Well here goes... Aluminium frame router design

[CharlieRam]

If your mostly cutting wood then this design will be fine but if your planning to cut aluminium often then I wouldn't use it.

Yeah I thought as much, What about this one, I have also reduced spacing on Z and Y by 50mm giving the machine an effective travel of 800 x 650 x 150. I thinK this should be a lot more sturdy.
I haven't thought much about the base yet tbh, I was thinking of either having it mounted vertically to save space (if I decide to make X longer) or just build an aluminium 8040 table, I am assuming I will need to brace up the bed of the MC to stop twisting?
For the M/C bed I was going to use 15mm MDF and then place a 5MM aluminium sheet on top of that, drilled and tapped for t bar/ slots?
What do you think, Still not sturdy enough or will it work?

[JAZZCNC]

Little better but still not what I had in mind if your seriously wanting to cut aluminium. Check out the forum for machines with base frames with sides raised and rails sat on top. This design is more router biased and it will cut aluminium no problem but not with the same abilty as machine aimed directly to do this.

Also that Z axis design is bad news. Again check out threads and look at Z axis designs that put the rails on the front plate and bearings on rear. Your design is a constant length lever that won't even reach the bed surface and will give the same amount of flex whether it's at the bottom of top of it's travel. With the other design the lever length is variable dependent on material thinkness or distance from tool.

MDF for the bed isn't ideal and I certainly wouldn't waste money putting aluminium on it. MDF changes shape from day to day so if your wanting accurecy then you'll need to surface it often. Personaly I'd try to find the most stable material you can afford for the machine base, Good quality ply will work then skim surface it flat and seal or put the aluminium on that. Then use MDF as a spoil board that you don't mind cutting away or into.

[CharlieRam]

Little better but still not what I had in mind if your seriously wanting to cut aluminium. Check out the forum for machines with base frames with sides raised and rails sat on top. This design is more router biased and it will cut aluminium no problem but not with the same abilty as machine aimed directly to do this.

When you say ability, do you mean accuracy or chatter or what?, I did see the designs where the gantry sat directly on box section but I was trying to keep the footprint down to a minimum (Lack of space!) and I thought the angle brackets would give less flex than a box section, obviously not though. Would it help if I were to increase the size of the angle plates or would you still say box section is the way to go.
I will look at the Z axis, I never thought about doing it that way but now when I think about it, it makes sense thanks, I would prefer just ally for the base but its pretty damn expensive for a sheet that size, I will look into ply.
Thanks, Charlie

[CharlieRam]

Your design is a constant length lever that won't even reach the bed surface

Just noticed this bit, It wasnt meant to reach the table because I was hoping to offset it with adjusting the spindle height and I left 150mm between base of gantry and M/C bed so I could hopefully fabricate a 4th axis on the unused part of the mc bed.

[routercnc]

I Charlie,

I wrote the original stiffness v5 sheet that you mentioned in the first post a few years ago, plus later posted a v7 which may be of interest.

I've tried to replicate your results but cannot match them (don't know all the dimensions etc that you used). So I'll just make some general comments.

The 13um in Z is bending due to the spindle weight applied to the centre of the gantry. This seems fine to me.

The 30um in X sounds like you've only noted the 'tool deflection' number which is only the bit due to twisting. There will be a bit more overall deflection due to X bending to give you a final total deflection in X. This is starting to sound a bit high (30-40um is 0.03 - 0.04mm) for aluminium. However, this is at maximum extension so you can reduce this problem as you've stated by raising the job in Z.

Biggest thing to point out though is that you haven't run the analysis on the gantry sides (plates) in the Y direction. Depending on your exact numbers this is likely to be similar or greater than the X deflection and because it is just bending (you can ignore Y torsion) and there is no work-around in terms of raising the job in Z etc. This is because all the tool forces in Y go through the ballnut and this is at a fixed height above the X axis bearings. This is why Jazz keeps pointing you to the raised X axis designs for aluminium since this offset between ballnut and X axis bearings is basically zero. You are then free to beef up the bed/raised X axis sides to react this force.

