Cylinder boring without a cylinder borer.

[tonyfoale]

A while back I had a couple of cylinders for my Aermacchi classic racer which needed reboring. There is nobody in my area who does such work that I know of, so I decided that I would do it myself, but I do not have a cylinder boring machine. The quill movement on my mill is a little too short, so I had to use the lathe. The obvious way would be to spin the barrel and use a fixed tool in the usual lathe manner but there are several good reasons to do the opposite, that is, spin the tool not the cylinder. The first stage was to make a really solid boring bar with easy diameter adjustment. My lathe has a no.5 Morse taper in the spindle which is plenty beefy. I had a length of 51mm 4140 bar of just the right length, so I turned a MT5 taper on one end and threaded the other end to accept a commercial boring head. To ensure rigidity right to the cutting edge I make a solid holder for a carbide insert, which is held into the boring head by two 1/2" pins. The next stage was to make a mounting to support the cylinder rigidly and accurately. This was welded up from pieces of steel from my scrap box and initially machined on the mill with a final truing cut taken with a fly cutter when mounted to the lathe's cross slide to ensure perfect alignment. The hole in the plate to locate the cylinder was also finished in-situ. What is commonly known as a "torque plate" was made to hold the cylinder in place. Now it is ready to go.


The bar made to accept the adjustable boring head. I like rigidity when machining. Note MT5 taper on one end and threaded for the boring head on the other.


Two sides of the boring bar fitted in the lathe and with boring head attached. Note the rigid insert holder in the first pic.


Firstly using a fly cutter to face off the mounting plate square to the spindle axis. Secondly tramming the mounting fixture after refitting to the lathe.


Cylinder mounted and clamped with a torque plate and then it is ready to do the important bit.

[Toolmaker51]

In my experience, few milling machine configurations have the intrinsic accuracy to bore at depth. Main issue is loss of quill support as it descends from the housing, and instances of swivel heads are rarely as square in 2 planes an indicator reads zero.
That can be improved by tramming with quill extended just shy of the stop, with apparatus to swing a larger diameter along X axis; move Y enough to fall within that range. I use a pair of 1-2-3's and 1 2" jo-block as surfaces, switching axis as needed. Wipe the lower surface on inside of wrist to insure cleanliness.
A lathe has all the accuracy built in, mainly as the part rides well aligned Vee's, quite like a jig bore. And a 5MT indicates a substantial machine. Effectively you were line boring. That is 100% possible if the bar is affixed to the tailstock and a center. Carbide tipped are most accurate, a well lubricated steel center is nearly as good. Live centers are often abused, inducing vibration or like harmonics via dry ball cage and races.
Your solution and dissertation are complete with accurate text and well detailed jpeg's. Those cylinders are so short! I'm guessing RPM range is low or mid 'teens? Is that an intake port in pic #4? Two Stroke and another couple thousand RPM?

[tonyfoale]

Is that an intake port in pic #4? Two Stroke and another couple thousand RPM?

I am not sure what you are mistaking for an intake port, that is a cylinder barrel, the ports are in the cylinder head. It is a four stroke. Here is a pic. of the top end complete if that helps. The length to be bored is around 200mm.

[Toolmaker51]

The mystery port now appears to be a DOHC chain-well, seeing side view of the cylinder. In pic 04, the supposed 'port' is peeking out above the torque plate, mounted to fixture proper.
Aermacchi's are triple uncommon in the US; though they supported the small motorcycle ventures of Harley Davidson [under AMF] awhile, in 100 and 350cc. That laydown cylinder is more than distinctive.
Thank's for photo's and clarification.

[tonyfoale]

Ah, now I get it, you mean the pushrod tunnel.
Actually, you are wrong about the 'macchis being so rare in the US. They are actually more plentiful there than here in Europe. HD in the AMF days owned the factory on the beautiful lake Varese in Northern Italy and most production went to the US.
I lived in the US for 3 years (I gave a seminar on motorcycle dynamics to the engineers at Segway and afterwards, completely out of the blue, they offered me the job of Director of Advanced Product Development and got me a 3 year visa so that I could sample the 'mercan dream for a while).
During my time there I collected a whole bunch of 'macchi stuff. Part of the deal with Segway was that when my 3 years was up they would pay shipping costs back to Spain, I had bought a bunch of everyday household items like a mill and lathe and other essential bits and pieces as well as the bike bits, which were all loaded into a container and sent home.


A view looking away from what was the Aermacchi factory (now MV) toward the lake. Aermacchi started out making sea-planes hence the need to be on the water. It is nice to see that with 2 or 3 changes of factory ownership the Aermacchi jet plane has been kept there. These pix. are from 2014 but little had changed externally from when I was there in 1984. The original slipway into the water is still there.
BTW. Aermacchi comes from Aer macchi which in English is Air machine.

[Toolmaker51]

Well, after my errant observations, this developed into a fascinating buffet of history and engineering. My favored diet; since it truly continues to feed me. Your contributions will be stellar Mr. Foale.

[tonyfoale]

Well if you like history here's a little more.
After Aermacchi faded from the scene the factory was used by Cagiva, new guys on the block. Cagiva also faded but the brothers who owned it took on and resurrected the MV marque and made the bikes in the same premises but with modern machinery and manufacturing methods. The brothers got into debt again and recently sold out to HD.
So it has gone full circle. HD during the AMF years owned the factory, then got out and Cagiva used it. that morphed into MV which HD have bought.
Here is a picture from 1984/5 when the factory was Cagiva and another near the same spot when I went back in 2014.

[DIYer]

Thanks tonyfoale! We've added your Boring Bar to our Machining category,
as well as to your builder page: tonyfoale's Homemade Tools. Your receipt:

Boring Bar by tonyfoale

tags: boring bar

[DIYer]

Thanks tonyfoale! We've added your Carbide Insert Holder to our Machining category,
as well as to your builder page: tonyfoale's Homemade Tools. Your receipt:

Carbide Insert Holder by tonyfoale

tags: boring, holder

[DIYer]

Thanks tonyfoale! We've added your Cylinder Boring Fixture to our Machining category,
as well as to your builder page: tonyfoale's Homemade Tools. Your receipt:

Cylinder Boring Fixture by tonyfoale

tags: boring, fixture, cylinder