Author Topic: Not a model engine!  (Read 5775 times)

Offline Allen Smithee

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Not a model engine!
« on: July 15, 2019, 05:56:45 PM »
PLEASE NOTE: This thread isn't about a model engine - it's about doing a favour for a friend and gaining machining experience. My primary reason for posting it here is to get comments on how I've approached the task, the setups I've used, and suggestions for alternatives so that I can learn from the expertise and experience of others!

So a friend of mine has a ageing boat whose decaying fittings can be troublesome to replace. One of those fittings (well three of them, to be truthful) is the catch system for windows in the lower hull. These are based on some mazak die-castings held to window frames with countersunk pop-rivets. The original (damaged) item looks like this:

AS
« Last Edit: July 15, 2019, 08:47:01 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #1 on: July 15, 2019, 06:01:50 PM »
As you will quickly see, the only flat/straight datum surfaces are the two outer faces (just to add some challenge). In my material pile I had some 20mm square aluminium bar, and a quick check suggested that the whole bracket would fit entirely inside a 20mm cube so I sawed off three pieces of the bar and trued them up to make said 20mm cubes. After that (I reasoned) it would just be a simple matter of carving away all the aluminium which was not part of the bracket...

AS
« Last Edit: July 15, 2019, 08:48:12 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #2 on: July 15, 2019, 06:13:59 PM »
Looking at the shapes I decided the first task would be to mill the 10.5mm wide, 12.5mm deep channel through the middle. I clamped the three cubes side-by-side so I could do this in a single setup. My mill is a tweaked Seig X2p, so this had to be approached carefully to avoid chatter, but with a 9.5mm slot drill it gamely chomped its way through in two 5mm cuts, following up with a 10mm four flute cutter to finish the width in two 10mm-deep cuts and then a finish run along each side to get a clean cut of the bottom. This worked well, but there was a tendency for the swarf to re-weld to the cut faces - does this mean I'm running the cutter too fast? When milling I tend to run the Seig flat out (2,500rpm) unless there's a specific reason not to, so perhaps I'm pushing it too hard here?

AS

« Last Edit: July 15, 2019, 08:48:50 PM by Allen Smithee »
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Offline mike mott

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Re: Not a model engine!
« Reply #3 on: July 15, 2019, 06:14:42 PM »
I'm surprised they were not bronze or stainless.

Mike
If you can imagine it you can build it

Offline Allen Smithee

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Re: Not a model engine!
« Reply #4 on: July 15, 2019, 06:21:26 PM »
I removed and deburred the parts, and decided that this would be a good time to drill the various holes (the countersunk one for the mounting rivet and the cross hole for the pin) as at this point I had datums I could work from. The brackets were clamped in the mill on parallels, using the holes spotted & drilled. Finally the countersinks were cut in the top surface.

Not happy with the countersinks - no matter how carefully I do this, and with decent tools, I can never seem to drill countersinks without chattering. Is there a knack to this?

AS

« Last Edit: July 15, 2019, 08:49:48 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #5 on: July 15, 2019, 06:33:59 PM »
Next I decided to cut the mounting face - this had a flat side and a bevelled side (presumably to match the profile of the window frame). I wasn't sure how to approach this, but I was "inspired" by the "milling by eye" approach Tug uses to emulate castings. So I marked the required angles with a scriber and carefully positioned the job in the vice, lining up the scribed lines parallel to the jaws of the vice by eye. I then cut the bevelled surface in a series of careful cuts by eye. I then repositioned the part flat against parallels to cut the flat face. Obviously I had to repeat this for the other two, and each had to be separately marked, positioned and milled.

I was quite pleased with the result - lining the parts up with the original shows the angles are all near enough for government work!

Mind you, this was one of those occasions when some angled-parallels would have come in handy so I guess they go on the wish-list...

AS
« Last Edit: July 15, 2019, 08:52:02 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #6 on: July 15, 2019, 06:44:25 PM »
The cross-holes were finished off by drilling one side to 6mm so that an M5 tap could be run through this hole (to align it) into the other side (didn't bother photographing this). The final step for these brackets was to cut away some of the material to bring them closer to the cosmetic profile of the original. Again, this was done "by eye" and again it had to be done separately for each part.  If I get time (and can be bothered) I'll make some filing buttons so I can put the radiused profile on the top, but that can wait.

AS

« Last Edit: July 15, 2019, 08:54:38 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #7 on: July 15, 2019, 06:55:45 PM »
Attention now moved to the cross pins. In the original these had just been a 3mm mild steel screw, and it had become severely bent by the tightening loads - it had to be sawn and drilled out of the original so I could measure-up the casting.

