DCC Loco Conversions - 23 - Dublo & Wrenn Brush Holder Replacement

This continues on from article 22 - Dublo & Wrenn Brush Holder Extraction

https://platform1mrc.com/p1mrc/index.php?threads/dcc-loco-conversions-22-dublo-wrenn-brush-holder-extraction.6017/

Brush Holder Replacement

First I discuss options for the various stages, my preferences, doing it and how it worked out and finally how it may be done better.

Options

How the replacement insulated tube is implemented depends on what approach you choose to take and the type of holder that was removed, formed or drawn, here are a few options I considered :-

Material Options

1) A removed formed tube could be re-used as only a few mm at the plug end would be threaded by the 4BA tap, however a similar paper type insulation would probably better than using heat shrink sleeving. This to be formed around the brass tube in a similar manner to the factory fitted insulated brush holder tube.

2) A removed drawn tube could at a pinch be reused but I would not recommend it, as the thread cut inside of it could catch the spring and also cause undue wear to a carbon brush.

3) Use a non-conductive material for the brush holder tube e.g. styrene tube or a plastic used in 3D printing.

4) Use brass tube with heat shrink sleeving shrunk onto it for insulation.

I did consider Kapton Tape or thin heat shrink sleeving but remember if this insulation is damaged a fried decoder is the likely result.

Once you have made your choice of tube material and if it is conductive the insulation it is time to enlarge the hole in the chassis to accommodate the new brush holder tube and it's insulation. A styrene tube is likely to have a greater wall thickness than the original brass tube, however a 3D printed tube may be capable of using the same wall thickness as the brass tube depending on your printer and the material used.

The tube does not need to be conductive as current will be transferred from plug to carbon brush via the spring.

Next, if the original hole requires enlarging there are options to consider here too.

Hole Enlargement Method Options

Which you use depends on your budget, skills and dexterity.

1) My ideal choice would be to set up the chassis on a mill or lathe tool slide so I can align and drill out accurately, but the nature of a wheeled chassis makes it difficult to clamp and would be different for each chassis type. At least with the Ringfield motors only a common motor end housing required clamping so only one simple drilling jig was required.

2) Drill out by hand using a DIY hand drill using drill bits that are checked for diameter as you work up to the required size. I used 0.1mm increment drills in an attempt to minimise cut depth on each size and in turn minimise the clamping required.

However it is still difficult to hold the chassis in position although with care and a very firm grip they can be held down by hand. Not for the timid as if the drill bit digs in (even with fine cuts) the whole chassis will easily be rotated in your hand, you have been warned, yep, got that tee shirt!

Potentially the chassis could be clamped upside down by the top of the chassis where the top bearing fits in a vice with soft to firm jaws. However hand drilling still has plenty of scope for problems including damaging the factory fitted insulated brush holder tube when you break through suddenly and run into it.

Also buying a set of 0.1mm increment drills is expensive if you only need a few of them for just this one job, I tried using them first instead of buying reamers which can also be expensive as an experiment.

It would be better to clamp the chassis inverted and be bolder with the drill sizes, then finish the hole with a parallel hand reamer.

3) Open up the hole only with hand reamers, this is much easier to control but still requires some dexterity to maintain alignment of the hole with its opposite number.

Taper pin reamers were designed to convert a drilled hole with naturally parallel sides to a conical one to accept industry standard pins for joining shafts e.g. Ref 23.1, we can use them to enlarge the hole and use a parallel reamer to finish the hole.

So far I have tried options 2 and 3 with mixed results and will describe them later.

My Preference

My preference is to fit an insulation covered brass tube, however the approach to enlarging the hole is common to all and the actual dimensions of drills or reamers depend on the outside diameter (O.D.) of your chosen replacement tube and it's insulation.

My Approach to Insulating Brass Tube

Taking a length of the 5/32 inch brass tube (Ref 23.2) cut the heat shrink sleeving (Ref 23.3) about 10mm longer than required and centre the brass tube lengthways in it. While holding one end heat from the middle and work towards the other end. To prevent air being trapped inside, smooth the warm heat shrink tubing in the direction of heating until it shrinks over the end of the tube.

From the middle again work towards the other end in the same fashion.

The grade of heat shrink sleeving I use when shrunk onto the brass tube is thick enough to be slightly compressible, has a relatively high friction coefficient, (grippy) and results in a nominal O.D. of 3/16 inch.



Reamers

Reamer parameter information is available in Ref 23.1 and were bought from the seller in Refs 23.4 and 23.5 who actually provides useful dimensional information on his listings.

