What's new
What's new

Advice needed; Circa 1940s tachometer

Darren McCarley

Cast Iron
Joined
Nov 24, 2017
Location
DFW
Would like to get yalls input. I'm restoring an old Wade 8a lathe. (Circa 1943) I'll be running it with a VFD and 3hp motor, jack shaft and permanent belt location(s). The VFD will be used to vary the RPM. While I'm primarily interested function, I do like to keep things true to the period if possible. The image attached is a later Wade tachometer. They are nearly impossible to find and expensive when they do surface. So, what are yalls thoughts on an alternative "period correct looking" tach? I'm not afraid of hot rodding the thing to use an old airplane tach, or even old Mac truck tach.... thoughts?
First pic - Wade tach (not mine) Thanks Wade8a.com!
Second pic - my Wade 8a before refurb/clean up.

69615.jpg


 
If you want to be authentic: L S Starrett Tachometer Model 104 High Speed RPM Indicator Gauge - Google Search

Modern contact & non contact tachs: electronic hand held tachometer - Google Search

Most VFDs have a readout giving a number. Use the hand held tach to get a specific RPM then enter that on a chart with what the VFD is showing.

My 1953 MG-TD had a mechanical tach run off a gearbox on the back of the generator using a speedometer type cable. A truck tach that ran 1:1 off the crankshaft would make life easy. My Cessna 150 also had a mechanical tach with the head that worked just like a speedometer.

Flying along over South Carolina one day, looking at the scenery, and the tach squalled and pegged. Scared me half to death. Some dirt or dust had gotten between the rotating magnet and the aluminum cup attached to the needle.

A recycled truck tach is likely to be cheaper than an aircraft one. A speedometer would work if you worked out the ratio of rpm to numbers on the speedo with a gearbox or belt and pulleys.

Paul
 
The underlying question in determining what type of tachometer to use is whether the lathe spindle rotation is reversible. If the spindle rotation is reversible, a mechanical tachometer will not work in one of the two directions and could have its movement damaged by rolling it in reverse rotation to what it was designed for.

This leaves an electrical tachometer. I believe Weston Instrument made some electrically driven tachometers which used small tachometer generators. These were old-time instruments with a "quadrant" shaped housing. A more modern approach is to use an electronic tachometer. If you can find a "Hall effect switch" (also known as a magnetic pickup) which works with the number of teeth on one of the gears on the lathe spindle, this would generate an input signal to the tachometer. Getting an old-time looking tach with this sort of movement might take some doing. Automotive tachometers and motorcycle tachometers will likely have too wide a range of rpm (doubtful you will need a tach reading to a high end of 7 or 8,000 rpm). Diesel engine or light aircraft tachometers might be more in the range/high end rpm your lathe will run at. Aircraft instruments are usually black face/white graduations and numbers, and are usually designed for a 4-screw panel mounting.


Some tachometers worked using ignition pulses as input. If you can find an electronic aircraft tachometer that would be half the battle. The gear teeth "cutting the lines of flux" from the magnetic pickup generate an alternating current. The frequency of the current (and likely the voltage produced) is a function of number of gear teeth and rpm. The tachometer may well be a frequency meter, or it may work on voltage produced. My guess is the magnetic pickup on an aircraft tachometer may well use the teeth on the ring gear (on the engine flywheel), so finding an appropriate magnetic pickup for the smaller number of gear teeth on the lathe spindle.

Older electric tachometers used small generators driven by the shaft whose rpm was being measured. If the generator produced direct current, then the tach might only read in one direction of shaft rotation. My suggestion is to focus on finding a tachometer which works using a "Hall effect switch" and see if you can get something compatible with the number of gear teeth/spindle rpm range. Back in 1979-80, I bought a new International Harvester Scout with the Nissan turbo diesel engine. I inquired about a tachometer and was given some literature about a steering column mounted add-on tach. This instrument worked off the engine's alternator, using either the frequency of the AC current or voltage produced at the stator connections (before the alternator output went to the diodes) as input. The tachometer was adjustable for a fairly wide range of rpm since alternator pulley size and engine rpm ranges would vary with different vehicles. This particular tach would be bi-directional but getting the right AC input current for it from your lathe would be a very difficult proposition, IMHO.

I'd contact Faria Instruments in Connecticut. They build automotive and marine instruments including tachometers. They might have something that would work on your lathe application.
 
