Y axis wacko on my PR

[waynec]

Well, I've done it this time. I just welded my gantry corners on my PR R&P machine, and now I can't get the Y car to ride the rails no matter what I do.

I'm not convinced there is a cause/effect relationship- i.e. I'm not sure its my poor welding skills or warping that are the problem. I moved the Y car and bumped it off its rails while in a welding frenzy. When I tried to put it back, it just wouldn't roll right, so I tried to re-square it. Things went straight downhill. After a full day of messing with it, I just can't get it to ride from one end of the gantry to the other without derailing. The rails also measure 1/8" narrow in the middle, which is the problem, I believe. If I set the wheels for the ends, they are spaced too wide for the center, and it falls off the corners. ARGGHHH!

Any suggestions on how to get it lined up?

After all that welding, it dawned on me that it did little good because its the tube and roller gizmo that keeps things square. Big fat bolts on the unistrut don't keep it riding square- little 7/16" bolts with inadequate holding power do. Time to think about replacing the rails and maybe a new gantry.

Meanwhile, if you have tips that will get me riding the rails again, I'll quite cussing and scaring my dog.

Wayne from White Salmon

[waynec]

I think I'm axially challenged. Its Z car that rides on the gantry that won't ride the rails, NOT the gantry on the X rails. The gantry is nice and square to the X axis , and rides the X axis rails beautifully.

Anybody got any ideas on how to make my gantry rails parallel?

Thanks

[supertigre]

Angle grinder?!

and next time start at one end and work towards the other setting the rails straight and the proper distance apart as you go. Half fun.

The X car goes up and down the length of the machine, the Y car goes across the machine, the Z car was build by Datsun / Nissan and requires way to much maintenance for my taste. YMMV

[bleeth]

You're in a bit of a hole mate. You need to determine if both rails are warped a 1/16th or one is warped an 1/8th or what combo because you'll need to have a go at straightening them. I would probably start applying bracing and a hydraulic jack.
For some ideas on a gantry and car that can't do that go to the Mechmate forum: http://www.mechmate.com/Forum/discus.cgi?pg=topics

[gerald_d]

Wayne, I have difficulty in understanding how your gantry is constructed and what might have gone wrong. Could you give us some pics? (mail them to me if they are too big to post here)

[waynec]

OK, OK, its the Y car ( and I really wanted a 240Z as a kid). The rails are equi-distant at each end, but narrow by 1/8" in the middle. I'm thinking I'll try a car jack to bend em apart. Can I apply a bit of heat from my torch to get it to bend some?

I sure like that gantry at Mechmate, and thanks for the link/tip. But I'm more and more convinced that I should just fix the PR back to stock, and think about buying a new machine to get more accuracy.

If I try more DIY remedies like building a new gantry, then I'll need a new Z axis, and probably a new controller, and then the X axis will be next, and what about vibration in the stand? For a hobbiest, this may be bliss. For me as a woodworker, its just time I'm not making money.

I'm gonna try to get this one back to stock performance and plan for a new machine.

Thanks all

Wayne from White Salmon

[mikejohn]

Wayne
Not only do the rails need to be the same distance apart along their length, they also need to be straight!
So heating and bending, or the car jack, needs to be done with great care and accuracy.

...............Mike

[gerald_d]

Wayne, an 1/8" is not that bad, considering that the gantry is flexible.....

Set your y-car's wheel spacing for the ends of the gantry and then let it push the gantry open when it is in the middle. You need good hold-down rollers to achieve this. But I don't think it will take too much force to open a PR gantry by a 1/16" on each side.

With welding, it is mostly possible to put a "balancing" weld on the other side to counteract distortion. With photos, I could make some suggestions for you.

The last thing I would use is a jack in the middle. If I did go so far as to consider a jack, I would use it nearer the welds. A jack's force will "kink" the rail at the point where the jack is applied.

[waynec]

Gerald, thanks for the timely input! I'll drop the jack idea. Actually, I tried it with a car jack, but chickened out after a few turns because I was afraid of an uncontrolled result.

I can't really use the wheels to push the gantry open because they ride on the outside edge of the rails- it would be more likely that they could push the outside rail ends inwards, but in actuality they just ride up as the rails widen. Pics will make it more clear.

I did manage to get it working more or less, square within a 16th over 92". I'm gonna try to cut a few cabs with it today. It did a test run OK.

I'll get some pics later today, and post em. Thanks for your followup.

A question for you about new rails- If I was to put say some new Bishop Wisecarver rails on a new gantry, how do I get the rails exactly square and parallel? I'm a woodworker, so I know about tape measures. But if the rails are gonna be accurate, won't they need to be installed parallel and coplaner within a few thousands? How do I get that out of a tape measure?

Both Bishop Wisecarver and HiWin have linear rails that I think would work. But I have to be certain that I can install the stuff accurately, and I can't figure out how to do that.

Wayne from White Salmon

[gerald_d]

The new rails won't be absolutely rigid (nothing is) and to some extent they will flex to accommodate the car's width. So, relax about chasing the last 'thou because it will be a waste of time. Light finger force will push those rails into, and out of "tolerance" Spend a day with "shims" here and there and trust your instinct more than your measuring instruments. As you said, you are a woodworker - if it feels right and looks right it is right.

The two most important measuring instruments are a reference straight line (extruded alu is okay) and a distance rod to feel the space between the rails. Then you could also have some feeler gauges to see how big the light gaps are and how thick to make the shims. A digital caliper is always useful around the shop and it can be used to get your shim thickness right.

Gerald from Black Africa