Thread: How to keep concentricity when working on both ends of a round bar

I have spent the whole afternoon making a die holder for the lathe. First the round bar was clamped in a 3 jaw chuck. I worked on one end and then flip around and do the other end. However, I can tell both side don't seem to be concentric. It's off by a few mm. Should I do one end. The other end needs to be clamped by a 4 jaw and dial in to keep concentricity?

One more question, the biggest drill size is 13mm from my drill set. However, 13mm hole could not fit in drilling bar, can I still use the bore to drill out a bigger hole. A large size drill bit costs a lot money, I am not sure whether it is very needed. If it is needed, 16mm drill size is the minimum in order to fit the boring bar.

Depending on the level of accuracy you are working to dialling it into a four jaw is the optimal solution. You can also put a dial on your material and tap it around in the three jaw.
If it’s mm out that suggests your stock was probably out of round before you started, I see that regularly when I use recycled or hot rolled.

You can make a smaller boring bar relatively easily with a piece of round stock, a grub screw and drill bit regrouped as a cutting edge. Or buy one, or buy a bigger drill bit. Thousand ways to skin that cat

Originally Posted by davekwok

A large size drill bit costs a lot money, I am not sure whether it is very needed. If it is needed, 16mm drill size is the minimum in order to fit the boring bar.

There ya go, cheap morse taper drills https://mcjing.com.au/morse-taper-shank-drill-bit.html

You could make a smaller Boring Bar. They are quite simple to make, there are many examples to be found with a google search. It's a reasonably quick project, you'll learn something new and have a tool you can use for other projects. Making tools to make tools to make something else is all part and parcel of this hobby...

Dave.

1) A few mm out seems quite bad. It could be the material you are grabbing, or maybe the lathe's chuck/jaws. Did you try to check the material concentricity (in the chuck) with a dial indicator before turning it down?



2) Yes, a 16mm drill is not cheap, and your lathe might have trouble drilling a hole that large.
A smaller boring tool or bar might be more useful.

Hi Guys,

If I need a hole bigger than 12 mm in a workpiece, then a boring bar is my first choice.

Actually I often find that my normal lathe tool turned to face the chuck is fine for larger holes.

Depending on just how close you want to get and how much material you have, turning between centers is one way to fix your problem but possibly a little OTT for a dia holder.

A few millimetres is a lot.

Get a bit of scrap about 60mm long (long enough that more than half sticks out from the chuck when fully gripped in jaws)
Face the first end
Turn down half the length to a known diameter.
Take out of chuck, flip.
Face second end
Turn down the second half / slightly more than half the length (turned areas need to meet) to the same diameter.

That will show the runout.

Make sure the 3 jaws meet properly in the middle. It is possible to have one jaw too far in or out (on the wrong part of the scroll)

Originally Posted by pippin88

A few millimetres is a lot.
.
.
Make sure the 3 jaws meet properly in the middle. It is possible to have one jaw too far in or out (on the wrong part of the scroll)

That will definitely explain mm of difference.