# Thread: How much deflection to expect...

1. Cheers Geoffrey, nice of you to say so - perhaps a bit of a toy compared to some of the stuff these guys are building, but it's a toe in the ocean and it'll be fun getting to grips with the software and making it move!

Wal.

2. Originally Posted by routercnc
If you can't be bothered with the maths - I just checked our kitchen scales (the flat digital type) and they go up to 5kg (50N). So if you have something similar handy use them to push against the collet until they read 5kg. If the DTI reads less than 0.05mm you are in business (>1000N/mm).
Good idea - I used 20kg hanging scales as they're exceptionally cheap on eBay and have a hook on the end which makes is easy to grab a bar/tool in the spindle to pull on. Do remember that, especially for a fixed gantry machine, we need to consider the stiffness of the bed.

You can think of the stiffness readings as spring constants, which can be combined as you would for springs. So in this situation you would have the same force applied to the spindle (i.e. Z-axis) and bed (i.e. X and Y axes), which is like having two springs in series - so to find the overall spring constant (i.e. stiffness), measure them both and use the following formula:

Then if is greater than 1000N/mm, you're in business. Unfortunately you can't really simplify that, but if one of the stiffness readings is significantly lower than the other (say 10*), then you can just use that as the overall stiffness wont be affected much by the other one.

Originally Posted by Wal
Anyway, one thing I will say at this stage is that the gantries move fairly smoothly without much effort when turning the screws by hand
If it doesn't, you're doing something wrong.

Originally Posted by Wal
although pushing the gantries around (back-feeding the screw) takes a fair bit of effort
That's normal.

Originally Posted by Wal
with a bit of binding towards the ends of the screw.
Not good! That implies the bearings on either end of your ballscrew(s) are slightly misaligned, so they're applying significant radial force to the nut when the ballnut is near the ends, but not in the middle as the ballscrew itself will just bend. Try moving the nut to one end of the screw, loosen the bearing mount (so it self aligns), retighten carefully, then do the same at the other end. You can also try leaving the bearing fixed and loosening the ballnut mount.
Do it now.. it's the sort of thing that you'll get away with for a while, but will cause problems soon enough as it prematurely wears the bearings/nut.

Originally Posted by Wal
I've noticed that the x ball-nut is a bit 'clicky'
I'd look into that sooner rather than later, before it's too late. If you dismantle the ballnut you'll probably find there's a ball in the wrong track, or bits of swarf in it. I've had that problem with new nuts more than once.
Last edited by Jonathan; 14-01-2014 at 11:13 AM.

3. >If you dismantle the ballnut you'll probably find there's a ball in the wrong track, or bits of swarf in it.

Heh, no - I'll find myself on ebay looking for another one as it becomes clear that I'll never put it back together again..! That particular nut is very easy to get to as it sits directly under the table. Thanks for the tip re: bearing alignment. I did take my time putting it together and nipping things up progressively whilst moving the gantry from one end to the other. I'll have another look at it now. Despite the bit of binding when pushing the gantry by hand, it's still pretty easy to turn via the screw...

Cheers Jonathan.

Wal.

4. Originally Posted by Wal
Despite the bit of binding when pushing the gantry by hand, it's still pretty easy to turn via the screw...
It's not too far off then, but remember the ballscrew gives a very large mechanical advantage, so it's easier to notice the bigger increase in force required to overcome the extra torque to backdrive than it is to notice the extra torque.

5. An update from the last couple of days, although I guess I ought to start a new build thread really - going a bit OT. I'll make it quick:

• Flipped the rubber - the protective flaps are working now - they look a bit kooky, but so long as they keep the worst off, I'm happy. Will also fit 'em to the table to protect X-running gear.
• Motors mounted and wired in (thanks to Clive S for the covers!)
• Monitor mounted to the frame and PC installed. Really nice having the DRO right next to the work-area.
• All gantries moving, a quick check of run-out along table X shows .07mm - the majority of which happens towards where the hint of binding was showing up - I'll whip the rails off and make sure there aren't any high-points.

All in all coming along quite nicely, still unconvinced by the clicky ball-nuts - I take it that they come pre-greased? Or should I squirt a bit more in..?

Photos:

Wal.

6. On my British made (Devon) machine I can get 1mm deflection at the collet with 1 finger pressure !!! That is why I plan a rebuild soon

BTW Wal - Your machine looks good, nice work !
Last edited by cropwell; 16-01-2014 at 12:41 AM.

7. Originally Posted by Wal
All in all coming along quite nicely, still unconvinced by the clicky ball-nuts - I take it that they come pre-greased? Or should I squirt a bit more in..?
No there'll be more grease in your hair than those nuts.!!. . . .Definately give them some grease.

Coming along nice and looking good. . .

If that screen is a touch screen then i'm not sure I'd have it that close to machine.?. . . . I wounce used a friends machine with a touch screen mounted next to machine but on a swivel head and while using the bloody thing kept Stopping, feedrate would change and all sorts of strange happenings.?? . . . Anyway after calling my mate's machine some nasty names and calling him over asking what the F'#k was happening he turned screen away and said try again.!!. . . . Yep flying chips where hitting it and changing feed over ride, feed hold etc. . . .Lol . . Was Very Funny.!!

8. >Yep flying chips where hitting it and changing feed over ride, feed hold etc. . . .

Hehe - nah, this isn't a touch-screen, although I ought to find some sort of cover for the keyboard - I can see those chips finding their way in between the keys.

Okay, I'll add a bit of grease - are there any tips on making sure that you don't over-pack the bearings?

Cheers guys, nearly there but loads left to do..!

Wal.

9. Originally Posted by Wal
Okay, I'll add a bit of grease - are there any tips on making sure that you don't over-pack the bearings?
Nah you won't really over pack them just grease untill you see it coming out from the ends.

10. Right, the build is nearing completion with homing and limits nearly all sorted, just need to machine me some new switch mounts - the ones on at the mo are rough fabricated jobs and I thought it would be cool for the machine to make nice ones for itself. Here's a vid of the mill cutting the switch mount profile shape into a block of semi-scrap 6082 I have lying about:

In the vid I’m using a 4mm square end-mill (2 flute) which is spinning at 12000rpm with the work feeding at 400mm/min. My initial depth of cut was 0.5mm with a 12.5% step-over. it’s a climb milling operation. Ignore the holes - they’re not part of the job, I used the plate to cut spacers from a while back… By and large, I'm pretty chuffed with how this test cut came out.

I've also had a go at perspex, which machines very well at the same settings as above (but with a faster feed-rate of 1200mm/min).

The other day I tried milling these switch mounts out of the ali that Apple use for their Mac Pro towers, which just shredded up like this:

I tried a few different feeds/spindle speeds, but to no avail. Earlier this evening I tried machining 'em using some old 3mm ali sheet that had been lying under my dads bench for probably the last 20-odd years, still no joy. Any recommendations on the grade of 3mm aluminium sheet I should be looking at for this kind of profile cut?

Cheers!

Wal.

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