How much deflection to expect...

Hi all,

Yep, I know the right answer is zero, but with home-built machines I guess we need to be a little realistic.

I mounted my z-axis yesterday, here it is:

Attachment 11262

No doubt it'll be on and off a few times more for shimming etc, but while I was at it, I torqued it all up and out of interest measured the deflection at the nose of the spindle with the z near it's downward limit. Pulling hard on the plate that the spindle is mounted to, my dial gauge read 0.05mm. Is that any good?

Vid here:

http://youtu.be/GJzGqhKwNSw

It's on TBR20 supported rails, how big an improvement would I see with profile rails?

Cheers!

Wal.

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Originally Posted by Wal

Pulling hard on the plate that the spindle is mounted to, my dial gauge read 0.05mm. Is that any good?

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Nobody can answer that question Wal to many variables mate.!! . . . . . And Anygood for what.?

Edit: Thou I will say it's an order of magnitude better than others will be getting from machines they paid several G's for.!!

It's a reasonably good starting point, but bear in mind this is only half the story as you need to add to this value the deflection between the bed and frame, for the same force.
If you want a less subjective answer, then measure the force you're applying and divide by the deflection. As a rough guide, if you get at least 1000N/mm stiffness you'll probably be fine for most things, but clearly it depends on what the cutting force and accuracy requirements are.

>And Anygood for what.?

Ah, sorry Jazz, was being a bit vague there - just ballpark any good for cutting alu plate. I'm not too bothered about crazy feed-rates and am happy with smaller step-overs. Reassuring to hear that it's a good starting point!

One more question, the pillow blocks which house the bearings that ride on the rails - they have grub-screws that you can tighten to get a snug fit between the bearings and the rails - how tight do you go with these? Just a nip front and side?

Wal.

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Originally Posted by Wal

One more question, the pillow blocks which house the bearings that ride on the rails - they have grub-screws that you can tighten to get a snug fit between the bearings and the rails - how tight do you go with these? Just a nip front and side?

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It's a feel thing but basically until they have a slight resistance with no play.!!. . . Thou if your planning on cutting Ali then you'd be better cutting your losses now at this stage before going further and buy some profiled linear rails. These rails are not really upto it and will need constant adjustment if you want any thing like decent accuracy.

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Originally Posted by JAZZCNC

Thou if your planning on cutting Ali then you'd be better cutting your losses now at this stage before going further and buy some profiled linear rails.

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I hear ya Jazz, it's a well known fact that the profile rails are the way to go for superior rigidity, but I think to swap out at this stage would mean too much heartache, ball-ache and expense on what was always going to be a learner build. I may as well build another machine, which I have every intention of doing..!

While I'd like to 'precisely' cut ali on this, think more ornamental than super high precision...

Wal.

You'll be fine with those rails to start with. I'm not saying the rails aren't important, but it's the whole picture not just the rails that affects the overall stiffness and the rest of your design looks pretty strong (as it's a fixed gantry) so you'll get away with it. For example, I cut plenty of aluminium on my machine before changing the Z-axis rails to profile rails and didn't notice much difference from the profile rails as the weakest point lies elsewhere.

You can look at it another way which is if the machine stiffness is 1000N/mm, and you are cutting aluminium (guide figure 50N load), then you would see 0.05mm deflection at the tool (50/1000). This assumes a rigid tool (as your data was from the collet), and ignores resonances which would add slightly to the deflection in a difficult to calculate way.

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).

Thanks routercnc, helpful info. I guess I could also hang 5kg's worth of weights off it..?

Anyway, today's progress:

Attachment 11272

All coming together reasonably well. I must say that I'm blown away by the minute measuring capabilities of some of these gauges. My dad's a retired engineer whose specialism was in measurement, he hasn't seen the machine and is pretty much bemused by what I'm building, heh - I guess you would be if you're used to measuring hydraulic gear to MOD tolerances... He's retired now and has given me a load of his old gauges, one of which is a Mahr with increments of .002mm - it's mad to see those otherwise imperceptible forces at work!

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, although pushing the gantries around (back-feeding the screw) takes a fair bit of effort with a bit of binding towards the ends of the screw. I'm not entirely convinced on the quality/consistency of the screws from China. Pretty sure that the z-screw is slightly bent and I've noticed that the x ball-nut is a bit 'clicky'... Not going to get too anxious about it just yet, I'm keen to see how it all runs under power...

