ok, first attempt using sketchup 8

[JAZZCNC]

Jazz,
I don't mind whether i use profile or not is there something else i could use

Well seen as your using steel for the frame then I'd probably look towards using that. Simplest way would be an I beam RSJ type beam.
Would get 180x100 for about £30 if your buying steel for rest of machine. You can buy 1500mm length from Metals4U for £37 plus postage.

This would be an easy option and plenty strong enough for your needs, easy to fasten rails, screws etc.
You will need to make sure you get a nice straight and flat piece then may still have to do a bit of cleanup with grinder/sander and straight edge to make it's flat but it makes a very strong gantry. (Don't be fooled into thinking profile is always straight or flat.!!)
The weight will be slightly higher but to be honest for a wood router if mixed with a strong spindle/big router the extra weight can be an advantage and allow deeper cuts which offsets the slightly lower feeds.

The other way is to use 80x40 box section in a similair setup to the Cad model using profile, just slightly cheaper than profile at £35 for 7.5mtr.

[deannos]

Well seen as your using steel for the frame then I'd probably look towards using that. Simplest way would be an I beam RSJ type beam.
Would get 180x100 for about £30 if your buying steel for rest of machine. You can buy 1500mm length from Metals4U for £37 plus postage.

.

I like the idea of using the RSJ, less for me to do. I assume i wouldn't need to box this in and it can be left as is, would it need support or stiffeners at each end.

[JAZZCNC]

I like the idea of using the RSJ, less for me to do. I assume i wouldn't need to box this in and it can be left as is, would it need support or stiffeners at each end.

Here's how I'd go about it.! . . .Miniumum welding and very strong. I've drawn it with welded corner braces but this could be built without any welding what so ever just drill and bolt if you used 19mm plate for the corner brackets.
Personally I'd weld 5mm plates in the corners and proably a few extra braces across it's length round the back side to just help reduce resonance but again these could easily be bolted, just weldings quicker.
I'd also probably incorparate a thin detachable cover plate on front with slot in it using brush's like in other design just to protect the screw from debris, esp if main use is wood. For the little extra work and expense it will save the screw big time. You could use flexible consertina type covers but they are expensive and can be akward to setup.


EDIT: OR you could just cut a slot in the RSJ and mount the screw on back side and cover with thin plate, something like clear perspex so you can see workings.!! Added pic (RSJ4) with slot in couldnt be arsed to alter sides or move motors but again you'll get the idea.
It will weaken the gantry thou and some extra bracing along it's length would be required.

[Jonathan]

The torsional stiffness of I section is very poor compared to RHS or box section and clearly the ganty twisting directly leads to tool deflection (proportional to the extension of the Z-axis) parallel to the Y-axis. For the same mass you should be able to make a much stronger gantry with RHS, or an equally strong but much lighter gantry. The difficulty is the position of the Y-axis ballscrew is sub-optimal, but you should be able to get it close to between the rails. That's why people often use two pieces for the gantry with the ballscrew between. That also allows adjustment to get the rail mounting surfaces parallel.

Routercnc's spreadsheet posted a few years ago on this forums lets you compare the sections easily.

With this setup you'll be ok for anywoods really so long as your not expecting to cut 20mm in one pass.!!

I've done that in plywood...pretty scary at first but the second cutter survived!

[JAZZCNC]

The torsional stiffness of I section is very poor compared to RHS or box section and clearly the ganty twisting directly leads to tool deflection (proportional to the extension of the Z-axis) parallel to the Y-axis.

There you go again Jonathan completely missing the point.!! . . . . YES AGREE there are stronger and lighter ways to build a Gantry BUT and here's the point.!! THIS WILL EASILY HANDLE THE JOB OF CUTTING WOOD AND BE MORE THAN STRONG ENOUGH . . very easy to build and cheap easy to source.

From what I under stand Dean intends mainly wood use so the Z extension will be minimal, or should be for best results, so given that it's only 1500mm long and twin screw driven being securely tied into both the X axis bearing plates and the gantry end plates and with a good steel thickness on RSJ then the twisting(torsion) will be minimal.

