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c cunningham
25-01-2011, 01:33 PM
Totaly new to this so pass a few ideas by you lot and see If they are good or crap!
Building a fairly big CNC for wood working so need a large bed size
In a perfect world 10ft x 5ft I do have the space in the workshop for that
BUT a bit over optimistic for my first attempt LOL so 5ft long x 3ft wide
With hopefully 2ftx4ft working area for MK1
The bed or base will be kitchen cabinates with a strong top as I am a cabinate maker
and so thats very easy for me to make
But the linear slides ,,, HUMM
Seen the skate berings and ali angle thing ,, works but
The V grove berings are better but at more than 12 each !!
Proper suported rails ECT is out of the question at the lenths I need
SO been brain storming and come up with ....
An aluminum I section thats 54mm x 25mm that has a 6mm hi x 4mm wide rib on the 25mm face running right along the lenth
I could use the skate berings in a V on ali angle , but its ugg ugg
I could use V berings but at 12 quid each and hardened steel on ali
would not last very long !
So skate berings with tyers? , replaceable nylon tyers
Ant thoughts or sugestions
thanxs

m_c
28-01-2011, 04:20 PM
Problem with using nylon wheels, is you risk losing rigidity due to flex in the wheels.

I wouldn't worry too much about running hardened bearings on an alloy extrusion. Provided you keep them clean and spinning, you shouldn't experience any major wear. You're only likely to experience major wear if things start sliding.

M250cnc
28-01-2011, 04:53 PM
Provided you keep them clean and spinning, you shouldn't experience any major wear. You're only likely to experience major wear if things start sliding.

That is very true and easier to accomplish when you have bearings with way wipers sliding on rails.

So unless you enclose all the running gear in bellows you will embed dust to the tracks/bearings that will give very stiff jerky movement.

Very expensive are bellows and awkward to implement.

Phil

blackburn mark
28-01-2011, 05:37 PM
skate berings with tyers?

i like the idea personaly, might be worth it on a large x axis.. bit bulky to impliment but if you have three way apposing wheels i imagine you could pre-load them against each other with enough force to prevent them flexing any more than your frame / gantry etc

the tyres (+ the larger diameter) would smooth out any of the smaller rail inperfections

might be a winner if your after saving the brass :smile:

its allways a gamble though when you try something new :eek:

blackburn mark
28-01-2011, 05:47 PM
chris, have a look at "routercnc" build log, looks like he is running plain bearings on alli profile, looks like a winner :)

he seems happy with it anyway

m_c
28-01-2011, 08:44 PM
That is very true and easier to accomplish when you have bearings with way wipers sliding on rails.

So unless you enclose all the running gear in bellows you will embed dust to the tracks/bearings that will give very stiff jerky movement.

One random idea I've had for such situations in a dry enviroment, would be a suitable air supply aimed to blow any dust of the rails either side of the bearings. For shavings/dust, a gentle air supply would be enough to blow any rubbish of the rails. Off course, there is the issue that you'd need a compressor to provide the air supply.

routercnc
28-01-2011, 09:11 PM
Hi C Cunningham,

As Mark points out I am running bearing races directly onto Ali profile, but I always knew it was a short term solution and a small witness mark is appearing on the profile where the bearing runs over it. Not the best long term solution, although a hardened running strip might give better service.

Stray dust and chippings do get engrained on the top ali surface, and aligning the whole gantry (90 deg to bed, same height from bed at each each etc) took some fiddling. Once the extractor shoe is working that should solve the dust problem. The top and bottom bearings are fitted to short axles which run in adjustable slots in the side pieces. This allows complete alignment adjustment vertically. Side to side location is from single bearings set into holders, with one side fixed and the other side on an adjustable pressure plate. A bit like these :

http://www.axminster.co.uk/axminster-universal-transfer-ball-prod21223/

Linear motion, cnc and cheap are a difficult combination.

Ross77
28-01-2011, 10:01 PM
Linear motion, cnc and cheap are a difficult combination.


Aint that the truth :smile:

I had promised my self not to post untill I actually start building again, but maybe I have an idea that might be worth exploring.

If the machine was solely for woodwork and the correct feeds and speeds could be achived with the rest of the system, then surley the loading on the bearings could be reduced and then a small nylon or delrin "Tyre" wouldnt be such a bad idea and allmost a cheap throw away maintainance part if wear occurs. (it could even have a rib that runs in the profile channel to proved some lateral restraint)

If the section was thin enough it shouldnt compress to much and if it was made wider than the bearing it could spead the load over a wider section on the ali profile.

Im not saying that it is a perfect solution but for this application and accuracy I'm sure it would be worth a try.

Later

blackburn mark
28-01-2011, 10:16 PM
(it could even have a rib that runs in the profile channel to proved some lateral restraint)

nice one ross, that sounds like a great idea!
that alli profle is pretty accurate, it sounds like it would work a treat :smile:

i cant see acetal/delrin wearing a grove in your alli to quickly

with the price of skate bearing being the way they are id be tempted to use two on each wheel and make the wheels as wide as possible

M250cnc
29-01-2011, 01:16 PM
As Mark points out I am running bearing races directly onto Ali profile, but I always knew it was a short term solution and a small witness mark is appearing on the profile where the bearing runs over it. Not the best long term solution, although a hardened running strip might give better service.

Stray dust and chippings do get engrained on the top ali surface,

Would you say it's the dust build up that's is causing the witness mark ?

Phil

routercnc
29-01-2011, 10:34 PM
Hi Phil,

Mostly I think it is the weight of the gantry, spindle etc. pressing down onto 4 bearing races which are only about 10mm wide. The contact patch (per bearing) is probably less than 0.5mm long, so in terms of area it is pretty small. I've not weighed the gantry system but it must be 20kg or more, so the stress (force/area) is pretty high. Aluminium is fairly soft so these lines are appearing. I think the cutting forces are tiny compared to the static weight running back and forth over many cycles.

Having said that, a few tiny chips have slightly pitted the surface where they've been run over. But I quickly stopped most of these occuring by using small screens along either side of the x axis.

I agree with Ross and Mark and anything which either increases the contact area (width or quantity of rollers) or increases the hardness of the ali rail (e.g. steel wear strip) should help. Wide nylon rollers are worth a go.

I have never had to do this but I suspect it is possible to calculate by using the force/area (gantry weight in newtons / area in m2) compared to some surface stress value for aluminium.