I decided to make the new pins rather larger and stronger. I had some 8mm "finest gash DIY store" stainless bar so my design would have a 5mm main shaft tapped M5 at one end, a 6mm head with a sawn screwdriver slot. None of this is rocket surgery so I decided not to take picture of the turning operations. The results worked quite well, although the "gash stainless" had an amusing habit of work hardening and then glowing bright orange sometimes while being faced and parted (but the cheap insert tools I used didn't seem to care). The results are shown below, together with the one that decided to harden along its whole length while being brought down to 5mm. It's a lovely colour, and is now hard enough to survive local thermonuclear shocks...

AS

« Last Edit: July 15, 2019, 08:56:41 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #8 on: July 15, 2019, 07:03:22 PM »
The final item was the main shaft. After the fun with the gash stainless I decided not to try to use it for these. As it happened I had some M8 stainless bolts (screwfix?). The threaded portion was exactly the right length (so close that I'm even wondering if the originals were manufactured from bolts rather than bar stock), so it was just a matter of parting to length, tidying up the ends and then cross drilling in a V-Block. These bolts machined beautifully, btw. Shown below together with the bolt they came from (only two are new - one is the original part that I've just drilled out to 5mm - there will be a brief round of applause for the first person to identify the original and point out what is different about it).

AS

« Last Edit: July 15, 2019, 09:00:28 PM by Allen Smithee »
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #9 on: July 15, 2019, 07:05:26 PM »
Finally an assembly shot to show they do all fit together. The one with the original shaft is fitted with the knob unit used in the boat. I still have to work out how to make two more of those. In fact if anyone has suggestions please jump in - the functional features are the metal flange at the base of the knob and the hand grip on the knob itself. The original is something like bakelite moulded over a stainless core but that's beyond my facilities. I have some 40mm stainless bar, but machining the whole knob from solid sounds like serious overkill (and would probably be too heavy).

Any ideas?

AS

« Last Edit: July 15, 2019, 09:06:31 PM by Allen Smithee »
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Online Jo

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Re: Not a model engine!
« Reply #10 on: July 15, 2019, 07:23:33 PM »
Well done Pete,

Jo

P.S. Could we have slightly larger photos say 600 by 800 pixels?  ::)
Enjoyment is more important than achievement.

Offline Allen Smithee

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Re: Not a model engine!
« Reply #11 on: July 15, 2019, 08:35:17 PM »
Sorry about that! I did a bulk-resizing of the pictures and I think overdid it - I'll see what size comes out at about a meg per pic and go back & replace them. Mind you, they're only phone shots so they mayt not be that great even then...

AS
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Offline Allen Smithee

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Re: Not a model engine!
« Reply #12 on: July 15, 2019, 08:45:32 PM »
I turns out the problem was that the app I was using defaulted to png format. Switching that back to jpegs made the files much smaller.

I'll swap them out now.

I've now swapped all the photos for bigger ones.

AS
« Last Edit: July 15, 2019, 09:08:29 PM by Allen Smithee »
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Offline crueby

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Re: Not a model engine!
« Reply #13 on: July 16, 2019, 12:27:22 AM »
I needed knobs like that for my car roof racks, the originals were cracking from being in the sun so many years. Found ones online, plastic knobs with steel threaded inserts. Much easier than making them. The woodworking catalogs have them for making fixtures and clamping rigs.

Offline Allen Smithee

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Re: Not a model engine!
« Reply #14 on: July 16, 2019, 10:10:18 AM »
The knob is a slightly more complex problem than it might seem. In use the metal flange at the base of the knobhooks in front of a u-shaped yoke (attached to the external frame) to push the window shut against its seals. So this flange sees a constant load trying to pull it off the rest of the knob. I'm going to have to make this base flange as a separate part, probably in stainless. I could turn a press-fit boss on the metal insert in the handle and then press/loctite the flange in place, but I feel that over time (especially with winter-summer temperature cycling) the press/glue joint may relax and fail. Having the window move back from its seals when berthed would be inconvenient, letting the rain in. But having it happen when under way could be a safety-critical hull breach, which the boat's owner considers a Bad Thing(TM) (damned sunny-day wimp sailors!).

Ideally I want to have the M8 thread of the stud in the flange itself, so that the knob/flange joint is just taking the torque loads of tightening/loosening the catch. So I'm looking for a knob that I can adapt for that sort of thing. I'm currently working my way through this catalogue which is looking promising...

AS

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