Boring on a mini-lathe

[SignalFailure]

I'm having some problems getting an accurate bore on my mini-lathe.

As the engines I'm making have cylinders in the 8 to 20mm range I have one of the smallest indexable-type boring bars (6mm shank) that will fit into a drilled hole around 8.5mm diameter. If I need to make a long (well, say 30mm) bore of 12.5mm dia I find that the resulting hole has variable finish, and worse, slightly variable diameter. I'm guessing that over such a length the boring bar is flexing and that's the cause of the problem.

I can see that maybe for a 12mm bore I should drill to 10mm then bore the rest but the bar I have is the only one that will fit in a 10mm hole and I'm sure it will get the wobbles on even light cuts when it's at 'full reach'. I'm not sure I fancy drilling holes bigger than 10mm on the mini-lathe!

The other problem is boring to the very end of the cylinder if the end is flat against the chuck (and for a 30mm piece of metal in the mini-lathe it pretty much has to be!)

I've tried making a boring bar to take a home-made silver-steel bit but couldn't get a good cut with it (http://www.truetex.com/boring.htm)

How does everyone else cope with these problems? Any tips or advice welcome before it all goes in the bin!

Paul

 

[mklotz]

Paul,

In no particular order, some things to think about...

At those small sizes, consider rough-boring to near size, then reaming to make a nice cylindrical hole. With the right speed and lots of lubricant, a reamer will produce a very acceptable finish for a model engine bore.

Ensure that only enough boring bar extends from the holder to reach the bottom of the bore. The less extension, the less chance for deflection.

When you near final size, take several no-feed passes with the boring bar to counteract the bar flex.

If possible, use power feed when boring to eliminate the vagaries of hand feeding.

Lock the cross-slide gibs when boring to make the setup more rigid.

Ensure that the boring bar is dead sharp. A HSS tool may work better than an insert tool.

Flex is a function of how much the bar extends beyond its holder, not how deep the hole is. You shouldn't be getting hole diameter variations even if the bar is flexing. How are you measuring hole size? Perhaps there's a problem with the measuring technique.

 

[Lew Hartswick]

I would put as First the last of Marves list:
quote
Ensure that the boring bar is dead sharp. A HSS tool may work better than an insert tool.

Especially on small holes on small lathes.
...lew...

 

[Tin Falcon]

One thing I learned from experience is honing /stoning tools makes a huge difference.For any HSS tool.
Use the larges BB you can that will fit in the hole and allow for clearance.
Tin

 

[SignalFailure]

Thank you Marv.

I'm doing most of those things except using power feed for fear of hitting the chuck.

Close inspection of the tool shows the edges to be chipped! Not sure how that could have happened as it's had pretty light use.

The bore in question is 16mm and I reamed it at low speed but without lubrication (it's brass) but I suspect the 'damage' was already done - I don't have any means of directly measuring the internal diameter but once I'd made the piston the flaws became evident.

I did have an off-the-shelf HSS boring tool but it wouldn't fit in these small dia holes.... maybe I should grind one?

Thanks again!

Paul

 

[SignalFailure]

Thanks Lew and TF - looks like a sharp and un-chipped tool is part of the solution!

 

[GailInNM]

Other things to be very careful of are the relief angle on the insert or boring bar and the tool tip height.

Some insert boring bars do not have enough relief angle if put into a hole that it just big enough to fit into. The boring bar makers specify the minimum size hole that can be bored and this is often larger than the minimum size the bar will fit into.

The tool tip needs to be very close to the center line of the spindle. Unlike external turning where if the tool is deflected downward it will move away from the work, on internal boring if it deflects downward it will dig into the work. I set my tool tip on boring bars with an indicator to be dead on center height as close as I can get it. Some people have better luck, particularly if the boring bar is not extremely rigid, by setting the tool tip a few thousands high. I have not found the need to do this, but my boring setup is quite rigid. But one thing is for sure. NEVER set the tool tip below the center line. Before setting up the cylinder, put a center in the lathe or turn up a scrap of metal to form a center and then set the tool tip to that with a magnifying glass. Better still is to use a dial indicator, but it a lot of work unless you have a dedicated setup for it.

Gail in NM, USA

 

[mklotz]

Re hitting the chuck...

When I'm doing something like that where I'm approaching, under power, some part of the machine closely, I use the dial indicator that's always fixed to the ways on my lathe. With power off, I note the DI reading at which I need to disengage the feed lever. With a tiny bit of practice, one can "hit the mark" within five thousandths every time.

Bob Warfield notwithstanding, I'm of the camp that thinks carbide doesn't have much place on small, less rigid, lathes. Chipped carbide is useful for scraping paint.

While a set of plug gages is to be desired for measuring both the size and testing the cylindricity of (small) bores, they may be a bit expensive for someone starting out. Get yourself one of the inexpensive 0.2-1.2" inside micrometers.

http://www.use-enco.com/CGI/INSRIT?PMAKA=307-3244&PMPXNO=7549594&PARTPG=INLMK32

Remember, due to the finite jaw width of calipers, you can't make accurate measurements of bores with them.

 

[Cedge]

Having recently had to tackle the same problem, a couple of things came to light.

Cheap brazed carbide tipped boring bars will definitely flex. They cut relatively well going into the bore, but the shallow angle of attack the tool presents during retraction will let the tool "climb" out of the cut, leaving a "bow in the middle" problem in the bore.

Taking multiple zero advance passes will help but might not cure the whole problem. As many as 20 or more passes will still be taking small cuts.

