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Thread: help needed 8' x4' cnc
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18th September 2012, 10:09 PM #1Senior Member
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help needed 8' x4' cnc
Hi guys
I'm thinking of building an CNC which would be able to work on full plywood sheet size... 1200x2400
After fair bit of research I'm thinking of using linear bearings for X and Y axis
To drive everything I am considering Homann G540 kit with 3 x387 steppers
Due to the size of the table, using ball screw would cause lots of issues... so I'm considering using combination of linear bearing rack and pinion...
General setup would be.. X axis sitting on two linear bearings (one on each side) with third one (the combination of linear bearings and rack and pinion) would go through the middle..
Y axis would be supported with one linear bearing and one combination of linear bearing and rack and pinion...
Z axis could be ballscrew or rack and pinion...
As a construction material I'm considering using mild steel as I might be able to get some mild steel at wholesale price.....
I have looked around and couldn't really find any designs with such size... Is there any reason?? Am I missing something??
Instead of spindle I'm thinking to make three different holders.. one to hold 1/2" makita router, one to hold 1/4" trimmer (like bosch GMR1) and one to hold dremmel...
I hope the Homann kit will be sufficient..
Any help, suggestion, information would be appreciated...
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18th September 2012 10:09 PM # ADSGoogle Adsense Advertisement
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18th September 2012, 11:56 PM #2
at least 1220x2440 ( measure a sheet ) , then add thickness of the largest bit you would like to use on the edge of the sheet , you will aslo have to look at the width of your yaxis ( centre of spindle ,to the back of the yaxis frame ) and allow enough so you can lower the spindle to the edge of the sheet , dont skimp on the yaxis foot print on the xaxis rails , the wider the better
nice rails , what will they set you back per meter , and whats the play in the gear like ? (backlash )
the pitch of the teeth on the smallest size is 6mm , on ball screws used by most its 5mm per revolution,direct driven from the stepper ( 200 steps , moves 5mm(0.025mm per step) ) , you are going to have to gear it down , adding more backlash , a 16mm thick 5mm pitch ball screw will not span 2400mm
http://www.mechmate.com/
when you weld steel , it pulls as the weld cools , causing all sorts of havoc to the unsuspecting non pro welder , if you know to set it up "out of square" so it ends up square after , you should be right , also welds leave behind tension in the steel that could come back to haunt you , "shot blasting " your welds will help with that , cold cut the material and bolt as much as you can ,depending on the manufacturer the steel could be more than 0.2mm out in size over a 1meter length , driving it home on the roof rack of your car , will probably make it worse than that
all but the last of them dont have a variable speed , so in slow areas you might "cook " your bit , or burn your material , a VDF/ spindle is well worth it, a router will slow down its RPM as the load on the bit gets larger ( not good) , a VDF/Spindle will up the amps going to the spindle ,. to keep it spinning at the RPM you set it at , there is a big difference , also noise , a spindle is nice and quiet( well sort of ), not like a makita router at all, more like the dremmel , but with the power of the makita
P.s almost forgot to ask , what do you plan on making with it/ use it for ?how come a 10mm peg dont fit in a 10mm hole
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20th September 2012, 12:59 AM #3Senior Member
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heya SawdustSniffer
Yeah.. the full size plywood sheet is slightly bigger than 1200x2400 .. was too lazy to type real size..
Found price for standar economy grade linear bearings..
rails are $110 per metre and bearings are $66 each
now they didn't have combo one.. was told ronson gears might have it..
thinking forward.. seeing the economy linear bearings ..they seem to be sturdy enough to hold the lot..without the need of a third one in the middle..
So next step will be to find the price and sizes of rack and pinion..
Had a quick chat to the sales guy at linearbearings.. had a look at aluminium profiles they are selling.. but price point is a bit steep.. starting price is $20 per metre and that is roughly 30x30 profile.. so thinking I might pursue mild steel.. construction wise i'm thinking of bolting everything together rather than welding it..
since I'm in the rental property.. welding such size table would make it very difficult to move it out... but bolting it together will allow me to disassemble it when the time come..
what do you guy think??
spindle might be good.. but i'm trying to use existing tools / equipment / materials as much as possible.. but at the end of the day if it has to be done then spindle it is..
Primary purpose is to cut plywood and mdf shapes out of sheet.. and do fancy edging.. (like a router)
on top of that I'm considering using it for more primitive purpose such as planning / thicknessing..
I have access to american oak and vic ash offcuts (american oak being rough sawn.. generally comes in quite generous thicknesses 48 - 55 mm thick)
Then I might use it to make square bowls / plates / fancy hampers
On top of that I'm considering using it to make PCB boards (that's where dremel tool comes in place)
And cutting some aluminium sheeting to size / shape (0.5 - 2mm thickness)
by the way.. this updated forum is starting to give me sh(@#@... every time I try to post it crashes so this is my third attempt in replying it.. (even posting this thread took several attempts)
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20th September 2012, 01:51 AM #4SENIOR MEMBER
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Rack and pinion / www.tea.net.au or Home / back in 2009/10 I purchased some R&P for a full size machine (from TEA) the cost was $300 delivered, it was custom ordered from overseas as well so it wasn't too bad price wise, they seemed to only stock 1M and 2M lengths as standard, that was back in 09/10 so things may have changed.
