View Full Version : Anybody used these linear bearings before?
07-06-2010, 05:45 PM
I've been looking at the RS Drylin N linear bearings for my A4 size wood router. It's my first build and I'm using it as a learning curve so I'm trying to keep the cost down as much as possible as I'll probably build a larger table in the future when I hopefully have more money.
I spoke to a guy at work who's used them before but in another application and he says they are a decent, I just wondered if anybody had used them on a homebuilt cnc router.
07-06-2010, 05:59 PM
These are the IGUS (http://www.igus.co.uk/default.asp?PAGE=DRYLIN)ones, generally quite well thought of. Supposedly very good in a dusty, woody, environment. I know several people have looked at them, dont know if anyone has used them.
07-06-2010, 07:12 PM
We fitted them some years ago to replace ball bushings on a laser cutter that was plauged by dust and grit from the cutting process getting into the ball bushing.
The drylin ones cured all this.
Somewhere I have some linear tables, with rails and bushings, not A4 but the rails could be extended.
07-06-2010, 08:30 PM
Oh would make a nice base for a PCB mill....
07-06-2010, 10:40 PM
Hi John, you have a PM.
Thanks for the link Irving. I do't know enough about my machine yet to fill in all of the ratings to get all the way through the steps. I'll save the link for future reference.
With regards to my own machine I was thinking for the bed and the side-to-side that I might need 2 rails with 2 carriages on each (4 in total) but when I look at other builds I think this sounds like an overkill. What do you guys think. My plan was to use 27's on the bed and the side-to-side, and one large one for for the tool up and down. (forgive my terms as I'm never sure what the x,y,z is lol)
Does this sound ok or overkill?
07-06-2010, 11:50 PM
X is usually the long axis, side to side, Y is front to back and Z is up and down.
07-06-2010, 11:51 PM
Generally, for a conventional mill, X is left/right, Y is front/back and Z is up/down. for a moving gantry device, X is usually the gantry movement, Y is the cross-gantry movement and Z is still up/down.
You only need 2 carriages if the distance from the rail to the load point is more than twice the length of a carriage. For two rails the carriage length must be no less than 1/3 to 1/4 of half the distance between rails. So, for an A4 work area 300 x 210 approx the X-rails will be ~250 apart (unless its a moving bed in which case they can be closer together). therefore the carriage (bearing) must be at least 60 - 80 long, and since they aren't you will need 2 spaced at 75mm. The parts you need for that configuration of x-rails are 1 x NK-02-27-2,350 & 1 x NK-02-27-2,350 LLZ costing about £40 and they'll be good for 2y worth of continuous running with a 15kg gantry/router load running at 1.5m/min traverse speed...
08-06-2010, 12:38 AM
wow, thanks for that irving. I think I understand the equations.
Do those 2 system numbers give everything that will make the "bed" slide :)
My sketches so far are drawn with the bed moving and the gantry providing the feet for the whole thing to sit on. This isn't based on anything other than the way I see my build going together based on dozens of different designs that I have seen. I can't work out any pro's and con's other than a moving gantry will take up less space?
Are there any general rules/pointers on this?
08-06-2010, 01:13 AM
Those 2 numbers give the two rails with the integrated sliders. The IGUS arrangement is a fixed rail and an adjustable one (thats the LL), not sure how the adjustment works tho.
The main difference between moving bed and moving gantry is one of loading. For a moderate sized machine a router can weigh 5kg or more and the all up weight of the gantry can be 20kg+, thats a lot of mass to move around, if all you're cutting is some small bits of wood. for an A4 sized work area the moving table is a better bet as the table is light and the workpiece cant be very heavy (size limited) so it makes sense to get the better rigidity by moving the table at the expense, as you point out, of a bigger footprint. One advantage of the moving gantry is that the downward cutting force is always midway between the bearings (although this can be acheived on a moving table by using the rails upside down and attaching them to the table, so that the bearings are stationary and the table/rail moves). the disadvantage of the moving table with bearing attached to the table but not at the edges is that cutting at the extremes puts a twisting loading on the bearing as the bearing nearer the cut acts as a fulcrum when the cutting point is outside the bearing footprint.
With a 350mm rail and bearing spaced 75mm the max working throw is 240mm approx (bearings are approx 35mm long spaced 40mm apart), so to get a real 300mm work area you need rails 410mm long for a moving gantry which adds about £4, but for a moving table you'll need them 600mm long (bearings placed at edges of table) although by not placing the bearings so far apart you could reduce the rail length to ~500mm with the disadvantage described above and an additional cost of £8 approx.
08-06-2010, 12:42 PM
Again, thankyou for the detailed reply irving.
Your point about the stability being better with the bearing "upside-down" is pleasing to hear. This was how I was thinking of doing it. The rails on the base and the bearings on the moving bed. I'd assumed it would be better so as not to get any twist or bow in the bed. I also thought that by doing this, the bearings would be protected from dust as best as possible.
I think I need to stop thinking so much and start cutting some wood. lol I get to the point where I am happy with a design idea in my head, but then I try and over-complicate things and it changes again. I have the remaining wood I need now to finish my benches in my workshop (garage) so if I can get them all finished over the weekend I'll be all set up and ready for some parts with this months pay :)
I'll be going with green MDF and some aluminium angle as my main construction materials. I'll start a blog as soon as I cut something. Nothing spectacular like some I've seen on here but for a first build for me personally it'll be good fun! ;)
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