Not every body has the skills or equipment you and I have so minimising the equipment required but still having a strong gantry doesn't get any easier than this.! . . .Drill and saw/grinder are the only tools required.

[deannos]

I would of also thought with the end plates it would be more than up to do the job i require. But i have to bow to knowledge of you guys who know more than i ever will. Was poking around on another forum and found this, but unfortunately i don't think there is a build log.

[routercnc]

Hi Deanos,

I've compared the following 1500mm long steel gantry shapes for tool deflection due to gantry twisting (when cutting wood in the X direction):
_____________________I beam__RHS____RHS
Width _______________100_____100_____100
Depth _______________180_____180_____180
Flange/wall thickness ___6________6_______3
Mass (kg) ____________26______38______19

Deflection (mm)______0.021___0.00037__0.00068

Clearly RHS is better, but I beam is also 'useable' for cutting wood. In the end it is about what is sufficient for your needs and what you can comfortably make.

[Swarfing]

The torsional stiffness of I section is very poor compared to RHS or box section and clearly the ganty twisting directly leads to tool deflection (proportional to the extension of the Z-axis) parallel to the Y-axis.

Jazz i'm with you on this one, i would love to see Jonathan try and twist an RSJ mounted on his machine? you are still talking 5/6mm minimum steel plate.

[Jonathan]

There you go again Jonathan completely missing the point.!! . . . . YES AGREE there are stronger and lighter ways to build a Gantry BUT and here's the point.!! THIS WILL EASILY HANDLE THE JOB OF CUTTING WOOD AND BE MORE THAN STRONG ENOUGH

Of course it will handle cutting wood well. A machine built from wood is capable of cutting wood but that doesn't mean you should make one.

My aim is to make as good a machine as I can, not one that's just meets my current needs.

From what I under stand Dean intends mainly wood use so the Z extension will be minimal, or should be for best results, so given that it's only 1500mm long and twin screw driven being securely tied into both the X axis bearing plates and the gantry end plates and with a good steel thickness on RSJ then the twisting(torsion) will be minimal.

A substantial joint at the ends, i.e. welded plates, will help but it's not the whole story. Good end fixity makes a big difference to beam bending but not torsion.

For instance, using the spreadsheet to calculate torsional stiffness relative to 180x100 RHS:
180x100 RHS section, torsional stiffness, 1
180x100 box section, 3mm wall, torsional stiffness, 103
180x100 box section, 5mm wall (same mass as RSJ above), 164
100x50 box section, 3mm wall (biggest from metals4u), torsional stiffness, 13.9
152x12.7 plate, torsional stiffness, 1.1

So you're talking 10-100x difference. The mass of the RSJ is greater than all of the above and the bending parallel to Y on all the above sections, except the plate, is negligible (much less than the torsional deflection).

So I suggest deannos should use the design Jazz suggested earlier with two 80x40 box sections, with the ballscrew between and a plate on the back. Easy to make with hand tools, no worry about parallelism of the rail mount surface and bending is less than all of the cross sections discussed above according to the spreadsheet. Unfortunately can't calculate torsional stiffness without FEA but it's clearly less.

[JAZZCNC]

I would of also thought with the end plates it would be more than up to do the job i require.

Yes Dean it will be more than up to cutting wood.! . . . . There are issue's which arn't Ideal like easy setup of parallel rails like Jonathan touched upon. . . But if like I said with carefull selection of material they will be very close and then It's not difficult to manipulate them parallel with filling/grinding and spacers/shims.
My experinece with Cheap chinese round rail is that it's not straight or consistant in dimesions along it's length, just as the bearings mounting surfaces are rarely level, flat or true so shimming is nearly always required to fine tune. . . BUT AGAIN . . . It's down to intended use and for machineing wood then the tolerences are much lower and forgiving so can be tolerated.
If you want to wear belt & braces (excuse the PUN!!) and just add some bracing along it's length.!!


Regards that Pic you posted I'm pritty I've seen that before and it was actually built to cut stone.?? . . . . But don't quote on that!!. . . . But I've certainly seen one built the same that was cutting stone and it actually came up for sale on Ebay.