built from 50x50x5mm steel, welded with the motion section bolted on

[BTA PLASMA]

Hitting those top speeds without issue is the big challenge. There are kits out there that will handle what you want to do without trying to reinvent the wheel. But if you have more time than money you can attempt a build that will max out at just under 400ipm. One of the challenges is using a linear guide close to the cut zone. Ask virtually anyone using a simple guide that has it within that magic 5" from top of plate and you will hear them say they wipe it down frequently. Even worse would be cold rolled steel where it will rust and pit from idling. We found great success in building gantry systems where the lower rail is a minimum of 8.5"s from the top of the plate. It makes a huge difference on a water tray but not so much on down draft. Extra wide rail can be done via V rail and true profile rail. Expect V rails to get rusty and bearings to get play and expect true profile rail in the wide format to be very expensive. If you go with the V rail or the profile rail you will be tasked with 2 lines of holes for mounting. The challenge then is machining the holes and keeping the panel you mount it to flat (If your not running a springs for pinion gear engagement...springs would not be good for 375IPM).

To do what you want to do and to do it right would be a process requiring some well thought out steps. Could be done but your quest might be a compromise in the upper speed range.

[davek0974]

Thanks

I am already seeing that this is range is going to be tricky to achieve, I am going for spending on the hard to upgrade bits and taking the rest as it comes after testing.

20mm lead ballscrew, direct drive from my existing 3.1Nm motors, nuts with wipers fitted, anti-backlash type.

Genuine HiWin 15mm rails with two carriages on each end of the gantry with scrapers and wipers fitted, covered from the top.

One wide HiWin on the gantry, one carriage with scrapers and wipers fitted.

50x50x5mm steel frame.

The gantry will be high-up and mounted right on the end supports with no upright so it will sit as high above the bed as possible, out of most of the muck.

The rest of the story will unfold as it goes i think.

[BTA PLASMA]

Looking forward to your build. It should be a fun build, especially for someone that has gone down the road of building already. Dont be afraid of having a machine shop dust your parts to keep them flat.

[asuratman]

We found great success in building gantry systems where the lower rail is a minimum of 8.5"s from the top of the plate. It makes a huge difference on a water tray but not so much on down draft.

What is the reason of your statement above, I made my cnc plasma is about 6" from the top of the plate?

[davek0974]

The end trucks for the gantry now looking like the attached sketches.

Should be very stable, i can gusset the beam to resist twisting and fitting the screw nut mount should be fairly easy. I can also fit a cover strip over the top of the screw to keep a lot of the muck off.

[BTA PLASMA]

What is the reason of your statement above, I made my cnc plasma is about 6" from the top of the plate?

Everything you need to survive motion with plasma last many times longer the further away it is from the plate. You need to have an ultra rigid rail and gantry system to achieve this. We can hit 400ipm cut speed with no wiggle at 11 inches above the centerline of the side rails. It isnt cheap but your components last many times longer. From the lower rail to any switches and gear rack/pinion combo.

[BTA PLASMA]

The end trucks for the gantry now looking like the attached sketches.

Should be very stable, i can gusset the beam to resist twisting and fitting the screw nut mount should be fairly easy. I can also fit a cover strip over the top of the screw to keep a lot of the muck off.

Your biggest obstacle is going to be ensuring the frame is strait, the ends of the screw are parallel and true within 3 planes of accuracy and the frame wont tweak later. A build like that should be done by heavy weldment, stress relieving and finish machining of all the critical areas. Its how a CNC router is made. Ballscrews are great but will bind on you and overload a motor easily if they are not held well. Look at the small torchmate table. They are using a large hole for the screw to bobble around and plastic washers for thrust. The screw cannot get enough speed to travel over 200ipm with antibacklash nuts with a setup like that. Make it a ballscrew and build the frame right and you could hit 10,000ipm with the right motor (But at that speed you would have to liquid cool the nut). You would be better served not using ballscrews unless your prepared to go all the way.