For aluminium profile check out 80/20 Australia Pty Ltd or Modular Components and Automation Australia - Item Aluminium although price wise they are all similar, an aluminium gantry isn't cheap but its strong, light, straight and you can easily bolt things to it.
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20th September 2012, 01:57 AM #5
30x 30 profile in aluminium , ????/ for a 1300 span for yaxis you should be looking min 80x120mm , even larger , did you look at the mechmate web site ?
how come a 10mm peg dont fit in a 10mm hole
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20th September 2012, 10:00 AM #6Senior Member
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Had a look at mechmate.. Think they are slightly overengineered (x axis)
Yeah.. I was just putting the size and dollar value just for comparison... For x axis Ilm planing to use 100x100 mild steel (4 legs) Y axis - I'm thinking 50x25 mild steel tubing.. or something slightly thicker.. (trying to keep the weight down)
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20th September 2012, 10:25 AM #7Senior Member
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Hi Ch4is
what size rack and pinion should I be looking for.... I guess size will affect the speed and precission??? WHich one is better.. more teeth ??? I have to admit that this part I havent investigated properly...
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20th September 2012, 10:54 AM #8SENIOR MEMBER
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I used M1 myself, its ~15x15mm rack
22T pinions, but I've read 30T is better as it has more teeth in contact with the rack.
The machine I purchase the rack for was never completed (lease ran out on the factory) so I couldnt see it running on R&P but I believe M1 is standard for most home cnc builds.
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21st September 2012, 04:57 AM #9Novice
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21st September 2012, 08:31 PM #10Senior Member
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Thanks Croxly
Depending of the stepper motor capacity.. (another thing I need to investigate) I might opt for mild steel gantry
As I'm planing to get the G540 with 387 oz.in I am hoping that 387 oz.in might be strong enough to move gantry...
Aluminium seems a bit pricey... although for the gantry it might be better option..
It might simplify things if I use same material throughout...
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25th September 2012, 10:14 PM #11New Member
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Hi Prle77,
I have built a Mechmate machine and are very pleased with it. Quite robust and tackles anything I have given it.
Chris
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26th September 2012, 01:02 AM #12Senior Member
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Heya Chris
Info info info...gomme everything you got!!!
What size stepper motors did you use?? wondering if the previously chosen homann kit would hold up
what about rack and pinion?? (I am assuming you went that way)
Can you show us some photos please...
Thanks in advance
Alen
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26th September 2012, 10:20 AM #13SENIOR MEMBER
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Hi Alen,
Sounds like a nice build if you decide to go that way.
I ran similar sized motors on a 7' x 4' (worked around some cheap linear rails i got) with gantry made from an 80/20 profile 6" x 3". Was ~$300 for 1500mm long section. Would have preferred mild steel but at the time it was easier than having to surface mill the mild steel as i am running linear rails and the surface was toleranced better. The motors were/are run on G201 drives, but the G540 and those motors will work fine. The drives had 1:4 gearboxes which gave 1200+oz/in which is plenty to move a gantry that size and take decent cuts. Its just a matter of knowing the limits you can push to get the results you want. I am currently upgrading the machine and going with bigger nema 34 motors to get more holding power to take deeper DOC. All were running 22tooth pinions (will be changing to 30tooth soon) on module 1 rack (X and Y Axis)from TEA. TEA now also stock/can get up to 4m lengths of rack upon request. Already looking at designing and building a new cnc if i could sell this one when finished just cause i enjoy it, and wouldn't change much with the drive gear.
One thing is, you linked the package for the driver and 3 motors, i'd recommend 4 for that size of a machine(2 on the X axis, 1 across Y axis and 1 on the Z axis) as a single motor that size i think would struggle moving the weight of the gantry, gantry supports, z axis and the forces during cutting. It would work but i think cutting quality could be effected.
What linear rails are you looking at using?
Start reading alot, on here, mechmate and cnczone sites. There is alot of detail out there with plenty of different ideas. You need to find something that suits your skills and machine needs etc. Also for me designing, building and trying is half the fun.
Daniel
P.S Also make sure you have plenty of room to store/run the machine, a big machine is nice but sometimes for hobby use is overkill.
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26th September 2012, 10:34 AM #14SENIOR MEMBER
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Just saw the rails/rack combo your thinking about. Hope your pockets are deep!!! The rails come is 25mm as the smallest and costs ~40c per mm or $400 for 1m. The bearing blocks are roughly $75 each and on top of that the pinions are custom and expensive (~$100ea) and you will need to design the reduction/mounting system for the pinion shaft/reduction drive. Also i don't think anyone in australia stocks it so shipping will also be expensive.
Personally, for that price i'd recommend buying NSK linear rails and separate helical rack and pinion setup or the new nexan roller pinion system. Although cheaper rails and standard rack would be fine for a hobby setup and if your just looking to run it as a hobby system is what i would recommend.
Daniel
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26th September 2012, 08:28 PM #15New Member
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Hi all,
The Mechmate has rack n pinion Mod 1 as per the plans for it, definite 2 motors for x at that length and width. Our rails and 65mm mild steel angle turned into 65 x 25mm V rail using v rollers, we found this is the same system used on some very high end CNC machines but obviously better quality hardened rails. All seems to running really well with good accurancy and good repeatability. I estimate that we spent around $5k all up on a pretty robust CNC machine as the end result.
That included all electronics like gecko drives, steppers, 2.2kw spindle and all the other bits n bobs.
Chris
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