Cheap DRO

[Doug_C]

Stumbled acrossed this stuff and thought I would clue you folks in on this little secret.

Here is a set of eBay parts that can make a DRO indicator all be it not very accurate for the pure at heart. It is cheap relatively speaking.

I have a setup like this on my Rockwell 10x24 apron travel. This tolerance is well within the capability of the machine. It is great just for point to point moves with reliable counts below 4 inches. Measure twice and cut again.....should never allow you to take a 4 inch move without leaving some excess for the sake of a shorter move reference. Less chance for error accumulated or otherwise. The nice thing is it can be set up on nearly anything for linear move applications much easier than the glass scale types. Worth ignoring the minor issues if it suits the bill.

The encoder

This encoder puts out quadrature signals for counting pulses through the pull of a cable(Steel fishing leader). The odd thing about it is 249.9 counts per inch in quadrature is 4 times this. That gives 999.6 couts per inch. That equates to .0004 deviation from 1000 counts per inch. .0008 at 2 inches. And so on to 25" or 50" of the cable length. So Theoretically it should be within .001 or one count in one inch. I just realized this was 2 encoders for this price. Sweeet deal!

This is the display. Now.... Red Lion puts out a ton of panel displays. Impressive little units considering the versatility they offer. The feature we want is quadrature encoder inputs. These can also make a tach or any other rate like SF/M etc. These are programmable rate displays. Even the decimal place. Some can RS-232 ASCII outputs strings. I used this feature on a group of protractor displays I made for adaptive voice out from another device. The more features, the higher the price.

The Display

The displays can be found in several voltage source options. This one happens to be 115vAC. The encoder however was 12v, so a small wall tranformer DC supply would be required. Most manuals for the displays are on the web. The encoder may come with the pinouts or easily obtained as well as a cable and connector.

Mount the display wherever convenient. In a good nema box keeps it clean and chip free. Mount the encoder at the right end of the bed and use a leader clip to hook the cable to the apron somehow. I used a strip aluminum "L" with a hole. A small eyebolt would do. The key is to keep the cable pull reasonably parallel to the bed for best accuracy. The cable is somewhat vulnerable. If there is a plastic coating on the cable that gets damaged, its accuracy in the damaged area can be suspect. Mine is below the leadscrew out of harms way.


Some times watch the prices and do your product research for the features you want. When everyone wants one, the prices get wild. Don't end up with a model version that looks like this but has a totally different purpose or is not signal compatible.

DC

[len]

Doug,

Interesting unit, but a Chinese DRO or digital caliper is cheaper, more compact, at least as accurate, and gives you built-in dispslay and communications ability. What am I missing?

len

[Doug_C]

Personal preference? Larger display? Versatility? Another simple option that keeps the display out of the chips?

I was only passing on what I know to work and what I've done with that experience Len. I didn't post it as a "be all to end all". What is there to miss? If you can't see the positives, you aren't looking for them.

Had I known the first response would be disappointment for the effort. I would have seen this as a waste of time too! If it don't pass the "Len test" it must be worthless? GEESH! LOL! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]LOL! [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]

DC

[len]

Doug,

Hope your response was really in jest, as I DO appreciate being shown cheap and innovative gadgets to use in our hobby. I spend (waste?) a lot of time on eBay looking for encoders and such, and I had never seen the item you pointed out. John at CTR Surplus has carried a similar reel-type encoder that has tempted me. But truth be known, I have a whole shelf of similar devices collecting dust, waiting for me to adapt them to my tools. But I doubt they will ever get used. And example is the linear encoder strip and pickup head I salvaged from my decomissioned HP660 printer. Nice idea, but too much work.

So please don't mistake the meaning of my response. Based on previous postings, I think we have similar interests. I love to scrounge high-tech, obsolete gems, like the precision disk-pack spindle I bought at a flea market for 50 cents. I still feel regret when I think I could have bought the second one the guy had for the same price, but I didn't want to carry the added weight around!!!

IMO the Chinese DROs seem to have trumped all the other homebrew setups I've seen for measuring position, and they are truly inexpensive. That's the point I was trying to make in my post. You like remote displays? You can buy those for the DROs now, or you could hook them up to an obsolete laptop with a little programming and some simple electronics.

Hope no feathers were ruffled.

len

[Michael_Az]

Doug, that looks like a nice set up but what about a guy like me. My knowledge about electronics is absloutely nil! Would it be hard for a novice to put it together? Thanks for the post!
Michael

[oheider]

I am interested in how the cable connects to the encoder. 2 wraps around a carefully made pulley? Almost sounds like a gear (say 100 teeth) and a simple counter could be slapped together easily. Or how about a poor mans' trav a dial? Just thinkin out text.

