Paddleduck problems

[Lposter]

Im building Bogstandards Paddleduck as many have before me.

Ive been using the CAD plans that are available and checking with the book itself as I go along.

I made the simpler crankwebs and was doing OK. The holes all lined up and they all could slide along the rods together freely.

I slit them (1 mm ) and used a M2 bolt to tighten them down.

But they do not grip the 5 mm rod - any of them. They hold the 4 mm like a vice but the 5mm can rotate. The 5mm hole was drilled to 4.9 mm and reamed to 5mm, the 4mm drilled to to 3.9 and reamed to 4. Both holes measure correctly, and the rods are 5 and 4mm upon measuring. The fulcrum hole is 2.5mm. Distances between holes are as in Bogstandards plans.

I could really go nuts on the bolt but Im worried I,ll wreck thevthread or bend the crankweb. I am petrified of having to make these again!

I was thinking of just soldering the 5mm bars ibto the crankwebs but thats going to mess things up later.

Could I use shims or is that going to throw things off altogether - apart from being a crude fix in itself.

Ive checked a bit but have not found others having this problem so Im a little worried Ive botched this.

Any help would be welcome.

L

[Jo]

  Lars, 


It sounds like for some reason when you cut with the slitting saw the brass might have closed up on the outer end more than the centre.

Are the two sides of the brass webs still parallel?  If not then you might try clamping it in a vice but it sounds like the centre has sprung out more than the end  If that has happened you will need to put a spacer in the slot before trying to clamp the piece in the vice to bend it straight again

Jo

[Lposter]

Hi Jo

The sides of the webs remain paralell and the slit (milled) is a fairly constant 1 mm.

I went loose on one of the webs and tightened it until the 5 mm shaft was locked. Tried this on one of the others and stripped the socket!

Im thinking I may not have been firm enough on tightening but the silver socket screws arent up to the strain. Theblack one didnt strip at all.

Is there a difference between the silver colored fasteners and the black in terms of strength? (That may be a stupid question but the black one seems better made)

I will replace all of them with black ones when i manage to get the stripped one out!

L

[Jo]

Hi L,

I suspect your silver one is stainless steel or nickel plated brass both of which are not very hard, the black one is high tensile steel so will be much stronger.

Is there any reason why you should not put nuts on the other side of the crank web? The Brass is not that strong which is why it stripped when you tried to tighten it.

The other trick is if you are using a hand reamer the end is tapered to help the reamer go in straight. You can use this to make the hole slightly undersized. Reamers are normally to size but some cheap ones can be found to be oversized   

Jo

[Jim Nic]

Hi L
It's been a few years since I built my Paddleduck but I remember having a similar problem with my crankshaft.  From looking at the model now my solution appears to have been to use hexagon headed bolts and tighten them, probably with fingers crossed for luck, until the web gripped both the 5mm shaft and the 4mm journal.  As I recall I had to get 8BA thread screws with smaller (9BA) heads to avoid the screws fouling the webs. 
It would seem that you have used metric fasteners so you will have to try tightening your black coloured screws and if the thread in the crankweb strips then put a nut on if there is room.
If things get really difficult you could try easing the 4mm holes a bit so that the web tightened on the 5mm before gripping the 4mm.
Jim

[Lposter]

Hello

Im looking to start the eccentrics on this engine which should be made from 18mm mild steel stock or similar. Which I dont have.

I do have a fair bit of suitable brass stock. I know that generally the same metals shouldnt be used in moving assemblies but is there any fundamental reason I cannot use brass for the eccentrics?

Lars

[crueby]

Hello

Im looking to start the eccentrics on this engine which should be made from 18mm mild steel stock or similar. Which I dont have.

I do have a fair bit of suitable brass stock. I know that generally the same metals shouldnt be used in moving assemblies but is there any fundamental reason I cannot use brass for the eccentrics?

Lars

As long as you use a different metal for the followers, bronze, steel, iron, even bronze, no problem using brass. If both the eccentrics and followers are brass, then you risk it galling and wearing out very fast.

[Lposter]

Many thanks! I think I will try and get some flat non-brass metal for the followers as its easier to come across.

Lars

[Lposter]

Hello

Regarding the steam control unit on this engine. Its 25x25x10 with a series of holes and channels and a large central 10mm hole for a valve which directs the steam for forward and reverse.

With the equipment I have, Im really struggling to make this 10mm hole in my mild steel block. I can get up to 8mm but then it starts to get bogged down and very messy. I dont have brass for this piece.

If I made the hole 8mm and resized the valve insery bit, could I get away with that? If push comes to shove there is another benchpress i can use but the bearings may be shot or something because it doesnt seem to run very true. So Id rather use my small but more accurate one.

