Hornblock cut out question

Simon Moore
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Hornblock cut out question

Postby Simon Moore » Wed Apr 07, 2021 8:18 pm

I have a question that cropped up in my head today & wondered if it could be cleared up.

When i built my primrose no2 chassis i had to mark out my own cut outs, no half etch on the frames. Somehow i cocked up the cut outs & made one slightly higher than the other 3. I fitted the hornblocks in & set the beam so the footplate was level. All the hornblocks have movement but i was wondering if the difference in height might affect the running of the chassis at all because i am having issues? I am going to rebuild the chassis i think because something isn't right. I am just trying to tick off things it might be causing issue.

Cheers

Simon

bécasse
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Re: Hornblock cut out question

Postby bécasse » Wed Apr 07, 2021 9:36 pm

Simon Moore wrote:I have a question that cropped up in my head today & wondered if it could be cleared up.

When i built my primrose no2 chassis i had to mark out my own cut outs, no half etch on the frames. Somehow i cocked up the cut outs & made one slightly higher than the other 3. I fitted the hornblocks in & set the beam so the footplate was level. All the hornblocks have movement but i was wondering if the difference in height might affect the running of the chassis at all because i am having issues? I am going to rebuild the chassis i think because something isn't right. I am just trying to tick off things it might be causing issue.


Providing that it isn't stupidly higher, it shouldn't affect the running at all as on reasonable trackwork the horn blocks shouldn't hit the top of the cut outs anyway. If you have any doubts, soldering a piece of wire of the appropriate diameter (filed down if necessary) to the top of the horn block (to limit the extent to which it can rise in the over-high cut-out) is an easy solution.

davebradwell
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Re: Hornblock cut out question

Postby davebradwell » Wed Apr 07, 2021 10:09 pm

We're getting our bits mixed up here - the hornblocks are soldered to the frames and the axlebox fits on the axle and slides in the hornblocks. Does this change anything so far?

Simon, I was thinking you might have a problem with the chassis when I saw the side view of this loco and the rear part of the coupling rod is drooping down. If you added a photo of same side but front centre we can work out an error for this side.

DaveB

Simon Moore
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Re: Hornblock cut out question

Postby Simon Moore » Thu Apr 08, 2021 4:56 am

Thank you for the replies so far. Dave the coupling rod is my own making i twisted the wheel a little to see if the wheel was out slightly & i then took the photo. It was only after the photo that i too noticed this.

When i get some time in the workshop i shall take some photos of the chassis. At the moment work is mental & i am not getting home until late.

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Tim V
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Re: Hornblock cut out question

Postby Tim V » Thu Apr 08, 2021 11:41 am

Difference in height of the hornguides (not blocks)? Should make no difference.

More likely that you haven't matched the centre distance between the hornblocks and the coupling rods, as you cut the holes yourself. This dimension is critical.

Before rebuilding the chassis, have a read of John Brighton's method of getting the centres to match, Scalefour News 142. You have nothing to lose. I have taken locos from poor runners to excellent runners using those methods.
Tim V
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davebradwell
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Re: Hornblock cut out question

Postby davebradwell » Thu Apr 08, 2021 2:44 pm

Strictly, a hornblock carries front and rear sliding faces in one component, "hornguides" refers to separate front and rear slides as separate parts, although in practice the two terms seem to be used quite loosely as the guides can be seen as part of the block. All are attached firmly to the frames. The big sin is to call the axlebox a horn-something which just causes confusion.

I agree, Tim, that John Brighton's description of the coupling rod fitting operation is probably the most thorough and intended referring Simon to it after finding the nature of the error by looking at coupling rods, but he would have had to find it himself. JB's opinion of setting jigs matches my own experiences, too. There's been a few builds that have skipped over this critical stage recently.

DaveB

Daddyman
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Re: Hornblock cut out question

Postby Daddyman » Fri Apr 09, 2021 9:51 am

If Simon is using CSBs (not sure if by "beam" he mean a springy one or a rigid one), surely having one hornguide too high would impact on the deflection of the spring? I got one too high on my first build and when the chassis was held in mid-air that wheel was clearly higher; whether that would have had any effect once the loco was on the track I don't know, but I suspected it wouldn't be beneficial, and re-did it.

Re the John Brighton piece, yes, it's very helpful, but the article is contradictory in places. It also doesn't help Simon in getting his cut-outs in the right place: after dismissing various bodges in paragraph 4, the next paragraph begins "John's approach was therefore to take out all of these variables" - but that approach, which was to buy a chassis with correctly spaced 6mm cut-outs, and then fit Brassmasters' blocks, is of little use to those of us like Simon having to cut out our own slots!

