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Tachometer.

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macmarch12/06/2011 18:26:22
147 forum posts
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Having read with great interest the 'cheap' tachometer for a lathe, I went out to obtain the bicycle computer. Big shops, little shops even Halfords, not to be found anywhere. I did find one that had an RPM option. This only went to 200 rpm and was a bit pricey. On rereading the article it seems that the author has had it running for a couple of years! I have located a laser version from a well known book site on the web at £14.99. A magnetic pickup type is, however easier to mount. Does anyone know where we can get such a computer of this type?

 
Michael Cox 112/06/2011 19:24:29
550 forum posts
27 photos
Hi Ray,
The AS 11G cycle computer is stocked by the DIY/Home/Garden chain Wilkinsons. They were in stock at my local branch (Widnes) last week.
Mike
Stephen Benson12/06/2011 19:27:15
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203 forum posts
69 photos
Well I bought a hand held laser one from Ebay direct from China for under a tenner including shipping works well enough although I found I had to colour the surrounding area black before it was reliable. So you need to fix the reflective tape (supplied) on a dark background maybe more expensive onse are less fussy.
 
 

Edited By Stephen Benson on 12/06/2011 19:28:16

macmarch12/06/2011 19:36:13
147 forum posts
1 photos
Thank you Michael. The number of times I find things at Wilkinsons. I buy tubs of dried insects for the birds. They cost less than a quarter of anybody else's offerings.
I'll get there tomorrow..
 

Michael Cox 112/06/2011 19:41:01
550 forum posts
27 photos
Hi Ray,
One other point. Despite what it says in the instructions the AS11G works accurately (checked against hand held reflective sensor tachometer) up to well over 2000rpm.
Mike
macmarch12/06/2011 20:11:12
147 forum posts
1 photos
I have just checked wilkinsons online and according to their website they don't stock the AS 11G. I'll go there tomorrow as I have found that they don't log all they sell in the shop online. I find this with Argos as well.
 
cheers
 

Peter G. Shaw12/06/2011 21:14:20
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1421 forum posts
44 photos
I assume you are referring to the article in the current issue of MEW (MEW 178).
 
To my mind, this article is much too brief: it does not give sufficient information, nor does it explain about calibration.
 
I have used a Halfords 9 Function Cycle Computer, similar to the one in the article, as a cheap rev counter to a) check my fixed lathe speeds; and b) to roughly calibrate my continuously variable milling machine.
 
My cycle computer can be set to either mph ot kph: I used kph since this gives a higher range of numbers. Furthermore, it has to be set with a particular constant representing the wheel size to which it is connected as the computer converts the pulses into distance and then by means of the internal crystal oscillator, calculates either the mph or kph. In order to get a direct reading of rpm, it is necessary to determine the figure which will give this direct reading: on mine this turns out to be 1667. The computer then gives a kph reading, eg 21.5 which can be taken as 215 rpm.
 
On my computer, this actually gives a maximum speed (rpm) of 99kph (990 rpm) which is a bit low, however by reducing the constant to 167, the range is multiplied by 10 giving a theoretical maximum of 9900 rpm.
 
To sum up, these are the ranges on my computer:
 
Calibration set to 1667.
up to 3.2 kph (32rpm): Too slow for the computer.
3.2kph (32rpm) - 29.9 kph (299rpm): Accurate readings
30kph(300rpm) - 59.5kph (595rpm): Computer rounds to nearest 0.5 kph (5rpm)
60kph (600rpm) - 75kph(750rpm): Computer rounds to nearest 1kph (10rpm)
 
Above 75kph the display stopped working correctly - obviously too fast! After all, it is a CYCLE computer and any bike travelling at 75kph is......
 
Reducing the calibration to 167 means that the upper limit theoretically becomes 7500 rpm, but I only managed to test to just over 1000 rpm.
 
Presumably there is an upper limit due to the time between pulses from the magnet becoming too short. I suppose that using gearing could lengthen this time interval and this extend the range - providing one could accept the rounding to the nearest 5 or 10 rpm.
 
