making a boring bar

[johan]

So I can now attach parts to the saddle of the lathe and use a boring bar to bore holes like for cylinders and piston valves. Only thing is I'll now have to make a boring bar. That would need to have something like 20-23mm diameter for the cylinders and 12 for the piston valves.

I plan to use some mild steel or free machining steel bar. Next, drill 2 centers and then machine it between centers so I get something that at least is running concentric on my lathe. But then I need to make a "hole" in it to put a piece of cutting steel in it.

Now the easiest is a round hole with a bit of thread at one side so the bit can have support and be "finely" adjusted. And another threaded hole perpendicular to it to "fix" the bit. But I see a few problems with that. First of all a single point to fix the bit isn't very secure, you need at least 2. But then the bar gets weakened a lot. And keeping the cutting edge well aligned when it can turn around in its hole isn't also always easy.

A square hole and square bit would be better in that the alignment of the cutting edge is the same when you adjust. And fixing it with a single screw is possible. On the other hand, a square hole isn't as easy to make a perfect fit and also not as easy to put a screw behind the bit to adjust the cutting diameter is something I don't really see how to do.

I found some designs where they drill the bar at an angle but those don't convince me at all. After all the way to measure the cutting diameter stays the same so the end result isn't more precise.

[chris vine]

On the subject of strength, the bar will be plenty strong enough, even with a hole in it. The stiffness, which is more Important, Will be hardly affected because the hole only affects a short length of the bar.
Not sure about your other questions, but you don’t have to worry about that!
Chris

[jasonb]

I don't have any problems with a single screw locking a round toolbit. Also no problem with a square tool in a round hole.

All my bars are drilled straight across for the toolbit, just some careful measuring is all that is needed.

youtu.be/AuWl-SZcC58?si=C8T-cWM1BjFvgIFK

youtu.be/haFzNA66Pmg?si=ko8owAq4MbLFOibZ

I also have a "Microbore" holder but don't use it that often, only on my largest bar. this has a fine adjustment of the carbide insert.



youtu.be/N8fHs6UCr3E?si=pkmFV2YSDGGDrSpv

[jem]

If you are worried about the cutting bit turning in the bar, grind a flat on one side where the screw holds it in place, much easier than cutting a square hole.

Jem

[chris vine]

Having never used a boring bar, how do most people adjust the cut finely so that it isn’t hit and miss??
Chris

[SteveW]

Nov 12, 2024 14:28:12 GMT chris vine said:

Having never used a boring bar, how do most people adjust the cut finely so that it isn’t hit and miss??
Chris

Was exactly what I was thinking as I read down this post.

A couple of options I've tried in the past is to have a couple of threaded holes either side of the tool and use a strap with/without a plug to push the tool in by three fifths of f-all every time before reclamping the tool and removing the strap. If there's room/clearance leave it attached. The strap can also provide a reference point for a feeler gauge or two in which case a couple of stiff springs between the strap and boring bar will help bias it up.

As for the cross hole better drill and file a 'V' to located the tool better on load and help reduce chatter which can be a problem if the geometry/stiffness is wrong somewhere.

[GWR 101]

Not the only way, but I drill a round hole almost through the bar the same size as the HSS. tool bit (I tend to use broken center drills). Then drill and tap the remaining portion a smaller size for a grub screw to act as an adjusting screw. Then the cross hole is suitably drilled and taped for the locking screw. I tend to grind a small flat on the tool bit to line up with the locking screw, which assists in keeping the cutting edge in the correct relative position. Of course not forgetting that the boring bar needs to be long enough to allow sufficient room both sides of the tool bit to allow for the component being machined. Regards Paul

[RGR 60130]

The boring bars I used for some cylinders are shown here -

modeleng.proboards.com/thread/11467/new-peppercorn-blue-peter-gauge?page=2

Reg

[tony9f]

Quite a few years ago, I made a boring bar from a length of one inch silver steel. It was cross drilled to take 3/16" round tool steel and half the depth of hole tapped 1/4" X 40 ME.
I made a threaded plug to go in behind the tool tip which had a slot cut in it lengthwise and a small micrometer dial to slide over it which was secured to the screw with a small pin so that it would slide while the thread was advanced.
The micrometer is removed when a cut is being made for obvious reasons, but when the cut is finished the micrometer adjuster is replaced and snugged up behind the tool, the clamping screw is eased off and the tool advanced to the depth of cut before reclamping the tool bit and removing the micrometer.
As with all these things, it requires a bit of time and effort to make but in use I have found it to be a very accurate and reliable piece of kit, and being made from a full length of silver steel, you get plenty of room to work with each side of the tool.

[jasonb]

For sizing, I take a cut and measure the bore. Then it is easy enough to work out how far the tool needs to be advanced. Put an indicator with a flat foot against the tip of the tool and it is fairly easy to see how much you move it out by.

Another option is to advance the tool more than you need and use your callipers to squeeze it back into the dimension you want, just nip up the screw so it is not loose but easily slides.

[johan]

Thanks for all the input. So if I resume:
- don't worry about drilling holes (within reason)
- put a flat face on round cutters
- put a grub screw behind the tool bit to adjust cutting depth. I'm not that much concerned about very high precision. After all the pistons have to be made after boring the cylinders.
- make sure the part to be bored can get both sides of the tool bit. Obvious but I assume just as often forgotten.

I'm not going to use a "full length of silver steel" as that is 2 meters which is about the the length of my workshop...

