Post-build review reveals one major defect (incompetence)

[Tony MartyrParticipant @tonymartyr14488]

I have just about finished my 3 1/2″ William – final assembly of painted parts to go, but nothing major.

Lots of lessons have been learnt about techniques and problems that I had not met during my previous ‘career’, when building model ic and stirling engines. Such as the fact that I needed much more heat than I first thought when silver-soldering the foundation ring of a loco boiler and once you close up the firebox it is VERY difficult to fix an internal boiler-stay leak.

But my major defect as a model engineer is the drilling, with sufficient precision, of matched holes in such items as cylinder blocks and their covers. I even made a steel jig-plate for the cylinder holes using a rotary table but still managed to get less than perfect alignment, perhaps the drill wandered(?)

I certainly need a better type of centre-punch (blame the tool!) as I frequently find the ‘dot’ is not exactly on the marking out cross.

Since I have always wanted to built a marine TE engine this defect in my work is rather critical as there are many instances where precise alignment of tapped and clearance holes on a precise PCD are vital.

Any suggestions other than better glasses?

Tony

[Tony MartyrParticipant @tonymartyr14488]

[StovepipeParticipant @stovepipe]

I’m sure others will come along with better suggestions, but I do find a pair of “Optivisors” (or similar) invaluable for locating the drill point in the punch mark. Your comments suggest that “better glasses” would be of great benefit, because they suggest that the drill is going into the punch mark because it is nearby, but in the process the drill is bending slightly and thus effectively moving the punch mark slightly.

That’s my two penn’orth, now over to those far better versed than I.

 

Dennis

[Gone AwayParticipant @goneaway]

An optical centre punch would do even better.

[NJHParticipant @njh]

Tony

 

My wife bought me a Dankroy’ Optical Centre Punch for my birthday. Definitly sorts out the aging eyesight! Have a search on ebay.

 

Pleased to hear that you have finished “William” I started this many years ago and progressed well to start with:- Frames, Wheels, Cylinders, Boiler and most of the Motion completed – and then life and work brought all to a shuddering halt! Today it sits on a shelf in the workshop looking reprovingly at me. Other things in hand now but maybe one day……?

I would be very interested to know how it steams.

 

Regards

 

Norman

Edited By NJH on 08/10/2011 18:53:54

[Jeff DaymanParticipant @jeffdayman43397]

My best advice about marking out/punching/picking up punch holes- is don’t, if you can avoid it.

 

If budget permits, I strongly recommend installing some sort of DRO system on your mill. If you make up a sketch with XY coordinates of all holes, then move to these positions and drill and tap without moving the table, you have the best chance of getting holes where they need to be, and square. Oh, and use a centredrill to start every hole, gently, once the DRO’s got you in position.

 

JD

[adyParticipant @ady]

DRO is good.

I shall assume you can figure out its abilities on your own

 

Something I got 2 days ago has been intriguing, particularly for marking out.

It cost me a tenner from Lidl and it’s a power engraver.

It has a tungsten steel tip and you can mark a tiny point with it or you can draw a line on

steel or glass as well as engraving with it.

 

I messed about on day one and “engraved” some cheap spanners as well as numbering up all my change wheels.

 

The possibilities for marking out on steel are almost limitless.

 

and as I said, a tenner

 

 

 

At the end of the day though, real accuracy comes with practice and experience.

There are no shortcuts.

Edited By ady on 08/10/2011 23:42:22

[BogstandardParticipant @bogstandard]

I was given two brand new American made optical centre punches, designed for use with steel.

 

I now know why they were given to me, the chap must have been an enemy.

 

They basically consisted of a round magnet with a hole through the centre where you placed the spying graticule. When you were spot on over the scribed cross lines, you replaced the graticule with a hardened punch, then did the foul deed of marking.

 

This worked fantastic for the first time, come the second time, I found out why they were given away.

 

Unless you keep the job, and the surrounding area, clinically clean, the magnet attracted tiny particles from everywhere, and because the centre hole in the magnet was a very close fit for the other two parts, they could not be put through the hole unless you spent half an hour or so cleaning the magnet off.

 

They are both now with new owners, FOC, and I hope they have better luck with them than I did.

 

All I can say is that if anyone ever attempts to give you one, politely refuse and run away, fast.

 

 

John

[Clive HartlandParticipant @clivehartland94829]

To clean a magnet that has attracted particles use some plasticine, it gets embedded in the plasticine and the magnet is clean.

Another way is to us adhesive tape which also takes up the particles.

 

clive

[NJHParticipant @njh]

Hey John

 

Look at it positively – what an encouragement to keep the workshop clean! Maybe the person who gave you these was trying to tell you something.

 

I’m not sure just what a magnet would add to an OCP anyway – but the non -magnetic version works well .

 

Norman

[NilochParticipant @niloch]

Here’s a good one.

[The Merry MillerParticipant @themerrymiller]

 

The Dankroy version is a brilliant bit of kit.

 

Len.

 

 

[Tony MartyrParticipant @tonymartyr14488]

Thanks for the comments so far – I have concluded that I will purchase a non-magnetic OCP and that starting each hole with a small centre-drill bit is vital. The small the drills shorter should be the exposed shank length, if possible, to prevent wander.

Jeff’s idea of using coordinates on my mill table may be worth thinking about – but when I was an apprentice in the 1960s the only machines that we were not allowed near were the jog-borers in their air-conditioned cells so I never learnt the practical skills required. I have not found my 6 months training on Maag gear grinders of much help in my model making.

