04-01-2013 #11Always bear in mind that your own resolution to succeed is more important than any other - Abe Lincoln
Ok, so I think I'll add the option for twin ballscrews on the Y axis, I mean X, or Y...
Stand to the side of your machine and make single finger gun with your left hand, Thumb pointing up = z, pointing finger straight out = Y, trigger (middle) finger pointing right = X. If you always shoot the axis's you can not get it wrong even if you can not remember which handIf the nagging gets really bad......Get a bigger shed:naughty:
Question: How should I go about limit and home switches?
My current thought is to only have home switches and use software limits. Is this something that can be done in Mach 3? In fact I am considering using Inductive Proximity Sensors for good repeatability. Any opinions on this kind of setup?
Just to re-iterate, I would use 3 (x,y,z) proximity sensors for homing and software limits for emergency stops...
Personally I use separate limits and home switch's on machines I build. Has well preferring them that way for flexibility of moving my home position around if needed, It's also due to the way I use either purpose safety relays or Standard relays to run my control box.
Having a safe control box means Limits should not be under software control and this means they can't be used for home switch's has they would trip the system every time you homed the machine. Being safe doesn't cost much money compared to surgery for replacing fingers so for £20-30 more doing it right and correct is a no brainier to me.!!
Proximity switch's are fine so long has they are accurate enough. Some require very close fitments of the pickup to achieve good accurecy and this can be a problem with damage depending on what your cutting.?
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?
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...
- 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.
- 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?
- 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?
- 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?
- 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?
- 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?
Last edited by biketrialsdave; 14-01-2013 at 08:20 PM.
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.
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.
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