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Old 3rd Apr 2017, 5:03 pm   #1
David G4EBT
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Default Lathe/drill/milling machine 'Tachometer' project

I know that a lot of forum members have metalworking lathes, woodworking lathes, milling machines and drill presses, so I thought this project might be of interest and use, and hopefully, not too far off topic, given that it is about electronics, and to an extent, about homebrew, albeit not on the airwaves. To say that it's 'homebrew' is stretching a point, in that it only involves drilling a few holes in an ABS box, soldering a few wires and making a small bracket.

So, with those caveats in mind, here goes:

Some time ago, I started building a project which had featured in one of the Model Engineering magazines which used a digital frequency counter display, and was operated by a disc which you had to make, with 60 slots in it, which fitted in and opto-interuptor, so with every rev, it gave a count of 60 to covert RPM to frequency. In the end, albeit I'd got it almost finished, I abandoned the idea as I couldn't find a way of conveniently attaching the 5" diameter slotted disk to the lathe headstock shaft.

Recently, in an internet search I came across a digital tacho which is triggered much more conveniently by a 'hall effect' sensor, which looked quite promising from a youtube video. At just £12.50 post free on a slow boat from China I thought I'd take a punt on one.

Its arrived, I've done what's needed to house the display and sensor, and am pleased to say that it works a treat. It's very responsive, and seems to be accurate, in that I have two 'non-contact' RPM meters which correspond with the readings on this device. One of the nice features of it is that it has 'leading zeros'. Hence, with the lathe at rest it reads '0' - not '0000', and at say 500RPM, will read 500 - not 0500.

The maximum speed of my woodturning lathe is 3,500 RPM, but is rarely run at that speed. As an aside, the optimum cutting speed for woodturning is 25 ft/sec. To calculate the RPM required to run at that speed, a rough ready reckoner is 6,000/turning diameter in inches. So for example, a 10" diameter bowl would need to rotate at 6,000/10 = 600RPM, whereas a 2" diameter spindle would need to rotate at 3,000 RPM.

To comply with forum rules, I won't give the e-bay link, but a search for: ‘4 Digital Red LED Tachometer RPM Speed Meter+Hall Proximity Switch Sensor NPN’ will find it. (Expect to pay about £12.50 post free). I housed the display in a 'Hammond Multipurpose ABS Plastic Enclosure' 87x57x36mm. Maplin Code: N56FK

For connecting the lead of the sensor head I used a Mini DIN plug – 4-pin: Maplin Code JX02C

For the sensor input socket I used a Mini DIN chassis socket – 4pin, Maplin Code JX08J

For the power input socket I used a '5.5 mm x 2.1mm DC PowerJack Socket. Female Panel Mount Connector'. Maplin Code JK09K

To power the unit I used a regulated 12V 500mA plug-top power supply from CPC, order code: 'PW03008' (£7.26 inc VAT with free delivery).

The first pic shows the items I used - the display comes with the sensor, a power lead, triggering magnet and the instruction sheet in pic 4 below. I've drawn a couple of sketches to show the cut-outs in the ABS case, and of the wiring diagram. Obviously others might have different ideas as to how to house the display.

To mount it on the lathe, I stuck a sheet of thin steel plate inside the base, then used small 50 grade neodymium magnets on the outside to firmly hold the display box on the front of the lathe. To house the sensor, I made a small bracket and again used neodymium magnets to fix it to the end panel of the lathe. I placed another neodymium magnet of the outboard end of the headstock shaft to actuate the sensor with each rev.

I hope this might prove useful to other forum members, doesn't sound too complicated, and that the pics and drawings make it clear. Both the power supply and the sensor head have quite long leads so the mounting position of both the sensor, the display and the power supply affords lots of flexibility. The sensor head needs to be well forward from the bracket as shown in the last pic, and about 10mm from the magnet to enable sensitive triggering. (It comes with a neodymium magnet to attach to the drive shaft of whatever equipment the tacho is to be used for).

I'll attach a few pics of the meter in action in another post.
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Old 3rd Apr 2017, 5:37 pm   #2
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Default Re: Lathe/drill/milling machine 'Tachometer' project

A few pics of the finished project are attached below.

