Making bushes

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: GlennW, Harold_V

Post Reply
charkmandler
Posts: 48
Joined: Sat Apr 29, 2017 8:11 pm

Making bushes

Post by charkmandler »

I have made 12 leaded bronze bushes (a one off operation), OD .501" ID 436" by an inch long. The ID is not a standard reamer size and OD/ID tolerances need to be .0002". I made them by machining the OD and then the ID, sawing off rather than parting due to the thin section of the metal, clean the cut. These have worked very well with a good internal finish, although taking far too long - I do not have a tool post with good repeatability. The bronze also did not cut the same every time although I only had one 'waste' bush.

Thinking how I could have done this more efficiently with the tools I have. Lathe, non collet chuck.

Method 1
Turn the OD and part all bushes
Make a slit collet in 3 jaw chuck
Mount each bush in the 'collet' drill and turn the ID
- This method would keep the same DRO settings for each operation. I would have concerns about concentricity that is critical.

Method 2
Drill and turn the ID / part off each bush
Make a very shallow tapered mandrel
Gently push a bush on and machine the OD - repeat for each bush
- I could have concerns about either stretching the bush while on the mandrel or having it move on the mandrel. Also making the mandrel would take more time than method 1.

Any suggestions?
Thanks.
User avatar
NP317
Posts: 4551
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon, USA

Re: Making bushes

Post by NP317 »

I would use Method 1.
Less chance of stretching the bushing when mounting.
~RN
User avatar
Harold_V
Posts: 20226
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Making bushes

Post by Harold_V »

For this operation, I wouldn't even consider anything except for simply turn and bore the bushing before parting off. I don't see any reason why you'd have to saw them, which is a waste of time. Second handling for the bore is also a waste of time, and most likely to introduce error (both in holding and in removing material, releasing stresses that offer the risk of the finished OD changing). There's absolutely no need to do them that way unless there's something I don't understand.

By doing them as you suggested you did, you can rough both features, then go back and finish machine (roughing and finishing are two distinct operations, neither of which should be ignored when working to tight tolerances). That guarantees concentricity, and you don't have to second handle the bushings except for holding to face and chamfer the end which was parted. Even that can be avoided if you are able to fashion a decent parting tool, which would allow you to part the bushings to finished length. In leaded bronze, you shouldn't have any issues with finish or taper. You would be left with a sharp edge on the bore (you can chamfer the outer edge before final parting), but I'd simply hold the bushing in a collet (or soft jaws) and chamfer the edge according to need. It takes but seconds and minor run-out won't make a difference, assuming that's of concern.

You talked about having only two tenths tolerance. That's a tall order for a reamer, which may or may not be capable, and it most likely won't be repeatable, depending on several factors. If the project was mine, I'd bore them, and forget about reaming. Unless you pre-bore each one before reaming, pretty good chance you're not going to keep concentricity, anyway.

I don't much care for reamers, if you didn't get that impression.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Making bushes

Post by earlgo »

Geez and no one mentioned soft jaws. That was the first thing that popped into my mind. They would have to be made so that they almost entirely covered the od to prevent distortion, but that is what they are good for.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
User avatar
Harold_V
Posts: 20226
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Making bushes

Post by Harold_V »

earlgo wrote: Sun Oct 21, 2018 8:59 am Geez and no one mentioned soft jaws. That was the first thing that popped into my mind. They would have to be made so that they almost entirely covered the od to prevent distortion, but that is what they are good for.
--earlgo
I actually did, as a second operation for chamfering the bore once the bushing is parted. My first choice in this case, however, would be a collet, as they're faster to use.

