Precision layout & machining?

[AjohnwParticipant @ajohnw51620]

With the type Rick linked to they give a focal length. That's the working distance for some one that has good distance vision. In other words the eyes will be focused on the horizon when they are at their focal length away from the work.

I used some RS do at work for electronics sometimes. The problem I had was eye spacing and head size. They would have been better if the eye spacing was adjustable.

The binocular loupes do give true stereo vision and various working distances so it's easy to judge distance moved once the brain gets used to the magnified view – even at pretty high magnifications.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Before I start feeling totally inadequate …

If I understood correctly; Dianne indicated that she was looking to acheive +/- 0.0005" accuracy on hole positioning.

… Is there anyone here that can honestly claim to do that with manual marking-out and drilling ?

MichaelG.

[EmgeeParticipant @emgee]

Michael, I believe the dimension was 0.001" and in answer to your question I am not claiming to hold that tolerance on a 8 hole bolt circle using my cnc mill, or manually with my rotary table.

Emgee

[AjohnwParticipant @ajohnw51620]

Posted by Michael Gilligan on 23/10/2016 16:46:44:

Before I start feeling totally inadequate …

If I understood correctly; Dianne indicated that she was looking to acheive +/- 0.0005" accuracy on hole positioning.

… Is there anyone here that can honestly claim to do that with manual marking-out and drilling ?

MichaelG.

If the holes are marked with squares via a height gauge the drill can be pulled to get it central so that when drilled the holes split the scribed lines indicating a precision of better than one thou. The scribed line is reckoned to be 2 thou wide. My first test piece as an apprentice. The boxes don't show that clearly. Not my dice either as one of the dots is worse than any I did. They went away for chrome plating. Maybe 3 months work learning to file.

Some can do it and split the line. Ancient tool making technique that belongs in the dark ages. So scribe boxes and centre point. Feel for the centre scibes with the centre punch, drill and correct as the drill goes in. It is possible to feel where the lines cross with a centre punch. Pulling isn't easy.

 Just add that by the time some one has done a couple they do start getting a feel for it.

John

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Edited By Ajohnw on 23/10/2016 18:23:06

[colin hawesParticipant @colinhawes85982]

The most accurate way to space pitch circle holes without a DRO is definitely to use co-ordinates and when working to linear dimensions always ​measure from a single first datum point to avoid accumulated errors. Colin

[Howard LewisParticipant @howardlewis46836]

+1 for using co ordinates wherever possible, whether using Height Gauge, Handwheel graduations or DRO, rather than marking out.

In fact, in some cases, I would back Handwheel Graduations against some of the cheaper DROs which are Digital Calliper based. (I use one on the mill, but have known the reading to move during machining, presumably due to vibration).

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dianne Best on 22/10/2016 19:05:28:

Okay, so maybe 0.0001" is a bit of an exagerataion but definitely to 0.001 +/- half a thou.

I have always envied those who can layout a bolt circle of 6 or 8 #4 screws on a 2" diameter with such precision that the cover fit in any position – I can't do that! I want to be able to.

.

Emgee ^^^ was my reference

MichaelG.

[GwilParticipant @gwil]

There is an interesting description of the accuracy (or not) of getting holes in the right places using marking out and co-ordinate methods, and also jigs, starting on page 20 of Tubal Cain's book 'Simple Workshop Devices'. (The message I took from that is that drilling is a distressingly unreliable business.)

[duncan webster 1Participant @duncanwebster1]

pressed the wrong button!

Edited By duncan webster on 25/10/2016 23:24:27

[John Stevenson 1Participant @johnstevenson1]

Think I need to read up on this lot.

Got a job with drawing in today for some steel bushes that need welding into to holes burnt out with Oxy torch on a digger bucket.

