I am wanting to true up the I.D of a lathe spindle and was wanting feedback on my potential plan of attack. The spindle bore is about 18" in length and 1.300 on the I.D. My plan was to do it like a line bore. I would first get a piece of rod stock 1"dia or maybe even 1 1/4" and a little over three times the length of the spindle. Drill a hole about 19" from one end and mount a tool in it. My next step would be build a couple of bronze bushings with an I.D to fit my bar. Now I would mount one bushing in the back end of the spindle and do what ever I needed to do to have that bushing run concentric. Finally I would mount my bar in my tool post and lock my cross slide so my bar was on centre. In short I would use the bushing in the back of the spindle to guide and steady my bar and I would move the tool on the bar to set my depth of cut. I would run my second bushing in the chuck after passing the tool into the spindle. I was hoping if I ran the lathe at a reasonable rpm and I kept my bushings well lubed I could get a reasonable finish. My motive is not to enlarge the spindle bore by any great amount but only to have it concentric with the spindle bearings. Please do not ask why I need to do this let's just say I have my reasons.
Your feed back will be very much appreciated
Good approach, but the spindle may be too hard to cut...Bob
"Why do you need to do this?"
There, it had to be said. This having
been gotten through, how much are you going
to remove off the radius?
How close are you going to come to the inner
bore of the roller bearings?
I think you would get a better finish regardless of the hardness with a toolpost grinder.
I mention this as I have only recently completed a 2AB to R8 Spindle conversion for a mill and ran into this same problem.
No matter how "gently" you turn, there will be ridges present.
I will disagree somewhat on the toolpost grinder issue, I have re-done ID tapers in spindles and have found several times that I could do a nicer job with a nice sharp tool. To be clear this is in situations where the rigidity of the setup was not optimum. If the setup cannot be optimum the tiniest chatter breaks down the grinding wheel rapidly, and I found it easier to work on tool geometry to be able to get a nice even cut. Polish with a bit of emery for a finer finish.
Once the spindle ID was bored concentric you COULD probably then lap it to as near perfect roundness, and surface finish as your heart desires.
Bob, I believe only the spindle nose is hardened. I touched the balk of the spindle with a file and it was quite soft.
Jim, I will only take out as much as I need to get it concentric. Maybe .025" max.
Ken, I do agree grinding is the best approach from a finish stand point but I think there would be no practical way to do this over 18" of length.
Willbird, polishing for final finish was what I was what I was thinking as well.
I used a different approach on a 20" P&W. It was already concentric - I just needed it a little bigger.
Bored as deep as I could using boring bar in tool post.
Made up a bar with a bushing behind a cutting tool , the bushing a running fit in the bore I had made, and the tool set to match the existing bore I had made.
Held this bar in the tool post, and fed it thru, had to reclamp once to extend it some more.
Back end may not have been perfectly concentric after this, but it was very close.
This was easy and gave great support to tool at all locations - something your "line bore" set up can not do.
Took out less than a 1/16"
Machined nicely with HSS.
John, I like that idea. That is the same principle as a gun drill is it not? you say it ran out a little at the balk of the bore, how much is real close? Thanks
A few thou. But it has been over 15 years now - who knows? The lathe is long gone to iron heaven.
How good is back end on yours now? You could always reverse the idea, locating in what is there now to make a clean up cut, and then the "cutter-follower" idea and pull it back towrds the front - right?
our atlast lathe had a spindle bore that was just a hair too small to put a piece of half inch black iron pipe thru, so we simply took a 7/8 drill bit with an extension mounted in the tailstock and opened it up, worked great, made the lathe a bit more usefull, but i dont know how true the bore was, never realy mattered much
Your aproach is exactly how it should be done. I had to do that to a Cincinnati N/C lathe back in the mid '60's so a bar feed would fit. Make the boring bar from a turned ground and polished pc of stock and it will fit the bushing very well. You mentioned the hole that is there now is not concentric, make sure the bar dia you chose allows for proper clearance. An oil-lite bushings should be available in somewhat close size. Take your time and it will work.
Thank you everyone for all the input. I appreciate it very much.
If you make the rear bushing like a step pocket that can fit loose on the OD of the rear of the spndle OD and fir it with 4 jacking grubb screws you can set your boring bar true to the rotation like a 4-jaw.
As to your method, should work fine. I have an automotive Line boring machine built by Berco and the bar runs on plain babbit bushings and it makes very round true holes. On the front end i would just clamp the forward bushing in a 4-jaw to get it true. You can then simply remove the chuck to check the hole and how she is cleaning up. That way it will repeat when doing the next cut. Make the hole for the boring tool through the bar , tap the back side and fit with a fine thread grub screw to "push" the tool up and hold its adjustment.....cross drill and tap for holding the tool. Use very little radius as chatter will be a problem. Final hone to finish to exact size and remove any dips and gouges. Good luck.
lego boy- I'm still waiting for a response to why it needs to be a 'precision' hole, just wondering...