Prepared by Steve Luck Originator BillyBoy

Tip description: How to get the correct size hole, specifically crankpins but also applies to holes in general

BillyBoy, Sep 26, 2018
I am now starting work on the 03 (also from DJH) and I have instantly hit a snag. I am wondering if anyone out there who has built the kit still has a colour set of the original instructions? My instructions (as received with the kit) are in black & white which makes it a challenge when certain holes (the red ones apparently) in the coupling rods have to be drilled 2.6mm and others (the green ones) have to be drilled 2.7mm.

Sandell10466, Sep 26, 2018
I certainly don't have either 2.6mm or 2.7mm drills. If such diameter hole were required I would resort to a 2.5mm drill and a light touch with a 5 sided broach until whatever had to fit in, or pass through, these tightly specified holes would do so.

StephDale. Sep 26, 2018
Or drill them all the smaller size, try the parts and open out the ones that don't fit…. Or measure the components you're supposed to be mounting through the holes and open up the holes accordingly…

Thompson21380, Sep 26, 2018
I would agree that the difference between 2.6 mm and 2.7 mm is so small as to be virtually negligible - even if I tried to drill out that size precisely I would be lucky to get anywhere near the correct size. Drill them out 2.5 mm and use a watchmakers broach to open them up on fit would be my advice.

BillyBoy, Sep 27, 2018
Based on the consensus I am going to go for the 2.5mm drill and broach approach. I did consider this approach but being a newbie at building model locomotives I was concerned by the prospect of introducing the dreaded “slop”. Slop is a mystery to me. I find it confusing as it seems I have read as many stories about how having too much slop is bad as I have stories about how having too little slop is also be bad. You must need to find the slop sweet spot - whatever that may be. I figured drilling the holes per the instructions would get me out of this conundrum. So I shall start by going tight and then gently opening up until everything seems to be running OK. I shall let you know how I got on when I am next on here asking more questions because I am stuck again. Now it is on with laminating the rods which I am sure is going to be a bundle of fun.

Buckley10339, Sep 27, 2018
The only thing you must do with rods is to get the holes in line first before any soldering takes place. The best way is to drill holes slightly undersize then once the soldering is done the hole can be opened to the correct diameter. This is when you are going to need drills the correct size.

Jim Snowdon, Sep 28, 2018
2.5mm, and I have never found a need to open the holes out any further from the “as drilled” state.

I drill all the laminations first, and then solder up, using the shank end of the same size drill to align them. A traditional wooden clothes peg comes in handy as a means of holding everything together whilst soldering.

Thompson21380, Sep 28, 2018
The only thing to beware of when opening up the holes in laminated parts, particularly rods, is that if you try to do it with a drill the drill bit has a habit of “snatching” and that will often cause the laminations to be torn apart.

It is far better to open up the holes using a hand broach which is more controllable.

Sandy Harper, Sep 28, 2018
From bitter experience, I would agree with Jim on opening out the crank pin holes. If you try to open them, after laminating, because they are now two, or even 4 times, thicker, it is possible for them to come apart, with the heat generated during the cutting process, melting the solder. It is much better to do the laminations individually. I never use a drill to open up holes. I always use a taper broach. If a drill catches it will distort the metal at best, or even totally wreck it at worst.

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Jim Snowdon, Sep 28, 2018
The other, probably higher, risk is that the drill bites suddenly and rips the laminations apart. Most of us are using drills that have their cutting edges shaped for steel, and are either using hand-held drills or pillar drills with probably more end float than is really desirable.

I found (the hard way) that the better tactic is to drill the individual laminations whilst they are still in the fret, which makes them easier to hold on to, and to drill through them into a wood block. Because each lamination is thin, the point of the drill comes through before the corners have started cutting and is supported to a degree by the wood as it drills into it. The harder the wood the better; birch ply and good softwood are alright from experience, but I would avoid anything truly soft.

Jim Snowdon, Sep 28, 2018
I don't have any broaches that size so I learned to drill them out of necessity. However, a round needle file, inserted in the hole and twisted as you would a broach, will do the job.

If I wanted a broach, I would go in search of a clockmakers supplier in preference to any of the “hobby” stores that would like our trade.

Thompson21380, Sep 29, 2018
Indeed, those are precisely the ones I use - from about 0.5mm up to about 5mm - if you can find them on the market you will find them invaluable - I got mine second hand at auction. They are tapered and fit into a pin drill holder perfectly.