Over the last few days, I have been pecking away at the cylinders doing a bit here and there. So this is the stage that they are up to.
I have come to realise that these cylinders are in fact the trickiest bits of the engine to build. The rest of the bits, although fiddly, are really just straight forwards machining exercises. The cylinders on the other hand are all to do with 'feel' to get them right. So hopefully, these bits I am showing here and in the past few posts will get you over the main stumbling blocks.
The first thing to do when using the topslide for the feed and very fine plunge cutting is everything has to be square to each other. If you don't do this, not only will measurements be out, but you will start to break the fine cutting tools.
The first thing I did was to zero up the topslide (compound slide). It was a matter of slackening the hold down bolts slightly, and either by winding on the topslide handle (laborious) or winding the saddle (much better), and gently tapping on the topslide sides, you get the topslide showing no deflection from straight. Very gently tighten up then recheck.
The area shown to check on isn't perfect, but unless you want to strip down to the dovetails, this will have to do you.
Again, this isn't the perfect method, but it works for me. You would usually set the lathe tool edge square to the rod in the chuck, but because I am using such small tooling, it is easier for me to do it this way.
A piece of rod was mounted into the chuck to make a holding mandrel. Once this is made, it will not be removed from the chuck until every machining operation on the cylinder has been carried out.
This mandrel will be used to hold each part in a perfect position for it to have repeatable machining operations carried out, without having to worry about resetting everything up and rezeroing each time.
A spigot was turned to be a nice sliding fit in the bore, and about 1/16th shorter than the cylinder length.
The end was drilled and tapped to take a short bolt with a washer fitted.
This is how it looks with a part fitted.
I will explain about how I mount the cutters first.
Because I am only using a 3/16" cutter in basically a 5/8" holder, it can be difficult getting the grub screws to grip while as mentioned before, getting the tool square to the chuck centre axis at the same time. This is how I do it, because it makes it easier for me, and the tool should end up to being spot on or very close to square
Now first off, I will explain about the tooling. Because I will be cutting hundreds of fins and slots, I made up three double ended tools. The cutting areas only have front rake. Because I am cutting brass, no top rake is required, and because I will be plunge cutting, hopefully square to the lathe axis, I don't use side rakes either. They were just ground up on my surface grinder, three ends at a time to exactly 0.025" width.
The text comes next. I worked out a machining sequence for cutting all the grooves and fins to exactly what is required on the drawing. If you don't want them to look the same as that, you will have to work out your own.
By having that working procedure, it was very easy to work out dial settings to religiously follow that at the end, I will have what is required.
This is all on the assumption that you have all your bits to the same length (or within a couple of thou), and the tools are set EXACTLY ON CENTRE HEIGHT, NOT NEAR ENOUGH, BUT EXACTLY. If you don't know how to do that, just ask and I will make up a little article on how to do it.
I used the first scrap cylinder to check that the tools I had made would have no problems plunge cutting to the depths required.
Now for the trial cuts. Using the second scrapper, I followed my machining routine to the letter.
This is what it turned out like, spot on.
When you cut the end fins down to size, small rings of brass are produce that sit in the grooves. The hooky tool is to get them out of the grooves.
So now that the finning has been proven, the flange area has to be completed first. So that is what I did, put the finning tooling to one side, and prepare for getting the flanges to the correct thickness and position.
Just a reminder, all tooling has to be sharp enough to shave with, have clearances that will work, and again, be EXACTLY on centre height. These are the very basic rules for lathe work, if you can't get them right, you need to take up knitting instead.
So what I did was stick one of the scrappers onto the spindle, with the correct end to be machined nearest the tool (it sounds obvious, but a few times I have done it the wrong way around, and ruined perfectly good parts before realising what I was doing).
Using the saddle stop, I machined down the flange until the spigot formed was spot on size. The topslide and cross slide dials were both zeroed with the tool in it's final cutting position.
The second scrapper was used to prove the settings. They again were spot on. So now I could work on the real thing.
One of the precious parts was mounted up (I only have one spare left). The tool was moved into the zero/zero position, and taking a cut towards the head at full depth, was cut until the saddle hit the stop, then with hand pressure holding the saddle against the stop, the cross slide was wound outwards to remove metal that was left by the angled front cutting face. This was repeated on all the remaining parts.
21 spigots cut in no time.
Now we come to what I consider the most difficult cut on the whole engine.
Because of where the fins end up near the flange, the widest cutting tool that can safely be used to cut the flange to thickness and depth is 1/16". Plus because it will be plunging to a depth of around 0.280", and an interrupted cut caused by the square flange and the four holes in it, the tool will require side clearances. As you can see, the root of the cutting part is very thin, and because I ground this up by hand, I don't want to have to make another one, so very easy does it.
As can be seen from the schedule, once the depth of cut and position of cut is obtained, everything will be locked up, and the cut will be done by the cross slide only. So cut to depth, retract, change part, cut to depth etc etc.
The tool was set up with the cutting face square to the lathe axis.
Depth of cut was found and the cross slide dial set to zero.
I have to mention that I could not use the scrappers for setting up, the interrupted cuts that they would have made would almost certainly have resulted in a broken tool.
The saddle was locked up and shallow plunge cuts were made on the back face of the flange until it was the correct thickness. Sorry about the quality of the picture.
Now ready for the cut. I had the machine running at 900 RPM, and fed fairly fast until I reached the main meat of the part, then I hand fed at approximately 0.002" per second, using a very smooth continuous two handed swapping action. A continuous steady feed is even more important than the feed in speed. If you are speeding up and slowing down or even worse, stopping, that is when you will get tool breakage.
This shows the finished groove.
I must have done something right, because I got them all cut, and the tool lived to cut another day
So the cylinders are nearly fin-ished, just got about 200 of them to do.
Bogs
