Author Topic: TSO GRS-16 and 4-cut/5-cut Calibration  (Read 4680 times)

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Offline Rick Christopherson

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TSO GRS-16 and 4-cut/5-cut Calibration
« on: May 19, 2017, 06:20 PM »
A couple of days ago Hans stopped by to chat over lunch. Same place we sat some time ago when he first proposed the idea for his GRS-16 guide rail square. What came out of that previous meeting, besides a full belly, was the concept whereby the square would never need adjustment nor calibration. So I was a little surprised when Hans told me the other day that a woodworker claimed that the GRS-16 failed on a 4-cut/5-cut calibration.

I said that wasn't possible. The GRS is machined at the same machine shop that I've been using for the past 6 years. Their default tolerances are too low, and the design leaves no source for error. So I agreed to put the GRS-16 through a 5-cut calibration. (Yes, for this test, I even did the 5th cut.)

I placed a piece of 18-inch plywood on Vacsys so I could spin it without undue handling. I mounted the GRS-16 to my 32-inch guide rail, and slid a Festool Quick Clamp down the rail to ensure the guide rail didn't move during each cut. I made sure the guide rail gib cams were properly adjusted on my TS55, with the standard blade.

The resulting offcut was 16-3/4" long, and it was pretty obvious that it was parallel for that long of a cut. So I brought it up to the office where I keep my most accurate caliper to measure. I got 0.825 width at one end, and 0.810 width at the other. That's just 0.015" per 16 inches, or 0.001 per inch. Pretty darned good. So I plugged the numbers into the angle calculator in my Kapex manual, and came up with an angular error of only 0.013 degrees.

That small of an error is what you'd expect if you had a piece of sawdust trapped between the square and the workpiece. More importantly, it's down in the range of machine shop tolerances, except we're in a wood shop. For a hand-held saw, with all of the unknown variables that can come into play with accuracy...that's a pretty astounding result.

So how could someone come up with a bad result? Well, down at this level of precision, even the slightest of variances can affect the result. This is a calibration, so you're not trying to make a routine cut. You want your cuts to be as absolutely precise as possible, and you want to remove as much human error as possible.

Two things come to mind that will result in erroneous results: Plunging the saw down into the beginning of the cut, or not having the blade inside wood on both sides of the blade.

If you plunge down into the wood, as opposed to plunging before the saw reaches the wood, the beginning of your cut may likely have a small amount of snipe that normally wouldn’t be detected, but is detectable in a calibration.

If you don’t have the blade completely buried in wood on both sides of the blade, the blade can drift. If you're only shaving the edge where only one side of the blade is actually cutting, then the ATB (alternate top bevel) teeth of the blade will cause the blade to deflect unevenly.

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Offline iamnothim

  • Posts: 1426
Re: TSO GRS-16 and 4-cut/5-cut Calibration
« Reply #1 on: July 17, 2017, 03:51 PM »
Cool testing and results

I'm considering purchasing a set.   

I'm wondering how it will work on Hardwood instead of sheet goods. 
Say cross cutting a piece of 8/4 stock 6" wide.
My reputation pre-deceases me.

Offline tjbnwi

  • Posts: 5400
  • Cedar Tucky Indiana
Re: TSO GRS-16 and 4-cut/5-cut Calibration
« Reply #2 on: July 17, 2017, 04:09 PM »
Cool testing and results

I'm considering purchasing a set.   

I'm wondering how it will work on Hardwood instead of sheet goods. 
Say cross cutting a piece of 8/4 stock 6" wide.

It works. A very slight learning curve to get your thumb to keep the rail in contact with the wood at the far end. You have to use your thumb to offset the weight of the saw on the rail when using a very short rail.

or------clamp the rail (for the very safety conscious).


Offline John Stevens

  • Posts: 800
  • Ardmore, PA
Re: TSO GRS-16 and 4-cut/5-cut Calibration
« Reply #3 on: Yesterday at 09:22 PM »
I just performed this test a few minutes ago. It’s been about  40years since I learned the concept of significant decimal places, but I think the result of the test is that whatever error there is, is immeasurable by my instruments. FWIW, I measured a width between 12.98-12.99mm at the front end of the fifth cut and between 12.92-12.93mm at the aft end, over a length of 59.0mm. Width was measured with a micrometer with a claimed accuracy of 0.02-0.03 mm in three samples ranging from 0-131.4 mm. Length was measured on a metal rule marked in increments of 0.5 mm, but sadly no evidence of accuracy.  The  max error would be 0.07mm and the min would be 0.05. The error as a fraction of the length would come out to around 0.0002- 0.0003 mm, but if we assume the length was accurate to within 0.1mm, I think I’ve run out of significant digits before I reach a non-zero number.
I’m not sure what if anything I’ve written here makes any sense, except that the TSO rail square is more accurate than I’ll ever need it to be for any work I’ll be able to do. Great job Hans. Yes, I’d recommend this to a friend.
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Offline tomp

  • Posts: 15
Re: TSO GRS-16 and 4-cut/5-cut Calibration
« Reply #4 on: Yesterday at 10:52 PM »
I made the handle shown in the photo using the connection bracket from the Rip Dogs Rip Guide (Thanks Richard). Shown on the rail being used for cross-cutting, but I primarily came up with the idea to use with the TSO square - as you're cutting, your weight naturally applies a force at the knob that's transferred along the rail to the square and eliminates any movement while you make the cut.