Linear Motion , CHEAP!!

[routercnc]

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...all-prod21223/

Linear motion, cnc and cheap are a difficult combination.

[Ross77]

Linear motion, cnc and cheap are a difficult combination.

Aint that the truth

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]

(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

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]

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]

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.