Straightening the rack on a drill press raising mechanism?

[HMF]

 

[Tony Wells]

I've seen those bent before, usually from a heavy table load and an unlocked table. The rack isn't meant to support the load. Couple of things to think of. It got bent cold, so shouldn't you be able to straighten it cold? I doubt it's hardened all that high, as it wouldn't have been bent so easily without breaking. Unknown material, known hardness.....I would do it cold. And it doesn't have to be perfect either, just close. A little patience, a little time, and it is done. You could use a propane torch to warm it up, say.....to 400 deg, and it will help with internal stress while straightening.

 

[HMF]

Tony,

How would you straighten it? Would you use a press with a piece of wood under the rack, or a piece of steel under it? The wood gives, making it straighten more slowly, the steel doesn't, so you risk a crack. What about an arbor press, again with some support under it?

Would heating destroy the temper?

It's so badly curled, I would worry about how to do it exactly.


Nelson

 

[Tony Wells]

Straightening stuff is kind of an art. I learned it at the first job shop I worked. Lots of dump truck hydraulic cylinder rams, and multi staged centrifugal pump shafts. It has to be done in stages, not attempting to get it all in one move. From the photo, I see 2 major bends, one fairly large in radius, one small.

If it were brought to me, I'd soak it in the oven, if it would fit, for an hours at 400-450 f, then hit the smaller of the bend points first, using a hydraulic press.....because of the slow, controlled pressure. I'd keep a scale, or some sort of straightedge real handy so as not to take too much time and lose the heat. A hardwood block might be a good idea for some things, but the direction of the bend makes the bottom, where the wood might go, present the teeth to the support. The teeth would have a tendency to sink in, and grab. You have to remember than when you flatten an arc, the piece grows in length. It must not be constrained from moving, and that might happen with a wood block.

I would bend a little, then release pressure to gage how much beyond straight it must be pushed, and allowed to spring back. If it is truly a hardened piece, it will spring back a great deal, and then you must apply a little more localized heat with a torch. It also helps sometimes to use a torch to keep from introducing more bends not centered on the apex of the starting bend. Smaller readii require the supports underneath to be relatively close together.

If all you have is an arbor press, you still can do it, but I'd recommend some support under the center of the bend a little taller than the end supports. This will limit the overtravel of the ram, which might happen under just hand pressure on the press handle. You might overbend without it.

On the cheap, quick and dirty (CQD method), a machine vise that grabs the straight section, and a strong arm is applied to the rest of it, and pulls or pushes against the bend. I prefer to pull, and I can catch myself on one leg easier that way if something breaks. If more control or leverage is needed, a pipe can be slipped over the piece.

That reminds me, I straightened hundreds of shafts from PW drives for a local boat shop. I never did see how they got bent, but they brought them by the dozens to be straightened. Usually got them with 0.005 TIR anywhere along the length, which was 3-4 feet.

On the lost temper (pun intended) issue, unless you heat it to color, which isn't necessary here, that won't be a problem.

 

[HMF]

Tony- thank you!

That was exactly the kind of instruction I was looking for. I have tried to do this kind of thing before using just brute strength, and messed it up. There was no way I got them back into alignment- always either overbent or underbent them. As you have demonstrated, it is a lot more involved than just "give it a try". For someone like me, making one of these is a bear.
Speaking of which, I need to take some photos of my new (to me) dividing head that I cleaned up the other day.


Nelson

 

[Tony Wells]

If all you have is an arbor press, you still can do it, but I'd recommend some support under the center of the bend a little taller than the end supports. This will limit the overtravel of the ram, which might happen under just hand pressure on the press handle. You might overbend without it.

Correction required here. The center support will need to be lower by a little, but not much. Just don't want to over bend. Then you can shim up the outer supports a little at a time to get the bend amount right. Or use a machinist's jack under the center and lower it a little at a time.

 

[Tony Wells]

Ought not take more than half an hour, really. It's not that the pressure needs to be maintained for a time, but the slow movement and control of the amount of counterbend you put on it that gives the advantage to the hydraulic press. The metallurgy behind it is this: Metals have two primary properties involved here. Tensile and yield. Tensile is the ultimate strength of the metal, the amount it can be stretched before or at the breaking point. When metal is bent, other properties come into play, but principally, the outside of the curve is being stretched, and the inside compressed. There are limits to which this is tolerated. Ultimate tensile point will fail by cracking on the outside. If the metal is hard, a bending test will fail sooner. Yield, on the other hand, is related somewhat to ductility. A low yield allows the metal to distort and stay distorted at a lower stress point. So, you have to bend the rack beyond it's yield for it to stay bent (or straightened in this case). Think of lead.....bend it and it stays pretty much where you leave it. A paper clip has a higher yield point than lead, and is noticeably more springy than lead. But if you stress it beyond it's yield point, it will retain some of the bend. A leaf spring on a truck has a little of both a high tensile for strength, and a high yield point so it won't stay bent under a load. Of course, most metals have a yield point lower than their ultimate tensile, which means we can bend them without breaking them, within their limits.

If you've done any automotive engine work in recent years, you know about TTY bolts. That's Torque To Yield. Idea being that more consistent load can be placed on say, a head, by using this method where the bolt is stretched to it's yield point, then a given additional rotation specified in degrees. The bolts designed for this are carefully graded to yield at close to the same point, thus loading the mating surfaces very evenly. More so than the old way of a torque wrench, where too many factors control the bolt load.

Anyway...that's OT.

Since it's not hardened, go for it. Just bear in mind that steel does cold work to a degree, and if you bend it back and forth enough, it will break.

 

[Starlight Tools]

Just did a similar job to a headache rack for a client's truck. The rear removable support had been hit, probably somebody backing up and was bent. Material was 1" tubing. I set the hydraulic press so that full extension of the ram was just under an inch from the top of the support blocks and pressed away. Support was on either side of the bend, ram pressing down at the centre of the bend. Worked it a couple of times and the tue ended up almost as straight as a straight edge.

Other point to remember is that those racks are quite inexpensive. So if you mess up, you will be abe to replace it for not much coin.

Walter

 

[Starlight Tools]

Those rings are a common part to break, I keep them in stock almost all the time, but your replacement sould outlive the drill press.

Walter

 

[The Liberal Arts Garage]

Tony- thank you!

That was exactly the kind of instruction I was looking for. I have tried to do this kind of thing before using just brute strength, and messed it up. There was no way I got them back into alignment- always either overbent or underbent them. As you have demonstrated, it is a lot more involved than just "give it a try". For someone like me, making one of these is a bear.
Speaking of which, I need to take some photos of my new (to me) dividing head that I cleaned up the other day.


Nelson

Put in the vise and see if it is springy. If it is soft, just bend it back in the press
Between two blocks of any thing. Positioning the blocks just right is the Wizard
Thing........BLJHB.