where to start?

[JAZZCNC]

Hey guys,
The machine I need will be mainly used for cutting 8' x 4' sheets of 18mm MDF using a 8mm bit and maybe thin perpsex but nothing more than that.
I don't know how realstic my budget of £1000 is but that's really all I have to play with.

Any advice or where I should start would be greatly appreciated.
Cheers in advance.

Hi,

First what tools do you have access too and what skill level or experience do you have working with metal.? . . . IE: Welding etc.

Building what you want is just about do-able for a grand but will need you to have the right tools and skill level, OR friends that do.! . . Along with some savy purchasing. Plus you'll need a rock solid design that you know will suit your needs along with good plan of attack because you'll have no room for error if your wanting to do it for around £1000.

Steel box frame and Rack & pinion on the X&Y Axis with ballscrew or lead screw on the Z axis is the route I would take for a Really strong accurate machine of this size.
Belt drive would also be an option which is relatively cheap and very accurate and lends it's self well to machine this size.

Best advice DONT BUY A THING untill your 110% sure the design you have choosen will do what you need, then have it checked out by others for reassurance.
Then and only then start buying materials needed and even then dont buy to far in front because plans have a nasty habit of changing on the fly very easlily rendering some parts or materials unsuitable purpose. . . . . It's like eating an elephant.? . . . best done in small bytes.
Like any good constructuion it all starts with good foundations(the frame), weak foundations will tranfer into a weak machine. . . . Equally poor materials will render poor results.
Also like any other construction the methods and skill levels of the builder will have a massive affect on the end result. I've seen some very good strong designs built with very high quality components that have been completely FUBARD due to poor skill levels built with caveman tools.
Equaly I've seen fantastic machines built with basic tools and cheap components when mixed with good skill and reasonabe tools, mix with a good dose of imagination and the results can even blow away some big buck machines.

Don't rush and ASK Q's then some more Q's and then more Q's not matter how dumb you feel or think they sound, keep asking untill you fully understand and are comfortable to go.

[trounce]

Hi,

First what tools do you have access too and what skill level or experience do you have working with metal.? . . . IE: Welding etc.

Building what you want is just about do-able for a grand but will need you to have the right tools and skill level, OR friends that do.! . . Along with some savy purchasing. Plus you'll need a rock solid design that you know will suit your needs along with good plan of attack because you'll have no room for error if your wanting to do it for around £1000.

Steel box frame and Rack & pinion on the X&Y Axis with ballscrew or lead screw on the Z axis is the route I would take for a Really strong accurate machine of this size.
Belt drive would also be an option which is relatively cheap and very accurate and lends it's self well to machine this size.

Best advice DONT BUY A THING untill your 110% sure the design you have choosen will do what you need, then have it checked out by others for reassurance.
Then and only then start buying materials needed and even then dont buy to far in front because plans have a nasty habit of changing on the fly very easlily rendering some parts or materials unsuitable purpose. . . . . It's like eating an elephant.? . . . best done in small bytes.
Like any good constructuion it all starts with good foundations(the frame), weak foundations will tranfer into a weak machine. . . . Equally poor materials will render poor results.
Also like any other construction the methods and skill levels of the builder will have a massive affect on the end result. I've seen some very good strong designs built with very high quality components that have been completely FUBARD due to poor skill levels built with caveman tools.
Equaly I've seen fantastic machines built with basic tools and cheap components when mixed with good skill and reasonabe tools, mix with a good dose of imagination and the results can even blow away some big buck machines.

Don't rush and ASK Q's then some more Q's and then more Q's not matter how dumb you feel or think they sound, keep asking untill you fully understand and are comfortable to go.

Thanks Jazz, that looks like a lot of great advice.
When it comes my tools I don't really have that much in terms of metal working. I want this machine to cut out cabinet pieces for my arcade machines so I guess most of my tools are woodworking.
I have experience with welding so that's not a problem, sometimes it doesn't end up looking that pretty but then I have my grinder to clean it up :tup:

I have a 110 amp Mig welder, angle grinder, bench grinder and bench drill along with the usual hand tools, drill, jigsaw, sanders, dremel etc.

I saw a very nice belt driven home build on youtube and I'm very tempted to use this on the X & Y axis, Im presuming that due to the size of my machine I would need two motors on the Y axis as apposed to just one on the machine I saw as it was much smaller?

[JAZZCNC]

I saw a very nice belt driven home build on youtube and I'm very tempted to use this on the X & Y axis, Im presuming that due to the size of my machine I would need two motors on the Y axis as apposed to just one on the machine I saw as it was much smaller?

Hi Neil,

Sorry didn't notice this bit, read that meny posts I tend to skim things and miss bits. . .Lol

Post the link and lets have a look.!. . . but NO you shouldn't need 2 motors just one decent size connected to a shaft linking the 2 sides.
With Belts you tend to have quit high reduction to get the resolution down to something usable (for your needs something in the 20mm pitch range) and this reduction ups the torque which helps the motor. The trick is getting the reduction right so the meat of the motors torque is delivered at the optimum feed rate (for nema 34 this usually around 600rpm) and still give good resolution.


