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DIY magnetic DRO

Building my own

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ChrisB26/07/2019 14:41:33
650 forum posts
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Posted by JasonB on 26/07/2019 14:14:34:

I think where you show the head in the last photo will clash with the nut when the cross slide is wound fully towards the operator.

I think I did not explain myself properly Jason. My intention was to install the head aft of the nut (approx 20mm aft of the grease point) The head would be installed directly to the underside of the cross slide and as the nut is also bolted to the cross slide they should not come into contact. My picture above is probably misleading, as the head should be under the cross slide (but then I don't have X-ray photo capability yet!)

I got your point now Jason, yes at the back in the area you point out in your photo the head should fit nicely I think. It's dimensions are 25mm long, 18mm high, 7mm wide. the tape and foil cover is 1.7mm thick and I need a gap of about 0.3mm. So I'd say the total height of the head and tape should be 20mm.

Clive Foster26/07/2019 15:32:40
2990 forum posts
105 photos

Watching this with great interest as am looking into a similar "concealed DRO" installation for my Smart & Brown 1024. OK I've a bit more room than you have and things are very much at the handwaving, pre back of the envelope, stage. Thinking in terms of putting the scale on a flat carrier affixed to the underside of the cross slide with the head in the space behind the screw as per Jasons picture. Longitudinal scale will go on an alloy beam sat in the gap between the taper turning attachment and the bed fixed to the slots that also hold the taper turning attachment bracket. Probably extruded L shape alloy with the scale underneath.

Intention is to hang both sensors off a single bracket or, if the shapes don't work, fix the wires to one and run them via an energy chain to the display for better control the angles of dangle when moving.

If you can't get the scale in flat to the slide is there room for an alloy "fin" hanging down beside the lead screw with the scale fixed to that. No real force on it so I guess L shape alloy extrusion with the side trimmed down to 1 mm would do.

If it almost fitted I'd have no compunction about milling a mm or so out for just enough room.

Clive

JasonB26/07/2019 17:34:22
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Posted by ChrisB on 26/07/2019 14:41:33:

I think I did not explain myself properly Jason. My intention was to install the head aft of the nut (approx 20mm aft of the grease point) The head would be installed directly to the underside of the cross slide and as the nut is also bolted to the cross slide they should not come into contact. My picture above is probably misleading, as the head should be under the cross slide (but then I don't have X-ray photo capability yet!)

That looks like a plan, there seems to be about 9mm between the top of the screw and underside of the slide so your head will fit there. Strip may be a bit close on the nut but easy enough to shave a fraction off one side of that if the strip needs a bit more room.

ChrisB26/07/2019 18:57:02
650 forum posts
210 photos

Thanks for checking Jason. I'll try to do a temporary installation and make sure everything fits, I'm confident it should work out - thanks for the idea.

Will report again when I progress further...got too many projects at hand (DROs, Mill Power feed, Solid tool post, Mill way covers...) but the will to visit the workshop in this heat is fading

Out of curiosity, do you have the dimensions of an emsi read head?

JasonB26/07/2019 19:07:06
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11mm on the side that faces the strip x 14mm x 36mm long

ChrisB26/07/2019 20:45:00
650 forum posts
210 photos

I see, so the magnetic strip is 10mm wide I guess. I'm using 5mm wide tape which makes the heads I'm using narrower. Hadn't thought of the nut but as you said, it's not a big deal to make it fit. Next week will try to make it a point and install both mill and lathe DROs

Clive if you build your own heads you could possibly design them to be even smaller than the one I have, the electronic circuit for the head is barely 10mm square, so the case need not be much larger.

Douglas Johnston27/07/2019 08:52:49
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763 forum posts
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I installed a magnetic scale under the cross slide of my Myford Speed 10 lathe a few years ago (a few pictures in my album ) and it has proved to be very nice and reliable. I covered the magnetic strip with a thin strip of non magnetic copper alloy to protect it and that has worked fine.

