Making a specialised D-bit

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Guy Rixon
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Making a specialised D-bit

Postby Guy Rixon » Fri Jan 22, 2021 11:00 am

I need a tool to "machine" home-printed buffers. Unlike those printed at Shapeways, where the 1.0 mm and 0.5 mm bores just need lightly cleaning out, the home-printed ones need more work. The 1.0 mm bore needs to be reamed to size and the 0.5 mm bore needs to be drilled as the printer leaves only a hint of this cavity. Naturally, the small bore needs to be coaxial with the large bore, and this happens only sometimes when I drill them by hand with ordinary drills.

I had thought to make a stepped D-bit where the shoulders cut the outer bore to size and the tip acts as a centre drill to start the 0.5 mm bore. Possibly this can be done by hand by cutting or grinding down a broken or blunted twist drill (got plenty of those), but I am clueless about the details.

Is is reasonable to make this thing by hand, or can it only be made with machine tools? I'm unsure about getting the tip centred.

Do I need to anneal the metal before cutting the profile? Presumably yes, if I want to file it, but if I choose to grind it? How exactly would something this small be annealed?

Do I need to harden it after shaping? If so, how?

davebradwell
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Re: Making a specialised D-bit

Postby davebradwell » Fri Jan 22, 2021 11:14 am

Why not drill the 1mm hole then slip in a piece of tube to guide the 0.5 drill? Actually you can use a 0.55 which is just a touch stronger. Certainly in metal the cone of the first drilled hole starts the second concentrically - I've done hundreds of them. Toolmaking at these small sizes would normally call for special equipment but you should be able to drill resin with an unhardened tool if yo want to experiment.

DaveB

Winander
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Re: Making a specialised D-bit

Postby Winander » Fri Jan 22, 2021 11:34 am

Guy,

Or put a cutting edge on the circumference of the tube, glue the tube and drill together and do both at once, assuming the 'edge' will cut the material cleanly enough. The drawback of this cunning plan is the management of swarf.

You could add a third tube or collar to act as a depth stop.
Richard Hodgson

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Will L
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Re: Making a specialised D-bit

Postby Will L » Fri Jan 22, 2021 11:51 am

I like Dave's suggestion of using tube as a drill guide.

Alternately print with 0.5 mm hole all the way through, drill that size all the way through then drill again 1mm for the require depth. That will be coaxial. I know its not easy to ensure the 0.5 hole goes strait down the middle but frankly, as long a it does come out at the end, even if not dead centre, it doesn't mater functional even if it offends the sensibilities. I've never had any trouble drilling white metal buffer shanks that accurately.

Alternatively again, drill 1mm all the way through and glue in a bit of tube to make the 0.5mm section.

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Guy Rixon
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Re: Making a specialised D-bit

Postby Guy Rixon » Fri Jan 22, 2021 12:13 pm

Thanks gents. I will try Dave's suggestion first, before trying anything too advanced.

Richard's idea is attractive, but the required depth of the 0.5 mm bore is too great for it to work. The drill would meet the resin before the tube entered the buffer.

Will, the 0.5mm bore is modelled all the way through, which is why the Shapeways prints work well. But the home printer doesn't manage it, filling in the bore with resin. There's a ghost of a hole to start the drill, but the resin is so soft that it's easy to drill at an angle by mistake.

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BryanJohnson
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Re: Making a specialised D-bit

Postby BryanJohnson » Fri Jan 22, 2021 12:16 pm

Will suggested
Alternatively again, drill 1mm all the way through and glue in a bit of tube to make the 0.5mm section.

Another option is to use a Gibson wagon buffer bush, short or long as required, at the back of the buffer beam. These are 1.0mm o.d. with a 2.0mm flange, drilled 0.5mm for the buffer tail.

Bryan

davebradwell
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Re: Making a specialised D-bit

Postby davebradwell » Fri Jan 22, 2021 2:12 pm

I'll round this off by saying it's better to start with the big drill as it's less likely to wander and in harder materials gives you less chance of breaking the small drill because it's making a shorter hole. Thank Dave Carter for publishing the tube idea in Snooze a long time ago when I was having trouble with buffers pointing in all directions and jamming.

DaveB

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Wizard of the Moor
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Re: Making a specialised D-bit

Postby Wizard of the Moor » Fri Jan 22, 2021 9:05 pm

+1 for Gibson buffer bushes. Much easier than drilling 0.5mm through whitemetal buffers, at least. It helps to use a 2mm drill to lightly countersink the forward face of the bush. This helps the buffer tail to find the hole.

