Some design ideas!

Hey guys, just registered on this forum. Looks like a heap of good info to be had here :)

I have been slowly progressing on my own laser cutter for just over a year now, and have tried a few designs, which I have not been happy with.

I'd like to ask you guys for some recommendations on how I complete my build, namely, the X and Y table configuration.

Firstly, I tried this setup for the Y axis:
http://dl.dropbox.com/u/203420/XY/DSCF2516.jpg

It seemed like the best way to do it, however I quickly ran into an issue. The Y support rails I was using were only 10mm diameter. I have now purchased some 20mm diameter rods in the same length, The X axis also uses 20mm rods.

You may thinking wwooaahh!! Way overkill for a laser cutter!! Yep, though I'm trying to make this machine as expandable as possible from the start, so if needed, I could just strap on a Z axis and a router and have a CNC mill. But, just working on laser for now.

So, my questions:

1. Steppers and belt drive. A while ago I purchased some belt for the machine. I was originally going to use leadscrews, but after finding the ones I had were warped, and that they'd probably be too slow, I decided to ditch them for a belt.

http://dl.dropbox.com/u/203420/XY/DSCF2273.jpg

As you can see, the belt is being direct driven from the stepper motor. At the moment, there are 2 steppers on each X axis belt, thus 4 steppers to move the X axis. Originally I was only going to drive 2 of them, and the other 2 would just act as idler pulleys. However, these linear bearings require a bit more starting torque to get them moving compared to the V groove bearings I used to have.

So, would you guys recommend some sort of belt/gear reduction on my stepper motors? I believe, due to the weight of all the components, inertia is going to play a big part in the final speed of my machine.

2. The Y axis.

As you can see in the first pic, originally my Y axis was mounted to some vertical pieces of decently thick angle aluminium. The reason I decided to go for a vertical setup was to make mounting the lens holder easier. It would also make attaching a Z axis easier. My question here is, whether angle aluminium would be sufficient for the side supports, or may I have flexing/twisting issues?

So basically what I'm asking is, should I go for gear reduction on my steppers, or save for some bigger steppers? I was looking at these: http://ausxmods.com.au/index.php?mai...roducts_id=199

Seeing as they're fairly pricey, and after just purchasing a DSP, by budget is running a bit low for this, I was considering maybe purchasing 1 sual shaft one for the X axis, and a single shaft one for the Y axis. However, I'm not quite sure how much force 269Oz-in actually is. Do you think 1 of those would have any issues driving my X axis?

And secondly, maybe some design ideas for suing 20mm diameter rods as the linear rail. I notice most laser cutters use some kind of square rail for the Y axis, but I haven't really been able to find that. Also, living in Australia makes shipping a killer sometimes!

Cheers,
Dan :)

Hi, the only part of a cnc that I liked the linear rail support mounted the way you have done it is for the Z on a CNC router as the length is short and it is the proper way to do it given you have forces generated in all directions.

One thing that I notice on commercial lasers is that the construction of the gantry is lightweight and when you see them doing engraving at the speeds they do it becomes apparent why they need to have a small amount of mass when moving around quickly.

I would think it you continue the construction the way you are planning that you will limit your options for the machine.

For the axis that the gantry runs along I would use the linear rail that is supported, not the 20mm bar that hangs as that span is too long and it will sag.

For the axis which runs along the gantry I would use some box section and smaller rail to run the carriage on. Try to keep it lightweight as you are only supporting a light head, there are no cutting forces as there are on a router. Any additional mass here is not required and can only give you problems with accuracy when doing fast work.

I think you are right about staying with the belt and not the ballscrew.

I would seriously thing about gearing it down or at least researching if that is common. Remember a router may directly drive a ballscrew but that in itself is a gear and improves accuracy. You don't want to see the steps of the motor in your cuts but not sure if that would be a problem.

I have never built a laser so this is just based on my observations of others.

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269Oz-in

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Depends on the nomenclature used, if it's per square inch, torque, pull...etc

That's 16.8125 LB's of force, the human Femur (the largest and strongest bone in the body) can withstand around half that per square inch before breaking.16lb is quite a lot depending on what it's being used for :)

best wishes

Dave

Thanks for the replies guys :)

@jasonharris22

Yeah, thats the one huge downside i'm seeing using such heavy rods. I've spent a bit too much money on them to not try them at least. If I do any raster engraving, the Y head will be a lot lighter, so I can use that for the scanning.

As for sagging of the 20mm bars, I've been told that the amount of sag will be virtually nothing for such thick rods over only 1 metre. I haven't had any issues with my X axis yet. But I suppose I'll see!

@Exsecratio

I went ahead and bought 2 of them anyway, too much torque is better than too little, and I'd rather not have to redesign and rebuild the whole thing because my little steppers aren't up to the task!

The pulleys I am using are around 2cm diameter, and are for a T5 16mm belt.

But yes, I think they will be completely overkill for my machine, but then again, skipped steps are a lot less likely at high acceleration.

As for belt/gear reduction, I can still take that route. I found with the pulleys I had and 1/16 microstepping, I could get around 21 steps per mm. Does that sound like a sane value for a laser cutter?