I have a machine broadly similar to your posts so far and it will cut aluminium but I you can tell from the sound it makes and machine vibration that it is approaching the limit. If I was cutting aluminium more regularly then this machine would not do. For the occasional piece it is OK.

Finally, feel free to experiment with the spreadsheet but do just use it to get a feel for general comparisons and where to place effort and material. As a like-for-like comparison of different gauge, sections, and dimensions it will give you direction, but don't expect to get the quoted deflections out as there too many simplifications. So long as you use it as just another way of investigating the design (along with reviewing the build logs and the invaluable subjective opinions of experienced builders) then you will be OK.

Good luck with it and welcome.

[JAZZCNC]

Just noticed this bit, It wasnt meant to reach the table because I was hoping to offset it with adjusting the spindle height and I left 150mm between base of gantry and M/C bed so I could hopefully fabricate a 4th axis on the unused part of the mc bed.

If I'm understanding you correctly then the Unused part is exactly that Unusable and putting a 4th axis there is pointless as you can't access it with spindle.? . . . . ( Edit: Ok Excuse that comment I'm not exactly firing on all cyclinders at the minute and realised you mean to put 4th axis head in that space and tail stock at other end running material down it's length.! . . . I was thinking material across the gantry.!!)

As for rest of machine then like I say it will cut aluminium no problem and if it's only occasionally and your not wanting perfection on the finish then it will be fine. If your wanting to cut deeper and get high quality finish the machine won't be stiff enough.
Personally I'd just go for it and if you find your cutting more aluminium than anything then build a machine just for this purpose because while a machine for cutting wood isn't 100% for Ali the same applies in reverse to some degree. Can't beat purpose built machines and Jacks of all trades always fall short in some department.

The bed and workholding on any machine can be a pain and often under estimated how important it is. This is esp true on machines that are intended to cut multi materials.
Problem comes from how you hold it down and the type of work and how you machine the material. If your cutting or drilling thru then having a nice aluminium bed isn't much use because it doesn't leave a nice warm fuzzy feeling when you chop into it.! So you need some spoil piece anyway and often this needs surfacing each time you use it if any decent accurecy is required.
Now IME the best bed is a combination of both.? So nice stable material for Machine bed that is surfaced parallel to the cutter with either all of it or part covered with sacrificial spoil board. If bed is large enough like mine is then having half n half works well and saves swaping between and surfacing for jobs that need the accuracy of the base.

[CharlieRam]

Also that Z axis design is bad news. Again check out threads and look at Z axis designs that put the rails on the front plate and bearings on rear. Your design is a constant length lever that won't even reach the bed surface and will give the same amount of flex whether it's at the bottom of top of it's travel. With the other design the lever length is variable dependent on material thickness or distance from tool.

Hi Jazz, is that Z axis any weaker the way I had it? I am struggling for room and didn't want to compensate by reducing my cutting area, I have gone back to Idea 3 but raised the rails on X. I would still like to use the fourth axis along X and this would allow that, giving me an effective work area of 150mm diameter by whatever length travel I decide on.
My X rails are 810mm apart, carriages are 250mm apart from outer dimension, Would I suffer from binding if I just had a central ballscrew or would you still recomend one on either side?
Thanks

[njhussey]

The normal convention for the Z axis is to have the rails on the front plate and the carriages/bearings on the back plate. This adds stiffness to the front plate which is where the axis most needs it. Also stick with 20mm plate if you can rather than 15mm. With regards to ballscrews two are way better than one, you undoubtedly would get racking with one central ballscrew at that width I would say.

[CharlieRam]

Okay so what if I were to make the spindle plate 30mm thick aluminium or 20mm+ thick steel ( I could grind the steel perfectly flat and parallel at work, well within a couple of microns anyway), would that be strong enough to have the bearing blocks the opposite way round or does that method just not work? in fact a lot of the plates could be made of steel which would be easier for me to grind flat as we use mag chucks, it would add weight but also a lot of strength.

Probably dont want to change too much though as the weight will increase substantially!

[CharlieRam]

Anyone? Is My idea 3 Z axis just not up to it, will changing the front plate to steel help or not? Just the Z plate, not the x rails as I have mounted them directly on top of the rails.