N.B. metric and imperial taper pin reamers are specified in different ways.

Metric are sized from the small end
Imperial are sized from the large end

They have different tapers, imperial 1 in 48 , metric 1 in 50 , although that does not matter in this application.

Enlarging the hole for the new insulated tube.

The aim is to open up the hole in the chassis to a diameter into which the insulated tube is a gentle push fit, tight may tear the insulation, a little loose can be fixed with a spot of glue.

Continued in part 2 below

 

Part 2

Doing It - Attempt #1

Ex-3 Rail Dublo Duchess

How to drill out is a thorny issue, ideally I would clamp it up and re-drill it either on the lathe or the mill, but having avoided that for the extraction process I decide to take a chance and drill with a handheld cordless drill and enlarge the hole 0.1mm at a time.

The chassis castings are not square, all sides tapered due to the casting process, and to remove (and refit) the wheels and valve gear is a big job especially if the wheel bushes and crank pins will not co-operate, disturbing 50+ year old bushes is not a good idea.

So I managed it after 4 drill sizes but beware, ensure you have some soft metal or a piece of wood in the armature space or you can run the drill tip into the original insulated brush holder and tear pieces out of it (add a ragged tee-shirt to my collection, ).

Extracting the damaged insulated holder is slightly more awkward as the tap wrench cannot be used, however by the time the tap started to bite the tube it released easily and a new tube was easily cut and fitted. But needed a spot of glue to secure.

Having fitted the new tubes, alignment looked okay and initial test running on the rolling road was positive.

So it worked but has high potential for failure which if bad enough writes off a chassis.

Doing It - Attempt #2

Ex-3 Rail Dublo 2-6-4 Tank

This was done using only reamers. On a Duchess and a 2-6-4T the distance between the entry point to the brush holder and the armature end of the opposing factory insulated brush holder tube is approximately 30mm which means that only the first 30mm of any reamer can be used.

The bore at the entry to the hole must be slightly more than the tip diameter of the reamer to ensure the reamer can be aligned and start cutting to enlarge the hole.

Metric taper pin reamers seemed to give me more choice for hole enlargement followed up by using a parallel imperial reamer to finish the hole.

The reamers I used :-

Metric taper pin reamers, 3.5 mm, 4 mm, 4.5mm
Imperial parallel reamer, 3/16th inch.
A sturdy tap wrench or similar is required.



Steps as follows :-

1) The 3.5mm taper pin reamer only cut metal 5mm deep before it contacted the opposing brush holder but it does increase the initial engagement of the 2nd reamer which does most of the work. I have actually considered shortening this first reamer so it does more of the cutting.

Update 1 (15/05/22): I have shortened the 3.5mm taper pin reamer, see the 'reply' below for details.

2) Followed by the 4mm taper pin reamer which cuts all the way until it reaches the opposing brush holder.

Update 2 (28/05/22): Reworded step 3.

3) Then the 4.5mm taper pin reamer is applied but only enough that the 3/16 inch parallel reamer can enter the hole by about 4 to 5mm at the most. Do not over do this step.

4) Finally apply the 3/16 inch parallel reamer to the entire length of the enlarged hole.

This approach gives you much more control over the process when enlarging the hole by hand, and with the brass tube and heat shrink sleeving I used allowed the insulated tube to be a light push into the chassis block but firm enough not to need gluing. Of course this may vary depending on how consistent the application of the insulation was applied and how well I reamed out the chassis hole. But it is a good start.

Update 3 (27/07/22): New note on cutting and fitting new tube

Cutting and Fitting the New Tube

To date I have found that it is better to cut the new tube to be 1 - 2 mm longer than the original, this may reduce brush force slightly but allows greater clearance between the brush plug and the chassis, these two must not make electrical contact with each other for DCC operation.

Frequently the spring force against the brush is excessive especially with new brushes.

Often the work using the final reamer can, when done by hand make the bore slightly tapered unless your hand is very steady, however this has been easily remedied by spots of cyano glue around the outside of the tube insulation after inserting the tube half way into the chassis.

To ensure that the tube is fitted quickly i.e. before the glue sets and to ensure the correct distance from the commutator I now place a piece of plastic against the commutator which is supported by the pole pieces. In other words the armature and pole pieces are refitted prior to fitting the tube.

The images below show the steps.







Could it be done better?

Probably, having better understood the parameters of taper pin reamers I could possibly have used two imperial taper pin reamers instead of 3 metric. But the limiting factor is that unlike using them for their intended purpose we are using them to open up what is in fact a blind hole which limits penetration depth into the hole we are enlarging.

