Machining cylinder from solid

Hi,

I have for a long time looked at LBSC's Minx as a loco to build. I am looking at the cylinder block then wheels as a starting point, if these two components are right then i have a good basis to progress with the rest. There also the biggest single cost next to the boiler. The cylinder for those that don't know is a twin cylinder inside frame jobbie.

So Cylinder first, am i correct in saying if its been cut from solid cast iron block that first it needs to be taken to overall dimension and squared up in the mill on all six sides before proceeding to look at marking out for cylinder bores, and milling the port openings.

The raw block will start at 180x90x75mm, and finished size needs to be 4.1/8" x 3.1/4" x 2.1/2" (roughly 105 x 82.55 x63.5mm) I intend on sawing off 60mm from one end for use another time or elsewhere. This is the closest size i can find to what i need available in barstock material.

Essentially what i'm asking is machining sequence. What order to do everything. Square up, mark out, line bore, then mill cylinder valve openings and drill for valve block mating surface, also do these holes need to be lightly countersunk to get a true flat surface after they have been tapped. I've read that tapping the holes will raise the surface slightly around the hole.

I already have a boring bar made, but need to make a fly cutter with a 3/8" shank to fit a collet on my mills spindle to square any work up.

Jon

I would have thought that purchasing a CI cylinder casting from one of the ME suppliers was the preferred start but respect your decision to machine from a chunk of CI. Your "raw block" seems to have plenty of machining excess which sadly, many commercial castings do not.
Suggest that first priority is to generate a flat datum surface (one that will be in contact with a frame side?) and do your marking out from that. Many critical dimensions and need for parallel surfaces so maybe generate next surface at a true 90 degrees to your datum.
Next I would then FIX the block exactly on the cross slide with that "90 deg" surface truly parallel to the face plate and bore out the main cylinders.
Almost certainly others will advise on a completely different sequence !

Have you looked at Minx postings here?: **LINK**

If you're going to c/s the tapped holes, do it before you tap them, not after.

Jon,

I have never machined a solid cylinder from such a large block but yes your sequence sounds good to me. However, I might be tempted to drill rough cylinder bore holes first and use them to bolt the block directly onto the milling table to ensure that the outer surfaces are parallel. You might have a really nice, large, vice meaning my suggestion is not necessary but your suggested sequence is how I would do it.

The cast iron should be ok but a tiny countersink wouldn't do any harm.

Phil H

Unfortunately you wont find the Minx castings for sale as Julian replied on a thread on the forum youve linked to they haven't been available for some time. After some searching i found which suppliers and castings would be suitable, then looked at the price. Whiich would be a none starter for me, hence my decision to machine from solid. AJ Reeves currently list the MOK cylinder casting set at £249.60, reduced from the listing price of £300. My lump of cast iron costs less than £45 + Postage, or i can pick it up for free. Yes the set contains other castings than just the main block. but you hopefully see my point. The lump sawn off can be used for some of these parts that would be otherwise included in the casting set.

Hi Phil,

I only have access at home to an ML4 lathe, and it has on its tail of the bed an Alomco milling attachment.

I should have enough reach if a flycutter is used though havent figured out how id mount it yet for squaring up i only have a small myford vice so will have to make a slide vice for the cross slide as i think the most the myford vice will accept is 3" (havent checked that yet)

Jon

Having cut a few cylinders from solid I would rough it out a little oversize say 1mm on all surfaces. Put the bores in first and at the same setting machine the piston rod end of the block which will ensure it is true to the bore, should be possible to do it with the block clamped to the cross slide so bores can be done with a between ctrs bar and then flycut the face. This can then be you datum surface to sit the block on while the four sides are finished and finally the other end.

Don't forget CI bar has rounded corners but with the excess in the 180mm direction that should not be a problem

Get a vacuum cleaner set up to deal with the mess.

You might consider squaring it up in the 4 jaw on the lathe.

That is something i hadn't thought about. I was just finding the next available size up from finished dimensions. Doesnt help matters having to convert to metric sizes when trying to figure out what would be the best stock to use. But hopefully i've got there.

Jon

I machined my 3 1/2" Evening Star cylinders from solid. Roughed out the material to a rectangular block, oversize, then bored the cylinder and valve bores in one setup, leaving slightly undersize for honing later. Then using the bores to align everything finished the square faces before getting everything shaped. I even reduced the material thickness around the bores which will be covered by the cladding. It all worked out well in the end.

Jon,

If you are using a lathe only, I would use the cross slide and a flycutter mounted to the face plate or drive plate. You can clamp the block to the cross slide with quite simple clamps (a few TEE blots and simple steel straps between them) - oh and packing if required. Once you have the first flat surface, you are away i.e., sit that on the cross slide and machine a 90 degree surface and so on. The face plate can align (with a square) the next surface.

