First design - Few questions

Hi all,
Working on my first design, having read various sites and postings I have the basics of what I want to do.
When looking at the Z axis design and pricing it up using square rails etc I noticed this pre built Z axis
http://www.cnc4you.co.uk/CNC-Kits/Z-...ess-plus-Mount

Is it worth the hassle and expense designing & building one from scratch, or just go with an item as per the link?
Unless I can get a clear performance advanage seems not...
Looking to machine woods and Aluminium, on a 1200x800x100 machine.

Thanks

I looked at that myself some time ago but decided not for several reasons.
It used unsupported rails, it was the wrong size for my design, the spindle I used needed a special/modified mount, I didn't like just one linear bearing per side (even though they are the longer ones), I thought I could build one cheaper and better, it has a direct drive mount for the stepper but I wanted belt drive.

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Originally Posted by 9designs

Is it worth the hassle and expense designing & building one from scratch, or just go with an item as per the link?
Unless I can get a clear performance advanage seems not...
Looking to machine woods and Aluminium, on a 1200x800x100 machine.

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Yes it very much is worth the hassle of building one has the Z axis is THE most important area of the machine as it's taking all the cutting forces so if it flexs the results show in finish quality. Doesn't matter how good or well built rest of machine is if Z axis flexs the results will be poor and inaccurate.!!

Regards the linked Z axis then it's Terrible design that as everything you don't want in a Z axis, namely Unsupported rails with not enough bearing support and overhanging front plate. (completely useless for machining aluminium with.!!)
Also the aluminium plate looks like standard hot rolled plate that hasn't even been surfaced flat and true. So any chance of accurecy is none existant as 99% of rolled plate isn't flat.!
For accurate and true Z axis you need surfaces machined flat and parallel or use Machine ground cast tooling plate which is what I use. It's surfaces are ground flat and parallel to each other also being cast plate it's very stable and doesn't distort when machined.

Thanks for the feed back... It's bit a chicken and egg situation, I could easily design something I can't make..... because I don't have a mill/router !! ... Challenge is keeping the design achievable, without spending a fortune on someone machine loads of stuff up for me. So those lovely fully machined parts will probably have to wait to the second build ;)

I have a layout with supported Hiwin rails and 4 carriages, with the motor spindle axis more or less between the rails in Y, looking to minimise the offset from the support to cutter axis. Some of the material/block size are a bit light at present so I'll keep tweaking the design before posting any screen shots.
Also looking how to tie the Z axis back into the gantry.

If it's any help I made my machine with woodworking tools (other than the welder for the frame), bandsaw (standard wood blades cut aluminium), router and standard carbide cutters for wood (cut aluminium okay, just use small cut depths), bench drill, small angle grinder with metal cutting discs, sabre saw with metal cutting blades, disc sander. Then the usual hand tools such as files, hacksaw, etc.

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Originally Posted by 9designs

Thanks for the feed back... It's bit a chicken and egg situation, I could easily design something I can't make..... because I don't have a mill/router !! ... Challenge is keeping the design achievable, without spending a fortune on someone machine loads of stuff up for me. So those lovely fully machined parts will probably have to wait to the second build ;)

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If you use machine tooling plate and profiled linear rails then it's not very difficult to build a very good Z axis with just a Bench saw and drill press.?
Tooling plate removes any need for machining surfaces true and parallel which is a big help in achieving accuracy.
Profiled linear rails provide smooth action and accuracy due to being built to much higher tolerances and stiffer than round type rails but they are also much less tolerant of error so tooling plate helps here also.

One issue with profiled linear rails is the lower profile means a channel needs machining in the plates to allow clearance for the ball screw but this is easily done with the table saw and router.?
First clear out most of the material using the table saw by stepping over blade thickness at a time so nibbling away material. Make the width & Depth just less than finish size. Then use the router to clean up and take to finish depth/width.

For accurate hole locations then make paper templates and stick to material. Then carefully centre punch and drill using drill press.
Same procedure can be done for shaped parts like gantry sides etc, make MDF and paper templates and use guided cutter in router.
Stick paper template on material then cut slightly oversize using a jig saw then fasten MDF template on material and finish to size with guided cutter.

Cutting aluminium with wood blades is easy, so much so that my old knackerd saw blades that struggle to cut hardwoods without filling shop with smoke easily cut 20mm aluminium. Thin kerf blades are best.
Any Carbide router cutter will cut aluminium but with router you'll want to take shallow depth cuts depending on router power and how brave you are.!! . . . Go too deep and it will jump and chatter like tigger so you'll soon know. :whistle:

Go for it you'll be surprised what you can achieve with hand tools and patience.!

Thanks for the tips and encouragement TBH I wouldn't have tried my router on aluminium... so something new to try.

