Fixed z axis to gantry design

[Robin Hewitt]

I didn't see that movie. It doesn't need a strut, it needs the rails fixing to the bottom of the bed and the blocks fitting to the gantry. Dump the overhangs, kill the vibrations. Simples

[JAZZCNC]

I didn't see that movie. It doesn't need a strut, it needs the rails fixing to the bottom of the bed and the blocks fitting to the gantry. Dump the overhangs, kill the vibrations. Simples

Sorry but you'll have to point these overhangs out because I'm struggling to see any.!! Or any that will make a difference anyway.?

I agree the Bearings on the base and rails on the bed would be a better option but it's not the weak link in the machine. The High gantry frame without support is far more a weak link and source of vibration IMO.

[routercnc]

Hi Robin, Lachlan,

Maybe this helps explain it. If you remember the days of 'free-body-diagrams' - the bed is part of the system. See picture which shows all the forces acting.

You can hopefully see how the extra brace can help. This is where myself and Jazz are coming from.

I think Wal created something like this (although smaller) and grappled with different layouts to make sure the 'tower' was stiff enough.

Can it work without the brace - of course. Is it much better with it ? - yes, significantly, and for little effort.


Thanks

[Tenson]

Why does this chap spend a long time adjusting the rails to be perfectly parallel and then use a lump of pine as the base for cutting? Seems a shame, since pine obviously isn't flat and stable!

[JAZZCNC]

Why does this chap spend a long time adjusting the rails to be perfectly parallel and then use a lump of pine as the base for cutting? Seems a shame, since pine obviously isn't flat and stable!

Ye did seem a lot of wasted time and to be honest I don't think he had a clue what he was doing judging on how he went about it.!!

[Lachlan]

HI EVERYONE,
Was surprised and happy to see so many responses. Thankyou! I think the triangular bracing of the gantry is a must for this build it makes sense as a gantry triangulated is so much stronger than a gantry just flapping in the breeze so to speak. I have some 50x50x4 box steel surplus, would that steel cut the mustard for this frame build?

[Robin Hewitt]

See picture which shows all the forces acting.

But the diagram doesn't show the rails being screwed to the bed and the blocks being screwed to the gantry.

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

[JAZZCNC]

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

Robin I agree the swapping the bearings and rails gives a good benefit because has you say the bearings are always in prime location providing Max support but the screw moving with the bed isn't needed and creates more Mass to move and complexity. Moving the Screw, Bearings/mounts, Motor etc. means the Negatives out weigh the advantages in this case.

Not being funny or picky here but please stop calling it the gantry it's the base Frame and I'm easily confuddllledd. .

[Robin Hewitt]

please stop calling it the gantry it's the base Frame and I'm easily confuddllledd. .

With both methods the rails have a length equal to the table travel plus the overall width of the bearing block assembly.

In this topsy turvey arrangement I am advocating the block assembly width is tiny in comparison, so the rails are short.

If you could bolt the "base Frame" to a workbench this could look ultra neat.

[routercnc]

But the diagram doesn't show the rails being screwed to the bed and the blocks being screwed to the gantry.

The nut should be under the gantry and the screw moving with the bed.

That puts the linear bearings permanently underneath the cutting head.

Looks a bit odd, but if you draw it you will see enormous benefits.

Hi Robin,
Yes, I totally get what you are saying. Putting the bearings on the bed and the rails on the moving part makes good sense but it is maximising stiffness in the vertical Z direction. It is similar to the debate around conventional Z axis where it is often better to put the rail on the moving part for the same reason.

However, the bracing on the tower is primarily to address stiffness in the X direction. I've re-drawn the sketch to show the moving rails and FIXED bearings but the forcing (for X direction) is still the same and suggests bracing of the tower is beneficial.

X cutting:


There is also some Z direction stiffness from the bracing due to moment created by the offset of the tool from the tower in X which is independent of the rail / bearing arrangement.

Z cutting/ plunging: