miter bar

Yes, I know that some might call this a woodworking problem, some metal.
A few weeks ago I noticed that the miter bar for the miter gauge on my table saw was bent. There was a bow in it of about 3/16 of an inch over a 20 inch length. Its a steel bar, not aluminum, for an Accu-miter gauge.
I know that I've never dropped it or abused it, so I'm wondering how it bent. I find it hard to believe that a 30 degree F temperature variation over the course of some years would do this, and given what it took for me to bend it back, I know that I've never subjected it to that much punishment.
To bend it back, I had to put it in the tail vise on my bench between three pieces of drill rod, then crank down to bend it almost 1/2 inch the other direction. After 4 passes or so at this, I've got the bow down to about 1/16 inch, as measured by a good straightedge.
Since others don't use my shop, any guesses what happened? Stress relief over 10-15 years of use?
Thanks, John
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John D'Errico wrote:

What are the other dimensions of the bar and how is it oriented and mounted?
dennis in nca
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Its 3/8 x 3/4 x 20 (inches)
Mounted to the miter gauge at one end by two screws, one about 2 inches from the end, the other at roughly 4 inches from the end.
John
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Hi John,
It could have been an internal stress that just worked its way out over time. If I had to guess, I would suspect that the makers chose cold rolled low carbon steel (1018 or maybe even A36). The stuff is great for welding, has a mill finish good enough to avoid any further machining, and it's pretty cheap. But, cold working the steel produces a great deal of internal stress. A good ME or Machinist knows to stay away from it for anything that needs to be dimensionally stable. Better to use hot rolled steel because it ends up being a lot more stable. Some (like myself) would refer to this as a "design defect" because a different material should have been spec'ed for the application. Alas, it's likely that the cost of materials outweighed engineering sensibilities.
Or, then again, I could be wrong. Even hot rolled steel can have internal stresses. It's a lot less common than cold rolled steel but it can happen. Stress relieving involves a rather lengthy heat treating process which would probably be out of the budget for such an application.
Bending the bar back using the "arm strong" method will likely inject some more stress (you are cold working it). So, don't be too surprised if it bounces back again. If you can heat the bar up to a dull red (1000F - 1200F), straighten it, and then let it cool slowly (like in the furnace) then it should be OK. Or, have a new bar machined from some hot rolled steel.
Ed Bennett snipped-for-privacy@ts-aligner.com http://www.ts-aligner.com
John D'Errico wrote:

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On 22 Nov 2006 09:57:50 -0800, snipped-for-privacy@ts-aligner.com wrote:

Just a little additional, as I've been picking away at forging a set of spinning tools, and have been researching this a little.
What Ed's talking about is called annealing, if you want to search for more info on it- it'll bring the steel to it's most malleable state, and releave the internal stresses. The most common suggestion I found for the slow cool is to bury it in kitty litter for a few hours once you get to that dull red color. Bear in mind that doing this will make the bar quite a bit softer, so you'll need to be extra careful not to drop it if you do that.
You can get it to that red with a propane torch, but it'll take quite a while, and you need to keep it moving up and down the bar. There's also a process for warm-forging that will releave some of the internal stresses by raising the bar's temperature to anywhere from 150F-700F while working it (i.e. bending it back into shape.)
Kind of fun to do, actually.
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I thing we're getting overly complicated here. It's just a 3/4" X 3/8" X 18 - 24" long steel flat bar, probably mild steel. It's not welded to anything else or constrained in any way, so there is no significant stress to relieve. IIRC the OP said it was a 3/8 inch or so gentle bend along the length of the bar. As long as it doesn't have a bad kink or tight bend that has deformed it in thickness or width, just straighten it out by whatever means is convenient, when it slides freely in the slot it's done.
--
Often wrong, never in doubt.

Larry Wasserman - Baltimore, Maryland - snipped-for-privacy@charm.net
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There are significant stresses. The stress is caused by the rolling process. For the intended purpose, these stresses can be ignored but, if the bar was cut, it would act much the same as a piece of "case hardened" lumber. It would warp.
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On Sun, 26 Nov 2006 21:29:38 -0600, snipped-for-privacy@fellspt.charm.net () wrote:

I agree- but playing with fire can be fun, too. I just chimed in because the concept happened to fall in line with a recent project.
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<...snipped...>

If your name is any indication that must happen pretty often!
--
snipped-for-privacy@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org
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In article
wrote:

