Formulae for Cycle Computer to Read Metres per Minute

[velocette]

Hi to all "Mad Modders" I have been restoring a vertical bandsaw and have added a worm drive gearbox to the drive system
for metal cutting.

Planning on adding a variable speed DC motor to be able to cut at between 20 metres a minute to 70 metres a minute.

what is the formulae to find the circumference to program into a "Cycle Computer" to be able to display Metres Min as
Km Hr  I. E. 30 Km hr or 3 decimal 0

Eric

[andyf]

Hi Eric,

I've done a few sums, and I don't think a bike speedo would work in this application. If the wheels around which the saw blade runs are 200mm diameter, you would need to program the cycle computer to think its actuating magnet was mounted on a bike wheel of an improbable 3.333 metres diameter. That is probably well over the permissible range of inputs.

Dividing by 10 (so a reading of 6.7 kph represents a blade speed of 67 metres per sec) would involve programing in a wheel of 0.333 metre diameter, which is probably under the permissible range.

Not would setting the thing to read in mph rather than kph help much, either. You would need to input diameters of  2.07 or 0.27 metres, which are still probably too large/too small.

Of course, your bandsaw wheels may be not be 200mm, but unless they are of substantially different size it probably still won't work.

Warning: my mathematics are not guaranteed to be any more accurate than they were when I was at school 50 years ago!

Andy

[DMIOM]

Eric,

Further to the issue which Andy has highlighted concerning the necessary input being outside what a standard velocimeter might accept - can you let us know what the limits are for your unit?
 
If if can't work as-is because of these limits, you may be able to bring it within range by using two or more magnets, so you have an electronic gearing.

Dave

[BillTodd]

Dividing by 10 (so a reading of 6.7 kph represents a blade speed of 67 metres per sec) would involve programing in a wheel of 0.333 metre diameter, which is probably under the permissible range.

How about adding another magnet?  i.e. two pulses per rev should allow a wheel of 0.66m or about 26"

[PekkaNF]

See

halfway down to this page:
http://andysmachines.weebly.com/dore-westbury-mill.html

This page:
http://mikesworkshop.weebly.com/low-cost-tachometer.html

http://psas.pdx.edu/ParaffinSpinCasting/BC500tachometer.pdf

I have one cheap one as lathe tachometer, just to check, not in permanent installation and my bike computer did not care how bogus unists i fed to it. I didn't bump into build in input value limits. Mine had reed-switch, but worked fine whatever I threw at it (under 1500 rmp). I have heard that some can't go very high rpm.

Pekka

[andyf]

Pekka, Mike's web page, mine and the paraffin casting one all relate to using the speedo as a tachometer, which is relatively easy as long as you aren't trying to measure such a high rpm that the reading is outside the kph range of the speedo (usually 0-99.9kph).

Dave and Bill - yes, extra magnets would do it. Two would halve the size of the imaginary bike wheel, but three would divide its size by 3 and so produce a result which is more within the general range of bike wheel sizes.

Eric, much depends on the actual diameter/circumference of your bandsaw's wheels. If you can come back with figures, I'm sure we can devise a suitable number of magnets and an appropriate circumference to program into the speedo.

Andy

[velocette]

Hi thank for all the valuable input this has my old grey head head on overtime.

I now realise that I have not supplied enough information by trying to over simplify with my posting

((Wheel diameter is 360 mm or 1.131 circumference  @ 30 metres minute = 26.525 RPM ))

Right now back to the problem (( Have taken a break to watch the sun bring a new late autumn day to Hawkes Bay ))

No success so far with the Math or to put it clearer not so much the math as my understanding of it.

Cycle Computer is a "CatEye Velo 5" for tyre size from 1020mm circ to 2333mm circ.

This works well as a Tachometer fitted to my drill press programmed to 1667 mm circ wheel.

This repeats the original post hopefully to save you from having to skip back to it.

""Hi to all I have been restoring a vertical bandsaw and have added a worm drive gearbox to the drive system
for metal cutting.

Planning on adding a variable speed DC motor to be able to cut at between 20 metres a minute to 70 metres a minute.

what is the formulae to find the circumference to program into a "Cycle Computer" to be able to display Metres Min as
Km Hr  I. E. 30 Km hr or 3 decimal 0""

Thank You all again for your help and advice will keep banging away at the problem

Eric

[andyf]

Hi Eric,

If the bandsaw wheel circumference is 1.131 m, then that is within the range of the speedo and you could program it in. The readout will then show the blade speed in kph; the blade will be moving at the same speed as a bike with 1.131m circumference wheels. If you think about it, the bandsaw wheels equate to the bike wheels and the blade equates to the road, the only difference being that rather than the road being stationary and the bike moving, the blade is moving and the saw is standing still.

But you want metres per minute, rather than kilometres per hour. 1 kph =1000 metres per hour, = 1000/60 metres per minute, which is 16.67 metres per minute. This means you would have to program in an imaginary bike wheel of 1.131 x 16.67 = 21.84 metres circumference. You can't do that; it's nearly 10 times what the speedo can handle.

However, if you program in one-tenth of 21.84 metres i.e 2184mm, that is within the speedo's range, and a reading of (say) 6.7 kph will equate to a blade speed of 67 metres per minute - just ignore the decimal point and imagine it says metres per minute rather than kph.

QED (I think) but you could check with a stopwatch. Paint a line on the bandsaw blade, set it going at a reasonably slow speed and note the speedo reading. Count the number of times the line comes round in one minute. Then, knowing the length of the blade, you should be able to work out how many metres of blade have gone past in that minute, which should coincide with what the speedo said.

Andy


PS Only one magnet on the wheel.

PPS I suppose that the above calculations are a bit out, because bike wheels are sized by the rim circumference and the speedo must make an allowance for the fact that there is a tyre present. If that adds say 50mm to the diameter of the bike wheel, it will add about 160mm to the circumference. It might be an idea to program in 2184 + 160 = 2344mm; that's slightly over the speedo's max, so 2333 mm would have to be used, giving a very minimal error.

[75Plus]

A couple of thoughts, since most of our counting is base 10 I use it in calculating the speed of my band saws. I too have what is known, on this side of the pond, as a 14" (360mm) band saw. Ten revolutions of the wheel moves the blade 11.3 meters. One hundred revs moves it 113 . I have made a chart, which I have laminated and attached to the blade cover, that shows the blade speed in ten RPM increments. Mine is actually in FPM instead of MPM but the idea is the same.

If you want to get very precise you can measure the distance between the blades which will give you the actual size of the wheels although plus or minus 5 or 6mm does not change the speed that much.

I too have been around the sun a few times, 78 at last count, and never got proficient in using the metric system however I can get by.

Joe