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jonnydeen
04-10-2012, 04:29 PM
im currently still at the planning stage for converting my centec 2 to cnc

one major issue with converting this machine is the sheer lack of space under the table and to even squeeze a flange 1204 ballnut through requires major metal removal to both the table and ballnut
since im not particularly happy about the amount of butchery necessary does anyone have any ideas on where to find smaller ballnuts for 8mm or 10mm screws that arn't twice the price of a 1605

failing that, any other ideas?

Gary
04-10-2012, 04:43 PM
1605 ballscrew is the most produced ballscrew so are the cheapest. Miniatures are harder to make and made in far less volume so expect to pay a lot more.

jonnydeen
04-10-2012, 05:29 PM
cheers for the reply gary, i was looking at your Gten FSDD ballnuts but the links to the datasheets were only bringing me to the zapp home page


i recon its just a tough choice of whether to butcher components to fit cheaper ballscrews or to just splash out on the more expensive miniatures :nightmare:

any who...

i thought i would post some solidworks screen prints to better describe my predicament (i know you guys love a little bit of imagery :biggrin:)


the first two is the standard table and the third shows how much material would need to be removed just to fit a chopped up 1204 nut in
7050 7052 7051




here we have a 1204 nut before and after 'modification'

70537054


just to show you the clearances involved
704870497047



100 points to the first person who finds 3 issues with the screenprints!

Gary
04-10-2012, 05:38 PM
Some of the links are not working at the moment, but you can get to all the download we have on the site in this link.
dirLIST - Index of: home/ (http://www.zappautomation.co.uk/downloads/index.php)
including solid works files of most of the Gten nuts.
We do have the 1205 RSW or 1204 RSW nut that are flangeless

WandrinAndy
04-10-2012, 05:49 PM
100 points to the first person who finds 3 issues with the screenprints!

Have only found 1 issue with the screenprints... Albeit a very serious one...

Your time format displays as for example 3:43 PM when I thought the whole world had agreed to use the 24 hour format.... it should read 15:43 !

Lol

jonnydeen
04-10-2012, 05:56 PM
thank you gary, that link is certainly very helpful

i want to get a bit more feedback on the subject first before i start ordering parts but i do think that zapp is the only reasonable choice for the smaller ballnuts IMO

so....

if anyone wants to share their thoughts or opinions, please do so or forever hold your peace

jonnydeen
04-10-2012, 05:59 PM
Have only found 1 issue with the screenprints... Albeit a very serious one...

Your time format displays as for example 3:43 PM when I thought the whole world had agreed to use the 24 hour format.... it should read 15:43 !

Lol

lol now thats just nit-picking

the 3 issues are a little more difficult than that

do you honestly think i would give away 100 imaginary points that easily????

i2i
04-10-2012, 07:11 PM
how long a ballscrew do you need?

jonnydeen
04-10-2012, 07:50 PM
how long a ballscrew do you need?
x axis is 375mm and y is 200mm

Jonathan
04-10-2012, 07:55 PM
The ballnuts from China are made from a fairly hard grade of steel. However I did check a RM1204 ballnut from Chai and I could file it, so you should be able to machine them.

There are some suppliers on AliExpress (http://www.aliexpress.com/store/group/ball-screw/609097_211302591.html) with small ballscrews - it might be worth asking if they have what you need although it's a bit of a long shot.

Could you not mount the ballscrew somewhere else, for instance to the front of the table perhaps using the convenient T-slot? It's clearly not as nice as having it under the table, since the force will be off centre and you'll need to protect it from swarf, but I don't think either of these are a big deal compared to what might happen if you mill that much out of the table. It wouldn't require modification to the machine in addition to being a lot cheaper.

i2i
04-10-2012, 11:40 PM
could you mount the nut on the edge of the apron for the x axis, and the front or the back for the y. That may lower the axis of the screw enough to get a reasonable one in there.

