Spine tester

[mzombek]

Good morning guys,
I gotta question about accuracy of spine testers. I just finnished a spine tester with a  weight pushing down the arrow from the top in the center of the arrow, with indicator on the bottom. I am using 26" length with a 2# wt.
Should there be much variation between what I had made and a Spine-omatic tester?. I have 6 arrows that I had purchased and each of the arrows have actual results from the place that I purchase from using the spino gauge. I had even spun the arrow and checked 90deg from each other not the same results as the ones that I purchased.
Any idea's
Thanks,
Mike

[Kelly]

So how far are they off?

A couple pounds or so is very common between spine testers of different manufacture. Even farther if in the upper spines like 70# and up. This is where most of the mass produced spine tester have an error in their scale.

[mzombek]

It is off about +/-3 to 4 pounds. I would think that using a digital indicator would be fairly accurate?
Is there a known wt, spined arrow that I can buy to check calibration.

[Lefty]

Some brands seem to vary more than others, at least that has been my experience.

[Cupcake]

Mike,

The drawback of a dial indicator is that the return spring pushes back against the 2lb. weight and introduces an error the reports a weaker than expected spine.  If the spring has 2 oz. of force when the weight is applied a 50lb. spine arrow would measure about 47lb.

I like to use aluminum shafts to verify accuracy.  A 2016 is .531" the way Easton measures (28", 1.94Lb.) and .438" with 26" and 2lbs.  Any aluminum shaft can be used.  Multiply the published deflection by 0.825 to get the expected deflection on your machine.

Kevin

[Bjorn]

The digital readout is only as accurate as what's giving it the info. Like Kelly said you will get a variance, and it depends too on the care the person used on the spinomatic. Like was the gauge zeroed between shafts. You could buy an aluminum shaft like a 2117 and look up the spine of that on a chart. I would make my tester agree with the shaft supplier's tester makes for an easier future.  Looks like Kevin and I were typing simultaneously!  

[mzombek]

Kevin,
You are exactly correct. After reading your comments about spring tention, I went to the basement to try something. I placed my weight on the arrow with the indicator shaft retracted. After the weight had settled, I slowly let the indicator shaft extend without spring tension on the arrow, just baely touching the shaft. Out of 6 check from a spino-matic arrows, only 1 arrow was over by 1 pound.
I have been working on this thing for three months, with out your help, it would have been chucked.
Thanks guys for your help

[Cupcake]

I was backwards in my explanation of the dial indicator spring affect.  It will decrease the deflection and thus the measurement will appear stiffer.  A 50lb. shaft woulf measure about 53lbs. if the the spring has 2oz. of force.