Lathe tachometer

Got one of these and it works well, not that I use it much.......

Martin

Neil,

"Just because we can". How very true of so many "new" and seemingly unnecessary products. eg *phone 9-10---X--Y and many other things.

I am sorry - I really did not intend to re-ignite the whole pro-con tacho debate again.

I just wanted to know whether the weight of the sensor magnet on the large belt pulley would seriously upset the balance.

Anyway I have done some calculations and reckon that the centrifugal force of a 1g magnet, 50 mm from the centre revolving at 600 rpm is 0.02 kg.f.

I am going to stop worrying.

Peter

Peter, on my S7 that hole is used for the spindle locking pin! Does yours not have one?

Oh ******** !!

I thought it was to balance the oil nipple cut out opposite. Learn something every day.

Anyway no great problem as I don't think I have ever used the locking pin.

Thanks John for solving my problem.

Peter

Why bother with a tacho

It just happens that some lathes have a mechanical variable speed drive, so the motor speed is constant, so not proportional to the speed.

That small hole is close to the axis, so will unlikely to affect the balance of the whole assembly to any discernible degree. Think of how the length of the grub screw would be considered at installation. Interrupted cutting, offset turning, or out of balance parts would have a much greater effect.  Lathe bearing sizes should be designed to be sufficiently robust.  To what degree are most (cheap chinese) chucks dynamically balanced?

However, I would like to be sure the magnet is: a) readily removable and b) unlikely to ever fly off (unlikely but...).

Edited By not done it yet on 08/01/2018 16:08:59

I'm not familiar with the lathe or the pulley Peter but I wonder if the hole is for balance; maybe it's a manufacturing artefact? I wouldn't expect a small hole or 1g magnet to make any difference at 600rpm.

Never mind accidentally re-igniting the great tacho controversy, it's all part of the entertainment!

Personally I find tachos moderately useful and am pleased to have them fitted on both lathe and mill. Although they can be used to pre-select theoretical cutting speeds, that's rarely what mine are used for. Now I have some experience I tend to settle on cutting speeds by informed experiment. The tacho's role is to tell me exactly what that speed is. Makes it very easy to go back to the same rpm, perhaps days after a job was interrupted.

Dave

Tachometers are quite useful if you happen to be a novice. They provide you with a visual guide to spindle speed and a link to the recommended operatingspeed based on material, object diameter and all the other factors that experienced machinists know off by heart. Eventually I won't need to use the tachometer - but at present, I find they provide me with a safety starting point on the lathe and milling machine. Rally drivers never look at their speedometers but novice drivers would be well advised to just glance at their speedometer before taking that sharp bend.

My mini lathe has operated for 10+ years at up to 2000 rpm with a small neodym magnet on the locking nut, held in place by nothing more than its magnetism.

The problem with a lot of modern kit is they have a speed adjusting knob that goes from say zero to max rpm with no real attempt made at the factory for calibration and a distinct lack of marking of intermediate values.

Couple of cases in my own workship with a Boxford lathe, with a 4 pole direct from mains motor running at 1400 ish rpm, the plate showing the pulley and backgear arrrangements closely follows the reality at the spindle.

For the Sieg X3 it's a knob a long reach away, only visible if you stand to the right side and towards the rear of the machine that does nothing for the first 20% of its travel and just a low and a high gear selection on the head. One gear apparently gives a top speed of 1000rpm the other 2000rpm, but confusingly with a minimum on both ranges is '100rpm' Work out the logic of that, a gear train that does nothing at one speed input but can also provide a 50% reduction at the top end.

Much of the work on the Boxford other than heavy drilling is performed either at the top 1300rpm or the slow backgeared equivalent that escapes me right now. Insert tooling lets you get away with that

The mill is much more sensitive to rotational speeds and while you can wing it, without a tacho you are just guessing. I'd class it as essential.

Just to be mischievous... how do you know the right speed before you select it?

