Entry posted by 45609
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Over the last few months I've had a couple of DJH Gresley A3s on the workbench. In itself there is nothing special about the two builds. However every now and then a small construction problem requires an unconventional approach. Having said that it is only unconventional if one is not familiar with using machine tools in their modelling. I certainly don't want this to be seen as an attempt to sneer at those that do not have machining facilities but rather I hope it will show people that do have the equipment or are even considering buying a lathe or milling machine the sort of things that can be done to lift the quality and finish on your models.
Credit must go to a friend of mine, Mick Moore, who I saw last weekend and he reminded me of the techinque I'm about to show.
Prelude
The problem is best shown in the photos below..... The DJH kit provides a pair of piston valve cover castings that have to be attached to the running plate castings. One is shown below in situ.
However the castings themselves are a little too tall and also a bit tapered from inside to outside. The photo is not as clear as I'd like (used my phone camera) but I think the basic problem is shown. The top of the casting should be flush with the top of the frame edge just to the left. Measurements with the calipers confirmed that the casting height was 0.25 to 0.35mm taller than it should be.
Of course it would be possible to attack the bottom face of the casting with a file but holding the part securely whilst trying to do this and get the face reduced in height, square and flat would be rather tricky. The amount of material needing to be removed was far more than just removing a few high spots. So I decided that this was a job for my Unimat 3 in milling configuration and the task has 5 distinct parts to it.
Part 1 - Making a fixture.
With such a thin and soft whitemetal casting clamping something directly into the machine vice is just not going to be possible. Even if it were setting the work up square to the milling cutter in 2 planes would also be very problematic. The answer was to make a fixture to hold the parts from a block of aluminium. Any suitable material, brass, tufnol or similar would be ok but it just so happened that I had a bit of aluminium the right size. After clamping in the machine vice the top face has been cleaned up square and a slot slightly wider than the castings has been milled down the centre of the block. The depth of the slot is just a bit less than the final thickness of the castings to give tool clearance and maximum support during the final operation. The photo below shows the finished fixture.
In this case there is a slight complication as the casting has a raised sandbox filler lid on the face to be located in the slot. A couple of counterbored holes needed to be put into the base of the slot so that I could locate two castings at the same time to fly cut the bottom faces.
Part 2 - Holding the work and setting up to cut.
As Mick reminded me last weekend when machining something soft like whitemetal or plastic it is quite feasible to use glue to hold the work. His recommendation is a 2 part epoxy. Now I think he uses the full on 24 hour cure stuff however I'm a bit impatient so I tried 5 minute epoxy but left it for at least 3 hours before machining again to achieve some strength and hardness.
This photo shows the parts glued in place. I've tried to avoid using glue in the bottom of the slot instead building up some glue on the edges in a few places to stop the part from sliding when the cutter traverses the work. You will also notice a black mark on one of the castings. This is the thickest part of the 2 castings, measured with a micrometer, and the actual size "2.07" millimeters has been marked on the fixture as a reminder.
Once I was sure the glue had gone off (about half an hour or so) I set the machine up ready to take the first cut. I planned to do this with a fly cutter rather than an end mill as this method is a bit more controllable and gives a better surface finish. Also whitemetal can clog the flutes on an end mill. After checking the tool bit was sharp and honed I set the flycutter into the quill of the machine and moved the tip of the cutter over to the black mark. I then moved the quill by hand over the mark whilst simultaneously moving the cutter down until it just contacted the work. It isn't that easy to see in the attached photo but a couple of bright marks have appeared in the black where the tool face has just cleaned off the surface. The zero position is now set and sufficient material will need to be removed to get the castings to 1.70mm overall.
Part 3 - Fly cutting the work
After what seemed like more than 3 hours (I really am that impatient) the job and glue looked like it was ready to take the first cut. I was careful to feed the fly cutter across the parts in a direction and position that pushed the castings into the rear edge of the slot. Any movement or snatch on the first cut would have been disasterous. The vertical feed was very cautious at only 0.05mm per pass but after the first one the castings had partially cleaned up as shown below.
After a further 2 passes (0.15mm removed) the faces had fully cleaned up. So far so good and a further 4 passes (7 in total) were taken to get the part to the theoretical size. (i.e. 2.07-0.35 = 1.72mm). Given the style of the fixture accurate measurement of the finished size is a bit of a leap of faith but you have to rely on the accuracy of the initial measurements and have familiarity with your machine. The parts looked like this after the final pass and I set about removing them from the fixture.
Part 4 - Removing the work
This is where I had a little bit of self made difficulty by not making the slot wide enough to get access with a small chiselling tool to prise the glue bead off. In the end I brushed a bit of cellulose thinner onto the parts to soften the glue but it still took a little bit of heat to get the glue to give up it's grab completely. This was a bit nerve racking as the parts are whitemetal. Indirect heat applied to the aluminium fixture with a small cook's blow torch whilst keeping my finger on the top of the block to make sure it didn't get too hot was the safest way to proceed. The glue softened and I was able to lever the parts off without damage with the tip of a craft knife blade. Phew!
Part 5 - Final fettling
With the two casting safely off the fixture all that was needed was a clean up of the edges to remove any machining burrs and excess epoxy. A check with the micrometer confirmed the thickness of the castings to now be consistently 1.69mm at every location I checked around the perimeter of the parts. Just about perfect! A dry fit was carried out on one of the A3 running plates and the results were exactly as required. Flush and square with the frame edge.
Whilst I've been doing this blog entry the second pair of castings have been mounted to the fixture and the glue is curing. So is it worth it? Well I think so but I have been known to go to extraordinary lengths in my modelling. Perhaps it has given one or two readers ideas of their own.....
Cheers.....Morgan
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