Member postings for Simon Williams 3

Well, many thanks as always to all those who took the trouble to reply, it's been hugely entertaining watching this develop over the course of the last 24 hours or so.

There seem to be three schools of thought -

1- don't fix what aint broke,

2 - you're going to make it a whole worse

3 - I've been there too.

The back story is that I bought this mic' back in the late 70's, and have hardy used it since, because I've never learnt to read it reliably. I'm not sure I understand why the same is not true of almost all of the imperial mic's in my toolbox, but this one seems particularly obnoxious. To me.

I was sorting twist drills yesterday, and took to this one to see if I could get on with it, and decided the answer was no. So I might as well modify it as throw it away. Yes, I completed the job using a digital mic', but there are times when a tidy little thing like this is easier to use.

Tubal Cain's video gives me some hope that I'm not going doolally - well probably I am but it's nice to have company - so thanks for that link, and particular thanks to Mike Poole who seems to have dismantled a perfectly good identical mic' just to show me what the internal arrangement is, which was my original question. I'm still humming and hawing over whether the game's worth the candle - but I've just worked out how to get the sleeve off so that gives me the option of taking a lick off the heel of this instead of the thimble. Less likely to get grinding dust in the measuring thread.

I'm keeping schtum about whether my metaphorical angle grinder is the way to do the job. I never said I was going to plug it in!

I've been chuckling all afternoon about the debate between accuracy and legibility. My warped sense of humour finds in this a direct analogue of the inability of the newspapers to resolve the same.

Regards to all

Simon

Good evening everyone, after some advice please.

x

I find this quite difficult to read, as the fiducial line is inside the thimble. This presently reads 7.50 mm, but the 0.5 line is obscured by the thimble sleeve. It can be quite confusing particularly at intermediate settings.

So is it possible to move the thimble about 0.2 mm to the right on the spindle? How is the spindle held in the RHS of the thimble?

How many heart attacks would I be responsible for if I took the angle grinder and took 0.2 mm off the left end (scale end) of the thimble, heretic that I am?

Thanks as ever in anticipation of The Knowledge!

Simon

Try Kidderminster Rewinds Hoo Farm Ind Est 01562 751820.

No connection, just a satisfied customer

Rgds Simon

Thanks John, looks like the couple of thou I added on to the DOC "just to be sure" were well invested.

Best rgds Simon

Good evening again all,

Here's my summary of the best choices I've found for the old style of gearbox on the S7

Myford Lathe Cutting Metric Pitches

(Old Style Q/C Gearbox)

The FF appended to the 0.5 pitch selection indicates to engage the fine feed drive for this ratio, giving an additional 9:1 reduction.

Best rgds

Simon

Oops, sorry for the double post. That appears to be something to do with having the cat sit on the keyboard (she wants her tea). I wonder what short cut she found! Shades of the Infinite Monkey cage!

Simon

Brian - my very sincerest best wishes for you and family. Please don't give this any thought for the time being.

In due course I'm sure we'd all like to hear your take on things here, but it'll keep until you've got the brain space to suit.

With best regards

Simon.

PS Just read your book, courtesy of the eager elves at Amazon. Thank you for this expertise, it's all there isn't it!

John -

Thanks for that, though please don't concern yourself on my behalf. I've got what I wanted, and compared with my alloy fake 30T gear the tooth shape of the one you've calculated for me can't be far out. Indeed, I'd venture that it's as near as I can make it with the resources I have available, particularly since (if you look very closely) there's something a bit odd with the width of the crests of the some of the gear teeth as shown in my photo's. Given that I've checked the size of each blank carefully, that tells me that either I've used the wrong cutter betimes (which I was very careful of) or the cutters perhaps aren't as accurately profiled as one might assume. Look at the difference in crest width between the 16 and 17T gears for a case in point, they are each cut with their respective cutters yet the crest width is quite different. What the hell, it works!

I checked that the cutter numbers were not reversed, as you suggested sometimes they can be. I'm pretty certain that's not the explanation, but given my ambitions are largely agricultural I'm very pleased with where we've got to. And it's been an interesting insight into what can be done with a bit of ingenuity along the way. If the lathe is a bit noisy I won't even notice - my impression is actually that it's quieter!

Having said all that, I (like you?) am intrigued to quantify the extent of the error implied in this mix of 14.5 and 20 deg pressure angles. I bet it's within the realistic manufacturing tolerances I can expect to achieve.

All the best Simon

Well, that's a low blow, what!

So I guess it's easier to buy the spares than a whole new set of cutters. Bother!

But that means that the transfer gears and my magic combo should all be 20 deg PA as well. Blow that for a game of soldiers. The change gears meshed with the tumblers, what's going on?

Looks like I'd do better to replace the tumbler gears and the one on the spindle. No rest for the wicked.

Evening John, (JS) - You've got me bang to rights guv.

Though that's not the whole of the story. What I haven't let you see is the hand filed teeth on the 17T pinion of the first transfer gear (just bottom of picture.