Do you think that the 0.5mm hole in your printed buffers fills up because it's difficult to clean the excess resin out before curing, rather than the tolerances of the printing process? If so, would compressed air help to blow it out or is the raw print too delicate for that?
James Dickie

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Terry Bendall
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Re: Making a specialised D-bit

Postby Terry Bendall » Sat Jan 23, 2021 9:31 am

Guy Rixon wrote: I had thought to make a stepped D-bit. Is is reasonable to make this thing by hand


A D bit can be made by hand but it is better done using silver steel which is then hardened and tempered afterwards rather than trying to grind a drill to shape. The steel needs to be the diameter required, so either a standard size or turned down in the lathe. The working end needs to be exactly half of the bar so it needs to be cut away with a file or on a milling machine. The end is ground at an angle. It then needs to be hardened and tempered. To harden heat to bright red ,900 degrees C, then quench in water. To temper. clean the steel with emery cloth, heat gently until the blue tempering colour shows, then quench in water.

To use a D bit a hole of the correct size needs to be drilled first. This then provides a guide for the D bit to finish it off. For a stepped hole it would be easier to make D bits of two different sizes.

Terry Bendall

billbedford
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Re: Making a specialised D-bit

Postby billbedford » Sat Jan 23, 2021 10:00 am

Guy Rixon wrote:I need a tool to "machine" home-printed buffers. Unlike those printed at Shapeways, where the 1.0 mm and 0.5 mm bores just need lightly cleaning out, the home-printed ones need more work. The 1.0 mm bore needs to be reamed to size and the 0.5 mm bore needs to be drilled as the printer leaves only a hint of this cavity. Naturally, the small bore needs to be coaxial with the large bore, and this happens only sometimes when I drill them by hand with ordinary drills.


The length of the 0.5mm section of the bore only needs to be about 0.5mm deep, which makes drilling by hand a whole lot easier.
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CornCrake
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Re: Making a specialised D-bit

Postby CornCrake » Sat Jan 23, 2021 10:47 am

Hi Guy, I have popped some AG wagon buffer bushes in the post to you.
Steve

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Guy Rixon
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Re: Making a specialised D-bit

Postby Guy Rixon » Sat Jan 23, 2021 12:05 pm

billbedford wrote:
Guy Rixon wrote:I need a tool to "machine" home-printed buffers. Unlike those printed at Shapeways, where the 1.0 mm and 0.5 mm bores just need lightly cleaning out, the home-printed ones need more work. The 1.0 mm bore needs to be reamed to size and the 0.5 mm bore needs to be drilled as the printer leaves only a hint of this cavity. Naturally, the small bore needs to be coaxial with the large bore, and this happens only sometimes when I drill them by hand with ordinary drills.


The length of the 0.5mm section of the bore only needs to be about 0.5mm deep, which makes drilling by hand a whole lot easier.


I respectfully disagree, Bill. With a fixing spigot 2 mm long, the overall length of a model wagon-buffer is typically 8 mm, with the length of the guide being about 6 mm. If the 1.0 mm bore goes to 0.5 mm from the end of the spigot, then the distance from the face against which the spring bears to the buffing face is 7.5 mm. I find this makes the springing too weak to return the buffer to its rest position. It also make the buffer prints much more fragile in the sense that the bearing face for the spring is likely to be driven out of the back of the spigot when cleaning the print with a drill. (Yes, I'm clumsy.)

I prefer to make the 1.0 mm bore to between 6 and 7 mm from the buffing face. My printed buffers make this length 6.25 mm. This makes the springing relatively stiff, but at least the buffers reliably pop back out. One can cautiously drill out the 1.0 mm bore a little deeper if required.

For this reason, I find that I have to be careful in using the turned buffer-bushes (but thank you Steve for sending them; much appreciated). They don't always give the correct depth of bore, and it's necessary to trim the spigot accurately to set this. However, this fitting may turn out to be easier than drilling for the buffer tail, and I shall consider home-printing buffers with 1.0 mm holes right through.

Jeremy Suter
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Re: Making a specialised D-bit

Postby Jeremy Suter » Sat Jan 23, 2021 1:04 pm

Hi Guy
I will start with. Making a D cutter. It would be easiest to take an old .5mm drill and grind off one side and the sharpen to a point either in the middle to make a pin point or one side to make a cutting edge. or both sided and make a paddle drill Making and hardening some silver steel is the other option Terry has already mentioned.
I do think making a D cutter is a none starter to go that deep 7mm even in soft resin I suspect it will snap before you get half way. Where a twist drill will lift out the swarf, the D cutter doesn't they are really designed for milling although I use them on the pantograph milling machine to do holes but the object being drilled is secured tight along with the drill unit so will keep in position.
Slotting the tube in is probably the easiest method.
Have you tried enlarging the hole on the print allowing for the over print will it leave the small hole in the centre that you require

billbedford
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Re: Making a specialised D-bit

Postby billbedford » Sat Jan 23, 2021 1:32 pm

Guy Rixon wrote:
billbedford wrote:The length of the 0.5mm section of the bore only needs to be about 0.5mm deep, which makes drilling by hand a whole lot easier.

