My second router design

[Ross77]

I'm just in the process of doing a post to explain the reasons why the rails work better that way. Looking at the Thompson guidance the load capacity is reduced by a quarter if the force is pulling the bearings away from the rail. whilst you probably wont exceed the max. load of the bearing it is the rigidity that is important.


Nice touch adding the wrap around z axis carrier, should be rock solid. I noticed on the Thompson site that they use a small bolt to squeeze the open ends onto the rail to pre-load them and remove any clearance to further stiffen the carriage. you could easily add this if needed ( note only very small amount and does mean the rails must be perfectly aligned as the factory clearance is to over come minor rail alignment problems.)

With regard to the the red sections I would stick with ali as you will then be able to have decent tapped threads for the rail mounts and backing plate. Normally solid bars are not the most efficient way of making beams as the centre part offers little in the way of torsional and bending resistance (parallel axis theory), but as you want a solid machine for Ali then the extra weight will help damp out vibrations. if you went for steel I would go for box section to reduce weight but these are known to 'ring' as certain frequencies, add the need to grind and its probably best to go for the Ali bar.

I would still be tempted to widen the y axis to create a squarer beam, as this will improve the torsion resistance when the cutter is in he middle of the beam. you could just add some angle to the back plate.

Again just my opinion, not saying you have to do it this way.

[Jonathan]

I would change the joint for the top Y-plate that holds two of the Y-axis bearings to make it easier to adjust the rails and bearings so that they are parallel. Currently you're relying on the plate at the back being both precisely the right height and having parallel edge faces, which is risky. If you change the joint round so the back plate is bolted to the aforementioned Y-bearing plate, then that plate with the bearings will self align to the correct position.

I strongly advise using timing pulleys (most likely 5M HTD) at the very least on X and Y since as you have chosen 5mm pitch ballscrews, you may need to use a 2:1 ratio between the motor and ballscrew to get decent feedrates for cutting wood. Since you're planning on mainly cutting aluminium, that's not so big a deal, however pulleys also help damp vibrations, so reduce resonance problems. Using pulleys also means you don't need motor couplings, which are often a weak point as you can see from the number of threads about which ones to choose. Pulleys easily tolerate a lot more misalignment than oldham couplings, so it arguably makes mounting the motors easier.

Have you considered not having gantry sides and making the machine frame such that the tool operates below the level of the X-rails, rather like your first machine? This tends to be much stronger as it replaces the flimsy gantry sides with the steel frame, which can be made as strong as you want since it's not moving. This is especially important since you want to cut aluminium, which as you no doubt know requires substantially higher stiffness than your previous machine. If you stick with the current design, then most of the time the Z-axis will be operating at near full extension, so the force imposed on the gantry due to cutting forces will be significant. Even if the material is raised up, you still have gantry sides which aren't as strong as a steel frame.

I would still be tempted to widen the y axis to create a squarer beam, as this will improve the torsion resistance when the cutter is in he middle of the beam.

I would too.

[Joe]

Thanks for your input

I have now changed round the Y axis plate as you said, this is one of those little things that will make putting the whole thing together much easier.

Ross what exactly do you mean by make the Y axis wider? doesn't this mean that the distance between the cutting tool and the back of the Y axis will be further?

As for the 2:1 belts, I had considered this but I wasn't sure if they would flex to much as in my old machine the belts stretched quite a lot under load. I was wondering what your experience of this was. And could I ask what sort of feet rate is possible from a 5mm pitch screw using 72V NEMA 23 drivers.

Yes I considered making the gantry sides much smaller but I was doing it this way so that the whole frame that the machine sits on could be fly cut flat so that I don't have any issues with the bed not being flat or to far off flat. but your right this would make the whole machine stiffer. But thinking about it just now as long as the surface that the rails sit on are flat compered to each other then the the machine could level its bed and it should then be fairly flat.

I will change the design slightly and put up a few more pictures

Thanks for all your help everyone

[Ross77]

Ross what exactly do you mean by make the Y axis wider? doesn't this mean that the distance between the cutting tool and the back of the Y axis will be further?

If you look at the last image of post #8 then you have dimensioned the beam as 126mm deep and & 70mm wide (guessing back plate is 20mm) the 70 mm is the bit increase. yes this will increase the distance from cutting tool to back of the y axis but you have gained a bit of space by rotating the rails to the top and bottom and you could allow the very back to over hang the rear gantry bearing a bit as the resultant twisting force from the z axis on the yaxis will act in the centre of the beam. as long as the rear X axis bearing on the gantry sides is behind the centre it will be ok.

If you are going to belt drive the z axis motor then it might also be worth mounting on the back of the z carriage plate so it is lower on the machine, whilst it is not going up and down with the Z axis it is still being accelerated back and forth on the Y axis, bringing all the mass closer to the centre can only be a good thing :o)

[Jonathan]

Ross what exactly do you mean by make the Y axis wider? doesn't this mean that the distance between the cutting tool and the back of the Y axis will be further?

Ross means increase the spacing of the Y-axis bearings. The deflection due to the bearings is proportional to the cutting force squared, so increasing them a small amount makes a surprisingly large difference.

As for the 2:1 belts, I had considered this but I wasn't sure if they would flex to much as in my old machine the belts stretched quite a lot under load. I was wondering what your experience of this was.

When the belts are used to drive the ballscrew the error due to belt stretch will, in most cases, be negligible compared to other errors. The difference with your other machine is the belt drives the axis directly, so the cutting force is applied directly to the belt. When driving a ballscrew however, the cutting force is divided by the effective ratio of the ballscrew drive, which put simply is a big number so the belt doesn't stretch much at all.

And could I ask what sort of feet rate is possible from a 5mm pitch screw using 72V NEMA 23 drivers.

Try this.

[Joe]

Right I have now changed the design quite a bit

The gantry is now much smaller and I have lifted the sides of the machine up to compensate. The main of the frame will be from box section steel, either 100 x 50 or 50 x 50 with the thickest wall I can get. I will try to weld it using clamps to keep it square and flat the best I can but it will then be fly cut to make sure that the top face is accurate and then the machine can face its base plate to ensure that is flat.

My plan is to have a second false floor that can be bolted into the frame which will be around 100 mm thick for machining parts which require less z travel, this should then make the machine much stiffer as the z axis will not be as far out.

As for driving the ballscrews with belts I am going to stay with direct drive at the moment to save some cost, but I am sure for what I am doing the slower rapid rates will not be to much of a problem.

If anyone else has any suggestions please let me know

[Ross77]

If anyone else has any suggestions please let me know

Well you could move......and then also......and put that over there..

Looks good. I know its a welded steel frame but you might want to look at the junction between the side walls and the bed, you might get flex there. Also can you add any bracing the ends or will that hinder your access? maybe look at at a removable end braces?

To help with the resonance it might be worth unequally spacing the bed supports or at least move the central leg off centre, if that makes sense? vibrations have harmonics at multiples and divisions of the base frequency so by having equally spaced connections you can unknowingly increase the vibration.

What is the bearing spacing on the y axis?