I recently tried to machine a Rob Roy cylinder in a large Myford 4 jaw chuck on a Myford Super 7 and I had no chance of doing it - it went on the miller and was bored on the lathe cross slide (boring bar between centres).

The face plate/ flycutter arrangement gives a swing that should cut your block. I have done it before on a Myford ML10 with a similar sized block but that was nice lovely machineable aluminium.

It sounds like you intend to bore the cylinders with a between centres boring bar? Just pack the block up to centre height and pass the block over the cutter. The cross slide can even be used to move from centre to centre using the dial.

The port slots could be done quite nicely with your Alomco attachment.

Phil H

I have just bought a Sandvik boring bar, it is 25mm diameter, but the cutting height is more like 12mm. I would guess that the minimum size of hole for it is about 40mm. It will have to have a custom mount made, and I will have to mill 2.5mm flats on three sides of the shank. Even this large size bar has a recommended max depth of only 100mm, 4X the diameter.

Old Mart,

I simply used a piece of 1/2" BMS bar with two angled cross drilled holes. One to take a 1/4 diameter piece of HSS for the cutting tool and a second tapped hole to take a grub screw to clamp the HSS. One end centred for the tailstock and I held the bar in a 3 jaw chuck at the other.

Jon could probably use a larger diameter bar because my bore was just 15/16". You simply loosen the grub screw and carefully move the HSS bit for each cut. The cut can easily be measured (the tool tip to the other side of the boring bar), do some arithmetic before you advance the part over the cutter of course.

No need for clever, expensive boring bars.

Phil H

Between ctrs bar will be better, you can do quite long and large bores with them As said cheaper than a large insert bar too.

This cylinder is something like 42mm bore x 320mm long

Edited By JasonB on 12/06/2020 17:37:39

Facemask essential, unless you want black snot for a week...

Neil

I'm curious as to your reply, why would I need a 25mm boring bar and why is it a such a problem to bore the hole? What is this custom mount that you speak of, as I was under the impression that between centres boring bar needed nothing more than a straight piece of steel with a HSS bit poking out. Between two dead centres and a dog to drive it. Please elaborate as for the woes.

Neil I'll buy a mask!!!

Actually I find cutting decent CI bar tends to produce far less dust than a scabby casting, swarf tends to come off in chips and sometimes even curls rather than dust.

On this type of milling I have an angle plate which is square on all sides and edges so that I can rotate the angle plate and clamp it in various positions without removing the piece bolted to the angle plate. I can clamp a piece, mill one side, rotate the angle plate and then bore a hole or drill knowing that the axis are square and it is easy to clamp an angle plate in various positions. I have drilled many holes in it to suit the job in hand but it works many times.

David

I don't know if this is any help, but I found it very encouraging and in fact it's the first thread I ever saved to favourites from this forum:

**LINK**

Some bloke who calls himself Stub Mandrel...

John.

Of potentially more impact on your well-being than inhaling some elemental iron and carbon is washing your hands at night after machining cast iron. If married or under some similar constraint, then next morning SWMBO will discover the bottom of the sink/basin is covered with myriads of rust spots, acrylic sinks being the worst as the iron particles lodge in the surface to rust overnight. A rapid application of Jenolite or similar to the surface will be needed to placate her. A dedicated, plastic bowl for hand washing is probably the easier solution but of course, you’ll need to empty it outside every time !

Between centres boring bars.
The subject has been aired several times on here. After I made the simple version with vertical cutter, its shortcomings were soon evident. The main irritant being accurate measurement when advancing the tool tip. I then made one to the Geo. Thomas design and had no further problems.
(There were several postings on here in early November 2018 about b-c boring bars)
There are 2 sources of his design:-

1. GHT's original article in Model Engineer. Model Engineer, 3rd June 1977 p. 615 (Vol 143, No. 3562)

2. That most valuable compendium of GHT's articles - "The Model Engineers Workshop Manual (vol 1)" 1992. Pub: Tee Publishing, ISBN 1-85761-000-8. It's on page 92.

Copyright of course, but herewith GHT's introductory paragraph. His comment about the normal, 90 deg tool resulting in flexure of a boring bar was well made IMHO. By putting the tool in the bar at an angle, that effect is very much reduced as well as resulting in very much easier ability to measure the swing with a micrometer. Geo. Thomas wrote “….the boring bar in its usual form leaves a lot to be desired. My main objection to it is the absence of any controlled means of advancing the cutter and all my design attempts to provide a suitable means ended in severe weakening of the bar at the point of maximum bending moment.”

IMHO it would time well spent to make one to the GHT design at the maximum diameter commensurate with your cylinder bores.

This was my GHT b-c bar boring out a 5" CI cylinder casting.

That brass item on the casting, under the boring bar, contains several small Neodymium magnets. It should have been within a plastic bag so that thecollected iron bits could have been emptied straight in the bin !