[tony9f]

Hi Johan,

I think you have misunderstood me. When I said 'a full length of silver steel' I was referring to a piece 13" (330mm) long as it is usually sold in the UK after all I am only using a Myford lathe.

Tony

[johan]

Sorry, my world is metric, we left the middle ages some time ago. That is what I get when I buy a "full length" My lathe is 450mm between centers and when I buy silver steel from my supplier I get a full meter length (other suppliers go to 2 meters). But I'll have to check about the last point, fitting the piece both sides of the cutter. So I'll probably have to use the maximum I can fit and I'll certainly have to check if I can use a live center or not. And that will be a possible problem. I'll have to center bore the bar. But if it can't fit it through my arbor then I can't center bore it. So I'll have to use the a size that can fit through the arbor. This is going to call for some thought.

I think that silver steel is a not needed for this, mild steel or even free cutting is "good enough". After all, Youngs' modulus isn't that different between different sorts of steel and that is what will decide the stiffness. Hardening will not change that.

[steamer5]

Hi Johan,
Another idea I saw somewhere is make 2 boring bars.
Use the first one to take out the majority of material. Then using the 2nd to sneak up on the hole size you want on the first cylinder, once you are there DONT change the setting. Do your 2nd cylinder with the first bar until you are close to size, swap to the 2nd bar & you should end up with 2 cylinders of the same diameter.

Cheers Kerrin

[jasonb]

We tend to get metric silver steel in 1/3rd of a metre lengths eg 330mm , 1m is not hard to find, 2m is usually a special order.

Having said that none of my between ctrs bars are silver steel

Beware of making bars too long particularly as the diameter of the bar gets smaller as they start to flex and you will get chatter. Also it does not really matter if one end is held in a chuck or collet, even if not perfectly in line the tool will still rotate in a perfect circle so you can "shorten" the bar for the job in hand by having some of the length within the chuck/spindle

When you do position your work on the cross slide also take into account machining the piston end of the cylinder at the same setting, I usually put a flycutter in the lathe spindle . so make sure you can get a full sweep and have the correct end facing the headstock.

[johan]

Nov 15, 2024 7:20:38 GMT jasonb said:

When you do position your work on the cross slide also take into account machining the piston end of the cylinder at the same setting, I usually put a flycutter in the lathe spindle . so make sure you can get a full sweep and have the correct end facing the headstock.

What exactly do you mean by "piston end"?

I have planned to measure this up this weekend. From a quick go with a ruler I think I'll have to use the full length of the lathe. I'd like to use a live center but I'm not yet sure that will be possible, it would make for about 50 mm less between the centers. Probably I'll not be able to use the chuck but a collet might work. Still not sure how I'll have to machine this engine block. From what I have drawn so far I think I'll have to bore quite a few holes. At least 7 so far: crankshaft tunnel, 2 cylinders and 4 piston valves. Bit of a drawback of having to construct it from parts.

[tony9f]

Hi Johan,

You can use any kind of steel you want to make a boring bar and even use square or hexagonal steel if it does the job. The reason I used silver steel is that it was convenient in as much that it was already ground to size and tends to be less likely to distort after machining. It has not been hardened, that is unnecessary.
I use this boring bar between centres and driven from the catch plate, it is easier to remove from the job when measuring and is resettable afterwards only having to move the tailstock. Also not having a chuck mounted gives more useable bed length.
Once you have your boring bar material, offer it up to the lathe and travel the carriage the full available length of the bed and note what area of it the cross slide will cover, the tool will need to be in the middle of that space so that when machining the bore you can be confident that you have clearance at both ends of the bore.
I apologize if I am telling you something that you already know but from experience I have found that it is often the obvious things that can catch you out.

Tony

[johan]

Nov 15, 2024 11:03:06 GMT tony9f said:

Hi Johan,

You can use any kind of steel you want to make a boring bar and even use square or hexagonal steel if it does the job. The reason I used silver steel is that it was convenient in as much that it was already ground to size and tends to be less likely to distort after machining. It has not been hardened, that is unnecessary.
I use this boring bar between centres and driven from the catch plate, it is easier to remove from the job when measuring and is resettable afterwards only having to move the tailstock. Also not having a chuck mounted gives more useable bed length.
Once you have your boring bar material, offer it up to the lathe and travel the carriage the full available length of the bed and note what area of it the cross slide will cover, the tool will need to be in the middle of that space so that when machining the bore you can be confident that you have clearance at both ends of the bore.
I apologize if I am telling you something that you already know but from experience I have found that it is often the obvious things that can catch you out.

Tony

All good points, thanks. As you say, a lot of things are obvious when you catch them after the facts...

[jasonb]

You want the end of the cylinder where the piston rod comes out to be square to the bore so on a Loco that would be the back end. If you have that facing the headstock then you can mill that flat and bore the main cylinder hole all at one setting which ensures they are square.

[johan]

Nov 15, 2024 13:03:24 GMT jasonb said:

You want the end of the cylinder where the piston rod comes out to be square to the bore so on a Loco that would be the back end. If you have that facing the headstock then you can mill that flat and bore the main cylinder hole all at one setting which ensures they are square.

Ok, got it now. For the cylinders and piston valves that is possible. For the crankshaft tunnel it will be complicated as that needs to be square at both sides.

Looks as if the engine I have chosen was not a very good choice. I'll have to do a lot of thinking before I even start cutting anything so I don't come to the conclusion that the order of machining was wrong late in the process. This is not something that I can start, then leave for a month or 2 and take it up again. Very likely I'll be here a lot...