[davek0974]

Hmmm, this is going to be tricky decision I feel.

I have people whose opinions I trust on here giving good advice and then I also have people whose opinions I equally trust giving good advice on a well known engineering site.

One favours rack and pinion, the other favours ball screws.

I do not know what way to turn now. I have priced up ball screws and the cost is ok. I have also priced up rack and pinion and it is cheaper as expected but also needs reduction drives making etc.

The only constant is that both will use HiWin linear rails.

Don't know what to do now.

[davek0974]

Just to stir the mud more,

Someone recommended the SwiftCut table, If i had the cash i would buy their 1250 table tomorrow! It is exactly where i want to be, has the full cut speed range I want, pity the pictures are not clearer

I see that they have R&P drive but use V-Rails all round, can't seem to find that stuff?

I like the below-table idea too for the transport, places more twist stress on the gantry support but obviously works.

The motors are not much above what i have - they use 4Nm and I have 3.1Nm, both hybrid so thats a good sign.

Now, can i build that table with Linear rail or does anyone know where I can source the V-rails????

[davek0974]

Torque and RPM...

Is it best to use a small drive pinion on the rack with higher motor rpm or larger pinion with lower motor rpm??

A 20t pinion will move 62.83mm/rev and at 3:1 reduction I will need 429.73 motor rpm for 9000mm/min travel.

A 32t pinion will move 100.53mm/rev and at 3:1 reduction I will need 301.58 motor rpm for the same speed.

I am fairly certain a larger pinion meshes better than a smaller one, but what size would be best considering my 3.1Nm motors.

My existing table is belt drive but works out at 100mm/rev travel and 300 motor rpm for 9000mm/min speed. When i did my high speed test it was running at 450 motor rpm so I know they can reach at least that.

I have a heavier gantry on the bigger machine, but also twice the motor power as i'm using two slaved motors.

Any views guys?

[BTA PLASMA]

Your using such small motors a larger gear with more tooth engagement will not be a direct advantage unless your looking to change you minimum movement (Which adversely affects top speed and acceleration rates). As your finding out, to design a machine to fit the book speed takes more than throwing simple components together.

[davek0974]

Ok, things stepped sideways a bit today

I am now looking at building first for direct drive rack and pinion with 15T on the shaft, this keeps the motor torque high and rpm low, resolution can be had with micro stepping.

I have also secured a package deal on bigger 4Nm steppers with DSP drivers and a higher voltage (48) PSU.

The new hardware means I can keep my table running right up to the last minute then just swap the z-axis and breakout board over, these motors are also stronger and well proven by the suppliers machine builds.

[BTA PLASMA]

Ok, things stepped sideways a bit today

I am now looking at building first for direct drive rack and pinion with 15T on the shaft, this keeps the motor torque high and rpm low, resolution can be had with micro stepping.

I have also secured a package deal on bigger 4Nm steppers with DSP drivers and a higher voltage (48) PSU.

The new hardware means I can keep my table running right up to the last minute then just swap the z-axis and breakout board over, these motors are also stronger and well proven by the suppliers machine builds.

The one thing that you may not hear is that gear rack pitch angle has a lot to do with how smoothly that small gear will run. Run away from 14-1/2` and similar gear racking

[davek0974]

yes, the standard seems to be MOD1 20deg PA so thats what it'll be.

[davek0974]

Reality check time...

How do you decide what table is the right one?

I know the 2'x2' i have now is too small, it works ok for most of my jobs but i can only cut with the plate square on the bed so no quick twisting it to get another cut out etc.

I can physically fit the 4'x4' (1250x1250) into my shop, thats ok.

I cannot lift a sheet of steel that size, have little possibility of lifting gear etc. as its only a double garage. A sheet of 3mm which is my most popular is about 36kg so only just about ok but the other one i use a fair bit of is 4mm and would be 50kg, no way i can lift that.

Would it be better to go 3'x3' instead?

Cost difference in build is minimal, but what makes sense - having the real estate and not using it or saving floor space etc??