[Doug_C]

Len,

Of course my response is in jest. No one can post something to suit all expectations of preference in hashing together what looks like junk. It works and in some respects has merit where the chicom stuff may not. Better for one does not fit all.


I am at a point now that I do not want to hoard any more junk I cannot use immediately. That is a tuff thing for a packrat like myself to walk away from cool stuff. Especially having a good friend that owns a scrap yard. He likes me pulling things apart so he does not have to pay someone. I store it for a while, then take it back to the yard.

You lost me on the disk pack spindle. What could that be invisioned for and where would it have originally been used?

Our interests do have similarities, considering we both hang out here to learn something, offer our experience and suggest alternatives for better or worse. Which I think that is hard enough to do without appearing pessimistic or over critical. Assuming my position is at the bottom of the pecking order, I have no feathers left to get ruffled. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

DC

[Doug_C]

The cable works like a yoyo tape measure. There is a clock spring that retracts the cable back into the spool housing. Some of these units use a traveling spool with a helix groove that keeps the current wrap at the exit hole. While others with shorter travel have enough room to allow the cable to ride at a slight angle. The risk is if the cable wraps over itself. The circumference of the wrap is very critical to translate rotary counts to linear motion. Which means that the spool must be a single level wind for the full length of travel. In this case 25 or 50 inch options.

Connecting these 2 units together for counting is very simple.

These encoders offered in the auction use a cannon connector which should be fairly common. Voltage and a pin out diagram will be needed from the manufacturer if unknown. The encoder will have a ground and positive to power it. They can be powered by a small wall transformer of the proper voltage or in some cases the display may output 12v conveniently if within the encoders voltage spec's. Some encoders are 5v, but still work in this setup using a 5v supply. The display itself must also be powered. Most are labeled as to the power source terminals and what is needed.

Gemini PDF manual

Some rate displays other than Red Lion are voltage specific on their inputs. A call to the manufacturer can verify quadrature signal voltage compatibility if not documented.

Two other pins from the encoder will be the A and B pulse signal outputs. Some have Anot and Bnot which are just inverted from their counterparts of A and B. When A goes to 12v Anot will be 0v and when A is 0v Anot will be 12v or at least near the supply voltage. Regardless of which pair, use one A and one B respectively to utilize them as quadrature signals. Do a web search to get a better detail on what quadrature is and how it works.

With only 4 wires from the encoder(one being the positive connected to the supply), connect only 3 to the display inputs. That would be the ground(which is also connected to the power supply) and the 2 quadrature signals. It can't get much simpler than that. The display manuals give a setup and guides you through the programming for encoder use. If the count is backward for your project, just swap the A and B input connection terminals.

Check out all the programmable features these displays offer. Programming a scaler for a direct reading display is very easy and can be used in many functions where rate counting can be had, given the relative sensor inputs.

DC

[John_Stevenson]

This is exactly how the sensors on the BW DRO system work, a cable pulling a encoder wheel round against a spring.

http://www.bwelectronics.co.uk/systems/361-362.html

The Shumate DRO can also use these. It was originally designed for the cheap chinese scales but Scott has produced a small $10 board that can convert quadruture counts into the Chinese scale code.
On this one you can mix and match, say chinese scales on the shorter Z and Y axis of a mill and an encoder on the longer and more expensive X axis.

http://www.shumatech.com/

John S.

[cba]

If you are after a really cheap, yet fully featured and accurate DRO, and do not mind putting in some of your time to assemble it, check out this:
http://www.shumatech.com/products/dro-350
I do not know of any better low cost DRO, its truly superb. On top, you get free software updates and support via the Yahoo group http://groups.yahoo.com/group/ShumaTech. Its a 3 axis DRO kit that can be built as lathe or mill version for something like $150. It uses cheap chinese scales or quadrature encoders for input. The pic shows it attached to my lathe. Chris

[len]

Doug,

You lost me on the disk pack spindle. What could that be invisioned for and where would it have originally been used?

The spindle is out of a mainframe hard disk system, dating back at least thirty years. In those days, the drives were the size of washing machines and the spindles used on them were high precision. The one I have, which I have mentioned in this forum a number of times, is about 3" in diamater and about 8" long, and has a mounting flange near the middle. The spindle itself is tapered at one end to receive the diskpack, and has a flat driving pulley at the other.

You could use this thing for a number of projects, like a precision grinder. One guy I know of used one to make a potter's wheel!

Anyway, there's a lot of high-tech junk lying in junk yards waiting to be mined. A lot of it appears on eBay, but I'm sure lots of it gets melted down or crushed.

Having said the above, I have to admit to passing over a lot of stuff on eBay that I once would have bid on. The fact is that I never really do end up using it--it just sits on my shelves as trophies. The high price that eBay bidders now bid is also a turn-off.

len