As I said in my first post, leading zeros are suppressed, so as will be seen from the pic of the display in action, it reads '612' - not 0612. It will read up to 9,999 RPM - way above the speed of a lathe. The first pic shows the inside of the ABS box, the second pic with the lathe at rest, third pic at just over 600 RPM, fourth pic at just over 3,500 RPM. The last pic shows the 'Hall effect' sensor head on the steel bracket I made, held firmly in place on the end panel of the lathe with small neodymium magnets. I attached a couple of small rectangular neodimium magnets (15mm x 6mm) on the outboard end of the lathe headstock to trigger the sensor with each rev. Being rectangular, they fitted the shaft better than the circular one provided. They can be see above the sensor head.

My NOVA woodturning lathe gets its model number '16-44', from the fact that it will turn up to 16 inches in diameter (that's 8 inches clearance over the bed), and up to 44 inches between centres. The headstock can be turned and locked in any position, so with an outrigger can turn as big a piece of wood as anyone is silly enough to want to turn.

I have to admit that this is a minor and frankly, needless enhancement, given that non-contact RPM meters that you just point at the lathe can be had for under a tenner, but well - boys will be boys, and small thing amuse small minds! I'm in my own little world, and they know me here.

Hope that's of interest.
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Old 3rd Apr 2017, 7:15 pm   #3
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Default Re: Lathe/drill/milling machine 'Tachometer' project

Interesting David,
I worked on some big industrial machines that used two such sensors at quadrature (spaced 90 degrees apart) this gave the number of rotations and rpm as well as direction information.
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Old 6th Apr 2017, 10:49 am   #4
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Default Re: Lathe/drill/milling machine 'Tachometer' project

Hi,

Thanks David for adding this interesting project. Excellent description and drawings; as you say it could be considered a needless enhancement but this isn't what hobbies are about is it; hobbies are where we are allowed to experiment and do what pleases us.

I'm always interested in all things lathe as you know and its useful to know the lathe speeds. Many lathes these days are variable speed through electronics which I dislike because I'll never ever trust anything with a circuit board installed.

Your "Tachometer" though could be used for anything where rotational speed needs to be counted; I have quite a few electric motors in stock without identification and a rev counter would at least determine the speed of the motor.

I restored/upgraded my Graduate woodturning lathe a few years ago adding a VFD and a 1.5hp inverter rated Brook motor; out of curiosity and with the motor on the bench the VFD could run the motor at over 11,500 rpm which actually is low speed for a router; I was surprised how sweet the motor sounded and it was unstressed.

I also like the small wooden enclosures you make to house your many and varied home made instruments; the comb joint making you've got off to a fine art and details of these little boxes would I'm sure be of interest to many members who need a small enclosure for their homebrew projects.

Kind regards, Col.
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Old 6th Apr 2017, 6:43 pm   #5
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Default Re: Lathe/drill/milling machine 'Tachometer' project

For just measuring the shaft speed of motors, lathes, milling machines or whatever, if a permanent display isn't required, the simplest and most convenient way to do that is with a non-contact hand-held digital tacho. You just put a white mark on the shaft (Tippex, small piece of white tape etc), and point the instrument at it for an instant readout, which can be stored in the memory if desired. (They're supplied with a strip of reflective marker paper from new).

These hand-held ones can be found on e-bay, typically called 'Digital Laser Photo Tachometer Non Contact RPM Tach' for about £6.50 post free. The display is wide ranging - in fact from 2.5 RPM to 99,999 RPM - way outside any practical requirements at either end of the scale. So, even 33/45/78 RPM is encompassed for those who don't have a strobe disc! The resolution is to 0.1rpm up to 999RPM, then 1RPM from 1,000 RPM upwards, with a claimed accuracy of 0.05%.

I've had one for some time and it's very good, but I wanted to put a permanent counter on my woodturning lathe, so opted for the one the 'Hall Effect ones that prompted me to start this thread. (I see that those are available on ebay with a green display from a supplier in Holland for £8.00 post free).

With this topic not being central to the aims and objectives of the forum, I only posted the thread for information to forum members who have lathes and other workshop machinery and who might not be aware of these useful RPM Meters. I guess that it's a more appropriate topic for other forums such as 'UK Workshop', but I'm pleased to note that it's been viewed 300 times, so I hope it's been of use to some fellow forum members.

I think that just about wraps it up, but if anyone would like any info as to sources etc, please PM me.
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Old 6th Apr 2017, 7:29 pm   #6
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Default Re: Lathe/drill/milling machine 'Tachometer' project

I added a similar sensor to a modelmaker's lathe so I can wind simple bobbin coils etc. In that case the sensor is attached to a counter.
I also designed a large-display led counter on a lathe used to wind industrial heating elements at a local element manufacturer, so the number of turns could be seen across the workshop.
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