John also dismisses axle jigs, and argues instead for using the crankpin, already attached to the wheel, and then to make up the rods with the wheels in the chassis. Again, all well and good if you have accurate ready-made cut-outs in the frames. But I'd question why a wheel in a hornblock would be any less likely to create slop in setting up than an axle jig in a hornblock, providing those jigs are tight in the block and tight where they narrow for the rods (as I was careful to make sure my home-made ones were).

While on the issue of setting up and soldering the rods on the crankpins, in the past I hadn't been able to understand why John opens the crankpin hole in the rod before doing this, as doing so creates a moving target: if the rod was left tight on the crankpin, I reasoned, that movement in the target would be removed from the equation. However, on reading the article 3 or 4 times this morning I notice that the text seems to advocate (paragraphs 9 and 10) using my system (tight holes during the soldering of the rods), while the diagrams and accompanying captions seem to advocate opening the holes before soldering the rods.

Personally, I did the following: (1) at the rod-soldering stage I used jigs not wheels and tight holes not loose; (2) I then introduced the wheels, and opened the rod crankpin holes to 1.55 to fit on 1.50 bushes - only then did I do the sighting experiment shown in the diagram (in fact, I think the sighting experiment is only necessary and/or possible once the holes have been opened). Anyway, I got there in the end (though I had to get heretical...), and agree that the John Brighton method (or one of them at least!) works, but the article could have been clearer.
Last edited by Daddyman on Fri Apr 09, 2021 11:54 am, edited 2 times in total.

davebradwell
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Re: Hornblock cut out question

Postby davebradwell » Fri Apr 09, 2021 11:02 am

I liked John's approach because, first of all, he acknowledged that the normal approach using jig-axles to set hornblocks doesn't achieve consistent, accurate results, or in my experience - never. The coupling rods, therefore, have to be adjusted to suit the actual axle centres and John describes a workable way of doing this although I've generally floated a bush (bit of 2mm od tube) into the already assembled rods. It was the thought process that counted.

It's all much easier with a mill and its dials but that doesn't help Simon.

DaveB

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Will L
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Re: Hornblock cut out question

Postby Will L » Fri Apr 09, 2021 12:05 pm

Daddyman wrote:If Simon is using CSBs (not sure if by "beam" he mean a springy one or a rigid one), surely having one hornguide too high would impact on the deflection of the spring?

No, what sets the level of the axle blocks in the guides is the level of the fixed fulcrums which should of course be in a nice strait line parallel to the designed axle centre line. The impact of getting one of the axle cuts outs in the frame slight too high would depend on what arrangements are made to guide the axle block, but just having a bit of extra space at the top of the cutout would not in itself be a problem. I think in this case we are talking about a compensated chassis(?) and, assuming we are not talking about a fixed axle, the ride height is set by compensation beam not the height of the cutout.

I got one too high on my first build and when the chassis was held in mid-air that wheel was clearly higher; whether that would have had any effect once the loco was on the track I don't know, but I suspected it wouldn't be beneficial, and re-did it.


When building a CSB chassis, I would recommend building it on 20 thou wires which will hardly flex at all with a bare basic chassis, and I would then check that, on the traditional bit of plate glass, all wheels are in contact with the glass. You only put in the correct wires when you know the models final weight.

Daddyman
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Re: Hornblock cut out question

Postby Daddyman » Fri Apr 09, 2021 12:31 pm

davebradwell wrote:I liked John's approach because, first of all, he acknowledged that the normal approach using jig-axles to set hornblocks doesn't achieve consistent, accurate results, or in my experience - never. The coupling rods, therefore, have to be adjusted to suit the actual axle centres and John describes a workable way of doing this although I've generally floated a bush (bit of 2mm od tube) into the already assembled rods. It was the thought process that counted.

It's all much easier with a mill and its dials but that doesn't help Simon.

DaveB

I suppose what I wanted to say is that John has chassis before he has rods, whereas I had (and Simon maybe has) rods before I had a chassis. And in those circumstances, the rods are one's only friend.

Will: yes, I think I was mislaid by the advice usually given that the cut-outs in the frames for HL hornguides don't need to be that accurate; I'd suggest they do need to be in the vertical plain; only side-to-side doesn't matter.

