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DIY magnetic DRO

Building my own

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ChrisB30/04/2019 22:10:54
397 forum posts
162 photos

I have been wanting to buy a couple for my mill and lathe for quite some time now, specially for my mill as it can get quite frustrating and confusing trying to keep in mind backlash, endmill diameter, cutting direction, the small handwheel scales etc.

I got a couple of quotes for a 2 axis and a 3 axis Dro sets with magnetic scales, but these cost roughly £1000 and to be honest I was put off for a while...Lately I have seen quite some posts on DROs, which re-ignited my interest in the subject.

I thought what if I could do something myself, at a fraction of the cost? I want magnetic scales as these are the smallest most compact there is and the least sensitive to dirt and oil, while offering very good levels of precision.

So here is my planned project:

I placed an order for five pcb encoders: **LINK**

and a pair of scales drivers from TouchDRO: **LINK**

I also purchased some miscellaneous items such as blank pcb boards and etching material, pins and sockets and wire, etc. I estimate the total cost to be not more than £400.

Will report on the progress and if it works or no...hope it does!

Nick Hulme30/04/2019 22:26:27
701 forum posts
37 photos

Quotes from where? -

**LINK**

ChrisB01/05/2019 07:39:07
397 forum posts
162 photos
Posted by Nick Hulme on 30/04/2019 22:26:27:

Quotes from where? -

**LINK**

Yes Nick, I saw those, they are even more expensive at £1200 for a 2 and 3 axis sets. I was looking at EMS-i, price wise they are just under £1000.

ChrisB01/05/2019 08:44:18
397 forum posts
162 photos

So until I get the encoders which are on order I'll start with the preparation. I need to print/etch a small board to mount the encoder on as I'm never going to be able to solder wires to the encoder chip (the contacts are less than 1mm apart!) Then I'll mill the encoder housing out of aluminium and try to make all fit inside.

Started off with some drawings to get a good I idea of how things will fit in.

Overall read head dimensions will be 25x20x7mm, compact size so it will not get in the way and hopefully makes installation easier.

dro1.jpg

Internals, encoder inside the read head,

dro2.jpg

The boards I need to build and the encoders which are on order.

dro3.jpg

I have a day off today (my birthday!) so I'll get on with these boards this morning - it's an experiment as I never did a board before but youtube and the internet is very helpful!

I'm using a photoresist pcb board, developer and a homebrew ferric chloride etchant... let's see how it goes!

Les Jones 101/05/2019 09:21:14
2092 forum posts
144 photos

Hi Chris,
I have not noticed any mention of the magnetic strip to work with the encoders. You will also need to make a guide system to keep the encoders the correct distance from the magnetic strip.

Les.

ChrisB01/05/2019 10:01:00
397 forum posts
162 photos

Hi Les, well noted. The encoder comes with a self adhesive magnetic strip and a protective stainless steel cover strip. It's about 1.6mm thick, the ride height is 0.1 to 0.7mm so there's a bit to play with. I designed the read head to get approx 0.5mm ride height. If you look closely at the first picture, you'll notice the read head sides extend beyond the magnetic tape, that is what will give me the 0.5mm gap.

I ordered the magnetic tape is over sized so I can cut to length as required.

Colin Heseltine01/05/2019 10:56:35
325 forum posts
77 photos

Chris,

Having Looked at the link you supplied where do you get the £1000-£1200 from. The three axis M-DRO set is only £520 complete.

I have used the Yuriystoys Touch-DRO software with M-DRO magnetic read heads on my Cowells mill and lathe.

Colin

ChrisB01/05/2019 11:14:40
397 forum posts
162 photos

Hi Colin, I need two DRO sets, a two axis DRO for the lathe **LINK** and a three axis for the mill **LINK**

If I had to get the read heads only, the cheapest they have is £140 each and those are 6" long I need longer ones. I'm sure the 5 individual read heads will add up close to the £1000 mark.

ChrisB01/05/2019 11:17:48
397 forum posts
162 photos

Works in progress: The PCBs are developed and being etched, so far so good!

dro6.jpg

dro5.jpg

Colin Heseltine01/05/2019 11:24:12
325 forum posts
77 photos

Chris,

I had not appreciated you required two sets of readouts.

Colin

ChrisB01/05/2019 18:22:17
397 forum posts
162 photos
Posted by Colin Heseltine on 01/05/2019 11:24:12:

Chris,

I had not appreciated you required two sets of readouts.

Colin

Thought so Colin, no problem.

The circuit boards have etched nicely, I will need to trim them to size and drill 1mm holes at the 5 solder points for the wires. Not sure how to cut them to make them identical...might take them on the mill rather than sawing them off.

dro4.jpg

Ian P02/05/2019 13:00:10
avatar
2171 forum posts
90 photos

I see there are instructions on the encoder data sheet that refer to soldering it immediately after unpacking it because it is moisture sensitive. I am curious why its not then moisture sensitive after soldering.

What was the actual part number of the RLC2HD encoder you actually ordered and how much do they and the magnetic strip cost?

Ian P

Andrew Johnston02/05/2019 13:11:49
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4852 forum posts
543 photos
Posted by Ian P on 02/05/2019 13:00:10:

I see there are instructions on the encoder data sheet that refer to soldering it immediately after unpacking it because it is moisture sensitive. I am curious why its not then moisture sensitive after soldering.