The whole scale being under the slide keeps it out of harms way and does not clutter up the right hand side of the carriage. I have the read head stuck out at the back of the carriage with an alloy plate to cover and protect it from swarf. All in all it is more work to install it this way but the benefits are really worthwhile.

Doug

old mart27/07/2019 21:57:58
3520 forum posts
216 photos

I would worry about all the ferrous swarf sticking to the strip all the time, or are you only going to be turning non magnetic metals?

SillyOldDuffer28/07/2019 09:48:37
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7918 forum posts
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Posted by old mart on 27/07/2019 21:57:58:

I would worry about all the ferrous swarf sticking to the strip all the time, or are you only going to be turning non magnetic metals?

That's a thought, but as attracting swarf would severely limit their practicality I guess it's unlikely they are magnetic in the usual sense. No luck here finding a description of them on the web though! Does anyone know how they work?

I suggest the tape isn't permanently magnetic like cassette tape. Instead the head might count transitions as a high-frequency magnetic field generated by the head passes over normally demagnetised iron particles embedded in the tape. Although that's also a magnetic tape, it wouldn't attract iron. But there must be other ways of making a magnetic DRO, induction perhaps? Help!

Dave

Michael Gilligan28/07/2019 10:08:40
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Posted by SillyOldDuffer on 28/07/2019 09:48:37:
[ ... ]
No luck here finding a description of them on the web though! Does anyone know how they work?

.

These guys: **LINK**

https://www.researchgate.net/publication/307610351_The_Front-End_Readout_as_an_Encoder_IC_for_Magneto-Resistive_Linear_Scale_Sensors

MichaelG.

SillyOldDuffer28/07/2019 11:08:31
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7918 forum posts
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Many thanks Michael, I don't know how you find this stuff! The paper led me to this web-site which - if I've understood it- describes another way of measuring linear position.

The magnetic tape (they call it a waveguide), is made of aligned magnetostrictive elements. When the head transmits a magnetic pulse, the magnetostrictive elements produce an ultrasonic pulse that travels to both ends and reflects. Measuring the time taken by both to return to the head gives the position of the head. It's like RADAR.

Scary how much I don't know. Every answer makesat least two new questions!

Dave

ChrisB28/07/2019 11:30:41
650 forum posts
210 photos

The magnetic strip is a magnet ( not a powerful one ) so yes some little ferrous fragments will stick to it. But I'm not too worried about it, the fragments that stick are few and small, and as the magnetic field is not strong it is very easy to wipe off. Besides that the strip is covered with a thin stainless steel foil which will protect the magnetic strip from abrasion and hot chips etc.

The area below the chuck is covered by a swarf tray attached to the carriage so not much will fall in between the ways on the lathe bed, and in any case I will have a better view if there's dirt accumulation and have better access to wipe it off.

If it becomes an issue I can always take off the tape and place it somewhere else, that's the beauty of sticking things rather than bolting! wink 2

Michael Gilligan28/07/2019 11:42:33
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Posted by SillyOldDuffer on 28/07/2019 11:08:31:

Many thanks Michael, I don't know how you find this stuff! The paper led me to this web-site ..

.

You're welcome, Dave

... but could you please reciprocate by correcting the hyperlink that you included

MichaelG.

old mart29/07/2019 20:09:30
3520 forum posts
216 photos

Are the guidelines regarding reusing the tape ok with the possibility of it stretching?

ChrisB29/07/2019 21:38:52
650 forum posts
210 photos
Posted by old mart on 29/07/2019 20:09:30:

Are the guidelines regarding reusing the tape ok with the possibility of it stretching?

Stretching? Didn't quite get how that could happen, maybe we're not referring to the same thing. The magnetic tape I'm using is not a tape pre se, not like a cellulose tape. It is a stainless steel strip with magnetic material bonded onto it with double sided tape applied to the stainless strip (by the manufacturer). I just peel the backing film off the strip and attach it to wherever I require it. If I should need to take it off, I'll slip a blade under the tape and lift it off. Can't think of a way I'd be able to stretch it, I think it would be fine.

