Well here goes... Aluminium frame router design

[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.