Z axis error woes

[HankMcSpank]

Hank

This is definitely a problem with the rails not being at parallel heights? Now i would check that the supports are equal, bearing cases are equal etc.

....but wouldn't leveling the bed at least workaround that?

Z Axis update: It's just too 'parky' in my workshop at the moment (no heating it must be nigh on -5c in there) 'm keen to sort this out but not *that* keen!

I'm beginning to think this is a combination of factors....

1. p1ss poor leadscrew.

2. poor stepper resolution combined with steps going awol (my Allegro A3977 IC based steppers no longer work at anything less than full stepping(1)....that's resolution of 0.0075mm per ste, which sounds fine it doesn't take many steps going missing to cause chaos when your target depth is just -0.12mm! I use an easy cnc 3 axis board.....on the face of it, it's nice, but I;ve had no end of problems with the IC holders (poor connections with the IC). I have a spare tb6560 3 axis board - made by someone that's partial to boiled rice - laying around that I can press into play over the coming days (while trying to source some more A3977 ICs for my easy cnc board!)...at least then I can go with 1/8 stepping.

3, too high expectations for an entry level machine....I was taken in by the promotional youtube video..... http://www.youtube.com/watch?v=TYAfNTvgTak (note - thru hole, therefore wider tracks, therefore you can go deeper with the v cutter ....SMD is hust too challenging for this simple CNC machine offering)

gameplan....

1. don't venture into the workshop until my tea can stay drinkably hot for more than 30 seconds.

2, Press my spare tb6560 stepper board into play & go with 1/8th stepping

3, Start saving for a proper machine and leave this one to those who make parts out of balsa.!

[Swarfing]

Hank you can level your bed all you like but as the table moves it will still go down hill or up?

[HankMcSpank]

Hank you can level your bed all you like but as the table moves it will still go down hill or up?

but all those uphills and downhills were present whilst the cutting tool was levelling the bed.....surely what's important here is the 'tool tip distance' to the machine bed - won't that be uniform after a bed levelling run?

[Swarfing]

To fix my issue i put a piece of rod through the bearings, sat this across two parallels before clamping in a vice then milled tho surface off to the lowest bearings. I took off .8mm off from the largest to match the smallest and that looks like the issue here. I also had the same with the supports even though they were all the same one of the holes was not machined to the same height as the others. They were from different batches and this was evident in the slight colour differnce in the anodising

[Swarfing]

ok imagine you are on one of those lifts at the seaside that take you up and down the cliff. You have a gantry across it when it is at the top of the hill and you level the roof off. You decide to move it along the rails a bit, is you table at the same height then? in you case it is the same except that on side of the rails is square with the gantry and the other side is on it's way down the hill.

[JAZZCNC]

No your wrong 2eOpoz the fact he's surfaced the bed means the distance from bed surface to the cutter tip is the same.

Yes what you say would apply if the bed was perfectly flat and level but because he's surfaced the bed he's effectively machined it into twist.
If the rails are not parallel the bed will be in twist as well after surfacing but the distance from spindle to bed will always be the same.

If the rails are not parallel you'll never be able to machine truely flat material without putting it into twist when clamped to the bed but the distance from tool to material surface will always be the same after clamping..!!

[Swarfing]

If the rails are not parallel then all he is doing machining it parallel to the twist. The bed will still drop as it moves along with the twist. I think you will find i am right on this one assuming that is what his problem is? Place an object on the bed at one end and check with a height gauge. Move the bed along and reposition the height gauge on the object and you will see it is different. The gauge will need to be set up off the machine.

[Jonathan]

No your wrong 2eOpoz the fact he's surfaced the bed means the distance from bed surface to the cutter tip is the same.

Agreed. The cutter when surfacing moves along the same plane as when cutting, therefore surfacing eliminates that error. It makes no difference if you move the cutter or the bed, the relative motion is the same. It could be that part of the gantry or bed has slipped after surfacing. It's strange that the error appears to be so large, 3.8mm in 100mm. Have you tried measuring over a much greater distance, say 100mm?

Unfortunately that doesn't really help HankMcSpank!

[Jonathan]

If the rails are not parallel then

If the rails are not parallel then it wouldn't be able to move, assuming you mean rails parallel to each other. If not, rails parallel to what?

[Swarfing]

We are talking about one of the rails being dropped lower or raised in one corner from the rest.