Although there are limitations with this method, it does work and within those limitations can be quite useful in a rough and ready sense. For my purposes, it was enough to check the lathe (9 fixed speeds) and then to roughly calibrate the milling machine control knob.
 
Regards,
 
Peter G. Shaw
 
 
blowlamp12/06/2011 21:37:22
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1614 forum posts
105 photos
Peter.
The cycle computer used in the article has an RPM feature and so doesn't need calibrating.
 
Martin.
Gone Away12/06/2011 22:36:16
829 forum posts
1 photos
I haven't read the article but there are potential cheap alternatives (that read RPM) on eBay such as:Cheep eBay Bicycle Computer
The Merry Miller13/06/2011 13:04:34
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484 forum posts
97 photos
 
Looking in my Wilko's this morning they had two versions on display, the standard version for £7.97 and for those of you who haven't got wireless, the wireless version for £15.
 
 
 
Peter G. Shaw13/06/2011 13:26:45
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1421 forum posts
44 photos
blowlamp/Martin,
 
Fair comment, I had not realised that.
 
In defence, there have been other tachometer designs published using standard cycle computers and I just assumed that this was the case and that Cox's design was just another one of these. Hence I had effectively written off the idea.
 
I think I might now have to go and have a fresh look at it.
 
Apologies to all concerned.
 
Regards,
 
Peter G. Shaw
Michael Cox 113/06/2011 14:11:34
550 forum posts
27 photos
Posted by Sid Herbage on 12/06/2011 22:36:16:
I haven't read the article but there are potential cheap alternatives (that read RPM) on eBay such as:Cheep eBay Bicycle Computer
This looks to be a possible alternative to the AS11G that I used. However, it is not possible to know what rpm range it would cover without buying one.
Mike
harold13/06/2011 14:30:27
35 forum posts
Up to 3600 rpm:
HTH
Gone Away13/06/2011 14:35:20
829 forum posts
1 photos
Posted by Michael Cox 1 on 13/06/2011 14:11:34:

This looks to be a possible alternative to the AS11G that I used. However, it is not possible to know what rpm range it would cover without buying one.

I just bought one so I guess I'll find out. Not a huge investment anyway ($3.43 including shipping).

HasBean13/06/2011 17:44:13
141 forum posts
32 photos
Slighly off topic, I purchased a non contact digital tachometer from China off the Bay of E for less than a tenner all in (strange how the likes of *achine *art charge a minimum of £30 postage to Jersey )
As it was it was less than a tenner well spent
 
From the chart on the side of my mil/drill it would appear that THREE of the belt speed positionings were transposed! For those of you who have one it might be worth a quick check.
I then tried it on my lathe, couple of rpm out on the lower ranges up to 50+ on the higher so not bad in all.
 
The idea was to check the speeds so I'd have a reasonable chance of getting the nearest appropriate speed for the operation in hand.
 
At the very least it gave me an excuse to be in the garage on a rainy Saturday afternoon
 
 
Roderick Jenkins13/06/2011 17:45:11
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2183 forum posts
608 photos
Lord knows what a cyclist wants to know RPM for. They are usually interested in cadence, that is, the number of revolutions that the chainwheel makes and thus the rate at which their legs are going round. This is usually taken from a second sensor on one of the cranks. You can't calculate cadence from the wheel RPM without also knowing which gear you are in. I suppose it allows them to quote an extra function on the packaging (up to 11!). Anyway, an interesting idea - I'm off to Wilkos.
 
Rod
Roderick Jenkins15/06/2011 21:01:27
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2183 forum posts
608 photos
AS11G purchased from Wilkos in Basingstoke lunch time today. Fitted to my Sharp mill (with inverter drive) in about 10 mins. Thank you Michael.
 
Regards,
 
Rod

Edited By Roderick Jenkins on 15/06/2011 21:03:10

macmarch15/06/2011 21:13:08
147 forum posts
1 photos
Went to Wilko today. No stock! I asked if they would be getting any more in. Their answer was "We are expecting a delivery next Wednesday, would you like me to put you on the
reserve list?" Apparently I'm not the only one to order this. ho hum.
 
cheers
ray

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