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kong
07-27-2003, 06:58 AM
I just bought some 16mm ID linear ball bushings, and was wondering just how strong the 16mm shaft would be? I'll be going for a precision ground/hardened steel shaft, but will it only be strong enough for the z-axis, or could I possibly use it for the y? Sorry, not used this stuff before, so I don't want a load of slop across the 800mm y-axis.

wjbzone
07-27-2003, 09:39 AM
kong,
I pluged in the your shaft to efunda.com
solid mechanics
beam
simply supported

If I did it right (before my free trial membership ran out)

I used a center center load of 40lbs (18.1kg) and it calculated a deflection of 0.033mm at the center.

I used that to get my shaft deflections when I built my machine.

Bill.

Forgot to add:
if you use 2 shafts, you will get about half that deflection for the same weight. plus your probably supporting the ends better then a simply supported beam, so deflection will be better.

kong
07-27-2003, 10:15 AM
That sounds good to me! i am bidding on some 25mm ones aswell, but the price of the shaft raises sharply for that size. Thanks for the info, I'm off to have a look around efunda!

anoel
07-27-2003, 12:14 PM
I picked up some 20mm and some 16mm From what I bet is the same guy. Nice bearings, and fit the Enco drill rod just perfect.

I was planning to use the 20 for my Y axis and the 16s for my x and z.

The rod is pretty strong but I don't think I'd use the 16mm for a long Y axis.

kong
07-27-2003, 12:35 PM
Good to know they are ok, the y-axis on my machine is the one carrying the z-axis, sorry, not sure what what the proper naming convention is! What is the drill rod like compared to steel shafting?

HuFlungDung
07-27-2003, 01:28 PM
Kong, the modulus of elasticity is a characteristic of a material that is (I think) an indication of stiffness, which is what you are concerned mostly with, in your application.

AFAIK, there is not a significant difference between any of the steels in this regard. The only way to improve the stiffness within the same material category, is to increase the diameter, or, of course, add stiffening supports.

A cantilever support on each end of the rod will improve stiffness quite a lot, as compared to simply butting the rod end against a flat plate. The cantilever support would be represented by a fairly long, tight-fitting shaft mount. I'm thinking something with a bore a couple of inches long that you would press onto each end of the shaft before you mount into your gantry. Of course, you do have the difficulty of getting off the shelf components that meet these close-fit criteria.

CAMmando
07-27-2003, 02:17 PM
I made the same design decisions for my project. I went with Linear rails for X and Y which are supported along the entire length. I decided to use 25MM dia shafting with ball bushings for the Z.

The thing to keep in mind is deflection is a Cubed function of length. So without knowing your Y axis length it is hard to say, but I cant imagine 16mm being stiff enough for more than the most modest sized machine.

kong
07-27-2003, 04:55 PM
Thanks gents, some good advice here. Hu, you're right, there's no way I could make anything accurate enough for a cantilever, but I understand the concept. As for the bar, I think it is best left to the z-axis for now, but I will be testing it out physically once i get around to the building stages. As I said, I'm bidding on some 25mm bushings too, so I'll use those for the y, hmmm, better go hunting for something for the x now!

anoel
07-28-2003, 10:54 AM
MY X axis rails Will be 26 inches wide and my Z axis will be 9 inches or so.

at 16mm that is a tick over 5/8" I've seen several machines using 1/2" shaft effectively over that length and the 20mm that I'm using for my other axis is over 3/4 of an inch so over 36" inches so I think that looks pretty good.

I'was thinking of picking up some more 16mm bearings and using them to build a little all aluminum 10"x15" router as well for doing PCBs and other delicate routing.

I'm not sure that the 16mm rail is enough for my Porter Cable 690 Router's weight but I'll build a mount and give it a shot nonetheless. It'll handle the Dremel Advantage with no problem I'm sure. But with an all aluminum X and Z I'm hoping that wil provide enough rigidity for the Porter Cable.

I scored some more linear bearings in pillow blocks, I think that they are 1" I.D. (Don't you love buying stuff a little blind?) But won't know for sure until I get them if so then I'll move the 20mm bearings to the x axis and use the 16mm on the Z and that will surely be enough for the PC router. I should have all of my parts gathered in the next week.

I'm thinking about getting a chopper board and using a set of 2.6v 3.1A steppers that I've got for the extra torque over the 4v 1.2 A motors that I'm using now. All thiat combined should make for a hell of a decent stepper based router. Plus I get to use my other controller and motors for a smaller router.

I just need to talk HomeCNC into making me a set of those kick ass collets for the Porter Cable.

kong
07-29-2003, 01:18 PM
Yay, I have finnished my purchases. I have 4x16mm bearing bushings, and 8x25mm. That's the three axes set up, but a quick question on mounting -
I found some pillow block bearings on ebay, these (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=26217&item=2547191286) to be precise, and was thinking on maybe using them to mount at least the x axes on. I know they are designed for a rotating shaft, but with something to prevent the bearing from spinning, they may make great little shaft supports. It just makes the alignment easier. Thoughts?

HuFlungDung
07-29-2003, 01:37 PM
Kong, those are tie rod ends for linkages. They do not make good rotary or thrust bearings, since they merely provide a loose fitting swivel.

kong
07-29-2003, 02:05 PM
It's lucky you're around here Hu! I presumed they were...well, you know what I thought! I've just been looking at the cost of new shaft support blocks, and they're a bit steep at 15 each over here! I thought of making some, but my MDF cnc won't be upto the job with the degree of accuracy needed.

ger21
07-30-2003, 08:49 AM
I'm making my z-axis with similar bearings, 3/4" shaft. I'm mounting them to Baltic Birch plates. What I did was first drill for my bearings on a Drill Press with a fence mounted. Then, with double face tape, tape the drilled piece to the shaft mounting plates, lower the bit into the predrilled hole, and clamp the pieces to the table. Switch to the bit for the shaft and drill the shaft mounting plates. I did the same thing with the mounting holes for my stepper and leadscrew support bearings. If your careful, everything should line up fine and be quite precise, although it is a bit time consuming doing all the bit changes.

Gerry