Drilling crankpin holes Heath Robinson style

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Guy Rixon
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Drilling crankpin holes Heath Robinson style

Postby Guy Rixon » Mon Jul 14, 2014 7:50 pm

I have on hand a set of Gibson wheels to go in an EasyChas for a Wainwright C class. The wheels are the old pattern of Gibson where the crankpin hole is not moulded but merely indicated by a dimple in the front face. I am trying to find a way to drill the holes accurately at the same crank throw in all six wheels.

I'm aiming to use the only decent machine-tool I have: a Proxxon drill-press with a collet chuck and an x-y table. I've turned down the end of a spare axle so that it will hold a wheel without slop but still allow the wheel to be removed without spoiling the final fit when the wheelset is assembled. "Turned" here means "reduced with a file while rotating slowly in a mini-drill". The jig axle is held vertically in the notch in a machine vice (a rather nice Eclipse one I picked up for half-price in a sale). The vice is fixed to the x-y table so that I can manoeuvre the wheels with the pin position under the drill. On top of the vice jaws, I have a piece of MDF packing to receive the drill bit as it breaks through the back of the wheel. I can see by inspection that the vice jaws close nice and cleanly such that their top forms a flat, working surface to align the plane of the wheel. I'm trusting that the MDF packing doesn't mess this up, and also the working plane is sufficiently perpendicular to the drilling axis.

The plan now is to position the x-y table by eye for the first wheel and to leave it in that position, then move the other 5 wheels onto the jig with the table locked. That way, the only differences in position between wheels should be rotations about the jig axle and they won't affect the crank throw.

Any opinions on this? Am I doomed? I've never done this before so am floundering a bit. But maybe a set of Gibson wheels is a suitable sacrifice for learning; they're cheap enough.

One detail is worrying me. The drill bit I have has quite a long fluted length - not a stub drill - and flexes easily. It looks like it will be pulled sideways into the dimple as the drill contacts the wheel. I worry that the radial deflection will vary depending on the lining-up accuracy in the tangential direction. Also, it won't be running quite vertically as it penetrates. Is it worth trying to fix this or would it be a non-problem? Because the drill is colletted, I think I can get away with pre-drilling with an #000 centre drill (which I have) and then switching to the twist drill without affecting the jigging. Does this sound plausible?

All advice gratefully accepted (even if it's "give up and buy better wheels").

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Will L
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Re: Drilling crankpin holes Heath Robinson style

Postby Will L » Mon Jul 14, 2014 8:13 pm

What you need is illustrated in this post. The simple jig means you can then drill the wheels by hand, though its better done in a drill press. You should be able to make up the jig with your drill press. It may be that you don't get the throw exactly right but so long as all the wheel are the same that matters very little.

There is a suggestion that society will producing a variable drilling jig to do this.

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Russ Elliott
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Re: Drilling crankpin holes Heath Robinson style

Postby Russ Elliott » Tue Jul 15, 2014 12:27 pm

Guy - there's probably not much doubt that some sort of jig as Will describes is the best way, but if you are worried about drill wandering if not using a jig, then go at the wheel surface very slowly (vertically) with an undersize drill first, and let the drill run for ten seconds or so when it starts to nibble somewhere in the dimple - this will allow the drill to find its least resistance centre in the soft plastic. Drilling from the underside of the wheel could improve the accuracy further, as there is no dimple present on that side.

Alan Turner
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Re: Drilling crankpin holes Heath Robinson style

Postby Alan Turner » Tue Jul 15, 2014 1:59 pm

Alternatively use a much bigger drill or centre drill, that is stiff, to just form a dimple and then use the correct drill to drill the hole. Also clamp the wheel down so that it doesn't move between drill changes.

regards

Alan

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grovenor-2685
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Re: Drilling crankpin holes Heath Robinson style

Postby grovenor-2685 » Tue Jul 15, 2014 3:17 pm

to just form a dimple and then use the correct drill to drill the hole.
But there is already a dimple, produced accurately in the moulding process, I have just used the 14ba tapping size drills directly without any problems. I usually use the drill press on the Unimat but turn the spindle by hand, just remove any pips or imperfections from the back of the wheel so it sits flat, if there is a rear boss then create a hole for that in the packing piece for the same reason.
Keith

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Flymo748
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Re: Drilling crankpin holes Heath Robinson style

Postby Flymo748 » Tue Jul 15, 2014 9:40 pm

Will L wrote:What you need is illustrated in this post. The simple jig means you can then drill the wheels by hand, though its better done in a drill press. You should be able to make up the jig with your drill press. It may be that you don't get the throw exactly right but so long as all the wheel are the same that matters very little.