Prices of reamers vary enormously and can be costly especially as they are used for precision fitting of parts, we do not need the level of precision they can provide. Those that I found on eBay are somewhat cheaper than many tool stores I looked at online but I considered that at about £34 including delivery for the four works out to be cost effective providing they last long enough to modify another 14 or so chassis.

In the precision engineering environment they would be replaced after a few uses especially the parallel reamer, but what we are using them for is very low precision and appreciable wear is tolerable. Also we are cutting a relatively soft material.

Summary

Subsequent use of reamers, this time on a Wrenn 2-6-4 tank was quick and easy resulting in a good push fit of the insulated tube such that glueing is an option, it is only likely to move when extracting or re-inserting the plug into the tube which is done rarely.

I will continue to use these reamers for the rest of my Dublo locos, the white of the insulation can be seen in the image below.




A final reminder: if enlarging the hole for an insulated brush holder by hand drilling protect the original insulated brush holder as it easily rips apart if hit by the drill bit when breaking out of the drilled hole.

Next - conversions of locos requiring insulation of these brush holders

I will continue to update and revise my articles as I progress with my conversion work.

Discussion always

Jim

Return to Index


References

23.1Taper Pin Terms and Reamer Dimensions

For a brief explanation to make it easier to buy taper pin reamers.

https://en.wikipedia.org/wiki/Taper_pin


23.2 Brass Tube - a supplier

Widely available, 5/32 inch O.D. 0.014 inch wall thickness brass tube.
These have a nominal bore of 0.125 inch (1/8 inch, 3.18 mm).

Supplier: Macc Models

https://maccmodels.co.uk/shop/5-32-brass-tube-pack-of-4-12-long-2/


23.3 Heatshrink Sleeving - a supplier

This tube is nominally 6.4 mm as supplied and will shrink down to 3.2 mm, although this reel is far more than most members would want to buy but you can at least see the specification and order smaller quantities elsewhere. It's the sort of material you find more and more uses for if it is to hand. It can also be shrunk with a cheap domestic hairdryer, a professional heat gun is not necessary.

Note: Substituted black here as the white I originally bought is discontinued at this time.

Supplier: Rapid Online

Description: UniStrand 6.4mm Heat Shrink 2:1 Black 25m Reel

https://www.rapidonline.com/unistrand-6-4mm-heat-shrink-2-1-black-25m-reel-03-0344


23.4 Taper Pin Reamers - a supplier

This helpful seller includes a useful table of dimensions for each one on his listing which I used to determine the reamers I required.

Supplier: Anchor Tool Ltd selling under the name 'yes.tools' on eBay.

Sizes Used: 3.5mm, 4mm, 4.5mm

Listing titled (as eBay listing number/link may change):

"HSS Taper Pin Reamer Metric Imperial 2mm 2.5mm 3mm 3.5mm 4mm 4.5mm 5mm 1/8 1/4"


23.5 Parallel Reamer - a supplier

Supplier: Anchor Tool Ltd selling under the name 'yes.tools' on eBay.

Size Used: 3/16 inch

Listing titled (as eBay listing number/link may change):

"Anchor 3/16" Spiral Flute Hand Reamer Straight Shank Imperial Milling HSS M2"

 

Update 1: Shortened the 3.5mm taper pin reamer.

Working on another Dublo 2-6-4 tank loco chassis today I decided to cut 15mm off of the small end of the 3.5mm taper pin reamer.

Easily done with a cutting disc but unlike a twist drill the cut end does not need to be shaped as it is not used, cutting starts further up the reamer.

The 3.5mm reamer now has a start diameter of 3.7mm and will now cut up to 4.4mm before reaching the opposing insulated brush holder.
This spreads the hole enlargement work more evenly between the first two reamers.

The use of the other reamers remains the same.

Jim

 

Whilst converting some more 2-6-4 tank locos it has become apparent that this step could be worded better especially as it is critical to the effective use of the final reamer, so I have added and changed the following text in the original posting above.

Update 2 (28/05/22): Reworded step 3.

3) Then the 4.5mm taper pin reamer is applied but only enough that the 3/16 inch parallel reamer can enter the hole by about 4 to 5mm at the most. Do not over do this step.

Jim

 

Thanks Paul, more to come as I progress through my collection of Dublo and Wrenn relics, Jim

 

Update 3, added new note - Cutting and Fitting the New Tube

Jim