X Axis using V Bearings?

[ratman]

First off I'm a newbie to posting in pretty much any forum. I've been lurking in the back ground for quite a while and finally decided to get off my duff and actually build a CNC table. I have read through must of the posts and did a search of posts that talked of using V bearings. I basically have a question concerning a couple of designs I have seen.

Specifically I'm looking at how Mechmate uses the V bearing on the X axis with a Machine gear below the rail, the drive moter is mounted on a spring loaded pivot. This seems to me a very simple and fairly low cost alternative to Linear bearings. I would like to build my gantry for the Y axis from either extruded aluminum or 80/20. My concern is that other than the spring loaded motor contacting the drive gear there is nothing to counteract any deflection when the router engages the material and I will loose some accuracy. I have to assume that the weight of the Mechmate gantry is enough that there is sufficent pressure that this is a non issue. It seems everyone that is using this desin is pretty happy. Does anyone have experience using this design for the x axis. I'm shooting for a 5' x 10' cutting area.

Since I need in excess of 10' of bearing surface this seems to be the most economical option for the X axis. The company I work for uses a HT2000 with a table that uses a V bearing design, I can see this working great for Plasma / Torch since nothing actually touches the work surface.

Any input either way is greatly appreciated or point me to a thread that has already discussed this.

Ratman

[Gerald_D]

Hi Ratman

2 issues at play here:

1. How much force does the router generate upwards? Most board cutting involves horizontal forces. When cutting horizontally with a typical spiral bit, the router is pulled down. Big upwards forces are generated when trying to plunge a big diameter cutter. Some cutters will refuse to plunge and then the gantry is lifted off the rails. We happily plunge 40mm [1.5"] ball nose cutters into hardwood without lifting problems. Do bear in mind that spindles and routers do not like big axial loads - one has to limit vertical loads whenever possible.


2. What counteracts the upwards force? The weight obviously. Plus the springloaded motors pushing up under the racks. Plus the dust collecter sucking down sometimes.

[ratman]

Gerald,

Thanks for the information, I do not plan to use large molding bits so it sounds like as long as I choose the correct bit designed to plunge and don't overdo it I should be ok using the V bearings. I never figured the dust collector into the equation, makes a lot of sense the more I think about it.

[Glidergider]

Hey Ratman,
For a newbee, you are asking the right questions. I have a JGRO router, and love it, but I'm really ready to grow larger, and more accurate. The mechmate is really a good looking design.

[ger21]

Originally Posted by ratman Gerald, Thanks for the information, I do not plan to use large molding bits so it sounds like as long as I choose the correct bit designed to plunge and don't overdo it I should be ok using the V bearings.

Be careful here. If you're not using rack and pinion on the bottom to hold it down, acceleration alone might lift it up. You didn't say how big your talking about, or how much the gantry will weigh, but there are a lot of forces involved here. Even side forces can be considerable, and more than enough to lift the gantry.

[ratman]

Gerry,

I hope to get a 4'x 8' cutting area, and if I use the V-Bearings I will definately use the rack and pinion machine gears. I have not decided if I will build the Mechmate from the plans or if it will be my own design. I still have a lot research to do, however I'm thinking of using a piece of 3060 for the Y axis. I like extrusions, Light weight yet very ridged. I thought that keeping the the weight down on the y axis would decrease any momentum problems that a totally steel Y axis could have. Once I have a little more information put together, I 'll do some estimates of what the entire gantry including the tool will weigh.

I realize without all of the information it's difficult to give advice. I appreciate everyones input.

Stay Low!!

Ratman