[Rading]

Hello everyone!

After studying this forum and others for months, looking at different designs etc. I have finally completed the drawings for my new router table.
But before starting the build, I would like to hear your opinions on my design. Are there areas that needs to be altered or improved to get the best stability/stiffness?

The size of the router bed is roughly 1200x800mm outer dimensions, with an expected working envelope of 1000x650x150mm.

Here are some information about the parts used, some have already been purchased, others are still to be ordered.

Linear components (these has been ordered from the Linearmotion2008 ebay shop):
Y-axis (base):
two anti backlash ballscrew RM2005-1250mm-C7, one for each side to get an even load distribution.
set of SBR20-1200mm supported rails, with SBR20UU bearing blocks.

X-axis (gantry):
One anti backlash ballscrew RM2005-800mm-C7
set of SBR20-800mm supported rails, with SBR20UU bearing blocks.

Z-axis (spindle):
One anti backlash ballscrew RM1605-350mm-C7
set of SBR16-300mm supported rails, with SBR16UU bearing blocks.

I intend to use aluminium for the entire construction, base and gantry will be created out of 15mm stock and 30x60mm extrusions. the Z- axis will be created from 10mm stock.

The spindle I have ordered is a 2.2Kw water-cooled spindle (80mm).

I intend to buy the following steppers and controller: Nema23 Stepper Motor Kit 3.1Nm x 4 Axis

My intention is to cut primarily wood, acrylic but also occasional aluminium parts.

What do you think of the design, is it okay?

/RadingAttachment 224040Attachment 224042Attachment 224044Attachment 224046Attachment 224048Attachment 224050

[ger21]

It's always a good idea to ask before you start making purchases.
Honestly, those 5mm pitch ballscrews are not the best choice. 10mm would have been much better, and would give you double the speed for the same cost.

As for the design. The gantry sides should be thicker, and much wider at the base, to spread the bearings apart. This will result in shorter travel, but a much stronger machine.

[dmalicky]

Good advice from Gerry, as usual. For stiffness, it's good you have dual X-drives, X rails near table height, a large vertical spacing between the Y rails, and your spindle clamps with good vertical separation. Still, there are number of very flexy spots on the design you posted; please see other recent posts for advice, for example:

- If using SBR, Z bearings need much more vertical separation, and Y and Z rails/bearings need to be oriented in opposite directions. Try the spreadsheet in the first link to see for yourself:
http://www.cnczone.com/forums/diy_cn...ml#post1435822
http://www.cnczone.com/forums/linear...ml#post1412778

- Make the gantry a large tube or tubular extrusion:
http://www.cnczone.com/forums/diy_cn...ml#post1430502

- For at least the Z axis, replace the SBR16 with profile rail and bearings. SBR20 is fine for the X axis, for your cutting needs. SBR20 is ok for the Y axis if the rails are oriented up and down. The xls spreadsheet in the link above will show the difficulties of using SBR for Y and Z.

[Rading]

Hi guys,

Thank you for your input. It is always great to hear from someone with experience before making bad desisions.
As for the 2005 ballscrews, I managed to change the order, so the X and Y axis now will be using 2010 ballscrews instead.
The reason for ordering the 2005 was that many of the designs I have studied, was using 1605's, and I just increased the diameter, thinking that the pitch was ok.

I was concerned that the gantry bottom was to small, and that i would have to insrease the distance between the bearings, I will change that from 100mm to 200mm. The gantry S shape was taken from another design.

As for replacing the Z axis SBR rails to better ones, that must be a future upgrade. I have already more than doubled my initial budget. Funny how one thing leads to another when making design desisions :-)

Gantry sides are made from 15x200 stock, as that is the thickest material that I have access to, can I bolt to pieces together, and obtain better stiffness that way?

I will have a look at the links posted, and redesign the gantry so rails are positioned in an Up/down orientation, and using 30x60mm extrusion to support the rails.

Again, I appreciate your input!

/Rading

[ger21]

I would consider bolting perpendicular braces to the outside of the gantry sides, rather than doubling them up.

[dmalicky]

Good to hear it's helpful. Yes, it's so much easier to change things before it's built. That's great you got the 2010 swapped in.

200mm spacing (outside to outside) for SBR 'feet' bearings is a little small for my comfort level, but probably ok for wood cutting. The S shape is good to keep the cutter between the front and rear bearings (in the side view)--that's important to prevent tipping when drilling or similar.