Problem with belts at the lengths you need for the long axis is keeping them tight enough they don't flap around but not so tight the stretch. . . . . So Something to consider could be switching from the conventional narrow gantry layout moving along the long axis and instead have a long gantry moving across the short axis.? This way you could use short twin motor driven belts on the narrow axis and R&P on the other long axis. . . . OR even ballscrews on the short axis.? Chinese ballscrews won't cost much more than belts,pulleys,bearing blocks etc so something I'd consider and ballscrews are very much easier to impliment.!

[GTJim]

JAZZCNC you have given some excellent advice in this and other threads and I am sure that myself and others will benefit for your experience.
A question, how does rack and pinion compare to ball screws in repeatibilty and accuracy, and does the pinion ever jump teeth under load?
Sorry if this sound dim, but curious.

[trounce]

One last question before I get designing....


As you can see by this image the sheets of 8 x 4 I'll be cutting get used almost right up to the edge of the wood. I will obviously need a frame which is larger so the router will be able to travel past the edge of the wood, I'm thinking 9 x 5 does this sound about right?

[JAZZCNC]

One last question before I get designing....


As you can see by this image the sheets of 8 x 4 I'll be cutting get used almost right up to the edge of the wood. I will obviously need a frame which is larger so the router will be able to travel past the edge of the wood, I'm thinking 9 x 5 does this sound about right?

Not a straight forward yes or no. It will depend on a couple of measurements of the machine to how much extra length and width past the desired cutting area.

First I call the long axis X (length) and Y axis (width) is narrow, Z axis(Up,down) runs along the gantry or Y axis.

Regards Y axis(width) you will want it Just a bit wider than the width of the Z axis, basicly half the width of Z axis at each side plus a bit of stopping room.
The Z axis width will more than likely be determined by the bearings and screw used or the width of the router mounting plate. Wider is stronger because it positions the bearings on the Y axis further apart but wastes travel needing wider Y axis and machine. 200mm is a good width and will fit most routers and spindles plus allow for resonable Y axis bearing placement without wasting travel. 150mm is about the minimum I'd use for a good Zaxis.

Regards the X axis(length) well this depends on the position of the gantry cross beam on Xaxis bearing plates and how far off the front of the gantry the Z axis sticks plus the spindle or router used.
This will ultimatly give you the cutters position relative to where it falls in line with the X axis bearings, this will give you the needed extra for the X axis.

I try to position gantry cross beam back slightly from centre of the X axis bearings so that the cutter position basicly lines up with the front edge of the bearings.
This keeps everything contained within the X axis bearing plate length and nicely balanced weight distribution on the bearings.
No matter how you do it you have to have lose some wasted space at one end or the other.! . . Some sweep the gantry sides back and half the loss at each end.?
I just keep the loss at one end. . . you can always use this space for 4th axis head then it's not taking any table roomand can be left on.?

Doing it the way I do basicly means adding the length of the X axis bearing plates to the required cutting area.

Hope this makes sense.? If not just ask.

[JAZZCNC]

JAZZCNC you have given some excellent advice in this and other threads and I am sure that myself and others will benefit for your experience.
A question, how does rack and pinion compare to ball screws in repeatibilty and accuracy, and does the pinion ever jump teeth under load?
Sorry if this sound dim, but curious.

Well Jim the honest answer would be they don't compare.! Infact if you asked me too place each system in numerical order of effeiciency, repeatabilty and accurecy then R&P would probably come 4th. . . . 1st ballscrews, 2nd lead screws close 3rd belt drive then R&P.!!

That said it's doesn't mean it's rubbish and for large format machines doing lower precision jobs but with high loads and cutting feeds then it's usually the best option.

Ballscrews (and Lead) are too whippy over long length's without taking special measure's IE: rotating nuts or excessive sized screws,(which brings other issue's) plus the accurecy and precision they produce hisn't often required for cutting wood and plastics etc which most large format machines are used for.

Belts are very accurate, effiecent with very good repeatabilty, virtually no backlash. But again they have there issue's over large length's and don't handle high loads as good as R&P. They can be made do-able for large machines but not ideal . . . More suited to large machines with low loads and high speeds.

So IMO R&P is the best all round option for a large format machine with high loads (IE: heavy gantry,deep DOC) needing medium accurecy with resonable repeatabilty and high-ish feed rates. . . Perfect for Cutting wood, plastics, stone.

Regards jumping It's not an issue if designed correctly with correct alignment and pinion tensioning into the rack. The issue's can come from debries getting in-between the rack & pinion but if the rack is positioned with the teeth facing down with the pinion engaged by spring loading into it rack from benneth and positioned out of the firing line as much as possible then it's not an issue normally.
The trick is getting the tensioning just right so it's not so weak it allows teeth too jump or have excessive backlash but at the same time not to high cause's binding which helps to reduce backlash (too a point) and jumping but cause's excessive wear. The required tension will vary from machine to machine depending gantry weight, cutting forces etc and a little setup & adjustment will be required at first but ounce setup it works good for this size an type of machine.

Again at the end of the day it's horse's for course's.! . . . . If you want super precise get a show jumper(screws), if you want light weight and speed get a stallion(belts), if you want a good strong all round work horse then get a shirehorse.(R&P) . . . . . If you want an old knackerd Donkey get some threaded bar and MDF. .:whistling:

[trounce]

Edit: double posted in error