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jonboycnc
04-09-2013, 01:05 AM
I'm trying to organise getting a CNC router at the London Hackspace, and I've been reading the really helpful DIY design advice on this forum.

We want to be able to cut big lumps of aluminium, as well as wood and plastic. We also want XY travels of at least 400mm x 400m, which rules out getting cheap mill as far as I can see - our budget is only around 1500-2000.

What we do have going for us is a very large workshop, on a 5 year lease, with lift access. So little need to worry about space taken up and difficulty moving the machine around.

This gets me thinking that we need a fixed gantry router. As I understand it, most router designs fare poorly cutting deep inside blocks of aluminium, and taking deep cuts, because of lack of rigidity. A fixed gantry machine could be made very rigid, especially if we didn't worry about space consumed and portability.

I've been looking at prestressed concrete lintels. They're concrete and huge so have fantastic vibration dampening, they're designed to take tensile load which concrete is usually poor at, they're precast so have very little shrink/creep left to do, and they're cheap. A lintel 215x140x900mm costs 30 and has a "safe working load" of 83 kN/m (whatever that means).

This leads me to the attached beginnings of a design for a router frame. It would have travels of 400x600x100mm and mount a 2.2Kw Chinese watercooled spindle.

99649965

To build I would:
1) build a heavy duty table to carry the whole lot
2) bolt & cement in place the padstones that are the support for either end of the gantry.
3) level the table top with self-leveling epoxy
4) Fastened together the 2 base lintels by steel straps at their ends, and epoxy level on side.
6) place the base lintels on the levelled tabletop, level side of the lintels down (level touching level)
7) epoxy level the front face of the gantry, cement the gantry in place using a digital spirit level to ensure the face is true vertical; then epoxy level the top face of the gantry if we need that.
7) epoxy level the other (now top) side of the base lintel pair.
8) Put a bolt though one corner of the base lintels, and hold the other corners down with bolted straps.
9) mount the gantry & Z carriages, wire up everything
10) mount the moving t-slot table
11) machine the moving tslot tabletop flat
12) adjust the base lintels (swinging around the bolted corner) to square up the machine, using test cut pieces and a micrometer to test for squareness.

QUESTIONS
(many not new, sorry ...)

1) Can someone point me to a design for heavy-duty X & Z carriages for the gantry? There's so much out there but little of it is fixed gantry and even most of that is still thinking much more lightweight. Given that I've got this monster frame, I might as well exploit its potential!

2) For this short 10mm Z travel, is supported rails better or worse than unsupported?

3) If I can afford to upgrade one axis from supported round rails to profile rails, which should it be? Would 15mm profile rails be beefy enough?

4) Which face/faces is best for the X axis rails on the gantry? I'd rather not have any on the bottom as that either reduces travel or increases gantry span.

5) Can anyone help me figure out how wide the X axis travel can afford to be before the gantry starts deflecting too much? I can always add a second lintel, bolted and epoxy cemented to the top of the first one, if that helps.

6) Am I barking up the wrong tree with this concrete lintel idea?

Any other advice?

thanks
Jonathan

JAZZCNC
04-09-2013, 02:27 AM
Hi Jonathan,

This would work well and I've considered something similar my self but space is my ruler so decided 150x150 steel box section filled with sand is better for me. I've actually got the steel it's cut and waiting just need time now but there will be a thread created when build starts.

Regards your questions then here my opinions/suggestions.
#1 You need to clarify what you mean by carriages.? Do you mean carriage plates the bearings fasten to IE: Plates which carry Z axis and run along gantry.? (this would be classed has X axis on fixed gantry)

#2 Do you mean 100mm Z axis not 10mm.? Either way then DONT use unsupported rail of any kind.
Supported round rail is the minimum and really if your after a monster machine then you want profiled linear rails.

#3 See above but if you only had enough for one axis then make it the Z axis has this is the most important has it takes all the cutter forces. Next would be the moving table (Y Axis) but Again really you want profiled on all axis for a proper machine.
15mm rail would work ok but 20mm would be better has the bearing blocks are larger and will give better support, use the wide type for moving table has they give best mounting and support.