For the SBR16 Z rails, since those are 300mm long (good choice), you can get a healthy 150mm vertical spacing of the bearings and also get 150mm of Z travel. If the rails are also oriented in opposite directions, the stiffness should be pretty good for wood. Yes, when starting out, or even later, budgets and schedules are often akin to fiction.

The best gantry extrusion is the Misumi GFS8-100200. At 200mm tall, though, it puts the SBR20 rails (up-down orientation) about 10" apart. Those SBR20 blocks need to be in a square pattern, so that makes them 10" wide, which cuts the gantry travel. So a better option is 8020's 3060 (3"x6")--it's less $, too, and probably a reasonable match to SBR16 in stiffness. 30x60mm is much too small, even many of them. Check out the posts; large tubes rock. To save $, avoid extrusion and get a regular alum or steel square tube. 5x5 alum would probably be good for this machine. Or a BB ply tube makes a stiff and low-cost gantry.

[Rading]

Hi David, I have been reading up on the posts you linked to, and looking into what materials I have access to.
The Misumi 100x200 is a bit to large for my project, due to both price and the required size of the y-car.
I can't seem to find the 8020 extrusion, when i look at their eBay store it is empty.

I have been looking at the alternative extrusions from both misumi and kanya, and have found the misumi HFS6-60120, the benefit of this extrusion is that the spacing between the T-slots is 30mm which alligns perfectly with the SBR20s. The question is if this profile has a to small crosssection to support two SBR20s upside down?

One thing I have been thinking of, is how you would attach the SBR20s to the T slots of the misumi 100x200 when the spacing does not match that of the rails, any advise on that?

/Rading

[dmalicky]

A 60x120 mm is about 79% the size of a 3x6 in. That seems close, but torsion and bending stiffness go by size^4, so the 60120 will have about 40% the stiffness of a 3x6". That's probably still much better than a 15mm thick plate, but I'd try to get the 3060 or something similar. Also consider regular rectangular or square alum tubing; powertapping is fast... cordless drill, spiral point, 1/4 or 5/16 (6-8mm).

Not sure if you are in the US: 3060 extrusion 15 | eBay

Yes, SBR rail attachment is an issue for many extrusions. It's easy to drill new holes in the SBR base, but it's only 45mm (1.77") wide, so won't reach the wider hole spacings. It barely fits the 3060; maybe sandwich the SBR base under long doubler bars for good clamping.

[Rading]

I just had a nother look at the 60x120mm extrusion, its material thickness is only 2mm, so I think it will colapse when forces are applied to it.

I live in Denmark (europe), so I believe that's why eBay turns up empty when it comes to the 8020 extrusions, including your link.

The largest alu tubing that I can get a hold of, is 120x120mm with a material thickness of 5mm. Or I could go with steel instead, but that would bring up the weight a lot, which I would like to avoid, due to all the other items that would have to be reenforced to support the added load.

Perhaps the best for now would be to create a rectangular box from 15mm alu plate, say 80x150mm and put some bulkheads into it to stiffen it? Much the same approach as using BB-ply, but with alu plate instead. If I were to use BB-ply, what dimensions would you suggest?

/Rading

[cpr]

That dimension of Aluminium is already bigger than what a well-stocked wholesaler/industrial supplier is usually listing in Western Europe.
You have a very good source!

[TSJ]

I am new to the CNC world, but am I way off base. Why not extend the gantry sides down below the table and mount the ball screws there. They would not collect as much dust or shavings.

[wizard]

I just had a nother look at the 60x120mm extrusion, its material thickness is only 2mm, so I think it will colapse when forces are applied to it.

Possibly. This metric dimension make me think about what exactly 2 mm is. Usually you run into other issues before you have collapsing problems.

I live in Denmark (europe), so I believe that's why eBay turns up empty when it comes to the 8020 extrusions, including your link.

The largest alu tubing that I can get a hold of, is 120x120mm with a material thickness of 5mm. Or I could go with steel instead, but that would bring up the weight a lot, which I would like to avoid, due to all the other items that would have to be reenforced to support the added load.

That isn't too bad at all for a machine this size. However I don't see a steel beam causing you a huge number of problems on a machine this size. You already need to make a few improvements to the machine no matter what you do with the beam material.

There are other issues that are important to understand before choosing a beams thickness, even if it is stiff enough. For example if a steel beam is the material thick enough to drill and tap for the linear rail mounting screws. If not is the beam big enough to reach inside to install mechanical fasteners. In other words don't run out and buy a beam until you consider assembly issues. Do you have a way to install beam stiffeners if needed. In other words once you select a beam that can do the job mechanically make sure it is suitable for the human element when it comes to assembly.