It's a production issue. The plastic encapsulation absorbs some moisture. Soldering on commercially produced boards will be done with a reflow oven, where the board with placed components is heated up to soldering temperature and back down again over a few tens to hundreds of seconds using infrared lamps. If the encapsulation has absorbed water it may well boil, cracking the case. It's not an issue for hand soldering, nor after the part is soldered down.

Andrew

Ian P02/05/2019 14:55:45
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2171 forum posts
90 photos

Well explained Andrew. having been responsible for getting boards manufactured in a previous existance it is something I should have remembered, although they were mostly flow soldered.

Ian P

ChrisB02/05/2019 15:18:10
397 forum posts
162 photos

As Andrew well explained, it's for production manufacturing, I'll be hand soldering so I don't think I'll have any major issues there. We'll see when it comes to soldering those miniature joints what difficulties will arise...got low temp lead solder which should help a bit.

The encoders I ordered are all the same and cost €36 and the magnetic tape around €50 per m

Below is the spec I ordered, you can get it with different resolution connectors etc

RLC2HD miniature linear PCB level incremental magnetic encoder

Product description
[HD] - Incremental, no line driver, 5 V [A] - N/A (standard) [2D0] - 2000 (1 µm) [B] - 0.5 μs (2 MHz) [00] - No connector [C] - Periodic reference as per scale pitch (2 mm) [00] - None (standard) [] - [B] - ±40 μm/m [M600] - Length in mm (less than 1 m) [B] - VHB back-adhesion tape, with cover foil [0000] - No reference mark [] -
Product part number
RLC2HDA2D0B00C00
ChrisB03/05/2019 19:32:29
397 forum posts
162 photos

Some more progress today. Milled the encoder housings out of aluminium, all ready for mounting the electronics. The encoders should arrive over the weekend, but will have to wait a bit longer to try them out and see if they actually work as the TouchDROs have to come from all the way from the States - might take a while.

First I machined the read heads in to the final outside dimensions and then made a fixture to clamp them.

This is the fixture held in a precision milling vice.

dro11.jpeg

Read head blank clamped on the fixture ready for machining. Clamping action is very simple, I tighten the screw, in turn the countersink pushes a "jaw" away which pushes against the work piece. I could have clamped the piece directly on the milling vice but was afraid the part would distort. Besides that, with this fixture I could get a good degree of repeatability ( had to make 5 pieces)

dro10.jpeg

First read head ready, four to go!

dro9.jpeg

Almost done.

dro7.jpeg

Ready. Also machined the covers which will also act as a guide to keep a gap of 0.5mm from the readhead to the magnetic tape.dro8.jpeg

Ian P03/05/2019 20:13:32
avatar
2171 forum posts
90 photos

Impressive progress in a few days Chris. You have got me tempted to go down the same (similar) route. My mill has three separate iGaging type scales and I think I will replace the three displays with TouchDRO running on a tablet. Once that's up and running I can replace the scales later.

How long can the cable be from the encoders? I see the description states 'no line driver' so presumably would not like too much capacitance. What type of cable will you use (and protect and anchor it to the encoder housings)?

Ian P

ChrisB03/05/2019 21:26:30
397 forum posts
162 photos

Hi Ian, you can wait till I get my TouchDRO to make sure it works, I'll be the guinea pig!

I'm using a 5 core felxible shielded cable like this: **LINK**

I the diameter on the housing is a close fit to the cable so that will slide in a tight fit. Then I'm filling the cavity inside the readhead with sealant so that will seal and keep everything together. Cable length is approx 1m, can be much shorter actually as the TouchDRO can be mounted on the saddle (for the lathe) and transmit to the tablet via bluetooth. But I kept the lengths standard so I can move them around and swap if I need to.

I could have used armoured cable but to be honest I think it would be an overkill, I mean I'm not in an industrial environment and with some care and proper routing the cables will be protected - It is chemical resistant so oil and stuff like that should not affect it much.

Next step is mounting the encoder, left Slovenia on Wednesday, so I hope I get them tomorrow, if not it will be next week frown

Ian P03/05/2019 21:40:10
avatar
2171 forum posts
90 photos

I am not in any rush but I have no doubts that what you are doing is going to work. A few minutes ago I installed the TouchDRO app on my phone just so I could see what it looked like. Obviously not anything I can use yet but can see it will be a big improvement on three separate displays (and on the two axis Sino on my lathe).

The cable you have selected, being polyurethane jacketed is pretty tough stuff and impervious to most things in the workshop.

Ian P

ChrisB07/05/2019 14:13:22
397 forum posts
162 photos

Some more progress : received the encoders over the weekend, so next step was to solder the encoder to the pcb and then install everything on the readhead casing.

They are indeed small and was a bit concerned I might not be able to solder them as the contact points are really close to eachother, but I managed. I checked continuity and all seem to be fine, the the proof of the pudding will come when I receive the touchDRO, so fingers crossed!

I anodised the aluminium read head casings with chromic acid, the material is 7075 and has a tendency to corrode although it's probably unlikely given it will live in an oily environment.

Next step is to seal the encoder inside the readhead casing, I'm split between resin or sealant, not sure which way to go. I would prefer resin as it will become one piece. On the other hand with resin, once cured there's no way I will open the thing again if I need to.dro12.jpg

dro14.jpg

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