There is a suggestion that society will producing a variable drilling jig to do this.


That is something that we are definitely looking at. It will hopefully use the same engineering firm that has recently produced the very nice jigs for turnout construction.

There may be some time that elapses before the jig sees the light of day, as the Committee's own CME is currently devoting a lot of his valuable time to a small event that happens at the end of September. However it is on our list...

Cheers
Paul Willis
Deputy Chairman
Beware of Trains - occasional modelling in progress!
www.5522models.co.uk

Alan Turner
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Re: Drilling crankpin holes Heath Robinson style

Postby Alan Turner » Tue Jul 15, 2014 11:04 pm

grovenor-2685 wrote:
to just form a dimple and then use the correct drill to drill the hole.
But there is already a dimple, produced accurately in the moulding process, I have just used the 14ba tapping size drills directly without any problems. I usually use the drill press on the Unimat but turn the spindle by hand, just remove any pips or imperfections from the back of the wheel so it sits flat, if there is a rear boss then create a hole for that in the packing piece for the same reason.
Keith


The original OP said - " It looks like it will be pulled sideways into the dimple as the drill contacts the wheel" from which I took it that the existing dimple was not where he wanted it to be. Hence my suggestion. If it was accurate then why would it pull the drill?

regards

Alan

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David Thorpe
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Re: Drilling crankpin holes Heath Robinson style

Postby David Thorpe » Wed Jul 16, 2014 7:51 am

Not having pillar drill, lathe, or suitable machined jig I drill the crankpin holes by hand using a pin vice, appropriate drill(s) and extreme care. This seems to work, although I imagine it would give problems if you didn't want the hole exactly where the dimple is.

DT

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grovenor-2685
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Re: Drilling crankpin holes Heath Robinson style

Postby grovenor-2685 » Wed Jul 16, 2014 8:06 am

Quoting the OP,
It looks like it will be pulled sideways into the dimple as the drill contacts the wheel. I worry that the radial deflection will vary depending on the lining-up accuracy in the tangential direction. Also, it won't be running quite vertically as it penetrates
. This indicates to me that his concern is not about the accuracy of the dimple but his ability to line them up accurately under the drill. I have used the pin vice method you describe but it is a bit easier to keep the drill vertical in the press, turning the spindle by hand allows me to get the drill and dimple lined up nicely by feel, and to check I'm not bending the drill. (the Unimat has nice big exposed pulley on the end of the spindle that makes this easy, the Proxxon might be somewhat more difficult but it should be possible to turn the chuck to ckeck alignment before turning the motor on.
Keith

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Jol Wilkinson
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Re: Drilling crankpin holes Heath Robinson style

Postby Jol Wilkinson » Wed Jul 16, 2014 8:29 am

Holding the drill bit in a pillar drill and rotating it by hand will reduce the likelihood of it wandering off centre. However, I think that one of the problems with using "traditional" drill bits into relatively soft plastic is that the drill point can wander owing to the forces at the cutting edges. As when drilling soft metals (copper, brass) the drill bit probably should have zero rake on the cutting edges to prevent it digging in and going off centre.

The drilling jig mentioned above probably helps reduce the likelihood of the bit wandering as it hold the bit vertical very close to the wheel centre, reducing bending of the drill. That's probably another good reason for using large shank drills.
Jol

allanferguson
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Re: Drilling crankpin holes Heath Robinson style

Postby allanferguson » Wed Jul 16, 2014 2:14 pm

As Keith says, where the wheels have a dimple, they'll all have come from the same mould, so the axle to crankpin spacing is unlikely to vary, and where I've relied on this I've had no problems. But if you want a different spacing, or are concerned about the drill wandering, then the jig mentioned above is fairly infallible and dead easy to make. Remember that what matters is not the absolute precision of the spacing, but the consistency across all the wheels under a locomotive.

Allan F

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Tim V
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Re: Drilling crankpin holes Heath Robinson style

Postby Tim V » Wed Jul 16, 2014 2:31 pm

Don't forget that a difference of a few parts of a millimetre on the crankpin throw could make a set of wheels unusable.