I have tried to clamp matching covers and blocks together and drill through both with an undersize drill but still found with the smallest items that the cover does not fit in all radial positions. I know the answer is to apply the upmost care and NEVER do the job in bad light or when I am tired.

A note to NJH re. William – I think the superheater ( an object of great beauty to me only) is a mistake, test indications are that it is a chronic grit trap and I doubt that its theoretical (heat energy) advantage is greater than its draught blocking disadvantage.

Tony

[Steve FParticipant @stevef]

Tony

Thanks for the interesting post. I to have been through these probelms with holes off centre. i kept blaming the machine. Eventually whilst looking on ebay i found some different centre drills called “spotting drills” which i believe are used in the CNC world. They have a very sharp point so its easier to see where they are going. The shank is much thicker (mine are 3mm) so they are stronger. Using them has improved my drilling accuracy.

Link to show the drill http://www.millerstooling.com.au/page.asp?id=54

 

hope this helps

 

regards

 

Steve

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Tony

 

All good advice before hand.

My personal experience with crappy off center holes was fixed by concentrating on grinding my center punch correctly.

If your punch is ground undevenly, then it will make an uneven mark on the metal. the most common fault is that one side fo the dimple caused by punching is higher than the other, causing your drill to wander

[WeldsolParticipant @weldsol]

I must admit I tend to use the digital readout then a centre drill but where I have to use a centre dot, once dotted file off the raised part (I was taught this many years ago) because if you made the dot at a slight angle you will have a raised edge to one side and this will pull the drill when it first bites.

 

 

Paul

[WolfieParticipant @wolfie]

Could you run a file over the dot thereby removing the raised part and leaving a smal dimple in the exact spot??

[Sub MandrelParticipant @submandrel]

> I know the answer is to apply the upmost care and NEVER do the job in bad light or when I am tired.

I discovered my patience suffered quite a bit as a reslt of the rugby – couldn’t do anything right!

 

Neil

[David LittlewoodParticipant @davidlittlewood51847]

Personally, I’m with the posters above who recommended fitting a DRO system. I fitted a 3-axis system on my milling machine 3 years ago and it has transformed the accuracy of my co-ordinate drilling. Before that I used an optical centre punch, and for really crucial holes I located the marking with an optical telescopic centring device like this one:

 

http://www.hemingwaykits.com/acatalog/Centering_Microscope.html

 

– mine is a different make, but same principle. The DRO is significantly better though, and allows holes to be located to at least 0.01mm accuracy, subject to the next paragraph.

 

However careful you locate the work under the drill, wander of the drill bit can spoil everything. I always start with a centre drill, and where the hole position is important I proceeed with a stub length drill. Bending of drills (and other tools) is proportional to the cube (IIRC) of the protruding length, so halving that length reduces the tendency to wander by a factor of 8.

 

David

[Ian S CParticipant @iansc]

Hi Neil,wot rugby? Ian S C Go the All Blacks

[Tony MartyrParticipant @tonymartyr14488]

Thanks to all contributors – I have my Christmas present list sorted.

A question to those who recommend co-ordinate drilling using DRO units:

Are there published tables or web based calculators for producing the co-ordinates or does one have to choose a datum hole and then work out the location of the others by basic trigonometry?

Tony

[Andrew BinningParticipant @andrewbinning37010]

The easiest way to produce coordinates is of course using CAD. My current project files are fortunately all on AutoCAD but I know this is too expensive for most people. TurboCAD is cheaper and there are “freeware” packages that can be had for the effort of downloading.

 

The learning curve to get a modest proficiency in a CAD package is time well spent. If you are working from paper drawings only put in the parts for which you need to produce a set of coordinates.

 

You will need to get an edge finder to pick up on the datum edges but the wiggler type with a ball are cheap enough. Use the edges as a datum and then all the holes for which you have produced coordinates will be in the right place as long as you use a spotting or centre drill before drilling. George Thomas (who wrote articles in ME and published several very useful books) always recommended drilling a hole well undersize before opening out to specified size.

 

Best of luck, Andrew

[Sub MandrelParticipant @submandrel]

Ian,

 

Sad proof that rugby balls aren’t designed for kicking

 

The Kiwis won’t have to do much to beat the French if they play like last weekend. Why do I think the runner’s up match will be more entertaining? Not knocking the All Blacks though.

 

Tony, yes there are calculators, but I usually knock up a simple excel spreadsheet and set the point size to about 28.

 

Neil

[David LittlewoodParticipant @davidlittlewood51847]

Tony,

 

When using a DRO, one usually locates a datum edge using an edge finder (well, in most cases, one locates two datum edges, say X and Y) and then simply uses the dimensions on the drawing to find the X and Y coordinates of the required hole location. No trigonometry is required. However, if you need to locate holes on the PCD of a round component, say a cylinder cover, then yes, you might need some trig, unless… If you have a “proper” DRO, that is to say ones like this:

 

http://www.machine-dro.co.uk/digital-readout-systems.html

 

rather than the ones that look like digital calipers with aspirations, then there is usually a built-in function for you to specify the parameters (start, diameter and number of holes) and it will take you to each hole location in turn.

 

David

Edited By David Littlewood on 18/10/2011 13:08:04

[AndyPParticipant @andyp13730]

Tony,

 

I don’t have a DRO but the suite of Model Engineers’ Utilities by Alan J. Munday has a calculator for pcd co-ordinates and lots of other useful stuff, I can’t remember (typical) where I downloaded it from but am sure Google (other search engines are available) can find it.

 

Andy

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