Perhaps the best for now would be to create a rectangular box from 15mm alu plate, say 80x150mm and put some bulkheads into it to stiffen it? Much the same approach as using BB-ply, but with alu plate instead. If I were to use BB-ply, what dimensions would you suggest?

/Rading

That would end up being heavier than a suitable steel beam or close to it. The thinking here being that a 5mm thick steel beam would be more than enough for many arrangements of your linear rails. An aluminum beam of the same thickness would likely work to if the linear rails are bolted in with pass through bolts. Beyond that more post processing would be required to square and flatten up the beam. If you have a decent shop to machine the parts of the built up beam it probably wouldn't be all that. If the beam is welded up you will likely need post weld up heat treat. Also it would probably be easier to go with a square beam.

The thing is a built up beam is a lot of work when you can buy steel and aluminum beams in so many sizes it is almost a joke.

[Rading]

Thanks for your comments Wizard.
I have been thinking alot these past days, and have come tomthe conclusion that I will drop the idea of making a box, it will simply be to time consuming and require precition beyond what I have tools for.

I am limited to the tools that I have my self, and I'm not so lucky that I know someone with a mill. What I do have is a good chop saw, a brand new precise floormounted drillpress, tablesaw and a whole lot of common hand tools. I also own a MIG welder, although it has been quite a few years since I last welded anything.

With that in mind, I think it will be much better to use alu/steel tubing, and simply bolt it together.

I will try and redesign my router, using a steel beam that I can drill and tap, for the rail mount. Just like you said, there is also the mounting issues that has to taken into consideration. 5mm steel will make a good base for that.

My goal is to utilize the power of the 2.2kw VFD spindle that I have ordered, And hopefully be able to cut hardwood and aluminium without having to suffice with very shallow cuts. I have no idea what to expect, but anything better than 0.5 mm cuts in alu that the momus design achieves will make me happy.

Once I have drawn up a new design, I will post back for further comments.

/Rading

[dmalicky]

Yes, 2mm thickness is slim -- good to avoid for a number of reasons.

Many decisions depend on the stiffnesss target for the machine, which depends on one's cutting needs and expectations. For cutting wood, a machine stiffness target of around 5k lb/in is probably reasonable. That machine could also do alum with many light passes. For high accuracy or heavy cuts in alum, I'd target about 20k lb/in. Since the gantry is 1 of about 10 components that affect overall stiffness-at-the-cutter, we want the gantry's stiffness-at-the-cutter to be about 10x the overall stiffness target. E.g., if targeting 10k lb/in, the gantry should have a stiffness of around 100k lb/in. For a more flexible machine, a good high RPM spindle (with carbide cutter) lowers the cutting forces a lot, which enables ~deeper cuts in alum.

Here are some FEA results, assuming 650mm-travel and a 150mm Z-clearance:

120x120x5mm alum tube, with end-caps (bulkheads) to stabilize each end: stiffness-at-the-cutter is 40k lb/in (on target for an overall machine stiffness of 4k lb/in). Without end-caps: 20k lb/in. With multiple internal bulkheads or a diagonal sheet: 100k lb/in.

120x120x5mm steel tube: Steel is 3x stiffer than alum, so multiply all those numbers by 3. Weight is 16 kg -- not much.

150x150x17mm built-up BB ply box tube, with multiple bulkheads: 40k lb/in. I would go bigger, except the SBR rails get far apart. Attaching sheetmetal to the outside of a BB gantry would block moisture and up the stiffness quite a bit.

By comparison, the SBR gantry bearings (oriented up and down, 7" apart on centers), would have a stiffness-at-the-cutter of about 50k lb/in -- also on target for 5k lb/in overall. With 2 components around 50k (SBR on gantry and Z), it would be difficult to get the overall machine stiffness to 20k. But 10k is feasible: we'd need some of the other components to be in the 200k range.

Also think about which option gives the flatest and straightest mounting surfaces for the rails. Wood is the most work to build-up, but the easiest to level if it needs it. But it would be more challenging to get the upper and lower surfaces super parallel for the SBR rails. For tube, if at all possible, inspect it with a long straightedge before buying. In the US, I find steel tubing of 3/16" wall (5mm) or thicker tends to be very straight and uniform (the sides are slightly concave, but the concavity is uniform down the length).

A 120x120x5+ mm steel tube is looking good.

TSJ, yes, screws below the table keeps them cleaner. The usual goal is to have the cutter, rails, and screw near the same height. So a little below the table would be fine.