As such, making a jig would probably save more time and money in the long run.

As someone once said "we don't have time to take short-cuts".
Tim V

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Guy Rixon
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Re: Drilling crankpin holes Heath Robinson style

Postby Guy Rixon » Fri Jul 18, 2014 7:27 am

Thanks to all for good advice.

The issue with the drill deflecting into the dimple was twofold: initially, I was trying to change the crank throw from 11" to 12" and catching the rim of the dimple. Subsequently, when I reset for an 11" throw, it was still deflecting, but that stopped when I set the job up in better light and with the drill-press repositioned so that I could inspect on two axes. So, accepting the 11" throw or drilling from the back, I probably could do it without the jig.

However, noting's Jol's point about the drill wandering regardless of the surface profile, I'll wait until I've made a jig.

billbedford
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Re: Drilling crankpin holes Heath Robinson style

Postby billbedford » Fri Jul 18, 2014 8:54 am

You could start the holes with something more robust than a ordinary drill, like a centre drill or even a round burr.
Bill Bedford
Mousa Models
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Paul Townsend
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Re: Drilling crankpin holes Heath Robinson style

Postby Paul Townsend » Fri Feb 09, 2018 5:17 am

Flymo748 wrote:
Will L wrote:What you need is illustrated in this post. The simple jig means you can then drill the wheels by hand, though its better done in a drill press. You should be able to make up the jig with your drill press. It may be that you don't get the throw exactly right but so long as all the wheel are the same that matters very little.

There is a suggestion that society will producing a variable drilling jig to do this.


That is something that we are definitely looking at. It will hopefully use the same engineering firm that has recently produced the very nice jigs for turnout construction.

There may be some time that elapses before the jig sees the light of day, as the Committee's own CME is currently devoting a lot of his valuable time to a small event that happens at the end of September. However it is on our list...

Cheers
Paul Willis
Deputy Chairman

I wonder if there has been any progress on this?
I ask because I recently found the rountuit engraved “locos”. After a long interlude due to house move, Highbridge restoration, Dartmouth building etc my various loco projects should get restarted in a few months.
A CP jig will be a must have so I am wondering if the Society one is coming soon or should I make my own ?

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Flymo748
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Re: Drilling crankpin holes Heath Robinson style

Postby Flymo748 » Fri Feb 09, 2018 6:46 am

Paul Townsend wrote:
Flymo748 wrote:
Will L wrote:What you need is illustrated in this post. The simple jig means you can then drill the wheels by hand, though its better done in a drill press. You should be able to make up the jig with your drill press. It may be that you don't get the throw exactly right but so long as all the wheel are the same that matters very little.

There is a suggestion that society will producing a variable drilling jig to do this.


That is something that we are definitely looking at. It will hopefully use the same engineering firm that has recently produced the very nice jigs for turnout construction.

There may be some time that elapses before the jig sees the light of day, as the Committee's own CME is currently devoting a lot of his valuable time to a small event that happens at the end of September. However it is on our list...

Cheers
Paul Willis
Deputy Chairman

I wonder if there has been any progress on this?
I ask because I recently found the rountuit engraved “locos”. After a long interlude due to house move, Highbridge restoration, Dartmouth building etc my various loco projects should get restarted in a few months.
A CP jig will be a must have so I am wondering if the Society one is coming soon or should I make my own ?


Hi Paul,

I'm afraid that there has been no specific progress with this jig. It's something that we might return to, but we are not looking at this at the moment.

We kicked a couple of ideas around, but nothing seemed to give a "universal" solution. If anyone has any design ideas that they think would work...

Cheers
Paul Willis
Deputy Chairman
Beware of Trains - occasional modelling in progress!
www.5522models.co.uk

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Le Corbusier
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Re: Drilling crankpin holes Heath Robinson style

Postby Le Corbusier » Fri Feb 09, 2018 7:28 am

Couldn't it be a circular jig with central rod and crankpin holes radiating round on a Fibonacci spiral with a locating mark on the line of each hole at the circumference. Perhaps a three part construction - a selection of circular base etches with voids to cover wheel sizes and facilitate the lining up with spokes/bosses. A 5mm deep machined cylinder accommodating the spiral holes, ensuring a true drill angle (with circumference markings) and a central rod to locate in the wheel.
Tim Lee

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Flymo748
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Re: Drilling crankpin holes Heath Robinson style

Postby Flymo748 » Fri Feb 09, 2018 7:47 am

Le Corbusier wrote:Couldn't it be a circular jig with central rod and crankpin holes radiating round on a Fibonacci spiral with a locating mark on the line of each hole at the circumference. Perhaps a three part construction - a selection of circular base etches with voids to cover wheel sizes and facilitate the lining up with spokes/bosses. A 5mm deep machined cylinder accommodating the spiral holes, ensuring a true drill angle (with circumference markings) and a central rod to locate in the wheel.