I'm starting to think that I know what happened.
When I withdraw it from the t-track on the saw table, the front end is captured in the track by a "washer". This prevents it from lifting during use. But when I take it off the table to use a different tool (like a tenoning jig or a panel cutter) I pull it out towards me.
The gauge itself is moderately heavy - about 11 pounds. So as I pull it out, I tend to lift a bit as I withdraw it. Since the end is still captured in the slot, this means I've been gradually cycling it through hundreds of small bends over the years, ALL in the same direction. This is consistent with the direction of the bow.
This feels like the probable cause to me as I think it over. If so, the solution for the future is simple. Slide it forward first to release the washer from the t-slot, then lift it.
Is stress relief with heat an option? I have Mapp gas or propane. Or do I just buy a new bar?
John
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Stress relief? If you did bend it as you suspect then there is no stress to relieve. Just bend it back. It's not like you are likely to distort the steel. You could heat it to bend it back, but you will likely end up causing more damage to the shape of it than if you just finish bending it back into shape as you have already done. Heat is a great friend of working metal, but if you don't know what you are doing or are not careful, that heat and the associated benefits can turn around on you and all you end up with is an interesting twist on a theme.
From what I've read John, you're trying to make too much of this problem. Just bend it back and carry on.
--

-Mike-
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Yes, its true that I may be making too much of it.
I have several goals here. I always like to learn what I did wrong (if indeed I did put the bend in it myself by my own methods of work) so that I can avoid a problem like this in the future. I wanted to gain some understanding of the mechanism of what happened. Fixing it myself, if possible, is a goal, but not a tremendously important one, since I could simply replace the bar for a nominal cost.
Thanks all for your feedback, John
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John D'Errico wrote:

...
Admirable goals, all of them. Very much in line with the attitude of most of us regular RCM'ers.
Do come back, you hear, Bob
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Just bend it back....as long as it slides well, thats all there is to it. Not exactly a super critical application. And take the stupid washer off the end....does nothing useful in actual use and as you've discovered it makes it harder to put the miter gauge on and off.
Jeff
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John D'Errico wrote:

Yep 3/16 inch bend in a 20 inch length means that somebody bent it. Your description sounds like you found the problem. Personally I would throw that washer away, then you could lift the bar out any time you wanted (unless there are other considerations, since I have never used a saw with a T slot). There is absolutely no reason to consider a new bar (a steel bar isn't going to exhibit any kind of movement from stress, What stress?. The only thing that will make it move is temperature change and you will never detect that and it won't affect anything you do. If the bar now lies flat on your table top, you are good to go. If not give it a few appropriate tap (not against the table) to get it to lie flat within a few mils.
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Hmmmm.....
Well John, as you know, from your experience of trying to bend back the bar, even dead soft cold rolled low carbon steel is very elastic at room temperature (modulus of elasticity is something like 29*10^6 psi). It seems unlikely to me that the pressure of removing your miter gauge from the slot, even many times over many years, would result in gradually bending the bar as much as you say (50% of the thickness). I would expect the washer to have suffered far more damage - especially since it's considerably thinner and on the short end of the lever. The process you have described would be best referred to as repeated stress and the results would be dependent on the plastic properties of the material.
In spite of opinions to the contrary, I'll stick by my original assessment. Stress relief (not "stress", as others have misstated) is the most likely cause. I base this assessment on many years of metalworking. I can easily understand why someone with little or no experience with CRS might consider it a ludicrous theory. If, like one of the responders, you consider your table saw to be a crude machine which is incapable of anything better than rough work, then just bend it back and don't give it a second thought.
Bending it cold will be a lot easier if you use an arbor press. You can improvise one by suspending the bar between two blocks along the edge of a table or bench. Then use a "C" clamp in the middle between the top of the bar and the underside of the table or bench. Using this setup will give you a lot more control in the process - minimizing the chances that you will overbend the bar or slip and hurt yourself.
Ed Bennett snipped-for-privacy@ts-aligner.com http://www.ts-aligner.com
John D'Errico wrote:

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On Thu, 23 Nov 2006 11:52:57 GMT, John D'Errico

That sounds like a winner to me.

You can try it, especially if the other option is to buy a new bar- at the very least, it will release the stresses caused by bending it flat. If you want to get really involved, find a cheap hibachi grill, fill it full of charcoal, and set something up to blow air across it, like a hairdryer or a shop vac.
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And don't forget that either now or later on if it starts to get a little loose, you can peen the edges some to tighten it up so it's workable.
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