Lathe tachometer

Hi all,

I have just fitted a Newton-Tesla inverter and 3ph motor to my Myford Super 7 and would like a tachometer so that I can set the spindle speed.

Has anyone done this?
I have seen a couple of threads but wondered if anyone had fitted one to an S7 and could give advice to save me making (the usual) fool of myself.

Many thanks

Richard.

I'm sure you'll find the conversion well worth while for the ease of use and the smoothness you get from a three phase motor but why would you need a tachometer? Isn't the speed control knob calibration adequate? Spindle speed isn't that critical outside of a production environment.

Russell.

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Hi Richard,

I have done similar to this on a Boxford. It's a simple job that involves fixing a small magnet to the spindle and arranging the sensor close to it. The rest is just simple wiring.

You should be able to see from this image how I arranged mine.

I would consider using a mains feed or you will be forever changing batteries. I 'think' it was an old phone charger I used for mine.

Nick

The Inverter drives on ALL of my machines have a multi-turn potentiometer to control the "speed". The base inverter frequency has been left at 50Hz and the linearity of the speed pot is sufficiently close to linear that if the pot reads 67, that is 67% of whatever the drive belt/pulley (or gear) ratio would give. Any closer than that in our use is merely chasing your tail! My one exception is my mill which has 60Hz so allowing me to use its faster upper design rpm should I wish.

Frankly if you can drive your lathe in simple mode of belt changes, then the new variabilty of rpm will give you an added choices and the best benefit is matching the rpm to say remove resonance chatter of the tooling (if any). You will also now have the ability to vary the rpm with diameter of workpiece to better match a desirable surface cutting speed whilst you are changing diameter.

Edited By KWIL on 11/02/2015 11:30:30

Thanks everyone for your help.

Russell - the speed control only gives me an idea of the motor speed but I have the choice of 2 x motor pulley speeds and the usual 4 x direct and backgear selection so the NT motor control does not tell me any thing other than the motor spindle speed. Hence my request.

Richard.

Nick - what have you got at the other end of the sensor connection? Wondering where you got the actual display unit from. Both my mill and lathe have multiple drive ratios and VFD control and it would be handy to have an actual rpm displayed. I like to start off with the surface speed at something like the right value.

Murray

Murray, Richard,

I added a tacho to my Warco GH1330 when I upgraded to a VFD controlled motor, bought from that well known auction site, like this. Just need to add a 12v supply, and a small magnet, works very well.

Graeme

It's this **LINK** one.

You need to get a box to house it in.

Nick

Glued a little magnet to the bull-wheel of my S7 and used a bicycle speedometer to read the RPm. Ignor the dot in the reading. Suits me well for lathe and drill-press using a Omron VFD for both and a home made control panel.

Jan

Nick. Looks reasonable and being low voltage, it's not going to be lethal!

Many VFDs have a 24V or similar sensor supply, so it's possible these could be powered from that without the need for an additional PSU. Must check the manual and get round to fitting them. I have a selection of neodymium button magnets that would doubtless fit the requirement.

Murray

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Murray. They come with the required magnet. Well mine did anyway.!

Nick

I used a cheap tachometer purchased on Ebay for my home built milling machine. It uses a proximity switch as a sensor and does not need a magnet. I reads off a hole 10mm D x 6mm deep on the edge of a pulley. It works well.

If you can access MEW issue 147 Feb 2009 its all there. I made both circuits one for my mill and the other for my Super 7. My inverters are very old no display, so the addition of a read out has been useful. If you send me a PM with your email address I'll email you a copy. Ted

And the 24V control supply on my VFDs can source up to 20mA. Hard to tell what the current consumption would be for this thing but it sounds as if it may possible otherwise I'm sure I could figure something out.

Hi Richard,
If you don't mind bulding one yourself there is a design on my website that would do wat you want. There is an example of the hall effect gear tooth sensor mounted next to the bull wheel on an ML7 or S7 on the page showing sensor mounting examples. It can be used with sensors that privide between 1 and 99 pulses per rev. I designed it about 4 or 5 yers ago so it is possible the display and hall sensor may need to be replaced with alternatives as it was designed so long ago. (It was submitted to MEW but not accepted.) I use one of these on my Chester DB10G lathe.

Les.

I've seen a really nice little application of an Arduino that take into consideration the number of teeth your sensor sees for one revolution. This is modified in the program prior to downloading it to the Arduino. For the life of me I can't remember what website I saw this but it looked very useful for this type of application. If your interested I suggest that you search for Arduino RPM indicator as a good starting point.

John

Edit... Dyslexia is a wonderful thing! 😨

Edited By Johnboy25 on 11/02/2015 14:15:51

Edited By Johnboy25 on 11/02/2015 14:16:21

I just keep a printed table of spindle speeds vs. pulley settings on the wall behind the lathe and then multiply in my head by the % calibration on the pot. Good enough for me.

Russell

I picked up a reference to this from another forum which I thought I'd play with on my ML7.

The inverter I have has a frequency readout which directly relates to motor speed and thence, by a series of graphs to spindle speed for each of the pulley ratios. I have the graphs on a single sheet pinned to the wall next to the inverter.

I do understand though that it's less fuss to have a direct tachometer readout and hence the eBay purchase.

Most of the time, actually, I just crank up the speed a say "yeah - that looks about right" (to no one in particular).

I fitted one of these to my ML7RB and it works just great:-**LINK**

I was just looking for a tachometer ,for a different application, and found a lot of cheap, hand held, infra-red jobs. Ideal for my use. Would these not do the job ? Only about £10.