Spindle Speed

[chrisJwardParticipant @chrisjward]

Spindle Speed Measurement using Variable Speed Motor Drive

[chrisJwardParticipant @chrisjward]

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 CoxParticipant @ericcox50497]

Have a look at this.

 Ebay.

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

[Ian S CParticipant @iansc]

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

[KWILParticipant @kwil]

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.

[mgjParticipant @mgj]

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 Wood1Participant @johnwood1]

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

[chrisJwardParticipant @chrisjward]

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 

[mgjParticipant @mgj]

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 CParticipant @iansc]

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 1Participant @lesjones1]

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.

Les.

[Gerald WeareParticipant @geraldweare85524]

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.

 

Jed

[Gerald WeareParticipant @geraldweare85524]

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…

 

 

Jed

[Ian S CParticipant @iansc]

There is a circuit for an electronic tacho on Tony Jeffrees web site. Ian S C

[KWILParticipant @kwil]

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.

[CirclipParticipant @circlip]

” 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.

[chrisJwardParticipant @chrisjward]

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 

[KWILParticipant @kwil]

Circlip,  What/who is a DRO’s mate?

[CirclipParticipant @circlip]

” 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 stephensParticipant @chrisstephens63393]

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.

chriStephens 

[mgjParticipant @mgj]

Moi?

 

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.

[mgjParticipant @mgj]

Passion for exact speed?

 

Possibly because a lot of people don’t have the confidence to know by look and feel how fast is actually right. So they want to refer to charts and speeds and feeds.

 

But those who do go off appearance and feel, and can adjust feeds to match speeds to give a balanced result should remember that perhaps at one stage they did not have that facility, and they too looked at their charts and feeds and speeds?

Edited By meyrick griffith-jones on 05/02/2010 18:18:12

[chris stephensParticipant @chrisstephens63393]

As one has come to expect, a reasoned and informative posting from the said West country gentleman.

I hope no one ever says you have to have DROs or VFDs, but if you have them, they do make work a little more pleasant and less fraught with mishaps. 

christephens 

[CirclipParticipant @circlip]

Aye, cynical with good reason pet. Many years ago I worked in the Textile machinery design trade and three customers making the same end product , each had exactly the same machinery but running at within 10’s of revs to each other to achieve cotton wool with NO discernable difference but all adament that THEIRS was the correct speed.

 

  My own lathe has eight speeds and my miller has four. Plenty to pick from, so if the new guys have a problem with such a limited range how the devil are they supposed to cope with zero to whatever??

 

  Regards, the cynic

[mgjParticipant @mgj]

Circlip _ I’m all for a bit of cynicism.

 

I used to stalk deer commercially, and if ever (beyond fishing) there was an occupation where the latest gadget was king – of which there was the most fantastic quantity, almost all imported from the US– and almost all completely useless and serving no purpose other than to lighten the pocket (of the inexperienced and enthusiastic) and make a simple thing complicated – then I never saw it..

 

So no, I wouldn’t change a cartridge, or bullet, or borrow a knife or bother to look through the latest binos with the greatest laser whatnot and GPS pre installed. I just stuck to very good quality simple kit which really worked. The test was only this. Was it useful? Would it actually make life easier? If it was then I never worried how much I had to pay for it. If it didn’t then I wouldn’t carry it even if it were given free. 

 

 

And so it is with a DRO. It passes the test – lathe or mill.

 

As for spindle speeds, personally I like geared or belt driven heads, because you get an increase of torque at low speeds. IOW, I don’t like my sine waves chipped. So I have levers on gearboxes which tell me my speeds. Thats fine. As you say, the odd extra or fewer revs will make no difference at all. But less torque is a crime. So IMO, varispeeds serve little useful purpose. But if someone comes up with a torque increasing controller at reasonable cost, then I’ll look at it.

 

(And for all those who have the latest and greatest controllers which amplify everything – I know. My MSc was to a great degree in control engineering. So we wont go down that route, because as we all know the basic principle of stiffness holds with high amplification. As you increase amplification to overcome the gaps, the more difficult it it to retain smoothness of transmission. (So says the Nyquist diagram or Bode plot for almost all feedback loops unless you resort to some quite clever and expensive programming) OTOH it is one  of those little things a Victorian gearbox does rather well.  )

Edited By meyrick griffith-jones on 05/02/2010 20:58:06

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