Hello and here I go

I've been reading this great forum and resource for just over a year and now it's time to raise my head above the parapet and commit myself to a build.
I have a joinery/wood machinery background (old school city and guilds) but have never built a cnc machine from scratch.

So, looking to build a 8 x 4 cnc router mainly for routing wood/mdf. My plans have gone through many changes, fully supported rails, 8020, etc etc
At the moment I'm leaning towards:
1. Mild Steel C Channel
2. Angle Iron
3. V-Groove bearings
4. Unistrut
5. Wooden base with mdf all round and storage underneath
6. Rack and Pinion

Hopefully, some of my sketches for the above will show in this first post.

I would very much appreciate some feedback from the very knowledgeable and helpful people here as to whether I'm on the right or wrong path.

Thanks,
Bob

If you have a woodworking background and tools, you might consider making a torsion box for the table and looking at the gantry design that ger21 came up with?

http://www.mycncuk.com/forums/linear...d-rails-3.html

Ok first why C channel.? Box section is cheap and much stronger.!

Would strongly urge you to stay well away from Angle iron/V-groove bearings route if your mainly cutting wood.? They stick and bind with the dust and resin build-up. It's one route you will regret I promise you that. Supported round rail from china will work out far less hassle give much better performance and probably cost less money in the end. R&P is ok for this size machine but it's very inefficient compared to other methods and any friction in the bearings will just compound the issue.!!

Some of the issues with R&P is driving both sides often means slaving motors so the design, alignment and quality needs to be good otherwise any binding or sticky-ness will stall a motor resulting in racking the gantry and potential damage. This means making sure the motors are running well below there corner speed while cutting ensuring the most torque is available.
Because the pinion dia will be large you'll need to gear the motors at least 2:1 or 3:1 to bring the pitch down to allow a decent resolution and help raise torque to combat the inefficiency's of R&P.
This means the relation ship between motor/drive/voltage choice to give a good corner speed along with pinion Dia/gearing to allow decent torque/resolution is important you get right. It also means the machine frame and build quality have to be first rate and any corner cutting soon shows has you have very little wiggle room due to the sticky nature of R&P and if shoddy will quickly Rob power resulting in stalling and head thrashing sessions.!!

IMO a wood frame hisn't a good idea for R&P due to movement which again will just compound to any inefficiency's already there.!!

Please don't think I'm trying to put you off R&P just pointing out some of it's quirks.!! . . . . I am 110% trying to put you off V-bearings/angle iron thou.!! . . DONT DO IT.

Hope this helps.!

Not really thought about the gantry construction yet, but that looks very interesting and a great alternative to aluminium or mild steel - thanks for the link D.C.

Currently, it's my choice of design and the non-wooden parts that I'm most concerned about.

When I worked through the cost of r&p some time ago the cost was comparable or even a bit more than ballscrews from china. I didn't shop around too much but I still know which way I'd go! Again jazz is right cost saving on angle and v bearing compared to Chinese supported rail does not make it worth doing.

I too was planning on mdf base as I haven't done much metalwork - I have decided to learn though! It takes a lot of effort to build one of these things and you might as well do it right?!

So nothing positive about my design then....perhaps my sketchup drawing was nicely coloured :unconscious:

Seriously though,

C Channel was chosen over Box section because I thought it would be easier and probably better for fixing and it would also provide room for adjustment later when getting everything aligned.
Cost wise, never really compared the C to Box. But, looking at Box it's double the cost of C - web prices of course. C was chosen purely because I thought it would work better.

So what you're saying is that 2900mm fully supported rails and ballscrews will work out cheaper in the long run.

Would I need 2 rails for each side?

So is it safe to assume that parts would be:
4no 2900mm fully supported rails for the long axis
1no Ballscrew 2900mm long
What do you think a rough cost of these items imported from china would be including any import duties?

I have welding experience and equipment so a metal base doesn't worry me. Just thought there was no advantage of using metal over wood for the base.

Please bear in mind, that this is a build for home not commercial use. The reason I'm building this is to learn about the processes of making a machine and to have some fun along the way. Of course, I'm going to fall over on some things but this is part of the learning process.

Thanks for taking the time to reply.

Ye nice colouring.. . Lol

Sorry to be negative but I'm just trying to save you some pain and money.

Being completely honest with you this size machine is not a wise choice for a first time build.? . . . Thou That does not mean it's not do-able because it most certainly is.!

Large machines change the game quite considerably when it come to linear motion and it becomes very important to success that the right choices are made. First time builders while they may have the skills required to accomplish the build don't always see or realise the importance of some of the things needed for success.!

The choice of linear rails and linear motion system like R&P etc become very important. Both these things are important to any successful build large or small but over a certain length, approx 1500mm, the choice and implementation raises the stakes and chances of failure rise greatly. This is why avoiding the weak designs like Angle iron/bearing linear rails becomes so important.
But the bigger issue over this length is the choice of linear motion becomes a much less simple affair.?
The usual simple choices like lead or ballscrews are suddenly very complicated.! The alternatives like R&P while more suited come with there own set of needs and things that have to be correct for success. . . . It's these that I was trying to point out before.

Now to answer some of your questions and hopefully set you down the right road for your needs and wallet.!

Seen has you can weld then it will be far better machine and easier to build from steel so that's the route I'd suggest every time.
Without seeing the full design then the use of C channel I can't fully comment on but IME box section works much better and far stronger. C channel is rarely flat or square on the inside and using the outside to have a decent level of strength requires a much thicker wall thickness which then makes it costly.

You only need 1x rail 2x bearings per side. 25mm supported round rail will be fine for this length and about the cheapest way to do it correctly.

To be honest the building of the frame is the easy part it's this next bit where it gets funky.!!. . . . . R&P or Ballscrews.?

Ball-screws are far far better than R&P but at this size they need a special attention and implementation is needed.! . . . Namely they need a rotating ballnut design.
This will mostly be beyond the first time builder to DIY build them selfs and far to costly to buy from a ball-screw supplier. The other option is to have another person make the rotating ball-nut assembly, Jonathan on here has a good design and I'm sure will gladly make for the right amount of beer tokens.!!

R&P then becomes the next choice has that doesn't have some of the issues long ball-screws have IE Whip but like I pointed out before they do have there own set of needs. Again Namely they need correct gearing ratio's, careful and accurate alignment of rack, usually larger motors due to inefficiency, larger drives and PSU to run the bigger motors.!! It also requires more maintenance than ballscrews and requires that the rack is kept clean and free from debris. . . Try to skimp and it will bite you with binding, racking, stalling and generally frustrate the hell out of you.!!

So has you can see not simple or ideal for a first build.!! . . . If you take the cheap route then be prepared to be frustrated and constantly chasing problems and unless you have a good grasp on what a CNC machine needs and the traps involved then not something I'd recommend at this size.

Do it correctly with the right components and careful selection of methods then YES it's very much do-able for a first time, easy NO but very possible.

Good luck.

PS The prices from china I don't know but the duty's will be VAT on the rated value on the shipping paper work plus a shipping handling fee of about £10. Often the supplier will put a lower value on the paper work so the Vat payed will be lower.! IE My last bought was £460 and I payed £20 vat. . :congratulatory:

Wow Jazz,
thats some reply and certainly lots to think about.
I prefer straight talk and no you are not being negative, just realistic.
It looks like I need to go back to the sketchup board and re-think my strategy.

Thanks for taking the time to help.
I will be back to pester you shortly.