Some general questions from reading and looking at some of the designs.... What strikes me is there is a lot over engineering in frames etc. Ok that's needed in context of cutting duty.
I'm thinking MDF, Plywood and Aluminium plate, with 3- 10mm cutters I assume.

Some of the frames are huge steel welded section, the likes which I've made/designed in the distant past to hold large diesel generating sets.... Then heavy steel gantries on top.... Seems OTT for such cutting duties.

I'm not expecting to machine steel billets, for a start I'm thinking of a Kress 1050 spindle, which I guess could be the limiting factor on loads.
I also want to use 3 4mn Nema 23's... So I have an eye on the weight.
Having looked at the designs here I'm reluctant to post my CAD design now, as its a way off what I see. For example running FEA analysis on say a single 1200mm length of 25mm x 200mm Marine plywood gives 0.0X range of deflection with a 50 Kg load.

So little puzzled at present....... I've kept it to aluminium, thinner sections and orientated in the direction I expect the load and bolted.... ie trying to consider the "I" value/Second moment of area.
I have an open C sections to refine at present, a brace across could do it.

That said some impressive machines on here.....
Happy New Year...

Quote:

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Originally Posted by 9designs

Some general questions from reading and looking at some of the designs.... What strikes me is there is a lot over engineering in frames etc. Ok that's needed in context of cutting duty.
I'm thinking MDF, Plywood and Aluminium plate, with 3- 10mm cutters I assume.

Some of the frames are huge steel welded section, the likes which I've made/designed in the distant past to hold large diesel generating sets.... Then heavy steel gantries on top.... Seems OTT for such cutting duties.

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Yes they are well OTT has my recent little rant tried to highlight for new users just like you.
If your into engineering then you'll have a good idea of whats required material strength wise so just apply common sense. The things to think and design around are resonance, flex along with plenty of adjustment built in to account for the DIY factor.
The most important area and under estimated is the Z axis. It's at the sharpe end taking all the cutting forces directly so any weakness here will show in the finished work.

Quote:

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Originally Posted by 9designs

I'm not expecting to machine steel billets, for a start I'm thinking of a Kress 1050 spindle, which I guess could be the limiting factor on loads.
I also want to use 3 4mn Nema 23's... So I have an eye on the weight.
Having looked at the designs here I'm reluctant to post my CAD design now, as its a way off what I see. For example running FEA analysis on say a single 1200mm length of 25mm x 200mm Marine plywood gives 0.0X range of deflection with a 50 Kg load.

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Going back to the Z axis and cutting forces. The spindle power is key to balance and all those OTT engineered machines you've seen are under powered in the spindle department, by long mark in many cases. That is just waste of money, resources and time IMO which actually reduces the machines capabiltys not improves. IE: Lower acceleration or rapid feeds.
For a serious DIY machine that will cut everything upto aluminium and do it for lengthy periods then don't use a Kress spindle. They are toys in comparison to the chinese Water cooled spindles and not much cheaper.

Weight wise then 3 or 4Nm nema23's with the correct drives and voltage using ballscrews and profiled linear rails will be ok upto 70-80Kg Gantry and give good speeds with correct pitch.

Regards your design then don't hold back post it up for us to see. It's the best way to iron out any grey areas.
I'm not a fan of wood machines for several reasons which have been said but what you have to consider is the main use of the machine. If soley or mostly Wood use then it will be fine if designed correctly. It won't be cheaper than steel thou and much weaker.

If your thinking to cut aluminium then just plane forget using wood it just won't work reliably over time and be repeatable. Cutting Aluminium correctly takes the machine into another level. Yes most any machien will scratch it away but thats not cutting and does take it's toll on machine and tool life if not designed for it.
For a first machine I'd recommend you concentrate on the wood use only and build it the best it can be to do that.

Go for it and DONT let those Massive builds put you off it's NOT REQUIRED.!. . .:thumsup:

Thanks for the encouragement, I did read and appreciate the little rant. I guess you could say I’m into engineering, apprenticeship trained and qualified, working as a mechanical engineer for the last 25+ years :)
So my gut instinct was telling pretty much want your posts have been leading to.

Anyway to try and put some fact & data to it and to defend some use of wood haha.
I compared two beams, one steel 50x50x4mm 1400 long. Constrained both ends and applied a 750N load in the middle. Deflection = 0.225mm

The same applied to a Marine Plywood 25x200mm beam. Deflection = 0.0345mm

http://www.mycncuk.com/attachment.ph...id=14279&stc=1


http://www.mycncuk.com/attachment.ph...id=14280&stc=1


OK the steel is the same in Z and Y.... The Ply only in Z. So perpendicular bracing required. (Ply also has lower thermal expansion and once sealed should be OK for humidity).

What would be a suitable spindle motor, single phase, from where? I can then look to see if I can adapt the mounting to take it.