The tachometer shown is an electric one. The first thing to do is see if the original generator is still there, assuming that your lathe also had a tach. Trying to enlarge the picture goes to a site selling all kinds of junk. Weston made AC and DC generators. An AC tach would work in both directions, Rivett used a DC generator with an instrument rectifier like multimeters used. I tried to calibrate a replacement meter for one but the meter he got was too low resistance for the rectifier and the whole thing degenerated into a mess and he went away mad.

Bill
 
Bill/Joe, great points and advice. Thank you. Just the kinds of questions and ideas I need to get my imagination working.
  • My lathe did not have a tach of any kind and all electrical has been "lost to the ages".
  • I am planning on using a VFD with a tach output signal. While still a VFD novice, I've seen VFDs that can take a parameter of pulley size and compute the output RPM. I assume some kind of conversion will be needed, analog and/or digital. However, I'll need to identify the tach first, then figure out the integration.
  • I'm moderately aware of hall effect signal generators, e.g. treadmill tachs, but am hoping the VFD can handle this aspect.

Good call on the car tachometer being too high. Although, it may be fun to put on a dummy tach and use it as a prank for my machinist friends. "8500 rpm? Sure, isn't that what HSS is designed for?" :)

I have a dream of an old US Navy plane tach. (Navy stamped lathe, needs a Navy tach....)
Something like this from
AeroAntique
IMG_6494_1024x1024.JPG
 
Most automotive speedometers show 60 mph at 1000 rpm. I have a cable drive military tachometer that I believe goes to 3500 - 4000 rpm . If you have any interest I will take a picture of it . Bill
 
Darren:

I think a Hall effect type of pickup to drive a tachometer is your best approach. This will let you utilize one of the gears on the spindle and the magnetic pickup is small and easily mounted.

I'd check the Faria website. They offer industrial tachs which work off alternators (for diesel engines) and off Hall effect pickups. These tachs may have what are known as "DIP switches" on them to allow configuring the tach for different ranges of input signals. Faria is a US manufacturer with an engineering department, and should be able to match you up with a tach and Hall effect sending unit. You can get the tach with a black face/white graduations & numerals. Maybe not so vintage looking as you'd like, but a working instrument and sending unit.

I did look on eBay and there are a variety of Weston electric tachometers and generators for them. The problem is this: no one is offering a tachometer and generator as a working package. Without a Weston tech manual or some specifications on the tachometer and generator, the odds are pretty good that you could wind up with a generator producing a current incompatible with the tach. You would need either a bi-directional tach/DC generator, or one-directional tach/AC generator. Finding that combination on eBay with any degree of certainty as to compatability of the tach and generator sounds like a crap shoot to me. The other downside to the tach generator is you'd have to find somewhere to mount it and drive it off the lathe spindle drive at correct speed to match the tach. Could mean using another belt drive.

The VFD can be fitted with a digital tachometer. I know this is not what you are wanting, but it is likely the most expedient thing. If you find out what kind of signal the VFD produces for the digital tach, maybe Faria can match up one of their analog tachs to run off it.
 
Most VFDs will convert to RPM using a factor. So, if you use the same drive system at all speeds (hard to do), you can get a correct reading.

If you do not mind a bit of electronic fiddling, you can use an LM2907 integrated circuit with a hall effect or other pulse output sensor, to drive an analog meter. works well, and could preserve the "look" you want.

Data sheet is here http://www.ti.com/lit/an/snaa088/snaa088.pdf

It seems to be still available, even though it is an analog device.
 
I added a tach to my 14 1/5" South Bend lathe by mounting a magnetic pickup to register the teeth on the large back gear. Red Lion made a digital tach that could be set to read right with any number of teeth and another that used the line frequency as a reference for considerably less money. The gear had 72 teeth when 60 would be the required number. I just lied to the tach, telling it that the line was 50 cycles and it read correctly. The pickup is not a Hall effect, only a magnet with a coil around it. The tach counts pulses and it doesn't care which direction it is turning.

Here is a link

Panel Meters | Best Panel Meters | Red Lion

Bill
 
Last edited:
Great feedback! Thank you all. I'm on business travel this week and can't stand posting from my phone. Will review, research, and respond once I get back.
 
Just for now and future reference for someone, here's an original Wade 8A tach.
 

Attachments

  • DSCN5622.jpg
    DSCN5622.jpg
    93.8 KB · Views: 96
  • DSCN5620.jpg
    DSCN5620.jpg
    95.5 KB · Views: 93
Just for now and future reference for someone, here's an original Wade 8A tach.

