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DIY magnetic DRO

Building my own

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ChrisB03/08/2019 14:08:32
430 forum posts
171 photos

 

Latest update: Lathe DRO installation.

Installed the read heads on the lathe X and Z axes between yesterday and today. The idea was to install the X axis read head and magnetic tape inside the carriage cross slide. While it was possible, it was a bit difficult to do given the limited space and the leadscrew. But following Jason's idea I managed to fit it somehow.

Here's how I went about it:

As I did not want to drill any holes in the carriage I glued a plate with an m5 stud to which I would mount the read head bracket. Here I had to take measurements carefully to make sure the leadscrew nut on the crosslide would not hit the head when it's at the end of it's travel. Also to be noted, that the crosslide cannot be removed by sliding it rear wards for the above mentioned reason.

20190803_110153.jpg

 

Here's how the X axis read head is attached to the carriage. It's a tight squeeze and a bit fiddly to get to the read head screws (M2.5 allen cap head), but not impossible. I glued a plate to the crosslide which has a dual purpose - the primary purpose is to attach the magnetic tape to the under side, the secondary purpose is to cover and protect the read head and tape from swarf, dust, oil etc. ( in the photo the crosslide is over extended just to show the setup - in normal operation the slide will cover the head.

20190803_112123.jpg

 

This is the magnetic tape on the bottom face of the crosslide. Worth noting that the magnetic strip is about 2mm thick, this means that I have to be careful when dismantling the crosslide. There is not enough clearance for the strip to slide between the carriage and the crosslide, so if I attempt to slide the crosslide off I'll damage the strip. The correct way to take the crosslide off is by first sliding and removing the gib strip, and then LIFT off the crosslide.

20190803_123559.jpg

 

Next it was the turn for the Z axis installation. I attached the read head to it's bracket loosely. Then the bracket to the carriage was glued, keeping every thing clamped and aligned and left overnight for the adhesive to set.

20190803_110228.jpg

 

This is after the adhesive set, next was to install the magnetic tape.

20190803_112211.jpg

 

Everything now in place, time to check every thing works and calibrate the readouts.

20190803_123436.jpg

All that remains is tidying up the cables and build a couple of cases for the touchdro circuit boards as they are still just a circuit hanging on cables!

In conclusion, I would say there was a fair amount of work to make a DIY set of DRO's for the lathe and mill, and if you're in a hurry it would definitely make better sense buying them new. But, for half the price or less, if you have time, it's worth a try to build your own - no special skills required, just patience - a lot of it!

Edited By ChrisB on 03/08/2019 14:09:49

ChrisB05/08/2019 17:20:16
430 forum posts
171 photos

I got s piece of nylon 6 black sheet from which I'll probably make the cases for the touchdro circuit boards. Any recommendation on how to mill the material, that is, feeds, speeds and cutter types - Iast time I tried milling plastic (it was HDPE I believe) I ended up with a blob of molten plastic and taking it off the endmill is something I'd like to avoid!

The choice of cutters I have are 4 and 3 flute endmills of various sizes (3-12mm)

Ian P05/08/2019 20:37:03
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2246 forum posts
90 photos

Main thing with machining plastics is to have sharp cutters, razor sharp new ones if possible. Too high a speed in conjunction with dull cutters, or cutters that do not clear the chips quick enough will cause heating and melting of some plastics.

I dont think I have ever milled nylon but I dont like using in the lathe, it produces stringy swarf which can get wrapped round the tool and job, Acetal by comparison is very easy to machine. Last week using a new long series 12mm 2 flute endmill I cut 100mm square holes in 60mm thick Delrin by plunging full depth before moving X and Y in one pass, then a finishing cut to size.

I am impressed with your DRO project and I will definitely by doing the same once I clear my current backlog of jobs.

Ian P

ChrisB06/08/2019 11:28:27
430 forum posts
171 photos

Thanks Ian, it's a very doable project if you have some time to spare - I'll be happy to help you out any way I can when the time comes for you to do it.

Re. Nylon, I have some new carbide 3 flute end mills - will try that first. If it does not work I get a 2 flute. Will have to wait anyway as I cant go to the workshop for now - got a nasty jellyfish sting on my wrist, can hardly move it sad

Clive Foster06/08/2019 14:10:22
1886 forum posts
59 photos

Chris

Splendid job so far.

Bin the nylon and splash out on some acetal / delrin for the cases. As Ian says Delrin machines like a dream with sharp cutters decent feed and not too much speed.

Nylon is (marginally) better to machine than HDPE but its still a miserable experience. Main saving grace is that it will behave given the right combination of fairly low speed, high feed and high depth of cut. Rather grade and condition sensitive in my experience. Sort of thing you can dial in on a lathe but not really Home Shop mill compatible. Especially if you hardly ever use the stuff and will have forgotten what works by next time.

Clive

Paul Liddle07/08/2019 09:44:56
7 forum posts

Hi ChrisB

Can you help me with the reading head selection please?

I think this head LINK (although a little bigger than your choice) means i don't have to make a custom board to mount the sensor onto. Can you tell me which option i should select? i think a 10um resolution should be ok but i have no idea about frequency or ref marks.

Also the MS05 magnetic scale, was this your selection also?

Any advice greatly appreciated.

Paul

ChrisB07/08/2019 13:44:55
430 forum posts
171 photos

Hi Paul, the encoder I used have a 1um resolution and 2Mhz frequency. The tape is the MS05, correct.

This is the spec and part no of what I used - the tape pn will vary according to the length:

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
RLC2HDA2D0B00C00-29.6€;MS05BM600B0000-28.28€

Whether or not the the one you linked will work, I cannot tell - but they do have similar specs.

