Cheap Idea for a DRO

[z3t4]

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*** NEWB ALERT*** 1st post***
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I'm in the process of putting a DRO on my X3. Given that a Yorkshireman has been defined as a Scot deprived of his generosity it prob'ly comes as no surprise that the reading heads for the scales will be these, at £2.99 a pop:
http://www.virtualvillage.co.uk/150m...01481-065.html
I've gutted 3 of these to get just the reading heads (ie no case, no readout, no battery) and replaced the batteries with pairs of capacitors to reduce jitter. They talk nicely to the capacitive scale strips I'm using (ex Wixey tablesaw DRO) so it looks all good to go. Readout is a Shumatech DRO550.

Please understand that all this is written with the unshakeable confidence of the totally ignorant. I know nothing about machining but I'm having enormous fun learning.

Regards, John

[irving2008]

**************************
*** NEWB ALERT*** 1st post***
**************************

I'm in the process of putting a DRO on my X3. Given that a Yorkshireman has been defined as a Scot deprived of his generosity it prob'ly comes as no surprise that the reading heads for the scales will be these, at £2.99 a pop:
http://www.virtualvillage.co.uk/150m...01481-065.html
I've gutted 3 of these to get just the reading heads (ie no case, no readout, no battery) and replaced the batteries with pairs of capacitors to reduce jitter. They talk nicely to the capacitive scale strips I'm using (ex Wixey tablesaw DRO) so it looks all good to go. Readout is a Shumatech DRO550.

Please understand that all this is written with the unshakeable confidence of the totally ignorant. I know nothing about machining but I'm having enormous fun learning.

Regards, John

Good find...!

and welcome BTW !

[tumutbound]

http://www.virtualvillage.co.uk/150m...01481-065.html

There are the same ones I bought ($US8) and seem to work OK with the scales I bought from Shars
They're spares for when I gut the original heads on the scales prior to mounting on my X2 mill.

[1113562]

If you look at the multi-scale reader, I use an external PSU (wall-wart) to power the reader and off-set the supply to +3v5 and -1v5, so that the PC and scale grounds can be connected together.


My code handles both BCD and Binary scales automatically by measuring the clock pulses at switch on. It passes the scale data to the PC in string form, the PC/VB app determines which format and unit the data is in, then converts it to Inches (which is the inherit unit of the binary scales). Objects within the app convert the unit as required. I find that displaying MM and Inch (decimal and fractions) simultaneously is VERY useful.

If you're planning a display device make sure it's got an X2 (for lathe use) and a reverse button (easy swithing from absolute and relative is also very handy).

Hi Bill,

Thanks for this. I obviously did not look too hard at your diagrams did I?

The web site has been up and down over the past few days and I kind of gave up last night. I'll try to get a bit more time on it looking at your stuff.

I like the idea of using a PC or laptop for the readout's and controls. I've never really got into VB programming but I'm quite happy with PIC assembler (not C though...that scares me). I was told that VB interface to RS232 Com Port is not that well documented and is a bit tricky? Have you thought of a USB version seeing that RS232 com ports seem to be disspearing off the modern PC's (especially laptops)?

Did you have any thoughts on producing longer bar's/PCB for the read head to slide on? All the cheap Chinese calipers seem to come with a short 150 mm bar.

[irving2008]

i reckon the combs should be easy to mill out on a CNC PCB mill. They are 0.6mm thick PCB double sided but the reverse is plain. On the top side is the T-comb structure and a ground strip. Since the mechnaism is a capacitive vernier it would seem perectly reasonable to manufacture this in say 150 or 200mm approx lengths where the end is exacly halfway through a 'T' and then end butt them together ensuring electrical conductivity across the join with a solder infil. getting positional accuracy on the second or subsequent 'comb' is the critical factor (as well as the basic accuracy of the comb). Some sort of jig might do the job.

Alternately, buy 3 cheap ones and look to join the PCB together as per above. Milling a new channel from stainless might be tricky, but aluminium might do just as well.

It worth an experiment don't you think?

[1113562]

i reckon the combs should be easy to mill out on a CNC PCB mill. They are 0.6mm thick PCB double sided but the reverse is plain. On the top side is the T-comb structure and a ground strip. Since the mechnaism is a capacitive vernier it would seem perectly reasonable to manufacture this in say 150 or 200mm approx lengths where the end is exacly halfway through a 'T' and then end butt them together ensuring electrical conductivity across the join with a solder infil. getting positional accuracy on the second or subsequent 'comb' is the critical factor (as well as the basic accuracy of the comb). Some sort of jig might do the job.

Alternately, buy 3 cheap ones and look to join the PCB together as per above. Milling a new channel from stainless might be tricky, but aluminium might do just as well.

It worth an experiment don't you think?

Yes I fully agree an experiment is worthwhile.

For proof of principle it seems least effort to try butting two PCB's from two existing gauges together to see if this gives good results across the transition. The infill with solder may be tricky and may cause delamination and a nasty lump. Would it not be better to join down the gaps between T and comb in an L shape machining in the end of each strip.

