How much flex with 20mm shafting for linear bearings>?

[phantomcow2]

The linear bearing deals plus shafting from VXB is pretty good. I would like to buy some 20mm shafting from them, and support it (running open bearings). However they dont sell a 4 pack of open bearings, only closed. But a lot of people seem to use these closed bearing on this website, what are your experiences with flex?
The X axis would be over a 40" length. Two bearings per side for me, could i expect satisfactory results?

[psantangeli]

I can tell you in about 2 week :-) I am just finishing a machine based on those parts with a 50" X axis and a 48" y axis. The machine is "rolling" now - just finalizing the drive system. I was worried about flex when I first set it up, and there will definitely be a little bit, but it doesn't look like very much.

Note that I am creating a wood router, mostly for cutting operations though, so my tolerances to z-axis flex are fairly high. Note that my y axis gantry is pretty big/heavy too, as it has to carry the 20mm yaxis shafts, the gantry structure itself, and the z axis.

[phantomcow2]

Yea mine will not be too demanding i dont think. I am expecting 35-40 pounds for my gantry, its only 24". The router itself is a good 10-15 pounds (recieving tommorow), other than that, its all the aluminum that weights it down. Flex for the Y axis i dont concern about

[ViperTX]

Well the flex never looks like much until you actually cut something and then you notice that some surfaces vary due to the flex and it will be noticeable.

You should use a supported shaft with open bearings. Go to the Thomson Industries site and check their flex calculator.

[jeffs555]

ViperTX,
Do you have a link to the Thomson site and their flex calculator? I have Googled and can't find it. I agree with you about the need for supported shaft. Beamboy says that a 40 inch long 20mm shaft will deflect nearly 1/10 of an inch with a 40lb load.

Jeff

[Bruggles]

one thing that you need to remember about beam boy, is that if you are mounting the shaft in a solid mount, you need to use the canteliver mount, that will act like a clamp, and not allow the shaft to bend as easily, just like the clamp that will be used to hold your bar, in beamboy the standard mount is as if you were setting it on the mount, with no attatchment. I ran the numbers and got that with a 40lb load on that shaft you would get .0236 inches of deflection. In your situation it would be somewhere inbetween the two calcualtions, because the shaft won't be completely inmovable at the ends. I found the eqations that thompson uses on their website, they seem to be the same as the beam-boy ones, but without the canteliever support. Either of the calcualtions are just an estimate, since there is no way to simulate every possible situation with a simple eqation. I am not saying that I reccomend that you use the shaft, I just thought that I would help you out with your future calculations. If you have a 40lb gantry only around 20-30lb will be resting on it, depending on where the router is on the gantry. The forces that are the hardest to calculate are the cutting forces which can be surprising at times, but are highly dependent on feed rates.

They don't have a kit to do the open bearings but to obtain the shaft and the bearings should cost about the same to do out of a kit, as the closed bearings in a kit, give or take $20 for the each setup, but the supported rods require the mounts and also machining to drill and tap into the rod, and the will probably have to be spot bored to get past the rc60 outershell before they can drill and tap.

I won't tell you what to do, but I will say that I am making a similar size router, but am going to use continuously supported rails, because for me 20 thousands of flex just isn't as rigid as I want it. good luck.

[phantomcow2]

Thanks for that reply Bruggles.
Are you suggesting that i buy these from vxb?
http://www.vxb.com/page/bearings/PRO...Systems/kit989

I have access to a bridgeport milling machine nearly every day, and would much prefer to make my own pillow block.

[phantomcow2]

TO throw something else out there, what do people think of these?
http://www1.mscdirect.com/CGI/NNSRIT...PMT4NO=4751180

[DareBee]

I have not done this router build, but in my (other) machine building experience I have some input.
Lets say you dedide that 0.02" deflection is no biggie, and I agree that for thru cutting it wont matter; your BIG concern should be deflection and flutter under cutting pressure.
You get cutting along on alittle heavier material start getting near the middle of the rails and all of a sudden your Z and Y start flopping up and down and round and round at 1000 RPM.
This scenario at the least will likely break the cutter and scrap your part.
I have not built a router but this is what I see happening with this design, It costs more to build it twice than right the first time.

[wood_donkey]

I had the same vision, that is why .02" is not acceptable. I will only be happy if deflection is in the single thousands, .01 or below.
My original idea was to use skate bearings, maybe i will just persevere through and make it through.

[Bruggles]

You could use those vxb bearings, you could also make your own pillow blocks, I asked on the forum what people thought of the vxb bearings and knowbody had used them. What I am going to do is make my own blocks and use the thompson super smart ball bearings, they supposedly have excellent wear and load ratings, they are around $25 a piece, but without the pillow block that supports them. The shafting is very reasonable, but with pre-tapped holes it is around 3x what it is without. a good site for thompson bearings is http://www.qbcbearings.com/, I think the vxb bearings would probably be fne, I just cant gaurantee it. It will be hard to make the blocks with a bridgeport unless you have a reamer that is the od of the bearings that should be a very snug fit in the block.

As far as the liner from msc that is meant to line linear bearings similar to pacific style bearings. Which are also an option, i am going to use them on my z axis, they are cheaper and are excellent bearings, but with most of them the shafts need to be paralell whithin .001in. I definatly would recommend a supported shaft.

[Bruggles]

Oh, I just looked more at the liner, which is also an option, and it says it can be used with the bearings it was designed for or in a stand alone configuration, but it might be difficult to use as an open bearing.