Bad Rails?

[jimmythethief]

I noticed that the hold down bearings get loose only in one area that is basicly between two bolts on the y rail unistrut bolts. I tried to pull them in and retighten the bolts on the unitstut to keep the rail tight against the bearings, but the small distance of the problem area and the placement of the bolts makes it impossible to fix...I place a straight edge on both sides of the rail and I don't see any gaps of air...but that is my only guess that the rail may be defective...I doubt that the y cart is this hard to adjust normally...that is another clue to why I think I may have a defective rail...any thoughts?

Erik F.

[rgbrown@itexas.net]

Erik,

Good measuring tools should tell if the rails are worn or bad. A good "mechanic" might be able to bend the rails back into shape. But one could purchase new Unistrut for the services the "good mechanic" should charge. IF, you could find the good mechanic.....

But, the good mechanic would also quickly figure out what your problems are and recommend what to do about them.

BTW - I am using the term "mechanic" in the form it meant about a hundred years ago. Not the term used now for someone able to do more than change a tire.

Ron

[jimmythethief]

Well, I'm guessing not everyone has had as much trouble as I seem to be having...I ran some test patterns and kept stopping the y cart while it was working to adjust the hold down bearings to the best happy medium I can find...seems to be working better now. How much can I expect the rails to "work in"? Will the V bearings actually help to smooth out the variation in the track?

Erik

[gerald_d]

Erik, we welded our rails to the unistrut and then had them re-ground by a company that specialises in grinding machine tool rails. We havn't bothered to fit the so-called "hold-down" bearings.

[jimmythethief]

Gerald,
So you guys actually run with nothing holding the small Y cart down except good ol mother nature? I asked SB about trying that and they told me it wasn't a good idea. While messing around with the Bot and while I had the hold down bearins off, it seemed like it would take a pretty good jolt to rack the y car. The hold downs seem to be what have been causing me all the trouble, when I run it with none I get nice smooth dado's, but run the risk of racking the y car I guessing on heavy cuts. I still don't understand why they don't make use of a hold down bearing that would go under the rail as well as the ones that go on top...this makes more sense to me. If you could, please give me more info on you NO-Hold down bearing set up. Thanks.

Erik

[gerald_d]

Erik, the spring tension of the y-motor does a good job of holding down the one side of the y-car. Our single small router is mounted towards that side. Our reasoning was that we would mount a spring tensioned bearing on the other side (under the rail, like the motor) if we had a problem.

Our car has jumped the rail a few times, but every time it was because we did something really stupid - like hitting a clamp with the cutter. And in those cases we were sort of glad that the car wasn't held down more firmly. Otherwise we could have burnt the motor or driver . . . . . .

Our rails were not provided with a precision surface for the "hold-down" rollers to run on. We reckoned that it would be better to keep the rollers as spare parts for the x-rail rollers that carry a very heavy load.

Why have 4 expensive "Hold-down" rollers running on a rough surface? A couple of cheap ball bearings under the rail would do the job better IMHO.

[gerald_d]

By the way, this term "racking" needs to be defined, just to avoid confusion. Webster's dictionary says that racking means to become forced out of shape or out of plumb.

The y-car viewed from the top is a rectangle, made of 4 separate sides, bolted together at the corners. "Racking" means that the corner joints come loose and shift making the rectangle go out of square.

I can't see what the hold-downs have got to do with the potential for "racking" the y-car?

[jimmythethief]

I guess racking isn't the right word to use then, I'm talking about the Y car jumping off the rail or skipping teeth on the rack and pinion. What do you cut with your machine? Is it thin material? Thick? I only asked because I'm using mostly MDF and will be making some pretty heavy cuts...not sure if making cuts in thicker or more dense stock would leave more potential for the car to ride out of the rails.

Erik

[gerald_d]

A spiral up-cutter pulls the car down onto the rails. Some bits cannot be plunged straight down and we don't try and force a bit that wasn't designed to be plunged.

The only "risk area" is with spiral down cutters.

As I said, we have a small router, and our normal cutting bit is 6mm (1/4"). We sometimes cut 20mm deep spiralling down into hardwood at 0.75 ips with no problem. We surface the MDF spoilboard with a 1" cutter at 1.5 ips with no problem.

IMHO, using V-rollers lying flat, against an un-ground edge, of raw angle iron, is simply the wrong thing to do if you want to achive proper "hold-down". Maybe those edges have been ground since our early PRT, but I would still go for a bearing running under the rail - not against a side edge.

[jimmythethief]

Yes, my "hold down" edges are ground like the X and y rails...and I with you on the bearing under the rail...seems like having them too tight causes problems and having them too loose is like not having them at all...a sping tentioned hold down UNDER would be much more effecive.

Erik