Harold Halls Workshop Projects book?

[Bo’sunParticipant @bosun58570]

Reading through Harold Hall’s book “Model Engineers Workshop Projects”, on page 21 he refers to a technique for machining precise diameters.

It reads:  When turning outside diameters to precise dimensions it is common practice to set the top slide to an angle of 6 deg.  With this done, traversing the top slide by 0.025mm will increase the depth of cut by only 0.0025mm, a ratio of 10:1.  For even greater sensitivity………..

I can see how a 6deg. RA triangle will give roughly a 10:1 ratio between the two short sides, but how does it relate to the topslide traverse, or am I missing something blindingly obvious?  Probably.

[JasonBModerator @jasonb]

You don’t put the cut on with the cross slide, you use the topslide to put on the cut and as every division is 1/10th of the cross slide you get a finer adjustment

For example if you wanted to take a cut of 0.0002″ you would just need to move the topslide 2 whole divisions rather than split a single cross slide division by eye to 1/5th of the distance between two marks.

 

[bernard towersParticipant @bernardtowers37738]

Bosun, my lathe is permanently set at this angle it really does help when groping for tenths fits and using a skiving tool as well is the icing on the cake. If you have problem understanding it go to the workshop and set it up and you will most probably answer your own question, best of luck.

[Clive FosterParticipant @clivefoster55965]

There are two major pitfalls waiting for the unwary when using this oft cited method of generating fine cuts.

The first is accurately setting the 6° angle so the geometry corresponds to what you expect giving the expected relationship between topside feed and cut. In general the topslide rotation scales on machines of appropriate size and cost for the general run of amateur users won’t be exceptionally accurately calibrated relative to the machine axis. Doesn’t help that most aren’t particularly easy to read either. I’d not consider approaching a degree of combined error between reading and calibration error exceptional in casual setting. The scale on my Smart & Brown 1024 is accurate but still takes considerable care and the right angle of look to get it just so. Enough of a faff that I work around needing accurate angle setting wherever possible.

Checking with a dial gauge is fairly easy. Frankly I’d recommend setting up with a gauge in the first place to get things right. But that assumes you can do the all the work preceding the fine cut with the topside set to 6°.

The second pitfall is that very fine cuts need a very sharp tool if the amount of material removed is to correspond to the cut put on. A slightly blunt tool will generally take less of a cut than you expect.

Leaving aside bernards skiving tools, excellent and effective devices once the art of making and using is mastered, the very fine finishing cut will need to be made with a tool that has done the previous work of bringing to nominal size. Some wear is inevitable. Probably negligible if working with brass or free cutting steel but something to be taken into account when dealing with more obdurate materials. Some of the aluminium alloys can be unexpectedly abrasive, particularly when amateur tooling limitations enforce to use of less than ideal speeds and feeds.

A related gotcha is the frequency found difference in behaviour between normal reducing to size cuts and very light cuts. It’s common not to get exactly what you expect when going from deep cuts to shallower ones. Whether roughing to finishing or finishing to extra fine.

I reckon the best way to proceed is to establish a reliable and repeatable cut depth corresponding to the dial adjustment and use the angled topside to give a vernier style adjustment to bring a cut essentially the same as your reliable and repeatable one but set a fraction smaller on the dial up to the right depth. This sort of messing about is much easier on the brain cells if you have adjustable dials set so the last cut theoretically corresponds to zero on the dial

Reliably and repeatably working to tenths of thou / 2 thousandths of a mm is far harder in practice than the books make it seem. Most especially if you need to hit a particular one.

In practice the home shop worker uses very fine cuts to get things to fit just so when the “made to drawing nominal size” isn’t quite right. So dealing with very small cuts becomes a necessity. In such situations it’s probably worth doing a calibration check partway through bringing down to size to see what you actually get when the topside is used to dial in a very small cut.

Which all assumes you are set up to measure such small variations.

Another minefield if needing to match size for size.

Theoretically I’m set up to work to tenths (after £££!) but frankly dear I’d rather not!

Clive

 

[Bo’sunParticipant @bosun58570]

Thank you Gents,

I can see a little experimentation coming up.