jonnydeen
05-10-2012, 12:16 AM
Jonathan, kudos for thinking outside the box (or should i say table) i had never thought about doing it like that

i2i, i didn't quite grasp your concept but i would certainly like to hear you elaborate on your idea, btw im only focusing on the x axis at the moment

i had a look at some of the smaller nuts available and on average was coming up with a bill of 200 for 2 8mm nuts and 600mm of screw with 10mm screw being marginally cheaper
compare that to 1204 ballscrews which i can get all 3 screws and nuts for 85

i2i
05-10-2012, 12:25 AM
rather than having the nut between the table and the apron, you could mount it at the edge of the apron so all that has to pass between the table and the apron is the screw.

Jonathan
05-10-2012, 12:29 AM
:smile:
I think i2i was saying put the ballnuts on the ends of the axes, instead of near the middle. That way, assuming you have room for the 16mm screw under each, you would just need to use a longer ballscrew to get the same travel. It's actually rather like how I mounted it on my lathe cross slide (http://www.mycncuk.com/forums/lathe-build-logs/4497-micro-lathe-zero-backlash*-conversion.html). There I extended the saddle with a milled block of steel, then mounted the ballnut to that to get it outside. I also had to mill a fair bit from the bed to fit the RM1204 ballscrew in.

Edit: Didn't see post #13 before submitting this.

jonnydeen
05-10-2012, 12:50 AM
i think its was use of the term 'apron' that was confusing me. it just makes me think of an apron on a lathe lol

i have thought about it but i cant imagine how to implement it with much success. as far as i can imagine, i would have to mount the nut past the extremity of the table travel and at that distance i believe the mount would have tons of flex and the the overall rigidity of an overcooked supernoodle

mounting a 1204 nut on the side of the saddle would still require machining of the table

jonnydeen
05-10-2012, 01:03 AM
OK
just had a brain wave


mount the nut on the end of the table
mount the ballscrew to the side of the saddle and use a small pulley so i can lower the motor down out of the way:glee:

how does that sound?

i2i
05-10-2012, 01:25 AM
how about this.
THK AX ground ballscrew 12-5mm L380mm BALL SCREW Kugelumlaufspindel | eBay (http://www.ebay.co.uk/itm/THK-AX-ground-ballscrew-12-5mm-L380mm-BALL-SCREW-Kugelumlaufspindel-/140858488753?pt=LH_DefaultDomain_0&hash=item20cbd1f7b1)

jonnydeen
05-10-2012, 01:37 AM
how about this.
THK AX ground ballscrew 12-5mm L380mm BALL SCREW Kugelumlaufspindel | eBay (http://www.ebay.co.uk/itm/THK-AX-ground-ballscrew-12-5mm-L380mm-BALL-SCREW-Kugelumlaufspindel-/140858488753?pt=LH_DefaultDomain_0&hash=item20cbd1f7b1)

i have seen that one already, i have no issue finding 12mm ballscrews at good prices

i am however quite stoked at the possibility of the concept i mentioned above and i must thank yourself and jonathan for giving me the idea

cheers

jonnydeen
05-10-2012, 06:25 PM
i did a few drawings of my concept (has anyone seen this done before?)


the first 2 show the ball nut and a mount i designed to utilize the original leadscrew mount holes
70687069


i then proceeded to modify the old ballnut mount i designed into a ballscrew mount. notice how the new mounts are 0.01mm off concentric which i feel is negligible and with adjustment can be completely overcome
707070717072

the screw mount was then further modified to allow attachment of a stepper via pulleys. it was also extended to clear the lip at the back of the knee (see second pic below)
70737074

and finally with a nema 23 added for good measure
7075

still have to draw some pulleys, a ballscrew and a few other components. more to come soon


so...

any critical thoughts on this design?

