Optical Dividing Heads V Disc Type

Would appreciate comments for and against optical dividing heads v the traditional disc types please,

I've been offered a 'PG' type with a matching tailstock at what appears to be reasonable money relative to the more common disc types, it appears to be missing the light transformer and plug and I'm not sure if anything is missing from the spindle nose that would render it unusable without a lot of faff, but at £150 is it worth a punt anyway?

John

John,

PRO ... Orders of magnitude more accurate than most mechanical units.

CON ... Very expensive to repair the important bits, if there's anything amiss.

MichaelG.

Thanks Michael, that's very helpful, it does appear to be a far easier bit of kit to use as regards use for dividing (no wheels to change or holes to count) but I've no experience with these machines at all (yet), and there appears to be nothing to hold a chuck or faceplate, I gather from the owner that the spindle is threaded but only over a very short distance,

I believe the machine is in good order, but it's the missing bits that are bothering me, the owner knows nothing about it either, it's an estate sale so we're both winging it, it's too far just to nip over and take a look unfortunately.

John.

**LINK**

So it allows you to divide by applying an angular rotation. Very accurate as long as you are prepared to take the number of divisions you want, divide it into 360, prepare a table of cumulative angles, and very carefully set the machine to each one in succession. Much scope for error I suspect, but great for highly accurate work.

As it is driven by a precision worm and wheel, it might be a good basis for a digital DH where you rotate the worm (via the black knob with a handle sticking out) from a stepper motor - you could use a Divisionmaster, a Ward Divider, or the Arduino-based one recently described in MEW to drive that, taking away the need for tedious calculations and setting.

I was going to say the same as Michael G, in that it's likely to be much finer in scale but if anything went wrong with it, I doubt you'd find anyone who could fix it.

Michael W

Just found this: **LINK** which is worth a look.

http://www.nielsmachines.com/en/pg-optical-dividing-head.html

MichaelG.

.

John ... I have sent you a P.M.

Edited By Michael Gilligan on 28/02/2018 17:52:39

The only people (within the ME fraternity) who are likely to need the accuracy it can deliver would be people making bits for astronomers. Otherwise it is more of a burden than a benefit. It is much handier if it can take the same chuck and fittings as your lathe so you can turn to shape then divide as required.

Myself I would prefer a 4 in Victoria/Elliott universal which is a handy size, not too big and heavy but not a dinky clockmaker's size like the Myford, Sparey, or GT designs and capable of providing differential indexing or spiral leads if needed.

Many thanks all for the comments and links, I've got a better idea of what I'm dealing with now, think I'm going to have to get over to see the machine in the flesh and see what's going on with the spindle,

The transformer and bulb holder will hopefully be fairly straightforward to replicate, just need to find out what it needs if anything to fit chucks and faceplates now,

Point taken about divisions into 360 John, although to be honest I think I'd find that easier than working out which plates I'd need to get divisions needed, and chances are I'd miscount the holes at some point anyway,

Think I'm well beyond sorting out any electronic gubbins to fit it these days TBH, just the mention of Arduino CNC and programming and my eyes glaze over, too many wasted years hunched over broken computers I think, they've turned me into a luddite, I even hate my smartphone which thwarts all attempts to let me answer calls when the damn thing rings

So in summary, it sounds like a superb bit of kit that's more accurate than anything I'm likely ever to be capable of, and near impossible to fix without a lottery win if bust or has any serious bits missing,

As soon as the weather picks up, looks like being a trip to visit the beast and hope it isn't bust.

Have replied to your PM Michael.

John

You only count holes once when dividing, when setting it up.

Proper use of the fingers on a dividing head means you deal with the number of full turns and the fingers deal with the extra holes without any counting, very easy and very quick.

Thanks Nick, I'm sure I'll get to grips with them quick enough if I go down the mechanical head route, just seems a bit of a black art when you don't have the thing in your hands,

John.

Depends too on what you are planning on using it for. For most ME work, cutting squares, hexagons, and basic numbers etc, a simple spin indexer is all you need and much simpler to use. That's what professional machinists use for pretty much everything except actual gear cutting these days. You might want to start off with something like that and graduate to a dividing head when you know more about where your hobby is heading.

I built my own dividing head and have yet to use the discs and sector fingers part of it. Instead, 24 holes drilled in a circle around the main gear allows the direct indexing with a plunger  of 2, 3, 4, 6, 8, 12 and 24 positions. Another plunger made up to engage with the 60 teeth on that same gear allows also the indexing of additional numbers such as 5, 10, 15, 20, 30, and 60.  Only when I (one day...) get around to gear cutting will I bother with the discs and sector arms.

Edited By Hopper on 28/02/2018 23:59:04

Thanks Hopper, yes I'm heading for gear and pinion cutting plus a few gear wheels for i.c. engines,
I've been paying on average around £40 - £50 per raw un-crossed wheel from a pro' cutter, which hasn't been a problem when clients have been footing the bill, he charges around double that for crossed-out wheels, I've never bothered with those though because the work needs to match original parts,

I'm getting ready for a set of wheels and pinions for my Congreve but he's quoted me such an insane amount of money for them I don't think he wants the job, so I'll have to get my sleeves rolled up,
I've also got a couple of Skeleton clocks on the go that'll need them at some point, so plenty of work for it once I've got to grips with using one,
time isn't of the essence for any of these, so cost of the device and keep V cost of wheels and no device is a good argument with domestic management .............. we've all been there

John

It really isn't suitable for model engineering or clockmaking. You could almost make something more appropriate out of Meccano.

Yeah right, have you seen the price of Meccano lately Bazyle

Hi RJW

A while back I made an Excel spreadsheet up to calculate divisions in degrees minutes and seconds or decimal degrees.

If you enter the number of divisions you require at the top the correct list for that number of divisions specified will be be generated below.

Print the list and and the job is done.

**LINK**

Regards
John

OK I know its Captain Obvious speaking but when you print, or even write out, a list like the one produced by Johns program make sure its broken down into sensible short sections. Usual advice is 5 followed by a blank line. Maybe double blanks, shaded or whatever at larger intervals. But 5 isn't a law of the Medes and Persians. Often a slightly larger or smaller set makes it easier to keep track. Nowt wrong with uneven sets if that works better. Want to start or finish or both on a simple number.

And tick off as you go!

Used to do a lot of this "make a table of settings then use it for scientific / engineering experiments" back in pre-computer and early computer days. Learned the value of being anal over this sort of thing when an error in the table cost me about a fortnights work and many hundreds of readings in a dark lab. And all those turns of the handle on a mechanical calculator to make the table in the first place.

Clive.

John, Your spreadsheet is Superb, Many Many thanks for the link and passing it on, I'm sure many others will appreciate it too,

Thanks too for the tips Clive, hard earned from the sounds of it and well noted, Very much appreciated,

All I need now is for this crap weather to clear up and hoof it over to the seller and see the thing, hopefully in the next week, will update accordingly.

John