Drawing says the weld chamfer needs to be 63 degrees minus 0.00' and plus 0.15'

[julian atkinsParticipant @julianatkins58923]

Commander W T Barker described in ME 1953 his 'Aids to Accuracy in Marking Out. With Special Reference to Problems Encountered in Small Work'. For those not familiar with Commander Barker, he built some of the most intricate small scale working and beautifully detailed marine engines ever made. They are all museum pieces and highly prized. He had only very basic tooling and machines.

I would recommend these articles to Dianne if she has access to ME in 1953. If not, I can email the articles to her.

Cheers,

Julian

[Mark CParticipant @markc]

JS, that sounds suspiciously like the draftsman needs re-educating for the real world…..

Mark

[duncan webster 1Participant @duncanwebster1]

Posted by Mark C on 26/10/2016 11:45:31:

JS, that sounds suspiciously like the draftsman needs re-educating for the real world…..

Mark

I'm afraid this sort of thing is common nowadays. Possibly due to so called 'draughtsmen' not having any workshop experience. In my first proper job after leaving university the drawing office had a door leading direct to the workshop. If a foreman appeared clutching a drawing you knew someone was in for a rocket on the lines of 'How the *** do you expect me to make this you **** so-and-so?' This concentrated the mind somewhat.

The other tendency is 'I'll make it out of better stuff'. Had to scrap loads on EN8 which had been called up as part of a welded assembly. Scrapping and replacing with EN3 was a lot cheaper than the preheat/controlled cool which would have been needed for welding EN8. It was only a drainpipe!

[ChrisHParticipant @chrish]

JS – I agree with Mark C, has the draftee ever seen a weld prep never mind ever doing one? They need to get out into the real world more!

Chris

[JasonBModerator @jasonb]

I suspect JS was pulling all your legs with the angle on the drawings, he has been know to do this in the past

 

Either that or JS has been scaling off the fag packet drawing again

Edited By JasonB on 27/10/2016 07:53:52

[JellyParticipant @jelly]

By and large referential measuring using a datum point and accurate machine scales will take you where you need to be.

I’d point out that accuracy is not an end in itself, but a property which work should possess only as much as is required, if you’re drilling a bolt circle, then it’s likely to be sufficiently inaccurate due to the process to make laying out to ? a thou somewhat pointless, and if it’s being bored after drilling to achieve that accuracy why worry, the referential method used to position the boring head will make it irrelevant.

Achieving repeatability of tolerances to ?0.1thou (i.e. 2.54μm) or even ?0.5 thou is the realm of second-op work, holes need to be bored or reamed, cylinders and mating surfaces ground, lapped, scraped or planed and so on.

As has been hinted at earlier in the thread, the best thing you can do is to think about the design intent of the piece and which dimensions are critical and require that extra mile in terms of accuracy.

[Roy MParticipant @roym]

I'm surprised that no one has suggested making a drilling jig with turned register spigot, an drilled on the rotary table. The jig is then modified to suit the mating part and re used. You can then drill by hand to a high degree of accuracy. This system can be further improved by using slip bushes, and series drilling the holes. (Worked O.K. On aero-jigs!)

Roy M.

[IanTParticipant @iant]

Or Guy Lautards "Poor Mans Jig Borer" ?

This is essentially a worktable with two edge guides at right angles to each other – the 'work' is held securely in the corner formed by these two guides by screwed clamps/dogs. Either turned spacers (which can be accurately sized with a micrometer) or slip gauges are then used to accurately positon a drilling guide (also clamped/held by a screwed dog) in relation to the edge guides. The screwed dogs fit into a hole matrix drilled in the worktable top. Very simple idea really but obviously it would take time to set it all up and make the spacers (if you have no accurate gauges) – so unfortunately it's also a pretty slow process.

Never tried it myself but if you really needed something drilled 'exactly so' then this might just be worth the effort – and it's repeatable.

Regards,

IanT

[Rik ShawParticipant @rikshaw]

Just spotted this on WARCO web site. Cheap as chips and probably ok for non intensive use for those on a tight budget.

Rik

**LINK**

[NJHParticipant @njh]

Cheers Rik – just what I need – Order sent !

Norman

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