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Miter bars for Table Saw??

I've bought a few lengths of cold-rolled 3/4 X 1/4 over the years and I've never had one that wouldn't fit in the table saw slot. That's straight enough for me.

Reply to
Lawrence Wasserman
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Well, not necessarily. What fit snugly may not even be in the groove after the first four inches of travel. So you need a straight edge. That's the downfall of these fore and aft adjusting aluminum jobbies. tight goes to flop pretty quickly.

Reply to
George

I do see significant thermal expansion in the rails of my table saw. it's enough that I can't trust the scale on my biesemeyer. now, that's a lot more metal than a miter bar, but it's real. OTOH, the baltic birch ply sliders on my sleds don't give me any problems with humidity changes.

Reply to
bridger

Well, to be fair the thread was on mitre bars and I was commenting on the expansion of the significant (3/4") dimension but lets see:

The thermal expansion coefficients:

steel is ~ .000006 in/in/degF aluminum ~ .000012 in/in/degF UHMW ~ .00011 in/in/degF wood ~ .000003 in/in/degF (cross-grain) **

(** I found several references for this suprisingly small value for wood

- less than steel. It seems to confirm that the major problem with wood rails would be humidity rather than thermal expansion).

So for a 3/4" mitre bar over a 30 degF temp. range the expansion would be:

steel = .000135" (.000006 x 30 x .75) aluminum = .00027" UHMW = .0025" wood = .00007"

For a cast-iron/steel table, the diferential expansion of the bar to slot would be:

steel = 0" aluminum = .000135" UHMW = .0024" wood = .000065"

For an aluminum table, the differential expansion would be:

steel = .000135" aluminum = 0" UHMW = .0022" wood = .0002

Over this temperature range, I would suggest that only UHMW *might* (just barely) be noticeably affected by thermal expansion/contraction. (I still feel that humidity would be the controlling factor for wood. I don't offhand have any hygroscopic data for UHWW ... some plastics - nylon for instance - absorb a fair amount of water and can swell noticeably with humidity changes).

As far as your front rail goes, assuming it's ... what, 4 ft long? The expansion in that case (over the same temperature range) would be:

steel rail = .0086" aluminum rail = .017"

I assume the scale is thin and bonded to the rail so the rail would control the expansion. I'm not familiar with the Biesemeyer so I don't know the material for sure but the expansion is almost 20 thou for aluminum (but over the total length ... proportionally less for shorter distances). Whether that's a problem in your work, only you can say. (I think, in my case, my innate lack of craftsmanship would swamp it (:-) ).

Reply to
WoofWoof

If it fits in the groove, it is obvious straight enough. If it doesn't fit(has a bend) whack it till it does, then it will be straight enough.

Reply to
George E. Cawthon

the rail is about 6' long. and I live in Arizona. I'll figure the shop temp varies close to 100 degrees from a cold day in the winter to a hot day in the summer. that would result in a total variance of .043, or about 3/64".

I'm definitely seeing more than that, so there's probably something else going on. for instance, right now it's reading about 1/16" short in the 36" range. it's not particularly cold weather right now- brisk, but nice.

hey, and thanks for digging up those numbers.

Reply to
bridger

====================== No I have to differ with you on the need for an absolute straight edge...BUT I do agree with you on the adjustable "jobbies" since they generally only have a few "adjustable" points on the bar that are in contact with the sides of the miter slot.....

It is not at all hard to fit bar stock to fit the miter slot snug the ENTIRE lenght of the miter slot...

Bob Griffiths

Reply to
Bob G.

Is that all? My shop temperature in the mountains of Virginia varies by about 120 degrees from the lowest low to the highest high, as measured by the thermometer inside the shop. (Not by ambient air temps.) A lot of this is because it gets hotter than my thermometer can actually tell me in the summer. I've gone out there on a 20 degree day and seen it 55 inside the shop, just from solar heating. I really need air conditioning, but I'm not going to get it. :(

Reply to
Silvan

Seems to define a straight edge, or is your slot curved?