Jonathan
05-10-2012, 06:45 PM
I would mount the stepper motor to a block of aluminium since the posts are not particularly strong, wont conduct much heat from the motor and also look like a grandfather clock. Also you might have difficulty fitting the belt/pulley between two posts? It would certainly limit your choice of ratios.
Probably negligent, but instead of making the ballnut mount match the circular profile of the nut, I'd make the mount closer to a rectangle so it fixes to a greater area of the bed since that should be stronger.

jonnydeen
05-10-2012, 07:13 PM
I would mount the stepper motor to a block of aluminium since the posts are not particularly strong, wont conduct much heat from the motor and also look like a grandfather clock. Also you might have difficulty fitting the belt/pulley between two posts? It would certainly limit your choice of ratios.
Probably negligent, but instead of making the ballnut mount match the circular profile of the nut, I'd make the mount closer to a rectangle so it fixes to a greater area of the bed since that should be stronger.


fair points, i will certainly be changing the ballnut mount but the screw mount is, although debatable in need of more thought (access to y axis gib screws has been somewhat compromised)

cost also needs to be taken into account so i may well use standoffs just to get the machine running and then cnc a new mount

jonnydeen
09-10-2012, 01:54 AM
got some more feutering done on the x axis design this evening

firstly i modified the stepper/ballscrew mount by lowering the motor giving space for gib screw access holes and then added a rib on the back for some extra rigidity

70897090709170927093

then i drew a 1204 ballscrew and crude representations of thrust bearings for the ballscrew mount
a 450mm long screw leaves just over 38mm overhanging the table travel extremities

still have to decide on a pulley ratio so i can incorporate it into my design, any thoughts?


could the moderator please change the title of this thread to 'centec 2 ballscrews' as its getting far of the original topic and it would be a shame to have to start again

jonnydeen
15-11-2012, 03:52 PM
things have been slow to progress lately due to lifes consistent interuptions but on another note...
I now have ballscrews! :highly_amused:
2x 1204 & 1x 1605. 250mm, 450mm and 450mm respectively
also got my hands on a nice 20mm 2mt drill bit to drill the hole in the mills base casting for the z axis screw
7389

now that i have me some screws i need to get them machined. i have arranged for a friend to do this for me as the boxford doesn't have much in the way of change-gears and im afraid id screw it up (pun intended).

but before i can do that i still need to order bearings and pulleys so i can factor them into the design. this leads me to a few questions...

1) pulleys, is there really any difference betweel xl and htd or other types of pulley?

2) bearings, i see alot of people using angular contact bearings, wouldn't ball thrust bearings be a cheaper alternative or is their something im missing?

3) i see that using belleville washers is also quite common for preloading the bearings, would an easily available split washer not do the job?

Jonathan
15-11-2012, 05:12 PM
now that i have me some screws i need to get them machined. i have arranged for a friend to do this

Why couldn't you get them machined by the seller? Assuming you bought them from China it should be very cheap. It's not easy to do it yourself due to the hardness of the surface.

1) Different tooth shapes which are more/less accurate (backlash etc). HTD is generally considered the best for this application.
2) Could be, but I think they'd be harder to mount and pre-load accurately? Also possibly lower efficiency, but I'm not sure.
3) No since it wouldn't apply an even force on the bearing ring. Belleville washers may not be nessecary - it depends how you mount the bearings. The BK/BF blocks don't use them, instead they rely on the bearings being ground such that when they are tightened together the correct pre-load is applied, or they use shims. If you do use Belleville washers then you can put the ballscrew in tension, which is good since it increases the stiffness of the system a little (or a lot depending).

jonnydeen
16-11-2012, 03:05 PM
thanks for the reply jonathan

firstly, im getting the screws machined free of charge, why pay if i dont need to?

as for pulleys, i will take your word for htd and give them preference so long as i can find a set to fit the cramped x axis

now, the bearing selection is an interesting predicament...
the largest id magneto bearing i could comfortably implement would be E5 (16mm od)
i can easily get 8x16x5 ball thrust bearings

so, do i turn 12mm ballscrews down to 5mm to use magnetos or 8mm and just use ball thrust bearings. or in other words is 5mm sufficient if i ever hope for this machine to cut steel?

and as for bellville washers, point taken