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Tim V
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Re: Hornblock cut out question

Postby Tim V » Fri Apr 09, 2021 1:27 pm

I referred to John B's article, not for the use of the Brassmaster's hornblocks (though they are very good), but because by matching the coupling rods to the wheelbase (not the other way round!), a duff chassis can be rescued, even from the scrap bin.

I have done it more than once.
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Will L
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Re: Hornblock cut out question

Postby Will L » Fri Apr 09, 2021 2:24 pm

Daddyman wrote:... I think I was mislaid by the advice usually given that the cut-outs in the frames for HL hornguides don't need to be that accurate; I'd suggest they do need to be in the vertical plain;...

If too low possibly as they could restrict upward movement. Being to high they may weaken the chassis but won't affect the suspension otherwise.

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Will L
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Re: Hornblock cut out question

Postby Will L » Fri Apr 09, 2021 4:45 pm

davebradwell wrote:I liked John's approach because, first of all, he acknowledged that the normal approach using jig-axles to set hornblocks doesn't achieve consistent, accurate results, or in my experience - never. The coupling rods, therefore, have to be adjusted to suit the actual axle centres and John describes a workable way of doing this although I've generally floated a bush (bit of 2mm od tube) into the already assembled rods. It was the thought process that counted.


I'm sure that to a proper engineer, the axle jig approach to things may not be quite as as inherently accurate as some but there are plenty of modellers who have shown it is accurate enough to produce models that perform well.

I would have said that while John's approach (Click to see SN142 scan down to page 30) is good where you have to fit the rods to the chassis, as you will with Brassmaster style axle blocks running in accurately cut 6mm slots in the chassis, or for that matter, any chassis where you have no further say on the axle centres. Those who are fitting separate Hornguides, such as those from HL, are better served by setting those from the rods using a chassis jig if you have one or jig axles if you haven't. The logic, that the rods must match the axle centres, remains the same. Have to say that Johns method of using the crankpins set in the wheels strikes me as introducing another potential source of error. That said I'm sure he knows exactly what he's doing and it works well for him.

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Re: Hornblock cut out question

Postby davebradwell » Fri Apr 09, 2021 8:25 pm

Actually WillL, I have nothing against the idea of jig setting the axleboxes and in the days of hand drawn artworks it could compensate for differences in coupling rod length and off-centre holes in axleboxes. We used to do stuff like this at work to achieve close tolerances with cheapo parts but the jig would be made out of 1/2" cast iron plates and there would be precision bushes and the like - somehow the sound idea gets overwhelmed by a little looseness here and there and I, and others, just got fed up with the whole slow procedure which often required the careful fitting job afterwards to achieve success. I'm sure it can be made to work and I think the robust commercial chassis jigs are likely to be a significant improvement.

The key part of John B's piece is that when all is done and despite clearances, the pin should be in the middle of the hole, the axle in the middle of its hole and the axlebox in the centre of its guides. The jigs don't achieve any of this - they just put the parts somewhere within that clearance and it takes a degree of feel to improve on this.

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Jol Wilkinson
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Re: Hornblock cut out question

Postby Jol Wilkinson » Sat Apr 10, 2021 10:05 am

The "standard" 4mm hornguide cut out, based on the variety of kits I have built, and subsequently used on those I have designed for LRM, is 6mm wide and the top edge is 4mm above the axle centre line.

I have built all my locos with compensation, using either Perseverance stepped alignment jigs or more recently the LRM tapered variety. I prepare the rods to take the crankpin bushes before using them to set up the hornguides in the frames.

The biggest issue I have found in getting a smooth running "chassis" has been quartering (largely overcome by using a GW wheel press) and the looseness of crankpin bushes on the crankpin. That is with AGW crankpins and I find that it is quite possible to build the chassis, get it running perfectly, disassemble it for painting and find on reassembly it has a "tight spot. Taking the rods off to open out the crankpin holes to ease them often results in the bushes moving on the pins, just making matters worse. The bush bore is sometimes not concentric, adding a further problem. By loosening the crankpin nuts and running it up and down and the tightening the nuts this can be usually overcome but it usually takes a few attempts. Taking the rods off to open out the crankpin holes to ease them often results in the bushes moving on the pins when re-tightened, just making matters worse.

