View Full Version : Line boring bar setup on my lathe

12-18-2004, 06:01 AM
Looking for some insight on proper setup of
line boring bar to work center.
I am attempting to line bore a custom made
motorcycle swingarm's pivot section.
Here are my own results & thus my inquiry:
1. Mounted s'arm on toolpost fixture.
Mounted indicator in 5c collet on Hstock.
2. Indicated first pivot concentric to lathe
3. Mounted line boring bar in 5c collet to
Tstock live center.
4. Indicated axial relationship of bar to
to swingarm mounted in toolpost using
indicator mounted on toolpost.

Here is my problem:
Placing the swingarm pivots in position around the bar-- eyeball check clearly shows
the bar still in an offset position to the pivot center line. This offset error is a
front to back situation. I could move the
cross slide forward reducing the offset, thus
eliminating all of the setup work as described above. Also, the up/down was too close to eye, but could it not too, be in error?
Your wisdoms,considerations,suggestions would
be favorably received,

[This message has been edited by archimedes (edited 12-18-2004).]

[This message has been edited by archimedes (edited 12-18-2004).]

Spin Doctor
12-18-2004, 06:09 AM
One thing for line boring in the lathe. You need the tailstock to be absolutely on center with the headstock other wise you will get egg shaped holes. The other is use the heaviest bar that will fit through the hole and still allow you the ability to adjust the tool. Third is run it slow to avoid problems with the bar whipping. Question just how large is the finish diameter. For a swing arm I would think it can't be too big

12-18-2004, 06:14 AM
Greetings Doc,
Do you mean surface finish or the hole tolerance for the bores?
I am using needle bearings so the final
bore tolerance is fairly critical. Of course
the Locktite corp. provides me a final fit recourse!
My question is more about that indicator setup
process: Is it logical in its thinking and
thanks for your help,

12-18-2004, 06:17 AM
That pivot size is 1.375". I am using
a 1.125" boring bar diam.
My apologies in my previous reply-- I was
brain dead for a little bit.

[This message has been edited by archimedes (edited 12-18-2004).]

Ian B
12-18-2004, 07:04 AM

Sounds like an awkward job to set up and clamp firmly enough - boring bars, being single point cutting tools need more support than, say, a reamer.

Would it be possible to fix (tack weld if it's a steel arm, or bolt / clamp if it's alloy) a flat plate to the swing arm, under the arms and parallel to the eventual bore? About 6" square?

Then, mount the plate directly on the cross slide, with packing blocks beneath it. Check the height with a dial gauge in the headstock spindle, shim up until it's dead on centre height when you clamp down.

Next, if you can, mount 2 dial gauges on your boring bar - one just outboard of each end of the bearing tube. Adjust the swing arm back & forth until both are concentric, bolt up tight & recheck.

Alternately, machine up a pair of cone centres larger than the existing ends of the bore, and use these in the head & tailstock - wind the tailstock spindle forward to centre the bearing tube - should be close, as it's already on centre height. Clamp & recheck.

Then, bore away...


12-18-2004, 08:28 AM
Hi all,
I reread your original post, sounds like you are clamping your indicator on the toolpost and indicating the bar. You need to fix the indicator to the bar and indicate the hole. I use a small horseshoe magnet that has a short length of round stock attached to it.

Michael Moore
12-18-2004, 10:21 AM
Hello Archimedes,

Ian's idea of centering the pivot tube with cones and then clamping is what I was going to suggest.

I intend to do the same kind of work with my lathe. I'll try and spend some time coming up with a good fixture that can be used for a variety of swing arms, and have that bolted/clamped to the cross slide.

A needle bearing normally has a little "shake" in it and should tolerate a very small bit of misalignment. I like bronze bushings running on hard-chromed shafts and the bushings need to be dead concentric with each other, which is why I want to go with the line boring in the lathe. I've used taper roller bearings in swing arms and they seem to be reasonably easy to get aligned well enough to work.


12-18-2004, 11:26 AM
Greetings to all,
Thanks for your contributions.
My fixture is a v block I machined from
solid alloy steel. I use a KDK tool holder
as my toolpost. I machined the v block's rear
side with a dovetail & height screw that
duplicates the KDK male dovetail gripper.
Thus, my setup is flexible in work height
& follows the accuracy of the KDK tool holder
(so far).
The V block grips the cross tube & so far has
done an admirable job of rigidity under some
experimental "swipes" w/o need for packing blocks under the rails of the SwArm.
On the line bore setup-- I searched for Tstock alignment in this forum & found a # of
good discussions. I call these rules of 1st principles & I believe they are worth my evaluation for this particular job. I may be all day on a discovery process, but I believe its necessary.
I will of course still appreciate contributions.

[This message has been edited by archimedes (edited 12-18-2004).]

Bill Cook
12-18-2004, 12:07 PM
It may help to keep the swing arm close to ballanced over it's mounting. Weight hanging on the relatively short way contact of a lathe may make it difficult to set up and keep it positioned accurately.


12-19-2004, 01:49 AM
Normal cutting on the lathe will always put a down force against the ways. Boring as you are suggesting, if I understand correctly, will try to force the carriage front to back, and up and down, in a repeating cycle. It's easy to end up with an oval shaped hole, with a less than acceptable finish, if there's any play in the carriage on the ways. My lathe has a flat rear way, and I have to leave some small play there so the carriage can still traverse without undue friction. That means the carriage can rock if the cutting forces act to lift the rear of the carriage off the rear way. Just something to be aware of, it may not apply to you.