Would 8mm work for that valve?

Lars

[crueby]

Should still work fine - only change might be the top speed it runs at, if the passage for the steam/air is smaller at any point, it would just restrict the flow somewhat.

[Lposter]

Many thanks!

[Lposter]

Hello

Im working on packing glands in brass. In essence I have a 6mm  blind hole that needs tapping to M6 to  5mm depth. The hole is drilled to 5.5mm bore.

Any tap I have, including my M6 plug/bottom taps dont start cutting threads before 5mm. Ie they reach hole bottom before making threads.

I cut the top off a tap and ground it sort of flat. That worked sort of.....i dont think the threads are well formed.

Has anyone any tips? I dont have a thread cutting lathe.

Im confused as apparently the crossheads are the hardest part of this engine but they seemed to go ok.

This tapping of very shallow hiles on the other end is killing me. Ive scrapped 4 already!

Lars

[crueby]

The chart I have says for a M6x1 tap, the hole should be drilled 5mm, not 5.5.

[Lposter]

Oh oh........that might explain the lame threads.

I will try again.

In principle, is it ok to cut the bottom off a tap?

Lars

[crueby]

I've done it to make a bottoming tap when I could not find one locally, or to salvage a broken one and turn it into a bottoming tap. Ground the tip to give it just a little taper at the tip to cut better. It does work, though it can make it impossible to start the threading, depending how much is cut off.

[MJM460]

All the tables I have clearly suggest that the tapping size for M6 x 1 is 5.0 with the usual caveats about different materials and so on.  It always works for me.

I have ground the bottom of small taps when even the final cut version has a point on the end that prevents getting within a bull’s roar of the bottom of a blind hole.  But I go very gently on the grinding, with lots of dips in the water container for cooling so as not to destroy the temper of the tap.

I used to have a paper catalogue from a German tool making company and was surprised at how different the various forms of bottom tap, plug tap, final cut and so on, clearly these terms are strictly not just differences in local terminology.  And all have a few threads of lead in.  It also seems that a bottom tap of any form still has a slight taper but mostly it is intended to follow another tap for most of the depth, so it only has to clean up the form of the last two or three threads.  So care in cleanly entering the already tapped hole, but not intended to cut until near the bottom.

I have found to get a thread for a gland to really bottom out in the hole without any packing, that in preparing the end of the Piece to be threaded, I not only make a defined groove at the shoulder, and a chamfer at the end of the stock, but also a 15 degree taper for the length equivalent to two or three threads.  It produces a minimal amount of swarf, and seems to take an awful long time, but the time is quickly saved when it is screwed into the hole, and requires no further fiddling with a file.   But it also leads me to prefer the bolted gland design when it is practical, so only plain cylindrical holes are required.  But sometimes, a screwed gland is the only thing that will fit.

The other technique I use for the packing glands is to machine a short length at the end to about 80% of the tapping size for the length of two or three threads so the thread does not have to go to the bottom.

I don’t pretend to have the last word on this, so very interested in what others do, but that seems to work for me.

MJM460

[Lposter]

Hands up .... I should have checked the drill size. The only defence i have is that i have never tapped anything above M4 and up to that, a drill of 0.5 mm less than the tap diameter works pretty much everytime.

I'll have another crack at this with the right drill bit. With the proper size hole, the normal taps might be able to get enough purchase for a mm or two to give the ground off tap a fighting chance.

Seeing as we are talking about packing glands, I cannot find graphite string anywhere. Even ebay.

Can i just use pfte tape or something? Or a little rubber ring? Or can i make graphite string (i have quite a lot powdered graphite).

Lars

[Jo]

Can i just use pfte tape or something? Or a little rubber ring? Or can i make graphite string (i have quite a lot powdered graphite).

Use the pfte tape 

Jo

[Jim Nic]

Regarding the thread, you could start with the packing gland threaded portion longer, say 3 or 4 mm, to give you a good start for your tap and then trim it to the proper length when the thread has been formed.
I run all my engines on compressed air so I use O rings for packing glands these days.
Jim

[Admiral_dk]

Metric threads are kind of easy to calculate as you just subtract the pitch from the diameter to get the drill diameter.

M3x0.5 ; 3 - 0.5 = 2.5mm.
M5x0.8 ; 5 - 0.8 = 4.2mm.
M6x1    ; 6 - 1.0 = 5mm.
M8x1.25 ; 8 - 1.25 = 6.75 that you can't get so -> 6.8mm. (NEVER down, always up).

Some materials are better with a number or two bigger drill bit, so I sometimes use a 5.1 or 5.2mm for a M6 thread.