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Spindle Speed

Spindle Speed Measurement using Variable Speed Motor Drive

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chrisJward03/02/2010 09:29:23
3 forum posts
2 photos
I am using a Variable Speed motor drive on my Milling Machine and Lathe, these were fitted after purchase etc.
I wondered if anyone knows of an easy / economical way of measuring the spindle speed to check Cutter speeds etc. RPM measurement.
Any help would be greatly appreciated, Best rgds Chris
Eric Cox03/02/2010 09:55:38
526 forum posts
35 photos
Have a look at this.

Edited By David Clark 1 on 03/02/2010 12:23:53

Ian S C03/02/2010 11:30:52
7468 forum posts
230 photos
There was a artical in Model Engineer 6 Nov 1998 by Bill Stevenson in USA. His system is to use strobe discs (theres a page of them to photo copy),they work with 50cycle AC incandesent lamps.One site on the web also gives a single strobe disc that gives a speed range from 300rpm to 3000rpm in 10 stages. Ian S C

Edited By Ian S C on 03/02/2010 11:32:19

KWIL03/02/2010 12:48:41
3334 forum posts
63 photos
I copied these discs as a check on the inverter set up on my machines and they are a good way of checking, I also have a separate commercial RPM strobe machine now should I need it.
mgj03/02/2010 18:45:53
1008 forum posts
14 photos
Go to a model aeroplane shop. They do hand held battery powered tachos for £10-£15.
They work off the pulse made by the blade going past, so set the tacho for 2 blades and add 2 dabs of white paint. 
John Wood104/02/2010 15:00:27
116 forum posts
I have a similar situation with my retro-fit speed controller and have done approximate calculations which seem to get me by. Now Meryrick has come up with yet another neat solution which at that price has got to be worth a punt, so thanks to you, once again. I have gleaned so much information from Meryrick from various posts that I wish he lived next door! How useful would that be?
All the best,  John
chrisJward04/02/2010 15:55:28
3 forum posts
2 photos
Thank you to all for the information, I have follwed the ebay link to purchase a small tacho if it turns up and its any good I'll be very pleased indeed.
I have been really surprised and very pleased with the results. A good forum thanks
Best rgds CHRIS 
mgj04/02/2010 18:22:36
1008 forum posts
14 photos
John if you are looking at those tachos, just check the range at which the thing works. The engine ones are I would imagine optimised for 9-14000rpm. Bit quick for us.
However the helicopter  boys use more or less the same models ones to check rotor head speed, and there one is talking of hundreds of RPM.which is more helpful. 
And there are combination ones that will do both.
Ian S C04/02/2010 22:19:52
7468 forum posts
230 photos
As far as the strobe discs go there is a site on Google, put in "tony Jeffrees Model Engineering Pages" and look through that stuff. ypu'll find a lot of other stuff as well.The model aeroplane tacho sounds good, don't know if they are available here, must have a look. Ian S C
Les Jones 104/02/2010 22:50:37
2187 forum posts
150 photos
Hi all,
            I have a program that someone sent to me that is designed to produce the pattern for optical encoders. This could be used to produce strobe disks that can be printed out.
If anyone woulld like a copy send me your email address on this forums message system and I will attach it to an email.
Gerald Weare04/02/2010 23:11:01
9 forum posts
7 photos
A hand-held unit is a good thing, I have a cheap chinese one that I use in my business, it works fine, but you have to hold it, not surprisingly.  The other alternative, which was discussed at length in the recent thread called "Tachometer Design", is a built-in add-on device.  There was a kit offered ny someone on the web about 8 years ago, based on a PIC microprocessor.  Don't know if it worked, but I built one for myself, see the pictures in my album.  This has the virtue of being always on, versus the handheld unit.
A last alternative, since you have a VFD, is to use the speed output(s) of the VFD.  All VFDs I have ever seen have an output that gives the speed it is set to, or the speed it is actually going at.  This may be an analogue output, typically 0-10V DC for 0 to max configured speed.  Hook it up to a 10V meter, either a digital panel meter with a couple of resistors or a moving coil if you can find one (you could just use a multimeter too, they are pretty cheap these days). Other VFDs may havea digital output of some kind, but usually they are designed for use with CNC.
Both of my VFDs (one is Allen-Bradley, the other AC Tech) can be set up to display the speed on the VFD front panel itself (but some VFDs have no front panel).  You can typically scale the output (by setting VFD parameters) so that it displays any desired relationship between motor frequency (Hz) and the actual spindle speed.  Unfortunately, if you have a gearbox, or change speed device, this changes when you change the belts or change gears.  The fancier VFDs can accept a gear number input and use the appropriate ratio, but not the simple ones like mine.
What I did on my machines is to make the little PIC tachometer for the mill, as I change gears a lot but it isn't easy to do, so I use the variable speed feature quite a bit.  However on the lathe, there are a lot of gears  and it is easy to change, so I only really use the VFD as a 3-phase converter.  I did create a spreadsheet to convert the VFD frequency to RPM for each gear, but I rarely use it.
One last thopught:  if you have no speed output, you will have a speed input to the VFD.  This is usually a potentiometer.  If you measure this voltage (again it usually goes from 0 to 10V), you can see what speed is being commanded.
Gerald Weare04/02/2010 23:17:36
9 forum posts
7 photos
Oh, if you want strobe patterns, the best thing is to do them in CAD, just takes a few minutes.  As an example, there is a JPG in my album of some I made when I was messing with the tacho.  I printed a bunch on a single sheet, to see what worked.  Funnily, one pulse per rev is as good as any...