I have now purchased Ultrascale crank pins and will try those when the opportunity arises. I believe that the (now defunct) Exactoscale wheels had a turned bush which fitted into the back of the wheel and held the crankpin more accurately, which seems to be much better system

As a qualified engineer ( a long time ago but I've got a faded certificate somewhere), I see nothing amiss with using the rods to set the axle bearing/hornguide centres. I find it important to keep tolerances to a minimum and the biggest problem is ensuring the hornguides are vertical. However, the axle movements should be no more than +/- .5mm in action, so any movement of the axles in an arc (determined by the vertical movement of the rods about the crankpins) shouldn't be an issue.

Philip Hall
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Re: Hornblock cut out question

Postby Philip Hall » Sat Apr 10, 2021 12:58 pm

Tolerances are indeed the thing. I have no tolerance (!) for a crank pin bush that is not concentric and I just chuck any like that or which are loose on the crankpin in the bin.

Ultrascale as supplied have the bushes with a reduced end to locate in the hole in the wheel, so this section will need to be removed when fitting to an AG wheel. The bushes are a very snug fit on the supplied 14BA screw so another variation is removed.

Finally, I sympathise with the problem of dismantling a smooth running chassis which on reassembly doesn’t run so smoothly. My solution to this is not to do it! I paint a chassis before the wheels go on and almost always they only go on the one time. Sometimes this approach has come back to haunt me but generally it has worked.

Philip

davebradwell
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Re: Hornblock cut out question

Postby davebradwell » Sat Apr 10, 2021 3:50 pm

If you're fitting Ultrascale crankpins to AG wheels the last thing you want to do is cut off the location bit as that is the very reason for using them. Just drill the 1.25 counterbore in the wheel picking up on the AG crankpin hole for location. Use a depth stop on the drill to prevent it pulling in too deep. Measure the required depth from the crankpin with the washer in place. Finally drill out the crankpin hole to a 1mm clearance so there is no ambiguity about which feature is locating the crankpin. Yes it would be better to drill the 1.25 holes with a drill jig but if this doesn't align with the existing hole it may not go well as one location will fight the other. I've done this conversion many times. The big win with these crankpins is the recessed leading type, plus the improved location of course.

Loose or eccentric bushes are clearly no use so just chuck them away. It's fundamental to fault finding that everything goes back in the same position each time. If you can't take it part you can't tweak it to make it better.

DaveB

Philip Hall
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Re: Hornblock cut out question

Postby Philip Hall » Sat Apr 10, 2021 10:35 pm

Dave, you’re quite right, I didn’t think about the obvious opening up of the hole to take the locating part of the Ultrascale bush. So far I have just been happy to have the better fit of bush upon screw, but next time I will use your method.

Generally I only find a few eccentric AG bushes but also sometimes the screw threads are a bit shallow which doesn’t help the loose bush situation.

Philip

Daddyman
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Re: Hornblock cut out question

Postby Daddyman » Sun Apr 11, 2021 5:49 am

Philip Hall wrote: Generally I only find a few eccentric AG bushes


This is still happening then, is it? I thought it was something from the bad old pre-Colin days.

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Jol Wilkinson
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Re: Hornblock cut out question

Postby Jol Wilkinson » Sun Apr 11, 2021 10:11 am

Drilling plastic needs care.
I would certainly recommend using a slow speed pillar drill when drilling plastics. Dave B's recommendation of a drill stop to limit the depth of hole is also good. The standard drill tends to bite into the soft material and draw itself in. IIRC, drills for soft materials should have less angle on the helix and rake on the cutting edge to reduce this. However a quick search for suitable small diameter drills hasn't produced any results yet.

Philip Hall
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Re: Hornblock cut out question

Postby Philip Hall » Sun Apr 11, 2021 1:27 pm

Daddyman wrote:
Philip Hall wrote: Generally I only find a few eccentric AG bushes


This is still happening then, is it? I thought it was something from the bad old pre-Colin days.


Not sure how old many of mine are. I’ve got a pot for all the excess AG crankpin components that are left over from a job. Often I have opened a pack of ten when I need six and just tip the excess in. Perhaps I should start a new pot to see how it goes.

Philip

davebradwell
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Re: Hornblock cut out question

Postby davebradwell » Sun Apr 11, 2021 1:49 pm

I think I've always done mine by hand with a pin chuck. If you're not quite square it's unlikely to be a disaster in a shallow counterbore - better to be right, though. You might drill a hole in a piece of steel for a jig to keep it square. I went off using a pillar drill for precision holes in soft plastic after some came out greatly oversize due to a slightly bent drill but a chuck eccentricity would do the same.

DaveB


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