OK some rendered shots of my hybrid Aluminum/Wooden wonder.... :)


http://www.mycncuk.com/attachment.ph...id=14281&stc=1


http://www.mycncuk.com/attachment.ph...id=14282&stc=1



Gantry assembly complete comes in at around 77Kg. The center of mass is between the spindle and the Section8 cross beam. Cutter is inboard of the gantry end plates which are 275mm wide with two HGW25HC per side.

Width between gantry is 900mm, give a cutting width little over 700mm.
Bed length is just under 1400mm. Z clearance 130mm.

I'm sure it will raise a few eyebows :smile:

Looking very nice! Wood and aluminium together is just sooo easy on the eye :-) Interesting comparison of the wood and steel, just keep in mind that the force in the direction you simulated is actually very small in gantry routers. Because the cutter is located relatively far from the gantry the force on the gantry is more rotational rather than straight in Z and Y, due to the leverage created by that offset. Would be interesting to see the same comparison with that in mind.

I think the experienced guys on here might recommend two ball screws for your long axis to prevent racking.

I've often wondered about the way you have your rails on the gantry because it does sort of make sense to me. Not sure if someone has tried that yet but it might be a bit tricky to do the alignment. I guess the same applies to the way your Z rails are oriented, those Z parts will have to be perfect to get those rails parallel. For Z its also generally recommended that your carriages are fixed to Y and the rail to the moving Z plate.

I do agree with your comment that a lot of frames are way over designed but don't underestimate the importance of a good solid foundation for your machine. Some will likely disagree with me here but the stationary base of a machine can never be too solid. For starters, you don't want the whole thing walking around when its briskly moving that 77kg gantry during cutting:-) What you have there looks relatively sturdy to me but I'll leave it up to the pros for comment.

Ok got to be careful here don't want to sound 2 faced after the ranting but I agree with Mitchejc that the base needs to be strong and sound just like any well engineered structure the foundations are important.

Now this doesn't mean going bonkers but I'm afraid it does mean much more than you have drawn there. I'm not getting into the Steel/aluminium is better than wood argument because you clearly have your sights set on using wood so go for it.! . . But I'm afraid you'll need much more structrue than what you have there.!

The other things I see are classic under estimations of forces acting on parts of a CNC machine. Like the gantry sides are too flimsy for there height and when in use they will flex and resonate when short fast back n forth moves are happening while cutting. Just try shaking 20Kg back n forth really really fast to see how it feels and the effect it has on your arms.?

Single or twin screws depends on what your cutting but your border line on the width for using 2 screws. If your planning on cutting harder material with any intent of decent depths then you'll want twin screws.
If you stick with single then make the lower spanning brace much wider to help combat racking.

Z axis design is bad news and will be a proper bitch to setup. Why re-invent the wheel just use the proven concept of rails on front plate bearings on rear it works perfectly and is easy.
Y axis screw is way too high for my liking and you want to try getting it lower if possible.


Other things with design to think about is how your going to build it and allow for adjustment etc for setting up.! I don't see much.?
Also don't forget cable runs etc.

Hi Jazzcnc, thanks for the feedback.... I hadn't done much calculations of the side supports, so I've now gone back and tried to calculate some forces and deflections. The absolute values are perhaps hard to quantify but the level of improvement is at least relative.

Initial layout I applied a 500N lateral load to one side, ignoring the opposite side for sake of easy calculations. Load just applied to the top fixing hole of the Section8.
The analysis show stress concentrations as such.

http://www.mycncuk.com/attachment.ph...id=14290&stc=1



Adding some ribs and thickening the bottom plate that holds the carriages, also added the Section 8 to better simulate the load area.

http://www.mycncuk.com/attachment.ph...id=14291&stc=1

Stress plot looks much better etc.

http://www.mycncuk.com/attachment.ph...id=14292&stc=1


I hadn't correctly put the weight of the spindle and stepper motors in, so even with the extra material thickness I'm at 66Kg. Hopefully keeps with one X axis ballscrew.

The base, though not detailed is intended to be anchored to a side wall of my garage that it will stand next to. So it won't go for a walk and will further supported. That said I can always beef it up after testing etc.

For spindle is this the sort of item recommended?
http://www.cnc4you.co.uk/Spindle/Spi...VFD-plus-Mount
Is it worth it for a DIY machine?
If so I'll need to look at the Z axis anyway so will have another go at that. I was wanting to keep the offset from the Y axis down. Many designs seem to have a large distance, increasing the moment and forces from the cutter. I was looking reduce the offset.
I'll have another think.... and revisit the layout.

What sort of DOC is normal from Gantry type machines? Tying to get a feel for the forces to expect.

I can see the costs running away a bit on this project :disturbed: