Can this be done with a metal lathe?

[Enigma]

I have a bicycle that I am customizing and I want to use a particular stem. The problem is the bike requires the shaft of the stem to be .833" but this particular stem is not available in that diameter. It is .874. Can the stem shaft be turned in a lathe? I have talked to one machinist who said his lathe does not have enough swing. I also talked to a car guy who has a lathe but is not that experienced in it and said hes not sure how or if it could be done. It would be great to hear from any machinists on here. Please advise. Thanks.

 

[imagineer]

Yes, it can be done but you will probably have to make an mandrel for holding the stem from the inside.

Assuming it's a cast part, I'll guess the surface quality of the inside is not that precise, so you'll probably have to machine the inside first, then make a mandrel to fit.

 

[cvairwerks]

What's the current wall thickness. You're wanting to remove .0225" from it. Remove too much and there's no strength left in it.

 

[lilredex]

My old Southbend would not have enough swing either, but with no other choice, I'd cut at the green line, turn it down, and weld back together..........unless there is something special about the material, etc.

 

[imagineer]

Yes, it can be done but you will probably have to make an mandrel for holding the stem from the inside.

Assuming it's a cast part, I'll guess the surface quality of the inside is not that precise, so you'll probably have to machine the inside first, then make a mandrel to fit.

Another, less precise way would be to grab the bottom of the stem in a 3-jaw chuck, and grab as little as possible. Then use a live center in the bolt hole and lathe turn down as much of the stem as you can reach without hitting the chuck. Then manually, sand down the part at the bottom that wasn't lathe turned.

 

[Enigma]

What's the current wall thickness. You're wanting to remove .0225" from it. Remove too much and there's no strength left in it.

 

[slowtwitch73]

For sure doable.. I'd do a mandrel as mentioned.. basically taking the place of the bolt that would go though the quill.

But I would 100% not do it. It will break.

 

[DAVE94LIGHTNING]

If the lathe doesn't have enough swing they can do it on a mill with a boring head and clamp the piece fixed and turn the OD of that tube as it points straight up.

 

[Enigma]

Yes, it can be done but you will probably have to make an mandrel for holding the stem from the inside.

Assuming it's a cast part, I'll guess the surface quality of the inside is not that precise, so you'll probably have to machine the inside first, then make a mandrel to fit.

Its smooth on the inside. A long bolt goes through it. Its a tapered hole I believe.

 

[Enigma]

For sure doable.. I'd do a mandrel as mentioned.. basically taking the place of the bolt that would go though the quill.

But I would 100% not do it. It will break.

how can you be sure it will break. Its only like half a mm off each side. Is that enought to substantially weaken it? The force is coming from the wedge being drawn up against the bottom of the shaft which is now 1mm narrower. Do you think that will fail against the wedge? Because beside that its the wedge that is pressing against the side of the fork tube that holds it in place. but no I dont want to find out at 25mph going downhill and hitting a bump.

 

[dr_clyde]

If the lathe doesn't have enough swing they can do it on a mill with a boring head and clamp the piece fixed and turn the OD of that tube as it points straight up.

This is probably the easiest way, to be honest.

That looks like real headache to hold in the lathe, but a modest pain in the mill.

Get your boring head set up and indicate in the tube, both rotationally and vertically and take light cuts.

 

[Enigma]

This is probably the easiest way, to be honest.

That looks like real headache to hold in the lathe, but a modest pain in the mill.

Get your boring head set up and indicate in the tube, both rotationally and vertically and take light cuts.

Do you think taking that much off would weaken it too much like slowtwitch and Cvalrwerks warned?

 

[larry_g]

I don't like where your taking your measurement in the picture above. It seems that is showing the wall thickness of the counter bore for the wedge. What is the Id through the tube and is the smaller diameter seen deeper in the hole a pressed in sleeve or integral to the shaft? Either way you get a lot of your strength from that section of the tube so I don't see to much of a problem turning it down. Don't forget that you probably have to turn down the wedge also. If you were closer I'd say bring it over and we'd take a shot at it.

lg
no neat sig line

 

[Old Man Roger]

Is it possible to ream out the forks a little?

 

[Enigma]

I don't like where your taking your measurement in the picture above. It seems that is showing the wall thickness of the counter bore for the wedge. What is the Id through the tube and is the smaller diameter seen deeper in the hole a pressed in sleeve or integral to the shaft? Either way you get a lot of your strength from that section of the tube so I don't see to much of a problem turning it down. Don't forget that you probably have to turn down the wedge also. If you were closer I'd say bring it over and we'd take a shot at it.

lg
no neat sig line

Not a pressed in sleeve. I have a wedge from a .833 stem and they are identical to the wedges from .874 stems. I wish I lived closer too.

 

[Enigma]

Is it possible to ream out the forks a little?

I dont think so. The cro mo tubing is very thin.

 

[Enigma]

I don't like where your taking your measurement in the picture above. It seems that is showing the wall thickness of the counter bore for the wedge. What is the Id through the tube and is the smaller diameter seen deeper in the hole a pressed in sleeve or integral to the shaft? Either way you get a lot of your strength from that section of the tube so I don't see to much of a problem turning it down. Don't forget that you probably have to turn down the wedge also. If you were closer I'd say bring it over and we'd take a shot at it.

lg
no neat sig line

Oh and i took the measurement there because its the thinnest point. The rest of the stem is much thicker.

