Need help reducing shaft diameter

[gwin99]

Hi, so my team recently found ourselves drastically overweight. To shave off pounds, we need to replace our steel shafts with aluminum. However, the aluminum rods we got (because McMaster Al shafts were $60 each) are 4-6 thousandths to big, and don't fit our linear bearings. The shafts are to long to turn down without wobbling, and we've tried hand sanding and belt sanding with no success. We've also tried turning it and using sandpaper, which seemed to work, but a less dangerous solution would be welcome.

[bEdhEd]

Are you saying that you guys were trying to turn down the whole shaft as it is COTS, or did you cut it into smaller lengths for their specific use on the robot, then turn them down? Does the shaft have to be that long because it's part of your design?

[Mark Sheridan]

You can try loading the shaft into a hand drill chuck, spin the shaft while cupping the part with sand paper. Let the hand drill do the hard work. Go slow and keep checking the OD. also keep cleaning out the sand paper, your gonna get a lot of aluminum dust. It will take a while but its a low cost fix. I had to do this with a aluminum shafts a couple years ago. Make sure you got a fine grit sand paper too.

Also get in touch with 100 or 766 in your area. They should be able to help with this too.

[bEdhEd]

Whenever we use a drill and sand paper, we have someone hold it on the other side with a bearing. If the shaft is too big, try a bushing or a piece of pvc so you can hold on a little safer.

What seems sketchy to me actually is using linear bearings on an aluminum shaft that has been sanded or turned down. The aluminum may be too soft, and the surface of the sanded metal and possible bending of the shaft could cause binding.

This is not to discourage you, but it's just a precaution from what I speculate could happen. Ideally, linear bearings should ride on hardened steel. If this is what your team needs to get done, it may have to do. Have you lost all possible weight in other parts of the robot?

[gwin99]

Originally Posted by Mark Sheridan

You can try loading the shaft into a hand drill chuck, spin the shaft while cupping the part with sand paper. Let the hand drill do the hard work.

It's to long to do in a hand drill, which is why we tried a similar method with the lathe. I accidentally got my thumb pinched between the paper and the shaft, which hurt a lot, so I was wondering if there's a safer / alternate way to do it.

[Mark Sheridan]

Originally Posted by gwin99

It's to long to do in a hand drill, which is why we tried a similar method with the lathe. I accidentally got my thumb pinched between the paper and the shaft, which hurt a lot, so I was wondering if there's a safer / alternate way to do it.

You lay the other end of the shaft in a v groove, I used a pile of plywood. You have to go very slow. I cupped the paper in my palm and held it firmly to the shaft but I was not trying to grab the shaft. Keep the paper loose start slow, then you apply more pressure. Slow down if you loosing the sand paper. Make sure its a very fine grit. I can't recall what I used.

Let me know if you need a contact for 766.

[gwin99]

Okay, thanks, I'll give that a shot today.

[BeardyMentor]

I suppose you cant use a Steady rest or follower rest in the lathe?

[Mark Sheridan]

Originally Posted by gwin99

Okay, thanks, I'll give that a shot today.

I should add, when I did this I was not trying to go faster than a hand method but sustain that effort for a long time is tiresome, hence using the drill to make it easier. it will take a while, I think it me about a hour to fix 16 12 inch shafts. Slow and steady with this dirt cheap method.

Originally Posted by BeardyMentor

I suppose you cant use a Steady rest or follower rest in the lathe?

This would work too. 100 or 766 should have these on their lathes if you don't have this.

[gwin99]

Originally Posted by bEdhEd

Are you saying that you guys were trying to turn down the whole shaft as it is COTS, or did you cut it into smaller lengths for their specific use on the robot, then turn them down? Does the shaft have to be that long because it's part of your design?

We have 4 shafts that are 1/2in diameter and 38in long. They support the linear motion on our grabber. Changing them from steel to aluminum shaves off about 4 lbs, which is unfortunately necessary for us this year. The steel shafts we were using have a diameter of 0.498 in and the aluminum rods we want to replace them with are 0.504 in. The linear bearings don't slide on the aluminum rods, so we want to reduce the aluminum rods to the diameter of the steel shafts (.498 in), effectively shaving off 0.006 in consistently around / across the rod.

Even with a live center, and even if done 1/2 at a time, the shaft will wobble too much to use a normal cutting tool. Thus, we tried putting it in the lathe and running it while passing sandpaper along it. This was effective, but seems sketchy and dangerous. Any ideas on how to do this better / find cheaper Al shafts / do our sandpaper method more safely?