First chassis in P4

Help and advice for those starting in, or converting to P4 standards. A place to share modelling as a beginner in P4.
davebradwell
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Re: First chassis in P4

Postby davebradwell » Sun Mar 01, 2020 9:14 am

I've sent you an email, David, as trying to clarify the points you raised through a multi-way conversation is proving impossible. Once you get a set of axleboxes and hornblocks installed by whatever means takes your fancy it should be easy to put a beam across the top perhaps with a vertical screw bearing on the centre as an adjustable pivot. This would get you to where you were heading in the first place.

My point about artworks was CAD v hand drawn. Use of cad produces a much more accurate result and it's possible to match centres of axles and rods very closely. It's probably not possible to use hand produced stuff these days - the company that does my etching put their process camera in the skip at least 10 years ago. I would think your etches were hand drawn, like my earlier stuff.

DaveB

Daddyman
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Re: First chassis in P4

Postby Daddyman » Mon Mar 02, 2020 7:15 pm

I got the frames erected on Saturday, and am ready to start putting the compensation beams into the chassis. However, the coupling rods don't pass muster. They're very deep in section, have large bosses and are etched for 1.5mm clearance. I use Romford crankpins with Gibson wheels, and they're 1mm diameter, so 1.5 is far too much clearance - though it's the aesthetics of the oversize bosses that worries me more.
So anyway, the plan is to use some Mainly Trains J72 rods. These have the advantage of being close to scale profile with small bosses and holes etched at slightly under 1.2mm. The photo shows the two sets of rods (left = J72):

20200302_182213_resized.jpg


The major stumbling block, of course, is the different wheelbase of the two classes, meaning I'll have to do a cut-and-shut job on the rods. However, this shouldn't be any worse than making up universal rods by Gibson or LMS, and I think I have a plan for how to do it. I glued one of the compensations beams to a piece of wood fixed to my Proxxon coordinate table, and inserted an unwanted Gibson EM axle into the chuck of the bench drill. I then wound the X and Y handles on the table until the axle slotted perfectly into the bearing in one end of the beam, and zeroed X and Y. I then wound the X handle along until the axle slotted perfectly into the bearing at the other end of the beam. This gives me an accurate measurement of the wheelbase for the front two drivers (I'll do the same for the other two drivers with the shorter rear beam). I can now drill two 1.2mm holes in something at exactly the same distance as the bearings using the bench drill and the coordinate table to set it up, and then insert pieces of 1.2mm brass into the two holes to act as a jig on which I trim and solder the coupling rods to length.

X and Y dialled up and X zeroed:
20200302_181125_resized.jpg


End of the process - far end of the beam reached and an accurate measurement taken:
20200302_181647_resized.jpg


Four questions if I can prevail on you all:
1. What is the best material to mount the drill the holes in? Wood? Tufnol? I could even stretch to aluminium block, but not sure if a 1.2mm bit would break.
2. The holes in the coupling rods are etched at about 1.2mm (most of them!), and the crankpins are 1.0mm. If I broach the holes so that they are all 1.2mm it will make it easier to find brass rod to use in the jig. However, will 0.2 mm of play be fatal to good running?
3. Will I suffer either poor running or banishment if I don't join the rods on the knuckle but on the crankpin? The MT etches are only set up for the latter.
4. The centres on my beams might possibly be 24.1mm rather than 24.0. Should I make the rods 24.1mm long, or forget the difference? (I think I can hear Dave wincing...)

Thanks for your help!

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Tim V
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Re: First chassis in P4

Postby Tim V » Mon Mar 02, 2020 7:44 pm

You're over-thinking this.

Have a read of the John Brighton method of setting the rods (S4 News 142), so much simpler.

If it's good enough for the professional, it's good enough for us.
Tim V
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grovenor-2685
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Re: First chassis in P4

Postby grovenor-2685 » Mon Mar 02, 2020 7:53 pm

Regards
Keith
Grovenor Sidings

Daddyman
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Re: First chassis in P4

Postby Daddyman » Mon Mar 02, 2020 8:23 pm

Thanks to both. Had a feeling I might have been reinventing some wheel or other.

davebradwell
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Re: First chassis in P4

Postby davebradwell » Mon Mar 02, 2020 9:46 pm

David,

There's certainly some sound stuff in the JB article but I don't think you're about to fit his axleboxes, which is where it all started. Assuming you have some axleboxes, somehow:

0.2 clearance in rods will be terrible - 0.1 is bad enough. You should be able to drill out to 1.5 and fit bushes from 1mm bore tube. Tufnol is a good material for jigs but will wear your drill more than the aly - you should be able to drill the hole especially if you use a lubricant (paraffin for aly but anything will help). Don't forget to use a centre drill if you're spotting the holes at accurate centres with your table.