Cheers,
Dan

Its hard to see in pictures but if it is only one meter then not a lot to worry about.

21 steps per mm is ~ 0.05mm which is from memory the positional accuracy of a lot of commercial stuff so no need to worry about that then either.

300oz/in is a lot for something like this, two of them is even more :) Weigh up how difficult it will be to do it electronically and an software. Mach3 supports syncing them for at least one axis, if I get you right you are thinking of 4 steppers for a single axis?

Looking at pics of my soon to be here machine there is only one stepper for each axis, thats on a machine with a 1500x1200 bed.

Hehe nope, only 2 steppers :)

I bought a single shaft, and a dual shaft one. I'll use the dual shaft and some extension rods for the X axis, and the single shaft for Y.

The reason I have 4 steppers for the X axis in my pic is it was cheaper and easier to use stepper motors as idler pulleys. I don't really have any precise tools, so they were easy to mount and have nice bearings.

I was also going to try running all the steppers in parallel to try get a bit more power, but it seems they don't particularly like running in parallel.

As for software, I was going to use mach3, though I've heard it's quite a feat to get mach3 running properly with a laser cutter, to allow both engraving and cutting in the same run. Instead, I just went for the top and purchased a DSP: 2012 Commercial DSP CO2 Laser Engraving/ Cutter Controller. Support CorelDraw & AutoCad

Cheers,
Dan

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Also, living in Australia makes shipping a killer sometimes!

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Ha! Ha!.... is your VAT less than 20%?

Yeah, you've got us beat there unfortunately. I've had my fair share of ridiculous shipping costs from another one of my hobbies!

Anyway, back to the topic, jasonharris has sent me some pics of his soon to be machine, and the square tube Y axis system looks nice and lightweight. Does anyone have any ideas/pictures/designs on how this is achieved? I'm assuming it sort of has bearings on each side of the bar, which hold it on?

Cheers,
Dan

A 20mm unsupported span will sag by quite a bit (as far as I can see from the bag of the cigarette packet calculations)

I can post the numbers if it is any use to you?

The only awkward one to specify is the Youngs Modulus for calculation as the values can be anywhere between 201 GPa and 220 GPa depending on the actual makeup of the material.
A quick bit of number crunching suggests the deflection due to gravity will be more than the actual depth of field of a 50.8mm focal length lens.

A drawn aluminium beam may be a better idea?

best wishes

Dave

Yes, I'm starting to think a beam will be generally better too. Just having a bit of trouble figuring out the bearing/carriage design.

Cheers,
Dan

Want some pictures of my Chinese beam/bearing set up? it's on a 1200 x 900 machine so not small and doesn't appear to suffer with deflection?

best wishes

Dave

That would be handy, thanks :)

I see that most people are using some other sort of supported rail on the Y axis. Kind of annoying as that'd mean buying MORE rail and different bearings. Is there a way you can use the box section itself as the rail?

Cheers,
Dan

Just thought I'd pitch in here, I do actually have some v-groove bearings left over from a previous design. If I could somehow get these to ride on the corners of the box section, I'd be golden. The issue i'm seeing is I'd have to rotate the box section by 45 degrees, which would make mounting it to my X pillows a pain, as well as mounting the stepper for it's pulley.

Does anyone have any ideas for using v-groove bearings on box section as a Y rail?

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The issue i'm seeing is I'd have to rotate the box section by 45 degrees, which would make mounting it to my X pillows a pain, as well as mounting the stepper for it's pulley.

Does anyone have any ideas for using v-groove bearings on box section as a Y rail?

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Weld angle iron onto box section ie
Metal Angles | Buy Metal | Online Metal Supplies | Aluminium | Brass | Stainless Steel

Problem is, wouldn't I want to be using aluminium to keep the weight down? I don't have the right equipment for welding aluminium :(

It seems as though if I want to use v-groove bearings, I'm going to have to mount something at 45 degrees at some stage.

Cheers,
Dan

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Problem is, wouldn't I want to be using aluminium to keep the weight down? I don't have the right equipment for welding aluminium

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Its mild steel angle iron not aluminium and if you have good enough motors then there shouldn't be a problem unless you mean you have a problem weight in shifting the CNC?

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Problem is, wouldn't I want to be using aluminium to keep the weight down? I don't have the right equipment for welding aluminium

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Its mild steel angle iron not aluminium and if you have good enough motors then there shouldn't be a problem unless you mean you have a problem weight in shifting the CNC?

The problem I am thinking about is inertia. My steppers should be more than enough.

I suppose it depends on the thickness of the steel.

Something like this I was thinking:

http://dl.dropbox.com/u/203420/v-groove.png

If you look up joes 4x4 cnc router you will see many examples of using angle iron for running v bearings on.

You should get away with only one on top and one on the bottom. You can't weld the mild steel angle on to the beam and you can't use aluminium angle because it will wear out before you blink.

Joes design has the anlge screwed on to the beam through one side, not how you have pictures the mount and the v bearing runs along the edge. Works great for all the guys who have built his machine.