Ian S C05/02/2010 10:21:33
7468 forum posts
230 photos
There is a circuit for an electronic tacho on Tony Jeffrees web site. Ian S C
KWIL05/02/2010 10:51:35
3334 forum posts
63 photos
Why this obsession for knowing the exact speed? Once you have established the range available a close approximation is usually sufficient, does it really matter if you are at 1075RPM rather than 1000? 
With a insert tipped tool you know if it is the right speed by the surface finish and the swarf, if it is HSS then most amateurs run them more slowly than commercial speeds to achieve a longer life.
As Gerald Weare says, there is a potentiometer on a VFD, if you use a 10:1 dial drive on this your can have 100 divisions over the range and it is linear. 46 is 920 RPM if the max is 2000 for instance.
Circlip05/02/2010 13:03:18
1218 forum posts
" Why this obsession for knowing the exact speed?"
  Cos it's a new talking point now that the gloss has worn off carbides and DRO's mate.
   Regards Ian.
chrisJward05/02/2010 13:47:02
3 forum posts
2 photos
I seem to have stirred up a bit of a hornets nest on the subject of Variable speed measurements.
I have a new question, does anyone have a Speeds & Feeds chart for Metric diameters and Feeds in Metric Not inches? this will save me converting the old Clarkson chart, or swapping my DRO from inch to metric! which can be dangerous when you forget to swap back again before putting the next cut on!!! Best rgds Chris 
KWIL05/02/2010 14:03:26
3334 forum posts
63 photos
Circlip,  What/who is a DRO's mate?
Circlip05/02/2010 16:02:03
1218 forum posts
" Why this obsession for knowing the exact speed?"
  Cos it's a new talking point now that the gloss has worn off carbides and DRO's, mate.
   Regards Ian.
   SORRY, I missed the comma. But I soppose a DRO's mate is a collet set??
chris stephens05/02/2010 16:56:48
1048 forum posts
1 photos
Hi Circlip, you old cynic,
The speed control question has only really shown itself since variable speed has become available at an affordable price for the home engineer. It does show a slight lack of confidence on the part of the machinist, who might not be quite as experienced as you. A newbie will look at a milling cutter speed /size chart and think an exact speed is important to them, because he manufacturers give a precise speed. Don't worry they will soon learn, given the right encouragement not cynicism.
As for DROs, on a Mill they are the most useful of accessories, note I say useful not vital .They can save a great amount of mental arithmetic and worry about leadscrew backlash. If one wishes to drill a hole in the middle of the work there is a function for that, used surprisingly often, by the way. If one wishes to drill holes in a circle, there is a function for that too. This saves getting out the Rotary table or doing any form of accurate and time consuming calculations or marking. The list of benefits for a DRO on a Mill goes on and on. I would be very reluctant to give mine up, but yes I could, and did, work  for years without one.
At this point I can feel a certain Gentleman in the West Country  preparing to make the case for DROs on a lathe, I would not doubt his views for a minute but I have not had the benefit of them fitted to any of my lathes, YET.
mgj05/02/2010 18:11:58
1008 forum posts
14 photos
There is no CASE for a DRO on anything mill or lathe). You can, and I did, machine off the dials for years. Complete with errors, inaccuracies, some mistakes, bits of paper to remember  yesterdays settings and praying I hadn't wound one turn too far on a complex milling job.
DROs are just so accurate(with a sharp tool and properly calibrated) and make life so pleasant. So why not?
If people want  to go on struggling that's their choice. if people don't want to spend the money, equally thats their choice, but don't get jealous when I say truly, I can skim a job of say 3/4" diameter, take a micrometer measurement at that point, and then hit diameter to +/-.0002" half an inch further down. All I need is decent finish on that first surface.
Being a bit unkind, a lot on this forum regard a whole thou as good enough - in which case its a good micrometer, not a DRO they need. I regard 1/2 thou as a country mile, so perhaps a DRO with properly calibrated scales helps me be pernickety. (But then when I machine something - you can put money on it that it will fit dead right first time)
Does it make you a better machinist - define better. You still have to know your stuff to produce the right result, and since getting things to fit right is the business we are in, probably it does.   More accurate- certainly. Does that save hassle- yes of course. Is it worth it? Yes to me.
If people wish to hand grind tools and do their turning with a pencil stuck in their ear, that's their choice. However there is no point in getting Luddite about it. That was the attitude that had people breaking machine tools because they replaced working with a file, hammer and cold chisel. And when was the ruler better than the micrometer - because that's the principle we are discussing.
To summarise. Am I likely to produce a better, closer toleranced and accurate result more quickly with a DRO (particularly when using cheaper foreign equipment with less than brilliant leadscrews) - yes of course. Suits me fine.

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