My experiences with soft jaws dictate that the best you can hope for (reliably) in regards to concentricity is about a half thou----so if I had to hold two tenths, as was suggested, I'd avoid any second operation in which that feature could be compromised. Therefore, I'd do both bore and OD in the same operation. That way, assuming the material is roughed first, the resulting part would be as close to dead concentric as the bearings of the machine allow.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Making bushes

Post by earlgo »

Oops, my bad, you did mention soft jaws parenthetically and I missed it in the reamer comments. I agree with Harold: doing all features in the same setup and operation would be paramount, certainly if one had a longish tube from which to make the bushings; i.e. bore, turn, cutoff, advance, repeat.
Pressing the bushing into a larger component surely will alter the bore somewhat, so Loctite with a tiny press fit may be the answer.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
User avatar
GlennW
Posts: 7284
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Making bushes

Post by GlennW »

I use both ends of the rod.

Turn and bore one end to within .010" of finished size, then flip the rod and do the same on the other end. then go back to the first one. This lets the bronze cool off and relax a bit. Not as critical with leaded bronze, but aluminum bronze alloys move around quite a bit when machined.

Now cut the bushing to finished size on the ID and OD and flip the rod and do the same to the other end.

Then chamfer the accessible end and part off both bushings.

I don't know what the repeatability of your chuck is so i suggested .010". If it repeats well, rough it down a bit farther to start. Don't get too brave though, as it a lot easier to remove a few more thousandths than to start over!

I always put a sharpie mark on the stock and index it to the socket I use to tighten the chuck so it pretty much goes back in the chuck the same way as it was removed to aid in concentricity.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
charkmandler
Posts: 48
Joined: Sat Apr 29, 2017 8:11 pm

Re: Making bushes

Post by charkmandler »

Regarding parting off, I backed out of this having had ordinary bronze crumple - a few minutes testing would have shown that leaded bronze is a different creature - next time. Glad I did the bushes under the one turning operation for guaranteed concentricity although I did not rough out and feel that would have helped the inconsistency in the final cuts. Anyway, dob done and bushes fitted. Thanks for the advise.
Mark
User avatar
Harold_V
Posts: 20226
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Making bushes

Post by Harold_V »

charkmandler wrote: Tue Oct 23, 2018 2:41 am Regarding parting off, I backed out of this having had ordinary bronze crumple - a few minutes testing would have shown that leaded bronze is a different creature - next time.
If you use a properly ground parting tool, I can't imagine a case where you wouldn't benefit by parting instead of sawing. Even crumbly material will respond in an acceptable manner. Keep the tool sharp and on center. If the tool likes to self feed (often called hogging), you may have to play with rake a little. A rigid machine tends to be less troublesome than a light duty one in that regard.
although I did not rough out and feel that would have helped the inconsistency in the final cuts.
It's really important if you're working to close tolerance. I've never experienced material that didn't change when being machined, although the change is often negligible. However, when you work to tolerances under a thou, pretty much everything you do, including leaning on the machine, can make a difference. By roughing, the bulk of the material is removed before any finish cuts are taken. That allows the material to move, as stresses pent up in the material are relieved. Once you've removed the bulk of the material, the finish cut is light enough that any changes are diminished (for all practical purposes, eliminated), plus, if you've heated the material in roughing, it's had time to dissipate. Bottom line is if you learn to rough before finishing, especially if you have quantities involved, you'll achieve a higher level of precision, plus your tools will last considerably longer in the finishing operation.

Note that you often rough with a completely different setup. I tend to rough with negative rake insert tooling, while finishing with positive rake tooling, which cuts with a greatly reduced cutting pressure. Combine that with the normally slower feed rates, and the part is machined in an almost free state. I also tend to finish at greater velocities (surface speed), so finish is highly improved. In lathe work, I use soft jaws, so I can load parts without worrying about concentricity or length registration issues.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
charkmandler
Posts: 48
Joined: Sat Apr 29, 2017 8:11 pm

Re: Making bushes

Post by charkmandler »

Thanks Harold, that all makes absolute sense. I've been machining for years from racing motorcycle big ends to turning forged pistons (ovality etc) for the same racing engines. Its interesting that in those cases I always roughed out but in the case of the bushes I did not!
Post Reply