That was pretty much what we were thinking :-)

It's just the matter of someone having the time to produce a prototype and some drawings. There are one or two tweaks which even on your quick (but thoughtful) suggestion would need to be ironed out:

- if it uses etched (brass? nickel?) parts, is there the risk of wear, or misalignment if you fit them separately?

- What are the tolerances in a multi-part assembly?

- what size "disc" to make to cover a reasonable range of wheel sizes?

And so on. Not at all wanting to be negative. I could use one of these myself as I have my own couple of bodged up jigs at home. But it's better to think through the issues and solutions in advance. And then someone has to find the time to do it, of course.

Thanks!
Paul Willis
Deputy Chairman
Beware of Trains - occasional modelling in progress!
www.5522models.co.uk

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Le Corbusier
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Re: Drilling crankpin holes Heath Robinson style

Postby Le Corbusier » Fri Feb 09, 2018 8:22 am

Flymo748 wrote:
Le Corbusier wrote:Couldn't it be a circular jig with central rod and crankpin holes radiating round on a Fibonacci spiral with a locating mark on the line of each hole at the circumference. Perhaps a three part construction - a selection of circular base etches with voids to cover wheel sizes and facilitate the lining up with spokes/bosses. A 5mm deep machined cylinder accommodating the spiral holes, ensuring a true drill angle (with circumference markings) and a central rod to locate in the wheel.


That was pretty much what we were thinking :-)

It's just the matter of someone having the time to produce a prototype and some drawings. There are one or two tweaks which even on your quick (but thoughtful) suggestion would need to be ironed out:

- if it uses etched (brass? nickel?) parts, is there the risk of wear, or misalignment if you fit them separately? Just some initial thoughts ..Given Will's original jig referenced above - I was working on the basis that the etch could be relatively thick material as it is simply providing a level base across the wheel rims. It would need to take the form of some kind of spoked configuration to both support the cylinder and allow the drill bit plenty of scope to penetrate and also for you to view clearly the wheel spokes to eye in the positioning. So long as the etched base was of sufficient thickness wouldn't it be robust? The central hole to accommodate the rod would locate it in relation to both wheel and cylinder.

- What are the tolerances in a multi-part assembly? It might be necessary to produce another etch as a base beneath the wheel to take out the wheel boss from the equation. I had assumed that working off a piece of glass you could rely on finger pressure to keep things tight and level?

- what size "disc" to make to cover a reasonable range of wheel sizes? The etch could come with a selection?

And so on. Not at all wanting to be negative. I could use one of these myself as I have my own couple of bodged up jigs at home. But it's better to think through the issues and solutions in advance. And then someone has to find the time to do it, of course.

Thanks!
Paul Willis
Deputy Chairman


Thinking on the hoof here and maybe naive but I would have thought production costs would need to be low or people will just make there own? How often would such a jig be required?
Last edited by Le Corbusier on Fri Feb 09, 2018 9:44 am, edited 1 time in total.
Tim Lee

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steamraiser
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Re: Drilling crankpin holes Heath Robinson style

Postby steamraiser » Fri Feb 09, 2018 9:36 am

Personally I would welcome a crank pin hole drilling jig.

Gordon A

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Paul Townsend
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Re: Drilling crankpin holes Heath Robinson style

Postby Paul Townsend » Fri Feb 09, 2018 11:22 am

OK.
Thanks to Paul W ( Flymo) for quick response.
Having waited several years since the idea was floated and being aware of the practical difficulties surrounding it I am not surprised the project has stalled. Even a dynamic Society like ours has a shortage of roundtuits :(

I will add it my job list for this Spring to build my own MkII bodged version for a limited number of throws.
My MKI of years ago only did one throw and is worn out.


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