Need help aligning long rails

[BizCoach]

I got a set of rails on ebay. Didn't come with connector plates but I used some T-nuts and aluminum angle and mounted them along with some legs. (The legs are attached to a plate with a wheel so I can move the whole saw). Picture 1 shows legs Picture 2 shows angle bracket.

Then I mounted a sticky tape so I could measure cuts longer than 24 inches. Picture 3.

The fence slides smoothly over the joint where the legs connect. However when I lock the fence the measurement from fence to saw blade is off - but only on the new legs. The old legs (up to 24 inches) measure exactly. Picture 4 shows the fence set at 28 inches and Picture 5 shows the fence to blade measurement at 28 and 1/8.

Any thoughts on what I might do differently?

[dbhost]

Do you have a solid measure like a framing square that you could use instead of the tape? It may be something to do with the tape, but I am honestly taking a shot in the dark here... I don't have the extended rule on mine, and I double check my blade to fence distance out of paranoia all the time anyway...

[pelligrini]

First, I'd check the new roll of sticky tape you put on for accuracy. If it checks out I'd make sure that the extension rails are parallel and in line with the other. I'd also measure the distance from the fence when clamped out on the extension to the blade at the front and back of the tables, using a straightedge up against the blade avoiding the teeth. I'm thinking that the extension rails are slightly cocked forward making the fence further away from the blade, maybe...

[LCHIEN]

so , i don't think its right at 24 inches, either.
I'd make a chart of error vs. setting and see where it gets off.
Is zero right against the fence or 1/8" off?
is 12" right or 1/8" off?
etc.

I think maybe when you moved the saw you pulled the rail off so its all 1/8" off.

[BizCoach]

Thanks. Taking some more measurements the fence isn't square to the blade. So I'll work on that and see if it fixes the other problem.

[os1kne]

Looking at it, it seems like it might be wise to have some sort of cross-piece between the front/rear rails near the joint and again near the far end. I could be wrong, but it seems like a fence that long would put a substantial amount of leverage on the joint and could easily torque it out of square a bit.

Good luck!

[cabinetman]

I don't have that saw, and never used one, so my comments are based on my experience with other saws in adding long rails, and maybe it would relate to your situation.

With the length of those rails, there is likely some movement (front to back) due to not having a table top between them fixed to the saw, and keeping the rails parallel. That's where I would start. With rails that long they could be moved to be out of perpendicular to the fence, as they look in your pictures.

Align the fence to be parallel to the blade. Raise the blade all the way up and pick one tooth and mark it with a sharpie. Once that is done, then set the fence manually at a fixed distance from the right side of that particular tooth. Make sure that tooth is close to the saw top. Use your measuring device (it can be a tape measure), and rotate the blade so that tooth is at the rear of the saw close to the table. Take another measurement. If they are the same measurements, rotate the saw blade so you mark another tooth. Lets say your first marks were at 12 and 6. Your next marks will be 3 and 9 (o'clock). Measure again. If they are the same as 12 and 6, the fence is parallel to the blade (and the blade isn't showing significant runout).

Then install the sticky tape to the rails with the pointer at the measurement you used. If your cut indicator has an adjustable setting, use the center of the adjustment. HTH.
.

[LarryG]

Although I don't use my BT3100 any more, I still own it and used it for about four years before getting my cabinet saw. As you no doubt know, the BT saws are a completely different animal from conventional table saws and therefore require a completely different alignment methodology.

I think Erik (pelligrini) is on to one excellent possibility. Compare the graduations on the sticky-back tape to your regular steel tape and see if they agree. I once bought a sticky-back tape that had better than 1/16" of error over a section just a few inches long ... up to that point it was accurate, then it drifted off, then it drifted back to where it was supposed to be.

According to a VERY rough sketch I just drew up using CAD, if the front extension rail is cocked out of alignment by as little as one degree, you could easily get that 1/8" of difference you're seeing between 24" and 28". However, the far end of the added-on rail would then be maybe 3/4" out of parallel alignment, and you ought to be able to see that with the naked eye. Pull a string between the ends of the joined rails -- that will tell you whether there's a bend where the rails are connected. Do this first, as ensuring the combined rail sections are dead straight over their entire, combined length is the only way your blade-to-fence measurements will be meaningful at all points.

