Free hobbed worm wheel

[Howard LewisParticipant @howardlewis46836]

Beware of labouring under delusions of accuracy!

I do not believe Model Engineers who claim to be able to work consistently within "a tenth" or "a micron". Very many commercial shops would not make that claim. A Micron is the sort of clearance found in the Fuel Injection Systems of Diesel engines, often the result of a lapping together the two components.

To give a sense of proportion, a human hair is about 125 microns in diameter, a tenth of a "thou" is 2.5 microns.

We are making one or two offs, not spares for a Lunar Rover.

As JS says, accurate measurement can only be made under standard conditions of temperature and humidity, which are the conditions in the Standards Room of larger commercial engineering undertakings. The Calibration Room is even more closely controlled. In neither room will anything be measured until it has "soaked" to the ambient conditions for at least twenty four hours.

After all, the Calibration Room is where the measuring instruments and slip gauges are themselves checked for accuracy, (and slip gauges have to be accurate to a millionth of an inch). The instruments, there, will very likely be traceable back to a National Standard, such as those held at the N P L at Teddington in UK.

To minimise the effects of expansion, resulting from heat from the hand, micrometers now have insulating pads on the frame. Many measuring instruments are marked "at 68F" or "at 20C" to signify that they are only accurate at that temperature.

In industry, ball and roller bearings are placed on a hot plate for a time, before being placed on the shaft.

If you pick up an automotive piston and immediately try to fit the gudgeon pin, the chances are that it will not enter. Put down the pin and hold the piston in the hand for a few minutes. Then, the pin will be an easy sliding fit.

Gear trains often contain a "hunting tooth", to ensure that the same two teeth only mesh very intermittently, to reduce wear. Even with precision ground gear teeth, backlash and eccentricity will reduce accuracy and repeatability.

In our non climate controlled workshops, we are not going to match the end product of a heavyweight machine very carefully manufactured and assembled under carefully controlled conditions, and costing many more times than our lightweight, mass produced machines of lesser precision.

Howard

[AjohnwParticipant @ajohnw51620]

The bearings are heated because of the extreme difficulty the makers have even with the gear they use producing a relatively light interference fit. In fact I don't think standards list a light force fit any more. They can be found in machinery's.

All I would agree with in general terms is that micon's on micrometers are scotch mist. They are even difficult to use as comparators at the sort of resolution that implies. Not so 1/10 thou to anything like the same extent. The problems with both go up with the size of the micrometer.

Hunting tooth might confuse people – it just means 2 meshed gears without common factors – different base primes in other words but the smaller gear will always wear more quickly than the larger one, The same principle can be used to lap a gear as well.

As to the rest my workshop is always between around 18 to 22C when I am working. Working environments are often more controlled than that. 5C might matter on tubes of some size. The main aspect in this area for most on here is comparisons not actual sizes. It often is and it's amazing how closely people could work with a pair of ordinary callipers in that respect – all by feel and rather a lot of skill and years and years ago. Actually mic's need feel too as the ratchets just aren't consistent enough.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Howard Lewis on 09/04/2016 15:16:24:

Beware of labouring under delusions of accuracy!

 

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I labour under no delusions of accuracy … That's why I quoted GHT's modest words verbatim.

MichaelG.

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Edit: I did try to discuss the differing concepts of 'Resolution', 'Accuracy' and 'Precision' in an earlier thread … to no avail.

Edited By Michael Gilligan on 09/04/2016 16:56:20

Edit: Added 'Resolution' to the list of oft-misunderstood concepts.

Edited By Michael Gilligan on 09/04/2016 17:19:16

[AjohnwParticipant @ajohnw51620]

I've just pointed out the same thing in a different way Michael.

Things get more interesting when it comes to accuracy. Take me for instance. I was reckoned to be ok with a mic on round stuff but inclined to measure over size on flats. The differences were rather small but according to them were there. This was by people who had been using them for years.

John

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[HopperParticipant @hopper]

Wonder what happened to the OP? I'm still wondering what he needs a 12" wormwheel for.

I'm currently building my own fabricated version of GHT's Versatile Dividing Head, so after this thread I am just going to have to set up a dial indicator on a 12" long arm on the finished job and check actual movement vs the graduations on the micro dial, see how close to 1,000th of a degree it comes out to be in reality.

Edited By Hopper on 10/04/2016 05:06:43

[AjohnwParticipant @ajohnw51620]

That would be interesting Hopper. If reasonably well made I'd expect well under 0.001" variation on the flank positions of the worm wheel. A lot under actually. On the one I looked at and was offered there was no perceptual back play at all. It would spin all round with firm worm contact with the wheel and no change in the effort needed to turn the handle.

I could have bought it for £30 but refused as I wanted to make one myself. Later I didn't fancy drilling all of the holes in the plates so didn't get round to it. Later a local company was selling a sets of B&S plates very reasonably so bought those but still didn't get round to it. Then my DW miller came with a conventional smaller one complete with a tailstock. It's done for me so far even though either D or W decided to use none standard plate hole counts.

Curious, any time I have changed a machine it's come with something useful apart from the mini lathe. That's more recently and people seem to be less inclined to make their own tooling now.

John

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Edited By Ajohnw on 10/04/2016 10:04:40

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 10/04/2016 10:03:40:

I could have bought it for £30 but refused …

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… …

[AjohnwParticipant @ajohnw51620]

Posted by Michael Gilligan on 10/04/2016 10:08:17:

Posted by Ajohnw on 10/04/2016 10:03:40:

I could have bought it for £30 but refused …

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… …

I bought a number of things off him Michael. He was more of an odd jobs that came along from various places machinist type toolmaker rather than a model engineer who was giving up and advertised the fact in bargain pages. Pre ebay in the uk I think. He made all of his tooling himself including a cast aluminium shaper with metal guides.

When I passed on the dividing head, last item to sell and he wanted me to have it he said he wouldn't sell it and would ensure it remained there because I would want it one day. I have been tempted.

John

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[Neil WyattModerator @neilwyatt]

> If you pick up an automotive piston and immediately try to fit the gudgeon pin, the chances are that it will not enter. Put down the pin and hold the piston in the hand for a few minutes. Then, the pin will be an easy sliding fit.

<showing off mode>

I made a piston and cylinder a few weeks ago. Cold the piston drops gently through the cylinder, warmed in the hand its a light push fit and sits where its put.

But I didn't use any measuring device – just lapped the together, finishing with autosol.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 10/04/2016 12:50:14:

<showing off mode>

I made a piston and cylinder a few weeks ago. Cold the piston drops gently through the cylinder, warmed in the hand its a light push fit and sits where its put.

But I didn't use any measuring device – just lapped the together, finishing with autosol.

Neil

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Very satisfying, but; that's just how 'one-offs' are made, Neil

<showing off> would be making another piston to fit the existing cylinder.

[and, NO, I couldn't]

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 10/04/2016 15:05:31:

Posted by Neil Wyatt on 10/04/2016 12:50:14:

<showing off mode>

I made a piston and cylinder a few weeks ago. Cold the piston drops gently through the cylinder, warmed in the hand its a light push fit and sits where its put.

But I didn't use any measuring device – just lapped the together, finishing with autosol.

Neil

.

Very satisfying, but; that's just how 'one-offs' are made, Neil

<showing off> would be making another piston to fit the existing cylinder.

[and, NO, I couldn't]

MichaelG.

Oh, I could do that, no trouble.

But then the original piston wouldn't fit.

Neil

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