Concern with gantry lift

[CuttersCov]

Hi all,

I've been working on my table this weekend with what I am finding out is a crappy design (on my part) where the bearings on the xy rails are concerned. I'm examining whatever pictures I can on the shopbot and from what I can tell there isn't a point of contact for the bearings to protect against gantry lift (for lack of better terms). I am defining gantry lift as the force exerted to the gantry when the bit plunges into the substrate, and the tendency for the gantry to lift up when the bit is pushed down.

Tuesday I'm going to make some test Vee bearings out of aluminum and see how this works.

I'm just curious for those of you that don't use THK's/Pillowblock style bearings what your solutions are.

I might add that I think I would just use the pipe method with skate bearings mounted on angle iron, but my table is 10 feet long and the main supports are slightly bent. Though if things keep getting worse, ie I feel like this guy I might reosrt to that method somehow.

-Matt

[jcc3inc]

Sir,
You should calculate what force you expect the plunge to be, and see if it is greater than the weight of the cross carriage plus the router motor assembly. If it is, then you need something to couple it to the main carriage in order to have more "weight" to hold it down. Normally I wouldn't expect much additional weight would be needed.
Jack C.

[Bruggles]

i believe that the shopbot is held from jumping off the track by the spur gears on the track, but it still could jump a little if the down forces are greater than the weight of your gantry. This thought is just from looking at the pictures, so I am not sure.

[Geof]

Just relying on the weight of the gantry to hold it down may not be enough. In addition to the vertical cutting force tending to lift the gantry you also have a torque acting on it because the driving force will rarely be inline with the cutting load. I have attached a picture showing this.

Actually depending on how powerful your drive is just the acceleration force could make the gantry lift.

[spalm]

Matt,

Can you post some pictures of your machine? It would be easier to help you if we could see what you are doing.

Steve

[CuttersCov]

Guys,

If any of you want to take a look I updated my log. More to come.

-Matt

My Machine

[posix]

well, just rotating the bearings by 45deg around the rail eliminates this problem alltogether. if that's what you mean?

[posix]

ah ok that picture is better. well you either rotate the rail and bearings by 45 deg or you add a second set of bearings on the UNDER side of the rail. I will post a picture in a minute.

[posix]

here they are. so in first pic you rotate the whole arrangement by 45 degrees clockwise so you end up with bearings arranged like < rather than L (try and visualise)
in second picture you just add one bearing (I think one would be sufficient but if you have loads of bearings then 2 on each rail would be good) to the UNDERSIDE of the rail. and that's it. I think the second solution would be the quickest.

[posix]

Ah! remember this?

why did you do it? the lower bearings were there to address this problem.

[posix]

in this picture you even have provision for the extra 2 bearings for the under side! so make use of those holes and stick a couple of bearings in there!

[CuttersCov]

Posix,

I had bearings in there for the lift problem, but then it creates a new problem, and that's why I was asking about lift. (In hindsight I should have made myself more clear.)

If the bearings lower bearings are in there the gantry supports want to run into the rail itself when they are moved forward and backward down the rail. So I took them out.

As for the first design that had the square tubing that option will no longer work because of the legs of the table in addition to the main supports are bent so I have to use rails in order to eliminate the error of the bends.

I thought of something that I am going to try today. I'm going to put the lower bearings back in and remove the set of bearings on the inside of the upper set.

Another issue I have is if the bearings are all tightened down to eliminate skewing of any sort then they are so tight that A) the gantry won't move and B) the bearings will not run true and come off the rail or grind into it.

-Matt