#4 Front face

#5 Nope not me sorry but I'm sure you'll easily get upto 1.5mtr or so before it's own weight starts having an affect. There will be some specs somewhere that shows it I'm sure.

Here's some other things to consider regards Cutting Ali.
The 2.2Kw spindle won't cut great big lumps with larger cutters so if your wanting to shift large amounts of material with larger size cutters (above 8mm) then you'll need more HP/torque.
Also you want flood cooling so work that into design, you'll need to seal concrete.!!

~andy
04-09-2013, 09:30 AM
On a practical note, the concrete in these lintels can be a real bugger to drill and you have will have to avoid the steel inside.

I'd go with steel box section, filled with sand, so that you can break the machine down to move it.

You are going to need to build a decent table to support this monster anyway.

deisel
04-09-2013, 10:23 AM
i had a crack at a fixed gantry 400x400 mm work area,i,ll be pulling it apart soon and upgrading a lot of it.
i noticed your gantry is positioned on the halfway point of the bed but it will look very different once you factor in how far the z setup/spindle and mount pertrude,you want the center of the spindle to be at the halfway point of the bed.
9967

JAZZCNC
04-09-2013, 10:50 AM
i had a crack at a fixed gantry 400x400 mm work area,i,ll be pulling it apart soon and upgrading a lot of it.

That's a nice capable looking machine you built there Deisel can see there's some thought and care gone into it . .:thumsup:

Out of interest what you going to be upgrading.? The upgrade would make a good thread and this place is in need of some build or re-build threads so do use all a favor and post about it please.!

irving2008
04-09-2013, 06:33 PM
FYI 83kN/m means a 1m lintel supported under the ends can support a distributed load of 8.3 tonnes without cracking and deflecting less than a prescribed amount as set out in tables issued by the BSI and used by structural engineers & building control. For this purpose its massive overkill and, as has been said, you can't drill into it as if you damage one of the prestressed steel rods you can release some serious forces and I wouldn't want to be in the vicinity when you do!

deisel
04-09-2013, 07:00 PM
That's a nice capable looking machine you built there Deisel can see there's some thought and care gone into it . .:thumsup:

Out of interest what you going to be upgrading.? The upgrade would make a good thread and this place is in need of some build or re-build threads so do use all a favor and post about it please.!
cheers i wish id had a bit more patience though, changes.
the x axis is being stripped down
plan is to completely sheet off the bed like the one in the image below ,then 1 big 20 or 30mm thick plate running down the centre that will be slotted to take the slightly longer 20mm rail (that ive got to order),i should roughly get an extra 200mm of travel.

y axis im scrapping and starting again too much to list on the lads thread,so a new thread sounds like a plan ta
9969

Boyan Silyavski
05-09-2013, 01:03 AM
IMHO you should decide on bigger spindle and stronger supported rails, at least 20 size. And base your design on these and further develop it, not the other way around.

If i was building a monster machine and was tight on money, i would look at ebay for used roller linear guides in good condition. When i finish my current build, my next one will be a monster, so that's my conclusion after continuous research.

ramsbury
12-01-2014, 07:19 PM
Prestressed concrete deflects quite a lot in my experience.
Im a master mason and build onto prestressed concrete beams and they bounce like a rubber ball when heavy loads are put on or even men walking over them. they will deflect but usually when the wieght is put on and stays on.
thats beams at 225x150 at 2 metres deflects 10mm.



FYI 83kN/m means a 1m lintel supported under the ends can support a distributed load of 8.3 tonnes without cracking and deflecting less than a prescribed amount as set out in tables issued by the BSI and used by structural engineers & building control. For this purpose its massive overkill and, as has been said, you can't drill into it as if you damage one of the prestressed steel rods you can release some serious forces and I wouldn't want to be in the vicinity when you do!

EddyCurrent
12-01-2014, 11:26 PM
Put a catnic underneath it ?