Re the channel, is it essential to have a channel at all? Providing the reader is in close proximity with the PCB I can't see why it can't work. 0.8 mm (1/32") Shims on the reader half can be used to raise it off the bar.

Another question I have is "is it essential to use double sided pcb"? If the reverse is just plain copper the same capacitance would be achieved by bonding a single sided PCB to a SS bar or Aluminium or a brass bar if you prefer. I'm not sure if the design relies on the reverse plane of the PCB floating electrically from the bar in which case this may explain the need for a double sided PCB.

[irving2008]

Yes I fully agree an experiment is worthwhile.

For proof of principle it seems least effort to try butting two PCB's from two existing gauges together to see if this gives good results across the transition. The infill with solder may be tricky and may cause delamination and a nasty lump. Would it not be better to join down the gaps between T and comb in an L shape machining in the end of each strip.

Re the channel, is it essential to have a channel at all? Providing the reader is in close proximity with the PCB I can't see why it can't work. 0.8 mm (1/32") Shims on the reader half can be used to raise it off the bar.

Another question I have is "is it essential to use double sided pcb"? If the reverse is just plain copper the same capacitance would be achieved by bonding a single sided PCB to a SS bar or Aluminium or a brass bar if you prefer. I'm not sure if the design relies on the reverse plane of the PCB floating electrically from the bar in which case this may explain the need for a double sided PCB.

Good point about the reverse of the PCB. It definitely is some form of ground plane.... might go look in Aldi/Lidl whatever for some cheap ones...

[1113562]

I have just buzzed my PCB and the comb fingers are all linked together by the thin strip of copper that runs along the bottom so this would need to be joined across the strips

[BillTodd]

I like the idea of using a PC or laptop for the readout's and controls. I've never really got into VB programming but I'm quite happy with PIC assembler (not C though...that scares me).

You should be able to understand the code easily enough :)

I was told that VB interface to RS232 Com Port is not that well documented and is a bit tricky?

Doesn't seem tricky to me - maybe I've already done all the head-on-wall banging necessary ;)

Have you thought of a USB version seeing that RS232 com ports seem to be disspearing off the modern PC's (especially laptops)?

Yes I had thought about it, but VB has it's own problems with the USB driver interface so if I were to do it I just use a cheap USB-serial adapter.

Did you have any thoughts on producing longer bar's/PCB for the read head to slide on? All the cheap Chinese calipers seem to come with a short 150 mm bar.

It would be nice to be able to make accurate scales of any length, but then would require some way of accurately calibrating them, which is probably the hardest part. Given that long scales (upto 1000mm) are available for not too much money (600mm are about £70), unless you need very long scales, is it worth making one???


Since there seems to be some interest here, I've attached the PC app and PIC files to this post. The VB app has a sort of demo mode that'll allow you to add scales and adjust the value by dragging the mouse over a button. The current version is a fixed 640x480 window to suit my panel-pc, but I have a part finished MDI (multiple document - so I could add hole layout and utility functions) version that I would like to finish ... sometime.

[1113562]

You should be able to understand the code easily enough :)


Doesn't seem tricky to me - maybe I've already done all the head-on-wall banging necessary ;)

Yes I had thought about it, but VB has it's own problems with the USB driver interface so if I were to do it I just use a cheap USB-serial adapter.


It would be nice to be able to make accurate scales of any length, but then would require some way of accurately calibrating them, which is probably the hardest part. Given that long scales (upto 1000mm) are available for not too much money (600mm are about £70), unless you need very long scales, is it worth making one???


Since there seems to be some interest here, I've attached the PC app and PIC files to this post. The VB app has a sort of demo mode that'll allow you to add scales and adjust the value by dragging the mouse over a button. The current version is a fixed 640x480 window to suit my panel-pc, but I have a part finished MDI (multiple document - so I could add hole layout and utility functions) version that I would like to finish ... sometime.

Thanks Bill,

I think I may have managed to grab the demo VB DRO last night. I installed it and had to go and find/install the required inpout32.dll. After that its seemed to open just fine, but of course I have no multiplexed input to test it yet. The version I installed as far as I could work out could only open two scales (X and Y). Is that a limitation or me being stupid?

I will certainly make up the PIC board just to see that it all functions correctly with my Lidl/Aldi 150 mm callipers seeing I have most of the parts.

One question I have is whether there is a practical maximum cable length between the reader and the PIC board. There is some posts that suggest that EMC/noise issues exist at this interface and running single ended clock and data over long wires is not ideal for low pick-up. With all 4 channels on one board there will have to be a compromise about the best location of the board relative to the machine scale positions.

You may have a point about the long T/Comb PCB and bar if you say you can pick up 600 mm callipers for £70. My mill X axis is 18" travel so one of these would be just fine. It just seems wrong to buy the whole thing just for the PCB and maybe the bar when the readers can be purchased complete for <£4 as recent posts suggest.