View Full Version : Boring holes/tapers?

06-21-2004, 10:25 PM
Hi all, I need some help. I am boring a hole through a 1" thick x3" diameter aluminum stock and looks like to me I am actually boring a taper instead. I have yet to pull the part off of the 4-jaw and I only have calipers to measure the hole, but if I push the calipers inwards it looks like it narrows by a couple of tenths.
This is what I am using, 7x10 mini lathe, a boring bar a little over one inch and I am using the carriage feed to feed the tool.
So, what can I do to prevent this? I have bored some holes before and noticed this. Thanks for the help, Dean D.
Dean’s 7x10 mini-lathe page.

Dave Opincarne
06-21-2004, 10:36 PM
Two questions Dean, How are you infeeding and are you making consistent jumps when you reset the bar and by how much? OK, so thats 2 1/2 questions.

BTW, you said this has happened before. Have you in fact measured the holes to confirm there is a tapper?


06-21-2004, 10:39 PM
Sounds like your carriage isn't running on the same axis as your headstock. This can be caused by three things.

1. You moved/removed the headstock and there's something (dirt, grit) under it. Or the headstock is loose.

2. Your carriage is loose. (The four adjusting bolts on the underside.)

3. The bed is twisted. (Not mounted to flat surface.)

If this is an old machine, you could also have wear on the V-way close to the headstock.

Dave Opincarne
06-21-2004, 10:39 PM
Opps, sorry! I missed the line about the cariage feed. I'm assuming the head is square to the ways so tool path should not be suspect.


Forrest Addy
06-22-2004, 12:14 AM
Engine lathe alignment (and consequently straight boring and turning) is sensitive to bed leveling. There's also tool wear, chip build up, and heat to influence a simple little job.

You need a better way of checking bored than inside calipers. Don't get me wrong, I have hundreds of hours on them line boring pumps on a HMB. Inside calipers are great but you can "wish the reading" very easily. I suggest you get a set of telescope gages.

06-22-2004, 07:59 AM

I had the same problem boring some CRS a while back. I had a ridge about half way down the bore. I determined that it was the boring bar setup flexing enough for it to allow a lighter cut in the middle. I also ended up with work hardening in this area. I overcame it by running at a super slow speed with carbide boring bar, and LOTS of coolant (I used Tap MAgic). I got a super finish with a perfect bore. It is time conusming, but it worked for me.

06-22-2004, 10:56 PM
Thanks to all of you that replied. I took all of you ideas and really looked at it tonight. I tightened up the carriage, used a lot of cutting fluid, and I really now believe that calipers are not good at all for measuring holes. I finished boring the hole and it came out real well. Thanks again, Dean

06-22-2004, 11:18 PM
Calipers are good for getting you close, like .010 or so. After that break out the telescope guages and micrometers. You could also make a go-no-go guage for the particular bore you are doing.

06-23-2004, 09:50 AM
I agree with mochinist. You are using the wrong measuring tool.

There may be nothing at all wrong with your bore. It is very typical to get those readings using a caliper. It is especially exagerated if there is looseness on the caliper. Example: set the zero on your caliper with the jaw closed (most people do), then measure a "joe block" near the tip. I've seen some calipers off by more than .005", and it changes back to correct size as you move the gage in toward the slide.

If you are worried about the size down to tenths then you are definately using the wrong measuring tool. The inside measuring jaws on a caliper have a flat about .010-.020" wide. This throws off the measurement a small amount. The smaller the diameter - the greater the error.

Super Dave

[This message has been edited by suprdvn (edited 06-23-2004).]

06-24-2004, 03:37 PM
What often happens to me when boring is that the tool deflects inwards as cutting forces build up. This deflection stops when the force of cutting is matched by the resistance of the setup. So the initial opening of the hole would be larger, and at some small distance in, the hole has shrunken to the diameter it will be throughout. Making light finish cuts with freshly sharpened toolbits, properly angled, usually removes any taper. A large part of my education on the lathe has involved dealing with play in the compound and crosslide during a machining operation. Lots to go wrong there.