Sisyphus

[ian]

Hi All

I'm a new member, who recently joined the forum and am starting the journey. I've made some progress on designing my machine but have hit a few design issues and questions which I will appreciate help on as well as good general feedback on my overall design. I have experimented a little bit with a machine that I used as a proof of concept and have experienced some of the pains and joys of machining. I have little experience though and no point of reference for a build to know how strong/fast/accurate etc. builds, components or materials are.

My objective
I intend to build a CNC machine that will be used in product R&D (I'm building GPS tracking devices, and I will use the machine to build prototype cases and small production runs of a few thousand cases). It has to be fast and as accurate and precise (aiming for 0.01) as I can get for a budget of ±£3.5k or ideally less. I accept there may be tradeoffs and adjustment of my expectations! The machine will be used in my (small) office and will eventually be housed in an enclosure to reduce the noise. The machine will mostly cut hard engineering plastic sheets (good finishing quality is essential) and some occasional aluminium for custom moulds or jigs.

I know accuracy and precision are relative in context and does depend on the collective results of all the components. Still, I do want to endeavour to get it as good as I can (while keeping sane and being somewhat realistic) as poorly machined parts can influence the performance of my devices significantly. I've decided to use two ballnuts on each screw as a trial but still need to work out the detail and to determine how to pre-load them. In the smaller runs I've done this far I found that poor machining and poor machine accuracy/repeatability adds excessive post-processing and clean-up time and leads to a higher percentage of rejects and I want to avoid this. To this extent, I may be "overbuilding" my machine but my view is that and overbuild is less expensive than a rebuild, and I need a robust machine I can trust so that I can get on with my business. I also want to keep the build as simple as possible.

The target size is to be able to cut at least 500mm x 500mm and ideally 600mmx600mm ( so a machine size of 800mmx800mm). For the Z height, I don't expect to cut anything thicker than 50mm although I plan to have at least 125mm of travel.

I "inherited" some starting components:

X & Y
3 x 800mm 1605 ballscrews, C7
3 x pairs 800mm Hiwin 15mm rails and 2 HGR15CA carriages per rail

Z
1 x 300mm 1605 ballscrews, C7
1 x pairs 300mm Hiwin 15mm rails and 2 HGR15CA carriages per rail

I intend to use at least 4Nm Closed Loop motors or potentially 5.5Nm motors

I've not worked on the electronic and electrical design yet and will add this once I have the mechanical design settled

At first, I will use a Dewalt Router and later switch to a better spindle

I have limited space and tools for fabrication so intend to use aluminium profile for the build as well as 20mm aluminium plates (preferably something like Ecocast that is ground flat)

Frame

For the frame, I plan to use 80x80 and 80x160 profile. Please see the attached image for the frame.

Questions:
1. Are 4 cross-members on the X-axis too many - would 3 suffice?
2. Are corner brackets and the connection with t-nuts sturdy enough to give a rigid frame? Or should I tap the ends and drill M8 holes and fasten them from the outside of the Y beams?
3. Should I use the heavyweight or lightweight profile? (from KJN)

Fixture Plate

For the fixture plate, I plan to use 20mm aluminium drilled and tapped for M8. The plate will fix to the profile, and I will fit a 10mm - 20mm spoiler material to the top

Questions:
1. What grade aluminium would be best?


Gantry design
I've looked at different gantry designs and have looked at two different styles: Option 1 with the rails on top of the profile and Option 2 with the rails on the side of the profile. It is hard for me to gauge the difference between the two designs and which will be best or be "better" in the long run, and I will appreciate people's views on this. I attach images of the two designs that I have done. On both designs, the Y and X plates are all 200mm wide, and the Z plate is 180mm wide.

Option 1: Rails on top of the profile



Option 2: Rails on the side of the profile



Overhang
The router is set forward quite a bit and extend 40mm forward of the front face of the gantry plates. I've considered setting the X beam back by 40mm to offset the overhang but I will then have more issues with the gantry plate then interfering with the back motor plate. I know an alternative is to place the rails on the top/bottom of the X-beam which will reduce the overhang but I initially steered away from this it then involves more plates and a more complicated design (I may still have to do this).




Adjustability
I know it is important to have easy access to all the bolts/screw/nuts nd also to have adjustability so that the machine can be squared etc. However, I am unsure on which critical bits I need it and whether I should then just add oversized hole or slots in these areas? Any input here will also be appreciated.

Tapping
Where I connect motor mounts or bearing supports, I planned to tap the aluminium plate and not use a separate bolt - is this a good idea and how well does Ecocast tap?