I think I overdid it on the thickness of the rubber aprons I'm hoping to fit to shield the running gear. 4mm is WAY too thick. I think 2mm should do it, even then it might run under the gantry, I'll have to experiment. If anyone has a spare bit of 2mm rubber sheet lying about...

Attachment 11273

More updates as the week progresses...

Wal.

Wal. that really is looking like a nice sturdy little machine, "super job". G

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.

Quote:

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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).

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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.

Quote:

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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

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If it doesn't, you're doing something wrong.

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Originally Posted by Wal

although pushing the gantries around (back-feeding the screw) takes a fair bit of effort

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That's normal.

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Originally Posted by Wal

with a bit of binding towards the ends of the screw.

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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.

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Originally Posted by Wal

I've noticed that the x ball-nut is a bit 'clicky'

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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.

>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.

Quote:

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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...

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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.

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:

Attachment 11284 Attachment 11285 Attachment 11286

Wal.

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 :eagerness:

BTW Wal - Your machine looks good, nice work !

Quote:

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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..?

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No there'll be more grease in your hair than those nuts.!!. . . .Definately give them some grease.

Coming along nice and looking good. . .:applouse:

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.!!

>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.

Quote:

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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?

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Nah you won't really over pack them just grease untill you see it coming out from the ends.

Attachment 11494

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:

http://youtu.be/VR9hAaXSzto

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.

Attachment 11495

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:

Attachment 11496

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.

I tried various cutters, speeds and feed rates with ali and the best cutters I found were these :- Sorotec - Werkzeuge. Cutter geometry seems to be the crucial factor. Downcut spirals didn't work as the main problems seems to be chip clearance. Rapid and light cuts seem to work better, with a spray of paraffin. I tried 1-cal cooking oil but it just made me want a bacon sandwich :joker:.
Cheers,

Rob

Wal Looking like a very nice and cable machine, don't scratch it lol , I remember Dean saying more than once that some of the best machines are achieved by people with daytime jobs with no mechanical back ground because they will listen and learn. Well done. ..Clive

Yes, cutting nicely in the video, sound like you have some bagpipes going :highly_amused:

Clive, listening and learning is half the story - not only is this site a great resource, but I've said it before and I'll say it again - to have been able to come over to yours and have a look at your build and learn from you was an invaluable privilege - I've no doubt that this project would have taken me considerably longer and been weaker for it had you not spent the time and effort patiently talking me through the stuff I didn't have a clue about. Thanks again!

Wal.

Quote:

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Originally Posted by Wal

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:

Any recommendations on the grade of 3mm aluminium sheet I should be looking at for this kind of profile cut?

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Were you using any coolant? A little spay of anything wet can make all the difference. 6082 is a readily available grade that's easy to cut. Avoid 5xxx series grades.

It sounds like bagpipes :D

Heh, yeah - I guess it is a bit bag-pipey - should've had it sprayed tartan..!

I didn't use coolant on the profile cuts into the sheet, Jonathan. I may give it a go sometime, but to be honest the tearing was brutal from the moment the mill touched the surface, whereas the 6082 billet cut very nicely! I'm trying to get hold of some 6082 sheet (ali warehouse don't carry it) so if there's anywhere you can recommend then do let me know!

Cheers!

Wal.

Bear in mind coolant doesn't just cool - it lowers the coefficient of friction (i.e. it lubricates) between the tool and the metal. That means less heat is generated to start with...

True. Just a thought - I use EXACT cutting paste for drilling and tapping (looks a bit like a giant Pritt Stick) which has a slightly higher viscosity than grease - would a smear of that over the surface to be cut be of any benefit? Or is it going to turn into a grinding paste as soon as the chips starts accumulating..? I guess I should just give it a go!

Wal.

Wal, congrats on the machine !

I would think a grinding paste would be a bad move, what you really want is some of this: Castrol Cooledge Cutting Coolant I use this for drilling/tapping and so on.

.Me

Cheers Lee,

Will keep a look out for the Castrol - those end-mills aren't something you wanna be buying every other day..!

Wal.