Reply to
George

It is in fact "obvious" that this is not correct. Suppose the miter slot is

0.750" wide, and the bar is 0.740" wide with a 0.009" bow in it. The bar will fit in the slot, and fit snugly at that -- as long as both ends of the bar are in the slot. At both the start and end of a cut, though, when only one end of the bar is in the slot, there will be considerable slop in the fit, and hence slop in the cut.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.

Reply to
Doug Miller

One other thing that is going on is that the rail is tied to the table at several points so they fight with each other during expansion. The table will probably win so the rail will try to compensate by distorting (bowing or twisting). I wouldn't have thought that would be enough to give you the effect you're seeing .... but who knows?

You're most welcome. It was an interesting exercise (for me).

Reply to
WoofWoof

Not to mention - returning to the requirement only of a straight edge - if the slot is 0.75 and the _straight_ bar(s) 0.25, perfect cuts can be made by riding the straight edge against one side of the miter groove. A firm set can be made by attaching another in the other channel, so both ride either the inside or outside portion of the grooves.

Reply to
George

Well, there would be up to .009" variation in the cut. Surely suitable for woodworking. Nothing that a light sanding or a very light pass with a plane couldn't handle. I'm not arguing in favor or slop, I'm just suggesting that we might be too focused on precision that is beyond that which the tools we're using give on a good day, especially in light of the material we're working with.

Reply to
Mike Marlow

But only if they're straight (or reasonably close to it). And so we come back to my original question: how tight are the tolerances on the bar stock that comes from Home Cheapo? I'm quite certain that they're not as close to dead straight as the precision-machined miter bars from Incra, but how close are they?

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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Reply to
Doug Miller

A point well taken... but it still seems to me that it's important to know just what sort of tolerances we're dealing with, before recommending use of off-the-shelf bar stock from Home Cheapo. Incra miter bars are machined to a degree of precision that I'm sure we can all agree is more than sufficient for woodworking. But how close is the home-center bar stock?

To take what is probably a more realistic example than the one I presented previously, suppose that a nominal 3/4" bar measures 0.748" actual width. If your miter slot is 0.750", that bar had better be _no_more_ than 0.002" out from dead straight, across its entire length, or it's going to wedge in the slot before it gets all the way through.

And, of course, if that nominal 3/4" off-the-shelf bar from the home center is actually 0.751", it isn't going to fit at all, even if it's dead straight.

So... are the tolerances that good on the home-center bar stock? Personally, I have my doubts, after having purchased 3/4" aluminum angle from HD that turned out to be almost 0.020" out of straight over 24 inches. It's hard for me to imagine that the bar stock is an order of magnitude better.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.

Reply to
Doug Miller

My guess would be very close to yours. For those times when quality matters (tolerance being only one measure of quality), HD often falls below the mark. But, if it weren't for HD, where would all of those guys who really don't know much, go to work? And those mammas with the serious bubble butts - ya know they have to work somewhere with big isles...

Reply to
Mike Marlow

... er actually it might well be. Al angle is extruded ... the extrusion process often leads to a fair amount of bow. Steel bar stock is usually cold-rolled and the straightness is pretty good.

Reply to
WoofWoof

Simple test try first half or third in front of miter slot try full length in slot try rear third or half toward rear of slot If bar fits in all three cases and has minimal slop it is good enough to use Steel bar stock I bought passed this without difficulty. I am now using the bar in a circle jig for the bandsaw. John

Reply to
John

Thanks Doug, but it's obvious that you can't separate different problems. Straightness has nothing to do with the width being off. If the miter slot is .75 and the bar is .74 it doesn't fit period whether or not it is straight. Keep to the subject which is straightness. OTOH, if fit is a problem and it is only .74. whack it and soon it will be .75. Don't you adjust your miter bar to fit?

You might want to look at your sig.

Reply to
George E. Cawthon

Thanks yourself, George, but it's obvious that you're the one who has trouble separating different problems. You said "if it fits in the groove it's obvious[ly] straight enough." This is clearly false, as the example I provided demonstrates: a 0.740 bar with a 0.009 bow "fits in the groove" as you said, but it is very obviously not "straight enough".

You can't divorce straightness from proper fit.

Yeah -- but not by "whacking it until it fits".

Why? Is there something the matter with it?

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.

Reply to
Doug Miller

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