 

[Beerhippie]

Isn't the fact that it's an oval cross-section going to make turning on a standard lathe a bit of a fiddle?

I'm missing something here... never seen an oval stem or fork tube... but I've been out of the game for many years.

 

[Old Man Roger]

It wouldn’t be super precise, but you could get it done with a bench sander. Rotate it by hand while it’s against the stop.

 

[Beerhippie]

It wouldn’t be super precise, but you could get it done with a bench sander. Rotate it by hand while it’s against the stop.

To me, the problem would be where the turned-down part meets the original diameter. If it's a square shoulder, it's a stress-riser and likely to break. Stems take a lot of weight in hard riding.

 

[Old Man Roger]

To me, the problem would be where the turned-down part meets the original diameter. If it's a square shoulder, it's an stress-riser and likely to break. Stems take a lot of weight in hard riding.

Chamfer it?

Fast forward to 419 to see the concept. No need for the jig this guy built for a single job.

 

[slowtwitch73]

how can you be sure it will break. Its only like half a mm off each side. Is that enought to substantially weaken it? The force is coming from the wedge being drawn up against the bottom of the shaft which is now 1mm narrower. Do you think that will fail against the wedge? Because beside that its the wedge that is pressing against the side of the fork tube that holds it in place. but no I dont want to find out at 25mph going downhill and hitting a bump.

I've futzed with bikes for decades.. it will be a linear stress riser at the thickness change, and that will sit right at the (steel?) lip of the steerer tube and be torqued this way and that in the course or riding and will fail. The wedge type quill also pulls down on the stem.. pulling it into the presumably steel steerer.

 

[DocsMachine]

Ever see a "camelback" drill press? That "tree" casting was turned on a lathe to machine the column round so the table could ride up and down on it.

Doc.

 

[alfadan]

Ever see a "camelback" drill press? That "tree" casting was turned on a lathe to machine the column round so the table could ride up and down on it.

Doc.

In production, I imagine they had a jig to hold them to a faceplate to turn them...at 30 rpm! Lol

 

[The Cobbler]

Isn't the fact that it's an oval cross-section going to make turning on a standard lathe a bit of a fiddle?

I think it's an optical illusion . there's a wedge that mates with the bottom of the stem to force a tight fit when the bolt is tightened

 

[Beerhippie]

I think it's an optical illusion . there's a wedge that mates with the bottom of the stem to force a tight fit when the bolt is tightened

I'm aware of how bicycle handlebar stems work, and I'm also a photographer and struggling to see how that's a perspective illusion.

But I've gotta be wrong, cause it just ain't right.

 

[Old Man Roger]

I'm aware of how bicycle handlebar stems work, and I'm also a photographer and struggling to see how that's a perspective illusion.

But I've gotta be wrong, cause it just ain't right.

It’s the beer.

 

[Firebrick43]

If the lathe doesn't have enough swing they can do it on a mill with a boring head and clamp the piece fixed and turn the OD of that tube as it points straight up.

If a lathe has a T slot table basically the same thing can be done in a lathe with a boring bar held a 4 jaw chuck (or a boring head if its a morse taper lathe)

 

[Old Man Roger]

If a lathe has a T slot table basically the same thing can be done in a lathe with a boring bar held a 4 jaw chuck (or a boring head if its a morse taper lathe)

It would have to be pretty long.

 

[Firebrick43]

It would have to be pretty long.

6" is long to you?

 

[drmarkr]

Is there a reason you're locked into using that stem? Based on the size it looks like it must be French (22.2mm) and also pretty old? The size you're turning it to is, I believe the old BMX / stingray era stem, so I'm wondering what frame you're going to be using it in?

 

[Old Man Roger]

6" is long to you?

6 inches sticking out of a boring head? Kinda.

 

[Firebrick43]

6 inches sticking out of a boring head? Kinda.

Its only 4" sticking out, 6" OAL

 

[Old Man Roger]

Its only 4" sticking out, 6" OAL

Ok, I guess that wouldn’t deflect too bad. Probably have to take really light cuts, but you could probably get it done.

 

[Old Man Roger]

Belt sander would be easier.lol

 

[Beerhippie]

It’s the beer.

Nope. The beer can is round.

 

[Old Man Roger]

Nope. The beer can is round.

So is the goose neck.

 

[Beerhippie]

Belt sander would be easier.lol

If you hold the part from one end of the belt sander, so it overlaps the wheel, you've minimized the sharp shoulder and stress riser.

Now you have to anodize it again.

 

[Old Man Roger]

If you hold the part from one end of the belt sander, so it overlaps the wheel, you've minimized the sharp shoulder and stress riser.

Now you have to anodize it again.

I would sand it straight, then put a chamfer on it. Wouldn't bother coating it with anything other than a little anti seize. I’m a good enough kinda guy.lol

 

[Old Man Roger]

You used JBweld to get a brewing done, you know what I’m talkin bout.