Design Help Pt2 Required for CNC design/Build

[JAZZCNC]

I can make a few suggestions that you might consider having built a few machines very similiar for the same job and actually have one part built which I just don't seem to find time to finish.! I'll attach pics of it.

Looking at the drawings I presume you intended using twin screws.? If so then I would put a cross brace between the gantrys sides running under the bed, this will help make the gantry more ridged but it also then gives you the possibilty to run the ballscrews under the bed out off harmsway rather than exposed on the outside it also offers some protection from dust and rubbish.
On the machine I build the motors under there as well connected to the screws with belts. Other than the protection advantage the belts also allow geariing if needed and reduce the chance of resonance, but the main reason in my design was to keep the foot print as small as possible because the original machine was to live in a typical 8 x4 garden shed/workshop so space was precious. As you will see from the pics the Y axis motor/screw setup was inverted the same and the Z axis motor flipped to keep the height down so it could sit on a bench without hitting the roof, there are no components sticking out side the initial footprint.
I've built several versions both with single screws and twin screws, some with belts as mentioned, some with direct drive and motors on outside when space saving doesn't matter as it does ease the build cost slighlty slightly. Thats why some pics will look differant to others.

Regards the largish kink'd back gantry side, thou not a problem as such it's more material related.! . . Depending on the material width you intend to use you may want to check that you can actually get that much kink out the width's.? Unless you intend to use one large and expensive sheet then it could be tight using STD width's from the likes of Alu warehouse etc.? Most stop at 300mm width.!

Regards the Z axis I would run the rails on the Front plate and put the bearings on the backplate, this will help strengthen the front plate slightly. I would also recommend you at least beef up the Z axis with min 19mm plate. Remember it doesn't matter how well the rest of the machine is built if the Z axis flexs then accurecy goes out the window.!

Hope this helps and happy building.

[MikeyC38]

Thanks Jazzcnc for taking the time to review this. The photos are really useful. My drawings are incomplete at this stage as I will be using a single ballscrew on the long axis as per your photo with a plate underneath between the gantry sides . But the tip on adding the extra reinforcement on the long axis is taken and accepted. HiltonSteve's router/mill also had this. It also solves the problem of mounting the BF/BK blocks very elegantly and protects the ballscrew from muck! Many thanks!!

Again the comment on the material thickness on the Z axis is accepted and you given me some food for thought on the gantry side plate lay back. My original idea is to get the cg of the Z axis between the bearings on the side plate, but as you say well, cost of getting the plate for the gantry sides has to be taken into account!

I note your comment about reversing the position of the Z axis linear slides and bearings - I see in your photos you are using Hwin/THK slides which have better torque load characteristics than Supported round rail and linear slides. I'll have a think about that one.

Measure twice cut once is the adage!!!

Kind Regards
Mike

[JAZZCNC]

I spent some time experimenting with cutting MDF templates for the best layback and what you see in the pics is the outcome. I found it gives the best balance and can still be got from 12" x 3/4" plate from Alu warehouse with the minimum waste. Thou not exactly on centre of bearings it's pritty close.

Regards the Z axis I really would consider putting the rails on the front.! It does help regardsless of rail type used.
Remember the Z axis when extended is basicly a lever and a weak lever will bend and flex.!!
Like I allways tell folks.!!. . The machine is only has good as it's weakist part or component.!!. . . Unfortunatly I see lots make the mistake of building weak or inaccurate Z axis's. Which makes the rest of the machine no matter how well built or quality components used pointless because at the end of the day it's the Z axis which is at the sharp end taking the strain.

[MikeyC38]

Regards the Z axis I really would consider putting the rails on the front.! It does help regardsless of rail type used.
Remember the Z axis when extended is basicly a lever and a weak lever will bend and flex.!!
Like I allways tell folks.!!. . The machine is only has good as it's weakist part or component.!!. . . Unfortunatly I see lots make the mistake of building weak or inaccurate Z axis's. Which makes the rest of the machine no matter how well built or quality components used pointless because at the end of the day it's the Z axis which is at the sharp end taking the strain.

Good reasoning there JazzCNC...the placing the supported round rails on the moving part of the Z axis will add stiffness to that axis...

[MikeyC38]

Hi John S & JazzCNC

You guys have got me going now and its late (got work tomorrow :cry: ) but I have revised the drawings with your suggestions in mind... its begiining to take shape and make sense.

Thanks guys

Mike