In any case I started installation of the DROs on the mill, should be ready by tomorrow. I threw some iron filings from the bandsaw at the tape and some will stick, but it's very easy to dust it off and none will get under the readhead as there's no gap. Will post some photos of the installation tomorrow.

Douglas Johnston29/07/2019 22:48:08
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Posted by ChrisB on 29/07/2019 21:38:52:
Posted by old mart on 29/07/2019 20:09:30:

Are the guidelines regarding reusing the tape ok with the possibility of it stretching?

Stretching? Didn't quite get how that could happen, maybe we're not referring to the same thing. The magnetic tape I'm using is not a tape pre se, not like a cellulose tape. It is a stainless steel strip with magnetic material bonded onto it with double sided tape applied to the stainless strip (by the manufacturer). I just peel the backing film off the strip and attach it to wherever I require it. If I should need to take it off, I'll slip a blade under the tape and lift it off. Can't think of a way I'd be able to stretch it, I think it would be fine.

In any case I started installation of the DROs on the mill, should be ready by tomorrow. I threw some iron filings from the bandsaw at the tape and some will stick, but it's very easy to dust it off and none will get under the readhead as there's no gap. Will post some photos of the installation tomorrow.

Some double sided adhesive tape can be very difficult to remove, as I know from bitter experience. Better to make sure it is in the best place from the start.

Doug

ChrisB31/07/2019 13:45:42
650 forum posts
210 photos

Found some time yesterday and this morning to install the Mill DRO. Takes some fiddling with the read heads to get them aligned properly, but other than that the installation was realy easy. No holes to drill into the machine, some simple brackets, just tape and go!

Had to remove and modify the slideway covers to make space for the read heads, that took me some head scratching and time, but finally got something I was happy with - if anyone's interested and would like to know how I did it etc, I'll open another thread.

Here's some pictures of the progress.

Installation of the magnetic tape on the X-axis, I only needed to set up and align the read head. Then while traversing the table and peeling the backing tape, the read head aligns the magnetic tape.

20190729_152550.jpg

Same thing for the other axes.

20190731_094604.jpg

Y-axis installation. To avoid drilling and tapping into the saddle I glued a plate with a stud on to the saddle, then attached the read head on a slotted bracket which is in turn is held by a nut on the glued plate. ( I used scotch weld EC 2216 for attaching the plate to saddle - not going anywhere with it)

20190731_123456.jpg

This is the swarf cover for Z and X slideways. Had to remove the original ones to make the read heads fit. In a nutshell it's a sort of roller blind so rather than hanging around when raising/lowering the head, and when traversing the X-axis - the cover will stow itself like a roller.

20190731_123306.jpg

This is how the Y-axis looks like under the slideway cover. The bracket for the read head is bolted using an existing hole.

20190731_123528.jpg

The only thing which bothers me is the wiring. I placed the extra wires in a conduit, but the rest are hanging loose. The thing is the cables need slack to allow for the movement of the head and table, but it's not neat. I have to figure out something better.

20190731_123823.jpg

Clive Foster31/07/2019 14:36:26
2990 forum posts
105 photos

Commercial systems frequently use flexible metal conduit of 6 or 8 mm bore to protect the wires. Various types and finishes can be found. Typically £1 to £2 ish per metre in 25 metre rolls so not inherently silly expensive but finding folk who sell shorter lengths is a problem.

Clive

ChrisB31/07/2019 19:38:34
650 forum posts
210 photos

Unfortunately I can't do that at this stage, not unless I take the connectors off. I had a look at photos DROs installations and most of them have wires hanging freely. So I will find some split type conduit and try to route them as best I can.

Tomorrow I'll be doing the lathe, which should be easier.

JasonB31/07/2019 19:40:56
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Most hand freely in the flexi conduit which protects the wire from damage. It's a bit like small diameter shower hose.

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