In my case by a process of elimination

Brian

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Thus, if equipped with a tachometer, one can start the process of data-gathering ... and get to understand the characteristics of the machine.

MichaelG.

Using a very big book from Sandvik with lots of formulae in it , spinning the spindle up and tweaking the pot to somewhere towards the bottom of the recommended range.

I can get the CNC mill working fine with quite acceptable rates of metal removal, surface finish, dimensional accuracy and cutter life. But then it's not me it's the machine.

A lot of my difficulty manual milling stems I think from my height and the lack of it on the X3 with the factory supplied stand, giving a very restricted view of the cutting action compared to turning on a lathe (my brace of Boxfords despite being early 1950's and early 1960's have the chunky 80's era yellow chuck guard affording very good visibility) I can, from sight of the swarf and feel through the handwheels know if the feed and speed is right or not on the lathe I very rarely get that when milling. At least with the tacho on X3 I know I'm somewhere in the right area and can eliminate one variable.

Guess it comes down to experience, I've maybe a couple of magnitudes more turning than milling experience. Decades ago as a engineering trainee I was twirling handles on huge very tired mills with worn leadscrews, sloppy gibs and less than perfect HSS tooling, and despite cutting speed calculations ahead of time the results didn't always match the expectations. Since then I spent many hours being frustrated using a Boxford vertical slide and setups that were not even remotely rigid. The visibility of the cutting action is about as bad as you could ever get. The X3 was a huge improvement, but I'd prefer something at least half as big again with significantly more rigidity. A hefty multiple horsepower motor with a VFD would be useful too. I might have even bought something 'big' from Warco last year if they'd have done the Doncaster show.

Ah - so it's not just useful for novices. As an ex-scientist, I always want to observe, measure and record my actions. Not for me the "it feels right" approach - although I am starting to get that with experience. If I can duplicate settings from previous recorded observations - I'm more at home. I'm not denigrating those engineers who "fly by the seat of their pants" - it just isn't how I operate. And with CNC machines - there is little opportunity for a machining by feel approach - nearly everything has to be set up correctly beforehand - and digital appreciation of many of the factors in normal lathe and milling operations feeds directly into CNC operation.

Fair enough, although I think 'somewhere towards the bottom of the recommended range' suggests that we aren't really at odds

For Colin and Michael G. - even if you find the 'right' speed by experiment, the tacho is just providing repeatability, it's still practical experience that's helping you find it before you put it in your notebook.

There's no right or wrong way, it's what works for the individual. While advice and guidance is invaluable, if an approach works for someone, it works.

Neil

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Quote so, Neil ... I value both repeatability, and knowledge.

MichaelG.

For HSS tools I use 1000 inches a minute as my base calculation and choose the nearest speed to satisfy that. When you just have a pulley set to select your speeds you need to make a decision that is somewhere near the calculated value. There is no need to get your slide rule or calculator out for this as a bit of rough mental arithmetic will suffice. If the theoretical speeds are unsatisfactory then the problem is likely to not be the speed but tweaking the VFD pot can often improve things and is easy to adjust. Checking your tool setup and machine condition can also effect a cure. If you don't have a tacho then you have no idea where you are and cannot reproduce the setup. Just finding a setup that machines sweetly may be enough for most hobby men and we only have to please ourselves but being able to define a start point and even note a finish point has to be useful. For the sake of £10 and a bit of installation time why not have a tacho?

Mike

Three pages of tacho or no tacho.

Tachos (and VFDs, are a luxury compared to Grandad’s options.

Grandad did OK with six fixed speeds and maybe a back gear. Perhaps he saved hard for a new fangled myford. Or if well heeled, maybe a Raglan. But even with the variable speed Raglan, a tacho was a later luxury and few were ever fitted.

.

But Grandad would have needed to pay a disproportionately high price for a tacho !

We have the opportunity to buy rhem cheaply. Such is progress

As Mike Poole wrote, immediately prior to your post:

For the sake of £10 and a bit of installation time why not have a tacho?

MichaelG.