Making screws to replace screws that no longer are made, 27TPI.

jongig · Apr 20, 2019

I've just joined this forum although I've been lurking here for years. I have run into a problem that I can't figure out. I'm reconditioning a large French solid brass chandelier. It's very old and uses screw sizes that do not seem to exist. Everything on it is 27TPI but the screws themselves are of differing diameters. The original screws are steel and mostly rusted and I'd rather not use them. Since I have a lathe, it should be easy but I can't turn 27TPI on my lathe. Is there any ideas that I've missed?

My last resort I guess is to drill out the holes and tap them but it won't be easy and then it's would be modified which I'm trying not to do.

sheys · Apr 20, 2019

Many lamp parts still use 27 TPI threads, mainly 1/8 NPS threaded tubes/nuts and 1/4-27 nuts for on top of a shade harp. These guys may carry some of what you need, though it may be buried under one of their many categories:

Lamp Parts - Lighting Parts - Chandelier Parts | 1/4-27 Threaded Screws | Grand Brass Lamp Parts, LLC.

Otherwise you can likely buy or make the proper gear to get your lathe cutting 27 TPI. You maybe could use a 1/4 NPT die to get a passable OD thread if the diameter is near what you need.

4GSR · Apr 20, 2019

Don't know what brand lathe you have, most that do not have 27 TPI available, usually can be fixed by changing the tooth count of either the stud gear or the gear going inte the QCGB by two teeth. Of course to do this will require to buy or have a gear made to use for this pitch. If your gear train is made of common diametrical pitch and pressure angle, do some web searching and find you a stock gear. If your gearing is of Mudul (metric) pitch, good luck finding something.

billmac · Apr 20, 2019

Are you sure that you have 27 tpi and not 26 tpi? The reason I ask is that many brass light fittings have so-called BSB (British Standard Brass) screws. These range from 1/8" up to over 1" but all of them have the same tpi. The reason for this is that brass tubing used for lights has a roughly constant wall thickness regardless of diameter.

I came across this strange thread series when I made a special light fitting for a family member. I had to single point the threads, but of course 26 tpi is going to be a lot easier than 27 tpi.

L Vanice · Apr 20, 2019

You can buy lamp repair parts at hardware stores that have various size 27 pitch threads, male and female. You can use such parts as gages to see if your antique fixture uses those standard threads.

Victor has special taps and dies that include a number of different diameters in 27 TPI (and a whole lot of other odd sizes). Taps and Dies for Threading Metal from Victor Machinery

Note that 1/16-27 and 1/8-27 NPS are standard pipe sizes used in lamps and are listed with the other pipe taps and dies. Any other diameter, like 1/4-27 and 3/8-27 UNS are listed with the special threads.

Larry

jongig · Apr 20, 2019

Maybe it's 26 tpi

Maybe I'm wrong, maybe it's 26TPI. Picture attached is one screw with the calipers set to .5". I can cut 26tpi.

Thanks.

Rudd · Apr 20, 2019

Wait, you are threading things on a lathe and you don't own a thread gauge?

L Vanice · Apr 20, 2019

Looks like 26 TPI Whitworth form, also known as BSB.

British Standard Brass Thread

Larry

Chip Chester · Apr 20, 2019

27 tpi is also used on microphone stands and clips... 5/8-27.

plannerpower · Apr 20, 2019

If it is 1/4" diameter, as it might be, there are some candidates for 26 tpi;

BSF was used on British motorcycles whilst Cycle thread (CEI in the above table) was/is used in bicycles.

Your local bike shop might be a good place to start; I think there are also quite a few restorers of British motorcycles in the US.

In this application the thread angle shouldn't really matter.

EPAIII · Apr 20, 2019

It is really hard to tell from that photo. First, your caliper is not parallel to the screw axis, it is at an angle. Second, the left hand jaw TIP seems to be a bit further to the left on the thread while the right hand jaw's tip seems to be right on the edge of the flat.

A thread pitch gauge is not very expensive and is a far better way of getting an accurate reading. Even just one thread count off, as with 27 vs 26, will show up nicely: one will fit and the other will not. Get a thread pitch gauge. In fact, you are in the US so get two; one English and one metric.

As for cutting 27 TPI, it is just not part of the usual sequence of threads that a Quick Change Gear Box is set up for. So you will need an additional gear or two. This will be specific to your lathe so more details would be needed. Lathe make and model? Quick Change Gear Box: yes or no? What threads can you cut? What gears are normally in the chain leading to the Quick Change Box AND what separate change gears do you have (tooth counts)? And what is your lead screw TPI?

jongig said:
Maybe I'm wrong, maybe it's 26TPI. Picture attached is one screw with the calipers set to .5". I can cut 26tpi.

Thanks.

Mark Rand · Apr 21, 2019

jongig said:
Maybe I'm wrong, maybe it's 26TPI. Picture attached is one screw with the calipers set to .5". I can cut 26tpi.

Thanks.

It looks as if it could be 27tpi. Gas thread is that pitch, as opposed to brass thread, which is 26tpi. If you don't own a thread pitch gauge (buy one, they're useful), you can turn a short length of 26tpi thread on a piece of scrap and offer that up to the screws. If it fits perfectly over that length, you know it's the same pitch, If it doesn't you know it isn't!

If the thread is 27tpi, now it the time to learn about using additional change wheels to alter the pitches that you can get out of your current lathe/gearbox.

steve-l · Apr 21, 2019

I had that issue many years ago, but I forgot how I resolved it. So, I checked my lathes. I have 3, but only my L&S will cut 27 TPI. My 10" SB and my 13x40 SB will not.

jongig · Apr 21, 2019

Yes of course I own a thread gauge... The thread gauge doesn't confirm the TPI, the screws are just in bad shape. I frankly can't believe I had to come here for this as this should be easy, it's just a stupid screw.

LexD · Apr 21, 2019

The photo shows 13 threads in 1/2" or 26 TPI.

litlerob1 · Apr 21, 2019

jongig said:
Yes of course I own a thread gauge... The thread gauge doesn't confirm the TPI, the screws are just in bad shape. I frankly can't believe I had to come here for this as this should be easy, it's just a stupid screw.

Then you need a better Thread gage, if it's one of the ones that doesn't discriminate between Thread Pitches.

Single point some bar at 26 TPI, and see if it fits.

gregormarwick · Apr 22, 2019

Rudd said:
Wait, you are threading things on a lathe and you don't own a thread gauge?

I own multiple sets of thread pitch gauges, and routinely measure thread pitches with calipers anyway

The greater the quantity of different thread forms you work with, the more cumbersome pitch gauges become.

It's a perfectly valid way to do it, and if my pitch gauges were all lost tomorrow, I wouldn't really miss them.

OP, going by your photo it sure looks like a 26 TPI.

snapey · Apr 22, 2019

You say it was made in France so is there a chance it's a metric thread.

iwananew10K · Apr 22, 2019

Even if it wasn't metric it would be by the time I got done with it.

those things usually have f'ed up drunken threads anyway, do everyone a favor and standardize them, just make the heads look original.

1mm pitch.

EPAIII · Apr 22, 2019

That thread does look good enough to check with a thread gauge. You may want to hit it with a wire brush first. But if the gauge does not fit, it probably is not the right thread pitch.

jongig said:
Maybe I'm wrong, maybe it's 26TPI. Picture attached is one screw with the calipers set to .5". I can cut 26tpi.

Thanks.

Making screws to replace screws that no longer are made, 27TPI.

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