Chris

Paul Liddle13/09/2019 13:37:54
7 forum posts

Hi ChrisB,

In the end i ordered the same readheads as you and everything arrived today. I made the pcb's on a mini router. (i had lots of problems trying to photo-etch them!)

What technique did you use to solder the readheads onto the pcb? I was going to attempt it using a fine tip iron but someone told me that its impossible to do it that way.

As always, any advice greatly appreciated.

Paul

ChrisB14/09/2019 15:10:59
430 forum posts
171 photos

I'm not sure if the photo-etch pcb board have some coating which needs to be removed for solder to adhere.

For mine I applied solder along the traces of the pcb first. The readhead chip has exposed points on it's back side ( like little gold dots) and to prevent shorting them I applied some clear coat on it's back. I then put the RH chip in place on the pcb and applied a minute ammount of superglue to keep it in place for soldering. For soldering I have a Lidl variable temp iron, I replaced the brass tip with a diy pointed copper tip. Applied flux to the contact points and soldered on...pretty straight forward but quite fiddly as every thing is so small. Probably there are better ways how to do it, but this is my way round it.

Jed Martens16/11/2019 20:18:40
avatar
49 forum posts
26 photos

Hi Chris - how is your magnetic DRO system working? I've considering using the same components for adding a DRO to my mill. I think the version with integrated RS422 driver would works best my case (differential signalling having better noise immunity)...

My main question - do you see any issue with using the high accuracy versions (~1/4 micron per edge, 8192 edges per 2mm)? Assuming the electronics at the far end can cope with the data-rate of course. I know that the mill isn't that accurate (and the operator even less so) but I see no obvious reason for going with a coarser output.

ChrisB17/11/2019 18:58:47
430 forum posts
171 photos

Hi Jed, I went for the 1μm resolution as the most common type of commercial read heads are 1 or 5μm. Don't know if higher resolutions are better but for what I do, the 1μm resolution encoder is more than enough.

Jed Martens21/11/2019 12:35:19
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49 forum posts
26 photos

I've been thinking about how to mount the magnetic tape and head to my mill. My idea is to make an aluminium bracket and bolt that to the x-axis of the table, with the tape on the underside facing down. The head will be under that facing up. As in the diagrams at the bottom of this post (apologies for my amateurish modelling).

Are there any obvious draw-backs to doing it this way? It seems that the whole ensemble is better protected from crude (and me dropping things on it), and the read head is easier to mount. I'll make an enclosure to fit over the read head too, similar to what Chris has done.

dro1.jpg

dro2.jpg

ChrisB21/11/2019 14:51:47
430 forum posts
171 photos

I think it's a better idea to leave the magnetic tape accessible, small chips will find their way anyway and stick to the magnetic tape. So I'd rather have the tape mounted in a way I can easily brush or wipe off dirt and chips.

Also keep in mind that the encoder needs to have room for adjustment as it has to ride at a certain height and in the middle of the magnetic tape. I built the readhead case with this in mind so that I would require minimal setting up and adjustment.

Chris

Ian P21/11/2019 15:48:10
avatar
2246 forum posts
90 photos

I quite like the way Jed proposes to mount the tape and sensor as it really does protect the tape. There is no reason why the tape mounting bracket has to be a one piece component and I would bond the tape to a (say) square or rectangular beam and then have a length of unequal angle fixed to the top surface with the longest face coming down well below the sensor.

As I understood it, whilst the tape has a magnetic pattern in it, the amount of magnetic force is so low its unlikely any swarf or chips would stick to it, maybe ferrous dust might accumulate but with the protective stainless foil over, it would be easy to wipe off once every few years.

Ian P

ChrisB21/11/2019 16:13:57
430 forum posts
171 photos

Posted by Ian P on 21/11/2019 15:48:10:

As I understood it, whilst the tape has a magnetic pattern in it, the amount of magnetic force is so low its unlikely any swarf or chips would stick to it, maybe ferrous dust might accumulate but with the protective stainless foil over, it would be easy to wipe off once every few years.

You'll be surprised! Large pieces of swarf will not stick, but on a milling you'll be producing lots of small chips, and those will stick to the magnetic tape with ease. My reasoning goes: better have the tape visible and accessible where I can easily give it a wipe.

Chris

Jed Martens28/11/2019 13:08:18
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49 forum posts
26 photos

I've taken the plunge and ordered two read heads and tape, which I'll use for X/Y on my mill. If it's a success I'll roll it out to the mill Z axis, and maybe the lathe too.

I'm still pondering the pros and cons of the various ways to mount the tape to the table, thanks for the feed-back above. I like the simplicity of mounting the tape directly to the side of the table, and see the advantages of accessibility, but it makes mounting the read-head more challenging...

dro_side.jpg

I've ordered the aluminium to make the tape bracket, should I chose to go down that path, as it was cheap, but I'll probably wait until I have the read-head and tape before I decide.

For the electronics I'm planning to go full DIY. Something like...

* RS-422 receivers to convert differential signal from read head to 3.3v single-ended.

* Small programmable-logic card to count the pulses and present the data on an SPI interface. This is timing-critical "real time" stuff, which I don't like leaving to software.

* Raspberry Pi with touch-screen, reading SPI data and running a DRO GUI. The software will be written in Qt (a cross-platform C++ IDE with decent gui/widget support)

I've used similar set-ups for other DIY projects so I'm not having to figure much out from scratch.

On a slightly different topic, I note that RLS also make rotary magnetic tapes, which are used in conjunction with the same read heads. Given that electronic lead-screws are en vogue, has anyone used a magnetic encoder instead of rotary optical encoders to track the spindle position? I imagine that the rotary tape could be fitted directly to the back of the spindle, and monitored with no physical interface, removing the timing belts and pulleys that I've seen the guys on youtube use.

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