Making a bronze thumbscrew

[spike7638]

I've got some silicon bronze stock (a piece from a friend's slightly-bend propeller shaft) and I'd like to make up four thumbscrews, with knurled heads, a shoulder, and about a half-inch of 1/4-20 thread. I figure that this is a way to learn to do something a little more complex on a lathe, and also get some parts I need. It's a win-win.

The part I want to make looks something like this:

but with a little adjustment: I want the head to be wider (like almost the full 1" diameter that my stock has), and to be a little farther from the "shoulder". The shoulder being about 1/2" diameter is about right.

So my question is "what's the right order to do the machining?" I know that knurling applies lots of side-force, so I should probably do that before thinning down the stock at all, with a live-center pressed into the "free" end. Then perhaps I turn down the left-side to the shoulder radius (I don't actually need a "neck" between the shoulder and the knurled part). Do I then cut off (or face off) the right-hand side, and add the bevel. And then what? Flip the piece end for end, so that I'm grabbing the knurled part with the jaws (or maybe in a collet) and turn down the screw part enough to start cutting threads? I'm just spitballing here, because I have no idea whether cutting the threads is going to apply so much side-force that the material will bend, etc. Do I make the screw-part extra long, so that I can stick a live-center into the excess bit at the end, and then use a cutoff tool once the threads are made?

Any advice gratefully received!

 

[David S]

I think what I would do is chuck the long work piece and leave just the part length sticking out. Turn down for the thread, finish the details up to the large diameter for the knurl. Make the thread. Bring the stock out from the chuck a bit machine the with for the knurl and some relief on the left to clear the knurl rolls. Make the knurl..Part the completed part off.

David

 

[spike7638]

I think what I would do is chuck the long work piece and leave just the part length sticking out. Turn down for the thread, finish the details up to the large diameter for the knurl. Make the thread. Bring the stock out from the chuck a bit machine the with for the knurl and some relief on the left to clear the knurl rolls. Make the knurl..Part the completed part off.

David

When you say "turn down for the thread," you mean that you turn down for the thread on the end near the tailstock, right? So that when I'm making the knurl, the side-load is born by the full-diameter stock and the jaws, and the thread-end is just riding on the live-center or something. Am I getting that right?

Perhaps you can also help me with a related question:

"Parting off" always scares me. If you use a parting tool, it has to be exactly square to the lathe axis or it'll want to bind in the slot that it's cutting, and when you get to the point where there's almost no remaining material, the piece itself is likely to start wobbling, at which point it may hit the parting tool and mar the end, etc. Is there a good way to do this more sanely than what I'm describing?

 

[ThinWoodsman]

you mean that you turn down for the thread on the end near the tailstock, right? So that when I'm making the knurl, the side-load is born by the full-diameter stock and the jaws, and the thread-end is just riding on the live-center or something.

Yup, sounds like the right way to do it.

When you get to the point where there's almost no remaining material, the piece itself is likely to start wobbling, at which point it may hit the parting tool and mar the end, etc. Is there a good way to do this more sanely than what I'm describing?

First off, make a height gauge that allows you to easily set a tool on-center. This can be as simple as a cylinder the exact height of the lathe center that you place on the lathe ways. I prefer a stepped cylinder, so I can also measure from the carriage when it's not important to be overly precise.

For the wobble at the end, unless you're doing ring-shaped parts there's not much you can do about it. Some people finish parting with a hacksaw in order to avoid that bit.

 

[mikey]

Spike, not sure what kind of lathe you have but do yourself a favor and buy or make a scissors knurler. When you say "side-load", it implies you're planning to use a bump knurler that indeed puts a lot of load on the spindle, spindle bearings and the part. A scissors knurler will eliminate that.

I would make the part just like David S said. On a part this short given the OD of your stock, you do not need a live center unless you use that bump knurler.

Parting can be a problem on smaller lathes. One good option is the FoR Parting Tool from Eccentric Engineering if your lathe can run in reverse and doesn't have a threaded spindle. If your cross slide has T-slots then a rear mounted parting tool post would be ideal and will eliminate your parting issues/fears completely.

 

[WCraig]

"Parting off" always scares me. If you use a parting tool, it has to be exactly square to the lathe axis or it'll want to bind in the slot that it's cutting, and when you get to the point where there's almost no remaining material, the piece itself is likely to start wobbling, at which point it may hit the parting tool and mar the end, etc. Is there a good way to do this more sanely than what I'm describing?

Bronze should not be too hard to part. Try a couple of practice cuts. There are tonnes of resources out there about parting off--forum posts, YouTube videos, web pages, books, etc. I certainly read lots so I won't try to summarize everything here. In almost all cases, though, the main problem is rigidity. Anything that allows the cutter to move or flex will create chatter or worse. Most anything you do to improve parting off will also help the rest of your turning, so it is worthwhile learning process.

Re marks on the piece, that isn't a problem I've had. OTOH, I'm not good enough that the piece has a nice finish from the parting tool. I almost always turn the piece around and face the parted end to remove the nub.

Craig

 

[Bamban]

Part the almost finished product longer than intended length, keep the material you parted from chucked up, face it and drill and tap, screw the thumb screw into it, face it to desired length.

 

[Mitch Alsup]

So my question is "what's the right order to do the machining?"

In less than 1 minute of searching, I found dozens of the described parts available on the internet.

But, the right order is::
single setup in 3-jaw chuck the bronze long in the chuck; long enough you can easily part it off at the end.
face the front
turn the bar to knurled diameter
knurl the head
turn the bar to thread diameter
Thread
turn the other features
part off
flip-around and chuck in collet chuck
face the thumbscrew end.

 

[Cadillac]

One tip not mentioned is the grind on the parting tool. You will get a easier cut by putting a angled going on the end instead of a square grind. Grind your angle so it cleanly cuts off your part and leaves the nub on the material in chuck. Nub gets cut on next facing op. I usually get a near perfect finish on part. Of course make sure your blade is perpendicular to the work.
Another thing I do is start the parting go in about 1/4 of the way then switch tools and chamfer your part then switch back and finish the parting. I have a dedicated tool for chamfers that happens to be the same offset as the parting blade so no need to move carriage between tool change. With that it could be a one chuck operation no flippy.

 

[spike7638]

In less than 1 minute of searching, I found dozens of the described parts available on the internet.

Really? Your google-fu must be much stronger than mine. Because nothing that came back from this search is what I need:

silicon BronzE 1/4-20 thumbscrew - Google Search

namely, a 1/4-20 silicon bronze thumbscrew. Lots of folks make 'em in brass, stainless, and other materials. But those aren't suitable for use in the salt-water pump for which I want them. I've gone with stainless for a year or two, and the electrolytic corrosion has been only modest, but given the inconvenience and expense of replacing the pump, and the availability of the silicon bronze stock, I figured it was time to do it right.

But, the right order is::
...

Thanks for the suggestion about the order.