It could be that you have a combination of errors ... partly the fence out of alignment, partly the rails are not straight, partly a discrepancy between one measuring tape and the other. Off a little here, off a little more there; it all adds up.

[eezlock]

need help aligning long rails

I would not rely too much on the sticky tape measure on those rails for anything of accuracy. It would be better to use a a flat steel rule for reliable. accurate measurements. The stick on tape measure should only be used as reference or rough measuring....a lot more accurate!

[cabinetman]

I would not rely too much on the sticky tape measure on those rails for anything of accuracy. It would be better to use a a flat steel rule for reliable. accurate measurements. The stick on tape measure should only be used as reference or rough measuring....a lot more accurate!

I've used these Starrett tapes as replacements on Biesemeyer and Unifence rails, and found them to be extremely accurate.
.

[pelligrini]

I added a length of "sticky tape" (that is the name of the product) on the half rail extensions on my 21829. I found it to be very accurate. I hardly ever move my rails and I constantly use the fence rail indicator for setup. I've got my rails and fence indicator set exactly 10" to the right side of the blade cutting edge. I can get repeatable, accurate measurements just using the fence and rail tape.

[LarryG]

I both disagree and agree with eezlock. Like Erik, I preferred to use the front rail scale on my BT3100, and later followed suit with my cabinet saw. I too rarely moved the BT's rails, especially after I cooked up my Dual-Width Extension Wing, and with the rip fence properly calibrated to the scale I was able to make cuts that were dead-on accurate and repeatable.

But I agree in the sense that any measuring device, whether it's a steel tape, a sticky tape, a steel rule, or whatever else, is only as good as the graduations it bears. The sticky tape I mention above that had a 1/16"+ discrepancy out in its middle reaches was a Starrett item. Each individual measuring device must be tested on its own; reliance on brand name alone is not a guarantee of accuracy.

[pelligrini]

Some of the errors in those tapes might be in the manufacturing of the tape, or possibly in the application. When I was surveying the owner always used steel tapes. He said that the fiberglass ones had a tendancy to stretch. When I applied my tape to my rail I made sure not to pull on it. Not sure if it made a difference, but I didn't want to risk it either.

[cabinetman]

The sticky tape I mention above that had a 1/16"+ discrepancy out in its middle reaches was a Starrett item. Each individual measuring device must be tested on its own; reliance on brand name alone is not a guarantee of accuracy.

Sorry to hear you had a defective tape. As they say, poop happens. Tapes should be checked for accuracy before using, and if not accurate should be returned. I've only bought Starrett flat sticky tapes and there's been several, and all have been very accurate.

Other than being a manufacturing defect, which could be possible with any brand tape, an improper installation could be the problem. Once installed to the rail, if misplaced and pulled up, may account for the tape to read incorrectly.

If I need to buy another one I wouldn't hesitate to buy the Starrett tape. They have served me well.
.

[JimD]

I think your biggest need is an extension table. If you search my old posts you will find one where I explain how I make mine. I like to use melamine particle board for the top and 1/4 plywood scraps for the pieces that get the top to the right height. They are easy and cheap to make. On my big one, I have 3/4 plywood stiffeners under it. You can see that one on the article in the old articles section entitled Spruce and Sandeply. I have a router table setup under the large extension table which is something to consider.

I attached a more recent picture of my saw than appears in the articles. I had not finished the drawers or compartment doors at that time. I deliberately used different hardwoods for the drawer fronts and doors to have samples readily available. I also used different drawer contructions for the same reason. The fronts are Maple, Poplar, Walnut, Oak, and Walnut. The drawers hold stuff for the saw and a couple PC routers.

My way of attaching the rails is different, I like to use a piece of flat stock in the bottom and inside slots of the rails to clamp both rails to the base and thereby to each other. It aligns them and allows me to slide them (which I mainly do to tip up the saw to clean under it and lube it). If I need to rip a wide piece at 45 degrees, however, it is handy to be able to slide the rails to the left. You can see that in the articles. You need to tap holes in the flat stock but that is not very difficult.

Jim