How to measure hole when boring between centres?

[joeby]

You can, assuming the boring bar is already mounted between centers, clamp your DTI to the boring bar on either end. This will allow you to indicate on each end of the bore, by rotating the chuck, to get your part centered. I have a few small horseshoe magnets with short posts epoxied to them that work well for attaching the indicator to the boring bar.

A length of heavy walled pipe may work for a boring bar, but chatter may be a problem. It would certainly be easy enough to plug the end(s) and centerdrill them for use between centers, and possibly a donut shaped toolholder to slide over the pipe and be clamped into position. An added bonus would be the ability to position the tool anywhere along the length of the bar to suit the job at hand.

Kevin

 

[Jasonb]

I'll dig out some pictures of large traction engine trunk guides & cylinders being machined for you when I get back in from work.

Though if you are welding all the bits to a "tube " you may still be able to chuck the tube and use a fixed steady on the other end, still a lot of tool overhang though.

Jason

 

[SignalFailure]

Putputman:

Thank you, that's nice and clear. I've read about that type of bar with a screw to adjust cutter position along with a locking grubscrew but never seen one before.

Is there any disadvantage with using a (easier to make!) fixed point bar and using the cross-slide to alter the depth of cut instead? (I suspect the answer will be that the cross-slide may not move perpendicularly to the bore but I'll ask anyway!)

 

[tel]

No way that's gonna work Paul, as soon as you move the cross slide the tool will start taking material from one side of the hole and leaving the other alone. Totally useless unless you are trying to machine some egg-shapes.

 

[scoop]

when I measure awkward bores on a lathe I use a lever type DTI in a hight stand and stand it on the cross slide and find the highest part of the bore and zero the DTI then transfer to a surface plate and measure the hight with a tower gauge.Then do the same with the lowest part of the bore and measure that then subtract the small size from the big size to get the bore diameter.The boring bar may make the job a bit trickier but possible.

good luck Steve C

 

[putputman]

Paul, I think Tel answered your question very well.

To add a little more information on my set up. The first thing I did was to remove the compound from the crosslide. I then established a dimension from the crosslide to the center of the lathe. Very important. I recorded that dimesion for any future set ups.

Next I determine the distance from the base of the engine to the cylinder bore. The distance from the crosslide to lathe center minus the distance from the engine base to cylinder bore is the thickness of the mounting plate for this set up. This will establish the vertical height and vary from engine to engine.

The horizontal location was established by locating and doing a roughing cut on the mill. Then once it is set up in the lathe, I mounted a TDI in the chuck & zeroed in the location with the crosslide. That is when I set the crosslide indicator to zero also.

This sounds like a lot of work but once you do it you will find it doesn't take that long. You will also find that holding the long boring bar in a chuck with a live center on the outboard end will provide a very rigid set up and a very smooth cut. You should not have any chatter in the bore.

 

[SignalFailure]

Hmmm, this is rather more complicated and takes more equipment than I have at my disposal. It'll have to wait until way after Christmas

Thanks everyone for the help