That is a DC meter with an instrument rectifier. It could be fed by either an AC or DC generator. AC meters like a Simpson 260 have some non linearity in their scales but nowhere near as much as the Wade, implying a meter movement considerably less sensitive than the 50 microampere meters in 260s or a rather low voltage generator. A 260 has a similar non linearity in the 2.5 volt full scale range and much less in the higher ones.

Either they used a rectifier on a DC system to make it read in both directions or it had an AC generator. The presence of a capacitor implies that it is AC. A small permanent magnet motor can be used for a tach generator and if you don't mind the needle going backwards in reverse, you can have a linear scale. If the meter has a scale with the right number of divisions, you can sand the numbers off and replace with Press Type numbers. If you really want it authentic, you can photograph a scale with the Wade name and have a silk screen made. I made a lot of locomotive speedometer dials that way, but the setup cost was spread over a number of parts.

Bill

Addendum- I just talked to Jack Sherrer, who knows more about meters than I ever will about an instrument rectifier. He says they are copper oxide and all have aged by now. He says to use 4 regular diodes in a bridge. Probably the best would be so called hot carrier diodes for their low forward breakover voltage.
 
Last edited:
Interesting Bill.
The lathe itself is AC and I run it from a VFD.
The tach doesn't read correctly, it only goes up to maybe 300 in open belt.
Not sure if the VFD has anything to do with that.
Here are pages from the manual that show the V.S. Drive Speed Indicator (359).
 

Attachments

  • IMG_4143.jpg
    IMG_4143.jpg
    85.5 KB · Views: 65
  • IMG_4144.jpg
    IMG_4144.jpg
    83.6 KB · Views: 48
Interesting Bill.
The lathe itself is AC and I run it from a VFD.
The tach doesn't read correctly, it only goes up to maybe 300 in open belt.
Not sure if the VFD has anything to do with that.
Here are pages from the manual that show the V.S. Drive Speed Indicator (359).

I would first guess that the generator is AC with a permanent magnet field, which may have weakened or more likely, the capacitor is bad. Capacitors from that era often fail. Some radio collectors automatically replace them all. The generator appears to be belt driven and the spindle motor and drive would not have any influence.
 
Agree with 9100.

The only issue with the replacement rectifiers is tha they may not give the same non-linearity, and part of the scale may be "off". It is possible that if you can find some germanium diodes from way back, those might do a bit better, as they are low forward voltage, and also tend to have a "soft knee" characteristic similar to a copper oxide rectifier.

A 1N60 type would be a possibility, you would need 4. (now I AM showing my age)
 
About the only way to calibrate this unit is to drive the generator at known speeds and look at the needle. The generator output voltage is probably proportional to speed and the frequency is also varying so the coupling of the capacitor is changing. Then there is the generator waveform, likely to be sort of sinusoidal, but with a high harmonic content that couples through the capacitor more than the fundamental.

The current vs voltage curve of typical solid state diodes is logarithmic which would produce the non linearity shown. The capacitor would exaggerate this effect, having a high impedance at low frequencies, lower at higher ones so it would have little effect at the top end.

Bill
 
My 1953 MG-TD had a mechanical tach run off a gearbox on the back of the generator using a speedometer type cable. A truck tach that ran 1:1 off the crankshaft would make life easy. My Cessna 150 also had a mechanical tach with the head that worked just like a speedometer.
Jaeger was the make, similar rig, distributor shaft mid-point on the '59 TR-3.

Not hard to find an inexpensive mechanism. I have several. NOS S-W Marine/Diesel tacho @ mebbe $30 - $40 per, ebay. Quartet mill vertical spindle already has a drive, was OEM'ed without the option being populated with the actual tacho. Alzmetall AB/5s another planned host.

Those have 3.000, 3.500. 4.000 top-end where common gasoline engine ones go 6,000, 8,000, 2-wheeelers way higher, yet, redline at "Insanity, Squared".

But I said "mechanism".

You'd perhaps want a different size and look to the dial and hands? Hand is easy. Might have to metalphoto DIY an affordable dial. And re-calibrate. Not hard. Can be right tedious, though. Last one I did, a 1938 Packard Hearse S-W speedo the donor, and it wasn't all that old, yet.

How much joy is this going to give, second week of use and thereafter, to get to "period correct"?
 
Some great advice in here. Thank you all. First order of operation is to get the lathe working with the VFD. Ill use that read out until such time as I come up with a hall effect solution and a tach.
 








 
Back
Top