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View Full Version : Anyone using the millkins module to correct for skew..?



Wal
01-02-2016, 11:42 PM
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Hello all - hopefully the above images will give you an idea of what's going on: a 2 sided part and a holding jig. In the last two pics the second side has been machined and you can see how the part and the pockets don't quite match up and the holding pockets have had to be cut a bit bigger than you'd expect to accommodate for what I think is skew - but hey, there might be a bit of backlash in those Chinese screws so it could be a combo of both those things... (it's barely perceptible, but I get a click pushing on the table when the motors are energised and holding - and that ain't what I want).

Ok, it doesn't cause me a great problem with the kind of stuff I'm making, but it's something I'd like to (literally and figuratively) straighten out. I reckon I'll eventually upgrade the machine to linear rails and increase the work-area while I'm at it. I'll probably go for a better set of screws at the same time. In the meantime I'm thinking of adding the Millkins module - anyone on here done it? Easy-peasy or is there stuff I need to be aware of..?

Oh, and a vid of the part coming out:


https://www.youtube.com/watch?v=C_GrUVoFG04

Wal.

Clive S
01-02-2016, 11:57 PM
Wal Are sure it is not the BK bearing nut come loose. Try sticking a DTI on the end of the screw. Is it just on one axis? If you can hear a click it must be a reasonable amount. Clive

Wal
02-02-2016, 12:06 AM
Hi Clive - I've stuck a dial on the edge of the table and measured the click at around .05mm - needs pushing on a fair bit to get the click, but it's there. I'll stick the dial on the end of the screw to see if it's the bearing nut although something tells me it's ballnut...

Wal
02-02-2016, 12:08 AM
Oh yeah, Millkins:

http://wiki.linuxcnc.org/cgi-bin/wiki.pl?ContributedComponents#millkins_trivial_kin ematics_extended_by_XY_skew_correction

When you know how much you're out by use the module to have the machine compensate.

Wal.

magicniner
02-02-2016, 10:59 AM
If there's a click in axis movement isn't it a simple backlash setting issue?

- Nick

Wal
02-02-2016, 11:35 AM
If there's a click in axis movement isn't it a simple backlash setting issue?

- Nick

Hi Nick,

Probably not as a bit of force is needed for the 'click' - there are probably times in the cut where there had been no backlash and other times where perhaps there is - it's probably not consistent and where the cutting forces are slight there's probably no backlash at all... I'm not in a position to take a photo right now but there's an interesting observation I made on the part itself, I'll post later...

Edit: I get that the above might not make a great deal of sense as backlash is backlash and the reasons for it should be consistent regardless of forces... I'll double check the bearing nuts but I'm pretty sure I've tightened 'em in the past and the situation hasn't changed!

magicniner
02-02-2016, 11:51 AM
If you've got a mechanical error which only occurs occasionally surely any software solution will have to either detect or predict the error and only correct when it occurs?

Wal
02-02-2016, 12:04 PM
Well yeah - where would you even start with that..?! The (possibly intermittent) backlash is something that I can live with for the moment as 90% of my cutting involves pretty low forces. I really do suspect that the ballscrews/nuts are just a bit shite and it's a problem best sorted by spending a bit more on decent running gear... BUT as suggested I'm going to be nipping stuff up again during my next clean/service. The skew (again, miniscule on the sizes I work at) is something I'd like to have a go at dialling out.