The John Brighton article re-inforces the sound view that jointing rods on a crankpin is not good, especially if you're driving the rear axle.

You should aim to have the rods the same length as the wheelbase within a thou' or 2 which means trying very hard to get them right. If you know there's a difference that's a fail. If you still want to do it with numbers then best way to measure centres is by taking the average of measurements between and over the axles (with vernier). Searching for the centre with your bit of rod isn't so good.

You could set your frames very square using your co-ordinate table but you'd need a lever type dial gauge and this might be just a bit too much for the more conservative contributors.

You'll be realising that inventing your own way of making a chassis leads to a lot of questions. I'm with JB though - I hate those coupling rod jigs and could never get them to give decent results. Haven't used them for years.

DaveB

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Re: First chassis in P4

Postby Daddyman » Tue Mar 03, 2020 6:56 am

Thanks, Dave. Food for thought there. I'll measure across and between the axles as you say, bush the rod holes, and get hold of a centre drill. Point taken about the jointing of the rods, which will mean drilling the rods that I have (though I'm slightly worried about Bamburgh, further down the line, for which I have some RT Models MW coupling rods: these are over-etched, with the hole for the riveted joint unusable before I even start).

Would I save myself some trouble by using Gibson crankpins rather than Romford?

I'm no doubt missing something as I can't work out why John Brighton introduces play by opening the holes in the rods before adjusting the rods to length. Surely if the crankpins/jig uprights were an interference fit there would be no need to do all the sighting that he shows in the coloured diagrams? And why introduce a moving target by doing the process in situ using the wheels, which can rotate out of position in the middle of the job?

I can see it would work for an expert, but I can't help thinking a beginner such as myself would have more chance of success with a jig of the sort I'm suggesting which has the advantage of (1) an interference fit in the rod holes, guaranteeing absolute accuracy, (2) length accurately plotted off the axles, and (3) no chance of the wheels moving during soldering. And in fact, if I understand you correctly, you're saying my jig will work as long as:

(a) I calculate my rod centres with the vernier using the axles and not the method I used last night
(b) I get a centre drill.

Is that right?

Thanks so much for your help!

Philip Hall
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Re: First chassis in P4

Postby Philip Hall » Tue Mar 03, 2020 8:23 am

You would almost certainly have an easier time using Gibson crankpins, they are designed for the wheel, but do make sure that the hole in the wheel is vertical. The bushes supplied will be nominally 1.5mm (in practice perhaps a bit less) and the crankpin nut will screw onto the crankpin which is safer than soldering which I think you would have to do with a Romford crankpin. Also make sure that the hole in the bush is not off centre, but this is easy to see by eye.

Since, though, you are concerned about the size of the coupling rod bosses it may still be a case for trying to use Romford pins.

Philip

davebradwell
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Re: First chassis in P4

Postby davebradwell » Tue Mar 03, 2020 9:56 am

As Philip says, you're likely to be better off with the Gibson pins, especially if the Romford type still require a tapped hole in the wheel which can end up anywhere - the hole, that is.

JB's oversize holes are probably just one of those mysteries of the art of fitting. They would give confirmation that you're not pulling the pins out of position. Doing it on the job cuts out the middle man.

Sounds like you stand a good chance of success with your jig if you do as per your summary. Move the slide significantly before drilling the first hole to give the leadscrew nut a chance to settle and keep going the same way. Digital readout's the thing here! No I haven't!

Of course the centres may be different each side!

DaveB

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Tim V
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Re: First chassis in P4

Postby Tim V » Tue Mar 03, 2020 10:04 am

I have used the JB method of setting the rod length on the chassis with great success.

Plus, I've taken poor runners and brought them up to scratch.

The minor inconvenience of the wheels moving (and actually, they don't) can be countered by holding them.

The professional approach - do it once, do it right, and don't expect it back under guarantee.
Tim V
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davebradwell
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Re: First chassis in P4

Postby davebradwell » Tue Mar 03, 2020 10:21 am

I think a professional engineer would consider the measure and drill method - it depends if you're a fitter or machinist and both approaches are sound. A bit of equipment, vernier and drill table, certainly reduces the need for feel. It's down to personal preference, really. If it was mine, I might drill the holes in a set of blanks and make new rods if I was in the mood.