I bought the gearbox second hand from you know where-bay, fitted it ( the comments about a spacer behind the casting worry me, but I'll save that for another day) and found that it jammed in one ratio. Should have stripped it and checked first, but was far too excited with my new second hand purchase to be that clinical. Can't remember the details, but suffice it to say that the single tufnol gear that was fitted to the tumbler is still in the drawer with the rest of the set of changewheels from the days when I set it up long hand, and it's got three teeth chewed off it.

At the time I had a little Alpine simple dividing head, so the ally gear you see here was cut with a single point flycutter, probably of a crude and dubious shape, and has seen maybe 20 years service since the scrunch up. I'm fairly sure this was my first ever adventure into making a gear, so begging yer pardon if it's not up to snuff.

Now that I know a bit more about cutting gears, t'would be the work of a few moments to make another. Must get roundtuit.

To the best of my recollection the other tumbler gear is as original - or at least I haven't changed it. This lathe came out of Stroud Technical College in about 1976, so it's probably had a hard life even before I got hold of it.

Thinking about it, if I was going to have only one tufnol gear it would be better if it was the other one - which is in service all the time. Too late to worry about that now!

Thanks as ever

Simon

PS  The chewy chewy of the transfer gears wasn't me.  They were like that when I bought the gearbox.  20/20 hindsight says that should have told me I needed to strip the gearbox and investigate, but I didn't follow the clues.

Edited By Simon Williams 3 on 21/03/2017 22:40:09

Fruit of my day's labour ( the bronze bit centre stage):

Is a 29T gear pretending to be a 30T gear, with an integral 16 gear attached, meshed with the gear train driving the QC g/box on my trusty S7.

xxx

The proof of the pudding is in a 1.75 mm (approx.!) thread.  Q/C gearbox is set for 20TPI, I've only made a scratch pass just to prove the train.

With special thanks to John Stevenson for the geometry lesson!

Rgds to all

Simon

Edited By Simon Williams 3 on 21/03/2017 19:56:38

John - thank you for an impressive piece of work. I stand in awe of the expertise.

I'm hoping tomorrow to make the 16 1/2 tooth gear I promised you. Watch this space.

Rgds to you and the hound (hope he hasn't been chasing my cat!)

Simon

Evening all!

Well, bless my soul, that's a mighty pretty summary of where we've got to! Thank you John.

Do you have a spread sheet printout with the tumbler gear set to 33T?

Allan - request for explanation noted, really John's spreadsheet says it all, provided we don't worry too much about what's hiding inside the gearbox. It's only really the relation between the spindle and the carriage travel we're concerned with, and that's what the label on to top of the Q/C gearbox details. What John has done (bodger that he is) is to alter the input gear ratio driving the gearbox, which alters the amount that the carriage moves each revolution of the spindle.

It happens that the alteration chosen magically makes the carriage movement fit most of the useful numbers of the metric series of threads, hence being now able to cut metric threads - or at least a damn fine approximation thereto - with an 8 TPI leadscrew.

It's just a confusion that the imperial threads are traditionally measured in Threads Per Inch, and metric threads are expressed in the pitch between successive crests, measured in millimetres. They are actually the same thing expressed in two ways.

I've gone and confused the otherwise confusing logic of this by finding that the gearbox fitted to my lathe happens to run at half the speed of the one John's calculations were designed for, so all this stuff about 17 tooth gears is me trying to bodge my way round this. I suggest you ignore that for the time being.

The arithmetic is actually fairly simple - or at least I think it is though I'm waiting on Brian to tell me I've misunderstood something. The usual gearbox is fed from a 24 tooth gear. How the motion gets to the leadscrew doesn't matter, but the lever positions tell us the overall result, expressed in TPI. John's spreadsheet replicates this, AND adds the information of the thread pitch in mm. Remember it's the same information, just expressed in a different way.

What we've then done - not my idea, I take no credit for this, is to change the gear driving the gearbox, so that for the same speed of the headstock we have changed the speed of the leadscrew, hence the carriage travels at a different speed, and the TPI numbers (and thus pitches) alter. There's nothing magic about this except for the canny choice of the new driving gear, the gearbox itself hasn't changed and so the output variable - the speed at which the carriage travels along the bed - varies in strict ratio of the new driving gear to the old driving gear.

The original Myford set-up drove the gearbox with a 24T gear. We've decided that's not good enough, and now decided to drive it with a 34 tooth gear. So all the thread pitches (gearbox outputs) increase by 34/24, which is what John's spread sheets show, and the TPI's of course reduce by the same proportion. The clever bit is that someone realised that by choosing 34 as the new number many of the new pitches happen to fall neatly into the numbers you would want to cut metric threads. Eh voila! Metric threads on an imperial lathe.

This cunning plan was developed further by realising than some other useful (metric) pitches fell into place if said input gear was now altered to be 33 teeth.

So now for an apology, because I've come into this late, and presumed to write this up, but it's not my idea and I've got a sneaky feeling that the clever chap who did think of it is somewhere in the mix of this thread. Gent's I apologise unreservedly for my arrogance in taking this upon myself, I only hope I've got it right!