Honing /lapping will generally get you out of the crack, assuming you exercise patience. I use a piece of copper round stock with 2 relief grooves down the sides at 180°. This hand held tool and a fine polishing compound will soon have the bore slick and even.

Using the intended piston as a go no go gauge is one sure way to have the proper fit when done. Some will adamantly disagree, but as a final step, I've also hand lapped the bore using the piston and a bit of oil to get a nice close fit

Sorry, but I can't address the insert performance.... inexperience, doncha know.

Steve

 

[Circlip]

You've brought two points up there Paul, Boring bar rigidity, - the best I've found are the ones used in vertical boring heads, mine even have brazed carbide tips (PLEASE don't chastise me for saying the naughty "C" word). Although they have a shank dia of 1/2" you can make an adaptor to fit them into the toolpost.
Flat bottoming? The ultimate tool for this in "Elmer's" size bores has to be a "D" bit.
Just a final thought, bar overhang, if you're boring 30mm deep,you only need about 35mm hung out from the toolpost?

Regards Ian.

 

[BobWarfield]

Carbide? Works great on small lathes! :bow:

But, boring bars are finicky. Cheapies don't work so great for me. It was a revelation when I finally got my first "good" boring bar, a "Circle" boring bar. I would never have bought it at retail, but I was patient enough to get a flea bay deal. My Glanze turning tools were great, but the boring bar that came with that set stank. Those cheapy sets of a bunch of sizes with cemented carbide that are imported aren't much better. I've wound up stalking "better boring bars" until I have a small selection for the lathe and mill boring head that really perform. I've only got about 3--the Circle, a Micro100 that's pretty nice, and one whose name escapes that is purpose made for Criterion boring heads and is US made.

A few more points:

1) Use the biggest shank that'll fit your hole. ;D

2) If you once get started on a taper, a bar that flexes seems to want to follow that taper and it is heck to get it to cut true unless you have a very rigid very sharp cutting setup. BTW, you can get carbide inserts that are sharp. Did I mention--carbide good!

3) Angle the bar. If you do the trig, angling the bar gives a greater effective rigidity. Best way to be able to angle is to drill to nearly correct diameter and then bring out the boring bar to finish up.

4) Solid carbide bars are MUCH more rigid. I'm talking the material the bar is made of, not the insert or cutting tip. If you have to be many bar diameters out when cutting, you want a carbide shank on your bar. Marv, I'm not sure if you can get those with HSS cutting edges.

5) Silly tip: For some reason boring more than any other operation seems to make the QCTP pivot from the forces applied. Pretty soon you get that taper I mention and things go to pot. Be sure that sucker is tight!

Here's another one: As the guys mention, it's hard to accurately measure holes. Funny, it's easy to accurately measure cylinders with a micrometer. Hence, make yourself a plug gage if it matters. It ain't hard and it doesn't even take very long. Just grab some stock and turn yourself a piece that's close enough. What's close enough? Well, what can you easily measure for hole diameter? Accurate to 0.01"? Probably can do better. Accurate to 0.001" starts to be iffy for me with calipers. So I shoot for a plug that is say exactly 0.005" smaller than the desired hole size. I get to within 0.005" of the plug fitting with the caliper measurements, go slow until the plug fits, and then dial the rest on the dial by feel (i.e. my knowledge of plug fits and my lathe's ability to hit that diameter and my paranoia of being wrong!). This takes longer to explain than to actually do most of the time. If you need to hit it better than a few thou, invest in the right measuring tools.

Cheers,

BW

 

[zeusrekning]

Hey Bob. Ditto on the Circle bars! I had to make a brass bushing the other day and had no luck with the brazed carbide. Then I found a 3/8 Circle bar in a box of tools I bought.
Near mirror finish v/s the other which look like it was cut with a dull file.
Tim

 

[mklotz]

For holes too small to conveniently measure with an inside mike, a set of plug gages will run around $40.

http://www.use-enco.com/CGI/INSRIT?PMAKA=616-8125&PMPXNO=5215206&PARTPG=INLMK32

However, Bob's suggestion to make your own is good advice. I would expand only slightly on it by suggesting that you turn one end of the gage slightly undersize and the other end to the target size. Connect these two sections with an angled cut (i.e., a cone). Then the undersize end tells you when you're getting close to size and the depth of penetration of the cone in the bore tells you how close you are to hitting the target size.

 

[Cedge]

Marv
Some of your ideas are saturated with "why didn't I think of that". Simple, elegant and even I can use it...LOL

Thanks once again
Steve

 

[rake60]

I use the cheap brazed boring bars in my home lathe.

The whole set can be found for $20 on the internet.
They work very well for me. The holder was made in the tool post of the lathe so it
is perfectly on center for the machine.
Another bonus of the cheap bars is they can easily be ground to make an internal
grooving or threading tool.

To measure small bores I use ball type adjustable gages like THESE
from Enco.

For bores larger than .511" I use Telescopic Gages.

With either type is gage you can accurately mic most of the way through a small bore.

Rick

 

[Classic]

It's funny what will work if you don't know that it won't work

I once needed to bore a bushing that was too long for my little Unimat 3. My cousin was managing a business that had an old lathe that hadn't been used for years. There was no tooling with it.

I needed a boring bar, so I took a piece of 10mm round steel, welded some metal on to one end with hard face electrode and ground it to a cutting edge. I clamped it in the tool post at what looked like a good cutting angle. The bushing was for a cement mixer, so absolute accuracy wasn't important, but the bore turned out amazingly true.

Peter.