DaveB

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Tim V
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Re: First chassis in P4

Postby Tim V » Tue Mar 03, 2020 10:53 am

A professional engineer wouldn't start with what we have got.

I'm talking about the professional approach to model making.
Tim V
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Daddyman
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Re: First chassis in P4

Postby Daddyman » Tue Mar 03, 2020 5:16 pm

First of all, many thanks for all the replies - I'm extremely grateful to you all. I'm a firm believer in daring to ask stupid questions, so thanks for your patience.

Philip, thank you for the advice on the Gibson crank pins. As Dave says, it's probably a safer bet given that there'll be no (less?) need to open out the holes in the wheels.

Tim, thanks for your input. You saying that the wheels don't move makes me think that perhaps you experts are not reaming the bearings during setting up (leaving them stiff on the axles), whereas I'm opening them to running clearance straight away. It occurs to me that it might be more helpful for various reasons (not least the John Brighton rod method) to have the wheels an extremely tight fit on the bearings. Would I be right in thinking that?

Dave: if I understand well, the advantage of a centre drill is that it is less likely to wobble off true during the drilling process. I have two questions: (1) presumably the centre drill needs to be smaller than or the same size as (but certainly not bigger than) the hole I want to drill; (2) can I make my own centre drill? Mick Nicholson assures me I can: could I Loctite a drill of, say, 1mm into a brass tube with ID 1mm and OD 1.5, leaving just a couple of mms of the 1mm drill exposed?

Thanks again.

davebradwell
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Re: First chassis in P4

Postby davebradwell » Tue Mar 03, 2020 6:37 pm

If you just plunge in with a long drill the hole can be anywhere. You're right on both counts with the centre drill but what if your home made bodge wobbles? You could grind the shank off a drill and just hold the fluted bit in your chuck if you want to save the cost - or even wait until you break one!

For small sizes I go for the centre drills with curved profile at the ends as these are far more resistant to breaking than the standard shape which loses its nose at the slightest misalignment. Cost rather more but I'm sure there's a saving. It's just a consumable and I can't say my hobby costs me very much.

DaveB

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Re: First chassis in P4

Postby Daddyman » Tue Mar 03, 2020 7:46 pm

davebradwell wrote:If you just plunge in with a long drill the hole can be anywhere. You're right on both counts with the centre drill but what if your home made bodge wobbles? You could grind the shank off a drill and just hold the fluted bit in your chuck if you want to save the cost - or even wait until you break one!

For small sizes I go for the centre drills with curved profile at the ends as these are far more resistant to breaking than the standard shape which loses its nose at the slightest misalignment. Cost rather more but I'm sure there's a saving. It's just a consumable and I can't say my hobby costs me very much.

DaveB

That's all clear Dave, thanks. I don't mind the cost either. But could you be more specific about the curved profile please? When I search for "centre drill curved profile" on ebay all that comes up is a baseball cap! Similarly, on google I can find a pdf explaining the difference between curved and straight centre drills, with technical specifications, but it doesn't seem to be a term sellers are using. Do you by any chance have a link?



The other question before I take the plunge and use the drill in anger is: should I use a collet or chuck? I have both and they're both genuine Proxxon parts, but I've heard contradictory reports on which is the most accurate for our uses.

Many many thanks again!

Philip Hall
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Re: First chassis in P4

Postby Philip Hall » Tue Mar 03, 2020 8:47 pm

David, there is no need to touch the hole in the Gibson wheel. The crankpin screw cuts its
own thread in the wheel and you tighten it into the wheel. This is when you will find out whether the pin is vertical or not. If it isn’t, I just bend it straight!

Philip

Daddyman
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Re: First chassis in P4

Postby Daddyman » Wed Mar 04, 2020 5:42 am

Thanks, Philip. It's some years since I've used them.

Terry Bendall
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Re: First chassis in P4

Postby Terry Bendall » Wed Mar 04, 2020 8:46 am

Centre drills are available from many sources. Two examples are

https://www.maidstone-engineering.com/centre-drills

http://www.tapdie.com/html/centre_drill ... drill.html

The smallest imperial one (BS1) is 3/64 tip size - just under 1.2 mm and the smallest metric size is 1.0 mm. Varying prices from about £1.20 to £4.75 depending on the supplier. Certainly no need to make your own. If the required hole is smaller than the tip of the centre drill, just use it to get the hole started a little way in.

Daddyman wrote:should I use a collet or chuck?


Are you using a lathe or a drilling machine? All things being equal the collet is usually more accurate but a lightly used chuck should be OK. Depends on how well they are made in the first place.

davebradwell wrote:just hold the fluted bit in your chuck


Gripping just on the flutes may not always be accurate.

