Myford Super 7 screw cutting gears (metric)

Simon,

​I have another heavy day and there is much to read here and catch up with now anyway

I will cover your points later, I can also see the page I dropped in my earlier predictions and will put that right to your satisfaction.

​More later

Regards
​Brian

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

.

Weird shaped tumbler gears ??

And not Tufnol gears ?

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

To be technically correct the tumbler gears are 20 DP but 20 degree PA and not 14.5 like the rest.

Only the spindle gear and tumblers are this PA, done I presume to make the tooth form stronger and easier to produce than a corrected gear ?

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.

Sorry Simon, my bad in that not thinking I did the 29T gear at 14 1/2 degree PA.

As you have found out they will mesh, a lot of overthought is put into gears which are actually quite crude, hence your alloy one.

Where PA really counts is where speed is concerned and in turn noise.

Some years ago I sent a batch of 20 T 20DP gears in for some racing bikes to be cut and they cut them at 14.5 and not the 20 degrees on the drawing.

Not easy to see and i went ahead and built these engines up and they fitted with no problems given all gears need some clearance.

However on starting these engines up they howled really badly and it took a while to locate just what the problem was. A new set of gears were cut at 20 degrres and problem solved.

Later tonight I'll do the same exercise with 20 degree and a o 4 cutter in 14.5 to see what the difference is. Im betting not a lot.

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

Allan, Simon and John,

​You will perhaps have noticed I have been 'dark' for the last few days. Our son in law, who was skiing in Austria in January despite having lung cancer [ he was a non smoker] took a turn for the worse very recently and added two strokes to the battle and died on Monday morning. He was only 53 and up until then one of the fittest people I knew.

​I will get back on topic and put the maths together for your interest, but right at present we are rather pre-occupied as I am sure you will appreciate.

Regards
​Brian

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!

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

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

Simon,

Thanks for that chart, no doubt it will help other users of the early gearbox's but only if they can do a 30 /17 and 29/16 cluster.

I went back and examined the 14.5 and 20 degree differences.

Quite a difference. The one on the right is 29T corrected but at 20degrees PA with the original No 4 cutter at 14.5 PA superimposed. As you can see nothing like.

The one on the left is using a No5 cutter 21T to 25T range and cut 0.0065" deeper. Again loads of under cutting but the flanks are a good match.

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

Best rgds Simon

Quite late to this post, but I've just acquired a Super7. Are the metric gears still available? Thanks

Steve,

If you have the standard Myford gear box, 33 and 34 teeth gears are available from Myford **LINK**

HTH,

Rod

Brill, thanks

Does anyone know of a source of either a set 21, 33 and 34 tooth gears or just a 34 that will get me out of a current problem please.

TIA

Bob