CNC Router V1.0 (For Cutting Multiple Materials)

[JAZZCNC]

What configuration of switches do you use then? 6x Limit switches + 3x Home switches? How do you resolve the problem of hitting limit switches when homing?

Dave

Yep 6 + 3.!!
Limit switches are just that the Limit of travel, or just before it. HOME switches are usually just in front of the Limit switches, usually on left side but can be anywhere you like really.? By this I mean you could put the HOME switch in the middle of each axis then the centre of machine would be your X0,Y0 machine coordinate. Depending which side you place work material will determine if the Work coordinates are on the positive or negative side of MACHINE coordinates.
By far the most common and less confusing way is to mount the HOME switches on the extreme left. This way your WORK coordinates will always be Positive in relation to MACHINE coordinates.

If your using Micro switches and separate home switches then the switch needs to work on a ramp trigger.! By this I mean the HOME switch gets triggered by riding up a ramp or bump which it can ride over without damaging the switch. When things are correct and working fine and the switches are positioned on the left side then you'll never normally pass the home switch unless there's an issue and if something does go wrong and passes it then not far away will be the Limit switches.

OK, so I warned of a whole heap of questions when I first started this project. Here are a few of the ones at the top of my list... If anyone would be kind enough to answer just one that would be very much appreciated! I hope they make sense...

1. Should I rebate/slot the aluminium plate where the aluminium extrusion butts up against it? Would there be anything to benefit alignment strength wise? I have done this with the extrusion piece connecting the gantry sides at the bottom but was thinking of the main X and Y extrusions.

Yes it helps with keeping things aligned and registered. If you look at the pics below you'll see the pocketed end plates where profile bolt. The pockets give the profile a register to sit against keeping rails aligned perfectly parallel to each other. A close fit to the profile also resists any twist.

This is a general tolerance question. Should I perform a worst-case tolerance stack-up when calculating tolerances for holes etc? A particular question I have regarding this is the diameter of the bolt holes. I want a close-fit to aid with alignment. EDIT: So for example an M8 bolt, what clearance should I include and what is a realistic tolerance if I have the aluminium plate machined manually or on a CNC mill? I'm going to use the ANSI dimensions for hole clearances etc. Should I go for close or normal fit for bolt clearance?

Problem with close fit is that unless everything is machined absolutely perfect and to the same tolerances then the potential for miss alignment greatly increases. Being realistic and considering the DIY factor then you want some wiggle room so leave at least normal amount of hole clearance.

Following on from the above question...Another particular example of the tolerance question is where the Hiwin carriages are recessed into the gantry 'uprights'. The carriage blocks are 44mm high (I cannot see a tolerance in the datasheet...). Lets assume this tolerance is +/-0.1mm. Should I specify the slot in the upright to be 44.2mm +/-0.1mm? This would ensure the part would definitely fit, however if the carriage was only 39.9mm then this fit may not be tight?

Doesn't matter if the recess or slot is wider than the carriages. What matters is the slot has a Machined reference edge which the Carriage Reference edge sits and registers too. Again look at the picture of the Gantry uprights (bits8) the long slot with 4 holes is for the Carriages and the top edge registers against the carriage Ref edge but the slot is 1mm wider than the carriages. Having the slot a tight fit would make no difference other than making it harder to fit.!!

Would my design benefit from an Aluminium plate across the back of the two extrusions on the X axis? If so, should I mount this to the two gantry 'uprights' as well as the extrusions? Would 10mm thickness suffice?

Yes & Yes.!

In my current design I have the Y axis hiwin carriages mounted to the lower portion of the 80x80 extrusion profiles. Would there be much difference/benefit in mounting these to the upper portion?

Slightly better in the top has the distance from carriage mounting to top off gantry is reduced so having a similar affect to shortening the gantry height.

How do you determine how heigh to have your Z axis? What I mean by this is do you design around a typical flute length and add some? Should the Z axis be able to go through the bed?

Think more about drill lengths than End mills has they tend to be longer. Just workout the thickest material your likely to use then allow for either a longish drill bit or long end mill.
Regards cutting thru the bed then that depends, you want it to at least cut a few mill into the bed for surfacing.
Something to think about and I often do on wood routers is allow the spindle to pass the end of the bed, which often unless designed not to they often do anyway. Then make the Z axis with a little more travel so it can machine down past the bed.? This way you can machine into the edges of panels etc or cut over height material by clamping to end of machine.