I respectfully disagree, Bill. With a fixing spigot 2 mm long, the overall length of a model wagon-buffer is typically 8 mm, with the length of the guide being about 6 mm. If the 1.0 mm bore goes to 0.5 mm from the end of the spigot, then the distance from the face against which the spring bears to the buffing face is 7.5 mm. I find this makes the springing too weak to return the buffer to its rest position. It also make the buffer prints much more fragile in the sense that the bearing face for the spring is likely to be driven out of the back of the spigot when cleaning the print with a drill. (Yes, I'm clumsy.)



If you are printing your own housing the spigot can be any length you like as long as it's 2mm diameter.

Thus:
Buffer dim.png


And drilling the main bore at 1.1mm minimizes the chance of the spring binding.
Last edited by grovenor-2685 on Sat Jan 23, 2021 2:39 pm, edited 1 time in total.
Bill Bedford
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Guy Rixon
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Re: Making a specialised D-bit

Postby Guy Rixon » Sat Jan 23, 2021 1:53 pm

billbedford wrote: ... And drilling the main bore at 1.1mm minimizes the chance of the spring binding.


Noted. But if anybody has bought my printed buffers off Shapeways, please be very careful about drilling out to 1.1 mm as it may not leave enough wall in the print. In particular, the buffers with ribs (SER, SECR, LCDR) won't accept this. 1.05 mm is sometimes possible with extreme care and not drilling under power. In any case, the action is more like reaming than drilling, since SW form the bores quite well.

For the home-printed buffers, drilling 1.0 / 0.5 mm nominal does give free motion, provided that I get the holes in the right place. I got two good ones out of four attempts, and this thread is seeking a better success rate for less effort. This is with the same CAD, for SER coach buffers, that gets printed at Shapeways, in which a 1.1 mm bore is the wrong kind of breakthrough.

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Will L
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Re: Making a specialised D-bit

Postby Will L » Sat Jan 23, 2021 3:30 pm

Guy Rixon wrote:...I find that I have to be careful in using the turned buffer-bushes ...

First met these when Keen Magib first started producing this sort of sprung buffer and I was always afraid that they would get pushed off in a heavy shunt. For this reason I would prefer a piece of 1 mm OD tube superglued up the hole to these bushes. It would also give you the opportunity to tune the length of remaining 1mm bore while not having cut the tube until after fixing it in the buffer.

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Guy Rixon
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Re: Making a specialised D-bit

Postby Guy Rixon » Sat Jan 30, 2021 3:30 pm

A coda: I finally fixed this by tweaking the 3D model, not having the tube in stock to try the drill-guide idea. I told the printer to make a 0.6 mm hole through the buffer instead of 0.5 mm. Printer still says "Do'h" and closes up the hole, but it leaves a weakened channel in the middle. When I drill into the back of the buffer, the drill follows the channel and stays concentric with the main bore. I made six good buffers from a batch of eight prints. The two failures were where I harshed the front of the guide with the 1.0 mm drill, not in making the 0.5 mm hole.

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Will L
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Re: Making a specialised D-bit

Postby Will L » Sat Jan 30, 2021 10:38 pm

Guy Rixon wrote:A coda: I finally fixed this by tweaking the 3D model, not having the tube in stock to try the drill-guide idea. I told the printer to make a 0.6 mm hole through the buffer instead of 0.5 mm. Printer still says "Do'h" and closes up the hole, but it leaves a weakened channel in the middle. When I drill into the back of the buffer, the drill follows the channel and stays concentric with the main bore. I made six good buffers from a batch of eight prints. The two failures were where I harshed the front of the guide with the 1.0 mm drill, not in making the 0.5 mm hole.

Finding what works, that's my kind of engineering! :)

billbedford
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Re: Making a specialised D-bit

Postby billbedford » Sun Feb 21, 2021 11:18 am

Guy Rixon wrote:A coda: I finally fixed this by tweaking the 3D model, not having the tube in stock to try the drill-guide idea. I told the printer to make a 0.6 mm hole through the buffer instead of 0.5 mm. Printer still says "Do'h" and closes up the hole, but it leaves a weakened channel in the middle. When I drill into the back of the buffer, the drill follows the channel and stays concentric with the main bore. I made six good buffers from a batch of eight prints. The two failures were where I harshed the front of the guide with the 1.0 mm drill, not in making the 0.5 mm hole.


The bores in your buffers are almost certainly closing up because uncured resin cannot escape through the smaller hole. Chasing out the hole with a drill before the resin is cured works well and is easier than drill cured resin. What works better, though, is flushing out the uncured resin with IPA using a syringe.
Bill Bedford
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