Help with calibrating

[tierman]

My problem: I have a board that is 96" long, when measuring the board length, my machine only measuring 95.624". when I go to calibrate the length measurement my default is 890.368, I have calibrated it to 874.368 to get to where it measures now(the 95.624"). In trying to get to the 96" measurement, the machine will not let me go below the 874.368, I get "Calibration Range Error". The project requires a measurement closer than 3/8". Thanks in advance, any help is much appreciated. Rog

[bergerud]

If it is out of range, you should suspect something physically going wrong. That is a long board.

You may be interested in:
http://forum.carvewright.com/showthread.php?18432-resetting-X-cal-by-hand&p=156685#post156685

[bergerud]

I calculate 890(1+(95.625-96)/96)=886.5. Are you sure you need to go out of range?

[tierman]

Bergerud, it took me to the limit to get the machine to measure the 95.625". It was almost 2" short to begin with and I now have it down to 3/8". I am staying in contact with the brass roller all the way through the measuring process. It is consistently measuring the 95.625. Any suggestions on how to get it closer ?

[bergerud]

It must be something physical. Do you have the rubber damper under the tracking sensor? Try it without the O ring.

(Other things like head level, squaring plate square, O ring rubbing on rubber belt,...)

[tierman]

I'll keep trying and let you know.Haven't tried without o ring

[tierman]

Does not get a consistent measurement without o ring

[fwharris]

Does not get a consistent measurement without o ring


Roger,

How close is your calibration on a 3 to 4 foot board?

Is there any remote chance that your long board is loosing contact with the brass roller?

[bergerud]

Does not get a consistent measurement without o ring

I think this means that the brass roller is not being pressed hard enough onto the board.

Do you have the rubber damper under the board sensor? The brass roller should be about 1/8" above the plane of the belts and should be quite stiff to push down. You should see brass roller track marks in the bottom of the board.

[tierman]

Roger,

How close is your calibration on a 3 to 4 foot board?

Is there any remote chance that your long board is loosing contact with the brass roller?
Floyd,Bergerud,, Sorry I was out all day yesterday. I have measured the long board 5 ties and all are within .006 on all measurements but I will give a short board a try today, will also take out the brass roller today and check for the damper as suggested bergerud, thank you both for your input

[bergerud]

You should be able to just feel the tracking sensor assembly hitting the damper. It should feel stiff. What of the tracking marks on boards? Do yo get those?

[Digitalwoodshop]

Any chance of putting a strip of masking tape on the bottom of the board at the brass roller and see if that makes any difference?

AL

[tierman]

Any chance of putting a strip of masking tape on the bottom of the board at the brass roller and see if that makes any difference?

AL

The masking tape goes on ALL of my projects. Definitely a requirement with my machine.

[tierman]

You should be able to just feel the tracking sensor assembly hitting the damper. It should feel stiff. What of the tracking marks on boards? Do yo get those?


Well after tearing into the tracking senser I think you may have sent me in the correct direction bergerud, my damper was laying in the bottom of the machine. I have one of my projects running now (fingers crossed) have one more to do each being a 6 hour job. thanks all for the input.

[bergerud]

I hope that was it. It could have fallen out as you took it apart.

It would, however, explain why boards were measured too short. The O ring, not being compressed to the brass, would make the circumference of the tracking roller effectively larger. It would not go around as many times and so report a shorter board length.

[tierman]

I hope that was it. It could have fallen out as you took it apart.

It would, however, explain why boards were measured too short. The O ring, not being compressed to the brass, would make the circumference of the tracking roller effectively larger. It would not go around as many times and so report a shorter board length.

Before Itook it apart I tried to push the roller assy down and it didn't take much pressure so I am quite sure that it was out before I took it apart. I need to put this in my
memory for future reference. Thanks again for the input ,it was a great help.

[JLT]

I calculate 890(1+(95.625-96)/96)=886.5.

Dan, how did you arrive at this means of calculating the adjustment to the calibration. The manual (Rev 1.54, 01/07/13) states...

====================
To find the magnitude of the change to the current value subtract the two numbers (Length difference - _________inches). Although not an exact correlation, you should change the current value setting by 1 unit for every 0.03” (or for every 1/32nd of an inch).

So now you have a direction (add or subtract) to change and a magnitude (number of units). As an example: say you have a board that measures exactly 36” long by your tape measure and the machine measures board at 35.875”, adjust the X axis cal number by -4. If the current value was set to 890.000, change it to 894.000. Or if the machine measures 36.125”, adjust the X axis cal number +4.
====================

...so I had arrived at the following means of adjusting the calibration...

[New_Cal] = [Cur_X_Cal] + 32 * ([Measured] - [Actual])

...but found that upon measuring the length afterwards a few times, and continuing to recalibrate, that I was bouncing back and forth between too short and too long.

But your method was spot on (thanks!), and implies that the calibration is a ratio associated with the value of 890, and not an adjustment to the current calibration by 1 unit for every 1/32" the measurement is off.

Tx, Jon

[bergerud]

I do not know why the manual does not just give a formula instead of their "rule of thumb". (Which, if I recall, only apples to a 36" board.)

The approximate number of X encoder pulses per inch is 890. (1296 encoder pulses per rev/brass roller circumference ~ 890) The default X calibration number on the machine is this number "fine tuned".

You are right, it is just a ratio. Suppose the controller gets N pulses from measuring a board. The machine will report the length to be ML = N/(current X cal number). What you need is a new X cal number so that the real length RL = N/(new X cal) . Eliminating N from these two equations gives (new X cal) = (old X cal)*(ML/RL). This is the same as the formula I used except I took 890 as the old X cal and had the formula in the form of an added correction instead of a ratio. So, for future reference:



New X Cal = (Old X Cal)*(Machine Length)/(Real Length)



For example, if your X cal number is 892.5 and the machine measures a board to be 24.85" whereas the board is really 25", you would calculate a new X cal number of (892.5)*(24.85)/(25) = 887.15


EDIT: The "rule of thumb" calculations in the manual seem to actually be wrong. This explains why the the numbers jump back and forth. According to my calculations: (890)*(36.125)/(36) = 893 not 894.