Terry Bendall

davebradwell
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Re: First chassis in P4

Postby davebradwell » Wed Mar 04, 2020 10:24 am

My late post last night seems to have disappeared. It referred to Drill Service (Horley) website who have more types of centre drill than you can imagine - look for HSS Curved type. I've always found their service very good and no min price. They also sell genuine number drills to give odd sizes. Very small sizes of centre drill are listed and these are the ones that benefit from the extra strength of the curved shape. You'll have to decide for yourself what to consider small.

Plenty of advice on that now.

Sorry Terry, gripping on the flutes was a bit naff!

DaveB

John Palmer
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Re: First chassis in P4

Postby John Palmer » Wed Mar 04, 2020 3:27 pm

Dave, I've taken a look at the Drill Service (Horley) website but couldn't find any centre drills the shape of which was described as 'curved'. I think you are probably referring to what the website describes as 'radius type' drills but would like to be sure I haven't got my wires crossed. In particular I was drawn to the https://www.drill-service.co.uk/products/centre-drills-spotting-drills/centre-drills/radius-centre-drills/cdr-hss-centre-drill-radius-type/ range, which seems to go down to a 0.5mm pilot diameter. That can't be far off correct scale diameter for the bores in the end of axles, and as such would be of particular interest to me since my existing assortment of centre drills all have pilot diameters too big to reproduce this feature satisfactorily after I have faced off the axle end.

Edited to add that Drill Service (Horley) looks like a promising source of diamond point burrs such as those employed by Jeremy Suter and shown in his excellent thread on chimney turning at https://www.scalefour.org/forum/viewtopic.php?f=132&t=6350

davebradwell
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Re: First chassis in P4

Postby davebradwell » Wed Mar 04, 2020 5:03 pm

Very sorry, John, you're right. It was fresh in my mind last night but the repeat this morning failed miserably in its accuracy. You might want to check that teh cenre created by one of these drills matches your expectations of its appearance. They're not completely immune to breaking, either. I've also bought 60 deg single flute countersinks for clearing stray solder out of p-p brgs but it would be cheaper to chuck them away. You can deepen the cone, however although only with the smaller sizes of cutter. The range of stuff available is truly amazing. For some reason I buy my endmills from Cutwel who stock a further array of fine toys.

Back to centre drills, I read in Model Engineer that Spotting Drills are now preferred because they're stronger but have not been able to find any in our sort of sizes. These are really just a short drill (as you suggested, David) and used in automated machine tools although I suspect they are ground differently - I've never seen one.

DaveB

Daddyman
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Re: First chassis in P4

Postby Daddyman » Wed Mar 04, 2020 5:25 pm

Many thanks once again to you all - Terry, Dave, John and Dave again. Your generosity with your time is much appreciated! I'll have a look at the recommended sites this evening and get something ordered.
David.

Terry Bendall
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Re: First chassis in P4

Postby Terry Bendall » Thu Mar 05, 2020 8:28 am

The radius centre drill is something new to me and proves one of my beliefs that you learn something new every day. Put the term into your preferred search engine and it will bring up numerous suppliers. There are two uses for the centre drill. One is to dill a hole in the end of a bar held in the lathe, either to provide a hole for a lathe centre, or to provide a start for a conventional drill bit. The other use is in a drilling machine to provide an accurate start for a conventional drill bit. The theory behind the radius centre drill is that the bearing surface for the lathe centre is then recessed and will not get damaged. Not normally a problem for us since supporting the end of a rod on the lathe centre is not often required but it is certainly useful where a component has to be frequently mounted between centres. I have no idea of how the accuracy of the two types compares but I would have thought that here was not much, if any difference.

Terry Bendall

Edward45
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Re: First chassis in P4

Postby Edward45 » Thu Mar 05, 2020 12:31 pm

Using collets on my Unimate and cheap Chinese centre drill - about £5.00 for 10 - followed by Tungsten carbide drill - from a similar source - drilled a 1mm hole down 1.5mm steel. Did it to fit one of these cheap motors with a 1mm shaft.

John Palmer
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Re: First chassis in P4

Postby John Palmer » Thu Mar 05, 2020 1:15 pm

Thanks to Dave for confirming that his reference was to radius centre drills, and to Terry for his comments on their theory and practice. I can see that in principle they ought also to improve the centring of a component on which a taper is to be generated.

As to spotting drills, an interesting article on this can be found, amongst others, at https://www.harveyperformance.com/in-the-loupe/machining-techniques.


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