Digital angle readouts

[FlashGP]

By the way, I have recently talen to rounding the angles on the gemcad faceting instructions to the nearest 0.05 degree.  This is at most an 0.02 degree error.  So far I haven't had any problems getting the meet points.

When testing the change with Gemray there is little if any change in predicted output.

Dugite you can also reasonably guess 1/3rds.  Enough variations to give anyone trouble.

On the encoder issue above.  If the encoder is a potentiometer, it needs to be very low noise or the readout jumps about.  I have wet to see a pot that doesn't wear out and get noisy with use.  It will also need a very accurate and stable voltage supply as it relies on the digital encoder converting that output voltage to a number.

Hall have a dial guage calibrated to 0.01 degree.  The problem is in setting the zero point using the 0.1 degree vernier.  The manufacturer says that testing using engineers claibration equipment shows this can be done to 0.03 degree accuracy using a 10x loupe.  This means a Hall machine is accurate to 0.01 degree within a plus or minus 0.03 range.  E.g. you could be up to 0 03 degrees either side of the desired angle.  You also recalibrate the dial guage each time you change angles.  All the dial guage does is split the degree into hundredths.

My point being that two things affect accuracy.  The degree of granualation (0.1 or 0.01 degrees) and the calibration,  being the zero mark.

The bit most people don't think of is the accuracy of the zero calibration. 

Regards
Gordon

[Dugite]

Its all starting to get complicated open for errors . Manual sounds just fine for now.
Cheers

[MakkyBrown]

Hi guys i have only just joined the forum and just saw your post on the VJ machine.
I too have a VJ and was contemplating a digital readout. is saw that you say a VJ is good for 0.1 degrees. I have found with the right feel and finesse i can easily get 0.01 to 0.02 degree accuracy with no problems in manual mode.  So is the digital really much better or would i be wasting money.

Hi Dugite, Lefty a forum member had issued with his VJ readout aswell. Might be worth sending him a message.
Cheers
Andrew

[MakkyBrown]

FlashGP, Encoders are a bit different to potentiometers. Both should be fine to cut with but there is more risk of error from a potentiometer. An encoder when calibrated when installed should always read the same. It's basically a light gate and a wheel with little lines to trigger it.  In theory an encoder will always be more accurate than a potentiometer but in increments.  I basically use the trigger point of a the increments to cut to on my encoder. After using the encoder I've found quite a few machine factors to throw out the angle. These factors will apply on all machines.
Lap surface and general run out of lap spindel shaft. I actually find this benefical and it causes my encoder to flicker to another angle and helps find the point to cut to.
Lap and machine flex, hand pressure and using a hard stop. This is where big inaccuracies appear imo. I finish cutting each facet with no downward hand pressure and just gently move the stone side to side on the lap.
In my amateur opinion if you can cut accurately to 0.1 of a degree that's heaps, more importantly you are after repeatability, getting more accurate than that will require lots of looking with a lens imo.
Cheers
Andrew

[FlashGP]

Thanks Andrew. 

I couldn't agree more about the inaccuracies.  I use the flicker of the dial guage to tell me how far out the lap is.

The most use I have found for the dial guage is telling me when I am close to the meetpoint or when I have the facet parrallel to the lap when starting to polish.  From there I watch the meetpoints.
Regards
Gordon

[Shrek]

Gidday All,

I have had a VJ machine with a digital readout for about 3 years. After having it for several months, I found that the digital readout was starting to inexplicably creep out of calibration with the manual angle adjustment. Many discussions were held with Mal in regards to the cause of this and he mentioned that a couple of other users were having the same trouble.

Being an electrician, I was determined to find the culprit and after many dead ends, I believe that I have found the solution.

The cause was in the earthing of parts of the machine. i.e From the power lead to the control box, there is an earth wire to the motor (required). From there the earth circuit continues through the frame of the machine across to the mast and on to the encoder. The data wire shield is also earthed by an earth wire where it leaves the control box. This forms a closed loop earth around the machine. (control box - motor - decoder - back to the control box) This causes a circulating current to flow in this earth circuit which disrupts the pulse counter in the encoder, hence the minute discrepancies in the digital readout.

The Solution.
Open the decoder box and remove the earth wire to the end of the data cable which goes to the encoder. this open circuits the closed loop of the earth system and interrupts the circulating current. This earth is not required as it is low voltage. After doing this to my machine, it has not varied from zero in the last 18 months. Problem solved.

Regards Shrek

[MakkyBrown]

Thanks for sharing, I know Lefty has a VJ with the digital readout and has issues with it.

Cheers
MB