Lathe tachometer

Yes, they would work in many cases but you (it) need to be able to see clearly some singular feature on the spindle. Things like chucks, cutters etc often have 2, 3 or more features around the circumference and can confuse them so they read high by that factor.

A remote display allows you to mount the sensor where it works best (see Ian's and Norman's above) and the display where you can see it. If it's wired in and powered by the machine supply, it's there when you look up at it - it makes sense for my machines but it's also a personal choice.

I bought a "non contact Tach Tool RPM Handheld Digital Photo Laser Tachometer Meter Tester UK" from Ebay for £9.99. Had an RPM range of 2.5 to 99,999 and could be operated from 50 to 500mm from the rotating object - ran on 9v battery. Didn't really expect much for this price - but turned out to be excellent and versatile. Just stick the reflective strip to the lathe chuck and shine the laser on it. I used it to calibrate the variable speed control on my Chester Mini Multi Lathe/Mill. The RPM on the tachometer was within a few RPM of the stated maximum revs of the Chester lathe - so that was a good starting point. The Chester speed control knob had no markings so I replaced it with a guitar volume knob with 0-10 markings and made a chart up of RPM for each of the 10 positions for both the lathe and mill heads - this is good enough for my use and the Tachometer can be used for many other applications.

When I converted my S7 to VFD I developed the attached speed table , which I find suits my needs fine . The 800rpm column is highlighted because this is the minimum motor speed recommended by Newton Tesla for continuous operation.

Trevor

Hi Graeme,

I have just purchased a similar tachometer. However, connections on the 5 pin socket on the display are in Chinese. Did you get any information in English? How did you wire it?

Mike

Mike - this should help - I have wired up mine using this diagram.

Rgds,

John

Which translates to:

Wire 1 : +ve supply and brown wire from sensor.

Wire 2 -ve supply

Wire 3 Blue wire from sensor

Wire 4 Nothing

Wire 5 Black line from sensor.

I used an 18 v printer power supply for mine.

Rgds,

John

Thanks John. That is just what I needed.

Mike

I use a Wilko bike computer, £5 or less, comes with the magnet as well

So do I on my mill but it does not respond at the highest speed. I am planning to upgrade my mill soon and I am going to fit a better tachometer. The new tachometer is no more expensive than the old Wilko bike computer and will provide a much easier to read display.

Mike

There was good article in MEW a couple of years back. One version was to fit a ABS brake sensor adjacent to the Myford super 7 bull wheel it having 60 teeth, feeding its output to a LED display. I did this to my lathe and it works very well. The other method was to make a disc with 60 slots and use an optical slot coupler again to counter slots. I did the this to my mill using a piece of printed circuit board as the base for the 60 slots. Also I've seen on the ebay a unit for doing much as what I have just mentioned for about £6.John

Lots of good advice in earlier replies, I made mine for the lathe many years ago using a refective opto sensor and a PIC, just painted half the pulley black and half white. It's not that easy! Found that black gloss paint reflects just as well as white, so had to use blackboard paint, then found if it gets a film of oil that reflects as well, so every now and again I have to give it a wipe. I know, not hard, but on the mill I used a magnetic sensor. Just to make life interesting the pulley on the mill doesn't go at the same speed as the spindle, so I used an Arduino and LED display, more interesting than just buying it anyway.

I calibrated the numbered dial on my speed control by measuring the rpm by using a longish piece of all thread, a loose nut and a stop watch.

Measure and mark a suitable length on the all thread using a marker pen then time the nut running between the marks. The all thread is gripped in the chuck and for safeties sake the opposite end is in the tail stock barrel The lathe is run with the pointer on the various marks and the nut held by hand to run it down the thread whils timing. A chart can then be made.

Simples, may not be super accurate but good enough for setting a lathe speed.

As with several of the previous posts I have fitted an inverter to my Myford S7 but wanted a read out of RPM. So I purchased the cheap tachometer off eBay used by others.

The Hall sensor provided is far too big to fit anywhere in the belt drive housing so I replaced it with an A3144 chip housed in a short strip of carbon tube.

I noticed that the large belt pulley had a small hole drilled opposite the oiling nipple so I used this to hold a small magnet (about 8mm diam). The thin carbon tube holding the sensor is fixed to the housing adjacent to the pulley with a small clamp. All works fine.

But I am having second thoughts!!

How critical is that small hole to the balance of the spindle assembly? Am I likely to put undue stresses on the rear bearings?

Any views?

Peter

Why bother with a tacho?

You know the speed range of any pulley (or gear) setting, use a 10 turn knob on the speed pot and you have a 1/100 scale available. A lot of lathe users manage quite well with the fixed speed steps available without VFD.

p.s. I have a VFD on all my machines just to save some replies/comments.

As Kwil and others have said, why bother with a tacho.

My (geared head, nine speed) lathe has a VFD and I spent a couple of hours fitting one of the eBay type digital tachometers.

After fitting it about one year ago, I checked that it worked and can honestly say that since then have never even glanced at it!

I run the lathe in the one gear ratio that it is quietest at and use the unmarked pot knob to set the speed so that it feels right for the job. The only time I change ratio is for very large diameter work or screwcutting purposes.

Ian P

As mentioned a lathe tacho is not that useful in my view, however, this video shows a simple and inexpensive installation which a friend has used for his new mill;

https://www.youtube.com/watch?v=Y4WKVvz8JxU

Why bother with a tacho?

A number of reasons, and respecting contrary opinions, they come down to personal preferences.

I use the tacho on my mill all the time. I have developed a mental picture of what speed to start with for any particular combination of tool, metal or task. I felt I was lacking that info on the lathe.

However, and primarily, I fancied the project.

But back to my main question - how critical is the balance hole on the large pulley.

Peter

As it costs less than a tenner to fit a tacho it is not the end of the world if it is not as useful as you first anticipated. It seems that the speed is of interest as people have other strategies to determine the speed. With the wide speed range possible with a flux vector VFD you are going to check somehow what your speed is. Just going by feel requires some experience or you could be well away from optimum or safe RPM to not compromise tool life. I still check which pulleys I am using before setting my speed as I feel that adequate power is required as well as the right speed and if you reduce speed using the VFD then you also reduce the available power.

Mike

My new SC4 has a tacho built in. I fitted one to my mini lathe.

In practice listen and feel how the cut is going and look at the surface finish, and adjust the speed to get best results.

There are so many variables (tool style, tool material, tool wear, tool height, tool angle, lathe rigidity, type of bearings, adjustment of bearings, toolpost style, workpiece size, shape, overhang, diameter, material, use or not of coolant, cutting fluid, depth of cut, feedrate, your own style of working, even whether the lathe is cool or has been run for and hour and warmed up) there is no magic number for any job unless you have a controlled production environment where you control as many of these variables as possible. In a home shop, all a tacho can do is get you in the right ballpark, or repeat a setting you found worked earlier.

In the past, most people only had six speeds to choose from and got on fine, having minute control over our lathe speeds convinces us that getting the speed right to within a few rpm is essential, just because we can.

Neil