Hope this helps, best rgds to all

Simon

There was a similar query a couple of weeks back, which eventually was put to bed on the basis that the motor wasn't suitable for VFD drive, and the high frequency component of the VFD drive output was generating circulating currents in the motor. Sounds rather as if this might be another one.

Brian -

Not sure we're on the same page here!

24TPI selected driven by 17 T gear produces 1.5 mm pitch. Equally 10 TPI selected gives pitch of 3.6 mm. If nothing else this is pro-rata from established start point where 18 TPI selected produces a thread of 2 mm pitch as per the table published on page 1 of this thread.

Just to be sure I've gone out to the shed and tried both, can confirm as above.

Rgds Simon

John - Thank you for the update, I'm very interested to understand the process behind those figures. I think you can guess what I'm planning to do tomorrow!

Brian - Here's my attempt at the arithmetic.

For my old style gearbox, if I replace the 12T gear with a 17 T and select 18TPI, I get 12/17 x 18 TPI = 12.706. Pitch of 12.706 TPI = 1.999 mm.

For 2.5 pitch, I replace the 30/12 pair with a 29/16 combo gear, select 14TPI as per the table above. Now my lathe cuts 12/16 x 29/30 x14TPI = 10.15 TPI, or 2.5025 pitch. Pitch error is +0.1% (contrast error of - 0.2% with 33T gear on new gearbox.)

For 3.5 pitch replace 30/12 pair with 29/16 and select 10TPI. Lathe now cuts 12/16 x 29/30 x 10 TPI = 7.250TPI, 3.5024 pitch. Error is + 0.1%, error on "new" lathe is - 0.2%

So I think we're onto something here.

Obviously multiples and sub multiples of these pitches come out with the same error, so that's OK. Haven't looked at sub 1.0 pitches, probably never going to happen.

Would appreciate your confirmation!

Best rgds Simon

Morning John, Brian

JS - Complete set of 20DP 14 1/2 deg cutters in stock, they're only Garvin ones but all there and almost new. It's the only complete set I do have.

Also confirm I've got a 29 circle on one of my division plates.

If I cut 29 div's on a blank of OD size for 30 T (1.600 ins) how do I modify the tooth depth calculation (and choose a different cutter???) to be sure the otherwise oversize 29 teeth don't jam in the 30 T they mate with. Mind you, this 30 T is home-made (aluminium) as you may have spotted from the photo's, so it's a pound to a pinch of snuff the teeth were cut slightly over deep.

Do I cut 29 teeth to the depth they would be if the blank were the correct size = 1.55 OD less 0.108 depth of cut each side = 1.334 which is the PCD of the root of the 29 tooth gear cut normally?. That's 0.133 depth of cut each side from the 1.6 dia blank.

Brian - I was going to tackle (am just starting) the calculations to see how near the "correct" pitch this gives for the relevant choices. (Is it worth the bother of making the gear?) It's a wet and rainy day, so it's that or the hoovering...

Best rgds Simon

That metric banjo kit made nigh on £200!

Now, about this 16 1/2 tooth gear...

If I made another combo gear as 29/16T I'd have introduced a 0.3% error into the approximate calculation.

Following a train of thought from Mr Stevenson who was cutting gears in Delrin but making them oversize to make the tooth shape stronger, I reckon I could make the 29T gear mesh adequately with the tumbler gears. I'm working on the table of actual TPI against metric pitch, but I'm betting it's somewhere handy.

Just checking I've got a dividing plate with 29 on it ...

Simon

Having benefitted as I feel I have from so much of the various contributor's expertise, I thought it only fitting to put up a couple of photo's of what is under the cover on the RHS of the old type gearbox we've been discussing in such detail. This is what the diecast cover looks like:

And this is what it covers:

xxx

And (blowing of trumpets, fanfares, commotion) here is my first metric approximation. Thst's a 2.0 mm pitch thread gauge.

Looks like I'm not quite holding it straight, but in reality I couldn't see any discrepancy between the gauge and the thread (even if I didn't take a very good photo!)

Many thanks to everyone!

Simon

(edited for typo's)

Edited By Simon Williams 3 on 19/03/2017 20:57:49

Well, thanks one and all for the interest and advice, and I guess I really owe the OP an apology for hi-jacking his original thread.

But the rugby's over, Wales lost to France on a technicality, so while I've been sulking in my shed this morning I made this:

The gear on the left is the original 30/12 pair, and the pair on the right is my new 30/17 combination. I made them in bronze so I could soft solder the two parts together, and also when I rummage in the drawer through the accumulated selection of change wheels in a couple of years time I'll remember what this funny looking thing is for. For JS' benefit, when I've made the 16 1/2 gear I'll post a picture.

I haven't tried this out yet, I'll have a play with cutting whole mm pitch threads over the next few days, but I'm confident. What could possibly go wrong?

Regards to all

Simon