[Martin ConnellyParticipant @martinconnelly55370]

There is a reason why industrial machine shops use grinding when aiming for very tight tolerances.

Martin C

 

[Graham MeekParticipant @grahammeek88282]

I have always used this method when turning Morse tapers. Roughing to within 0.2 mm of size with a common or garden coated tipped tool and finishing, (depending on the material), with a Ground Sumitomo insert, (for Tool steel) or a well honed HSS, (for EN1A, or similar).

For general turning work, roughing and finishing in this manner saves a lot of tool wear on the finishing tools.

As regards the 6 degrees I have only ever set this by eye.

Regards

Gray,

 

[bernard towersParticipant @bernardtowers37738]

Martin, is it the tolerances or finishes?

[PeteParticipant @pete41194]

Just measuring to that accuracy is harder and much more expensive to make that possible than it seems, and surface finish would have a large effect. Mathematically that’s proven for how much the tool will infeed, and numbers don’t lie. But still a whole lot different than accomplishing it, and that still doesn’t factor in anything else for the mechanical realities. Possible for some, but still not easy. It’s worth understanding as another technique though since it’s been around for a long time. Even more useful for anyone with a decent tool post grinder.

Ignoring environment and work piece temperature fluctuations. How well is the top slide adjusted, its condition, amount of wear, and everything else on the whole lathe including spindle bearings? Depending on the lathe brand and condition, even it’s feed screw and nut accuracy might be highly doubtful to 10 times less accuracy or .0254 mm. Better than that over short distances of course, but there’s still lead and lag errors present in the screw and nut.

I’m not trying to be negative, it’s the practicalities and everything else that’s involved. I’ve tried and used it a few times, the measured accuracy for what the actual depth of cut was and what the numbers said they should be didn’t match as well as I’d hoped.  I don’t work in metric, but I found it faster to set up, easier, possibly a bit more reliable and accurate to just use a .0001″ dti for any tool feed numbers in that range. Even then, my lathes spindle bearings make it more wishful thinking than a reality. Better I think, but it depends on what lathe you have. Even simple lapping with a far easier to control and with better surface finish, round, straight and to size could certainly be a lot better than my own lathes.

[Graham MeekParticipant @grahammeek88282]

I think the object of the exercise for the OP was to comprehend the technique, which like Pete says is handy to know. When one starts to get into the rarefied atmosphere of close tolerances a whole other set of conditions come into play. Circularity, Perpendicularity, Run-out to other diameters, etc, the list goes on. Again Pete hits on another important factor as regards machine tool condition.

In practice the set over method does not have to remove exactly what is says on the dial. A trial cut of a couple of divisions will soon give a micrometer reading of what is actually being taken off. This information can then be used to establish the required size. One seldom relied on the dial even when doing grinding work.

While I spent most of my career chasing close tolerances, even into the Nano meter range. In general today, and this might come as shock to one or two on this forum, but the nearest 0.01mm (0.0004″) is near enough for me now when doing close work. Trying to hold anything better in a garden workshop is a complete waste of time in my book. (Temperature in there yesterday 32C a few months back it was 10C or less, seldom the NPL’s  20C).

An 8mm ball race slides nicely onto a turned 7.99mm shaft, (micrometer reading, no cut and try here), if it needs to be a permanent fit then use Loctite or a similar bearing adhesive. A cracked bearing is a useless bearing.

Regards

Gray,

 

[Howard LewisParticipant @howardlewis46836]

Graham is absolutely right.

It is easy to get delusions of accuracy, disregarding the condition of the machine, let alone ambient conditions. To take MINUTE cuts demand that the tool is SHARP, but the clearances in a machine can make a nonsense of that.

And the action of cutting generates heat so that what is measured (Assuming that the measuring equipment is accurate and consistent) immediately after machining is not what will be seen an hour afterwards, when everything has been unclamped and cooled down.

There have to be clearances, otherwise nothing could move, so an extra 0.01 mm of clearance somewhere could be more than what you are trying to remove.

Maybe a well protected machine and fine emery is what is required here; plus some good luck in the “suck it and see” stakes.

It might even be that the move is into lapped fits territory. (As used in Fuel Injection Equipment for diesel engines)

Howard

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