Oddball thread help on Kamakura 6” rotary table

[OCJohn]

I’m cleaning up an old rotary table I found last year. Though the tag says it was made in Japan, all the screws and clamp handles are SAE thread. (Was that common for Japanese manufacturers in the 50s, 60s and 70s??)

Anyway, it’s the table’s center hole that’s getting my attention. It’s threaded rather than tapered. The plug seats about 60 thou below the surface and seems useful only for blocking chips. I assume if I want any center alignment or fixture pins, I’m going to have to make them. That’s fine, but I need a sanity check trying to ID that thread…

Major dia. is 23.10mm/.9095in. Threads are 60° and a perfect 1.50mm pitch. (18TPI is too small, 16 too large and my Starrett No6 never heard of 17TPI.) I’ll be damned if I can find anything in my 15th ed. MHB or the engineer’s black book with specs like that.

I guess it doesn’t really matter if I’m cutting threads on the lathe. But it’ll be my first threading project and it would be nice to find an actual standard with tolerances. Just want to be sure I’m not overlooking something obvious…

Any help?



Looks like the guy who owned this 15th ed. before me had some difficulty with threads too…

 

[Jim F]

You have a sample, cut to match.

 

[Aaron_W]

Japan adopted the metric system in the 1920s, but they were a source of cheap manufacturing after WW2. There was also a much larger market for "Imperial" until the 1970s, so I imagine a lot of Japan's manufacturing for export in the 1950s-70s featured SAE fittings because that is what the end users wanted.

 

[Ischgl99]

Japan adopted JIS in 1949 I believe, so the threads would most likely be to a standard spec and not something weird. I’m wondering if there is a burr on the threads that makes them appear larger than they actually are. 0.1mm is only 0.004”, so it is just barely over the nominal size. Do you have thread wires to check the pitch diameter? I would compare that to thread tables and see if it is within the tolerances of an M23x1.5 thread. JIS should be the same as DIN/ISO, so any of those tables would be fine to reference.

 

[MrWhoopee]

The 70s vintage Taiwanese 4x6 bandsaw that I converted to vertical was an interesting mix. Threads were imperial, the bars that supported the blade guides were imperial, but the heads of the fasteners were metric. Built for export to the U.S., but fastener heads sized so the assemblers wouldn't need special tools.

 

[NCjeeper]

My 1981 Jet lathe is a mix of Metric and BSW hardware. I wonder if it is whatever the manufacturer feels like using?

 

[Dabbler]

@OCJohn You seem to have a 23mmX1.5mm thread in your table. It is not uncommon to cut a thread with plenty of tolerance for fitment.

I own a lathe with an 82mmX8TPI spindle. So there is worse out there!

 

[Jake M]

Major dia. is 23.10mm/.9095in. Threads are 60° and a perfect 1.50mm pitch. (18TPI is too small, 16 too large and my Starrett No6 never heard of 17TPI.) I’ll be damned if I can find anything in my 15th ed. MHB or the engineer’s black book with specs like that.

Somebody making tools (and threads) from scratch are under no obligation to use standard thread forms, or standard thread dimensions. Practicality says that if they can use a standardized part, it will be more efficient and cost effective. But if they have a need for something that's not standard, well, they're making stuff from scratch anyhow.... Very often if they have to do that, you'll find a non-standard thread pitch and/or a non standardized thread form on a standard sized piece. Or a non standardized diameter with a standardized pitch. Basically, changing ONLY the variable that they needed to take liberties with to achieve the end goal. Or, manufacturers can and so sometimes make the whole thing up from scratch. But that's custom tooling, custom programming, job specific setups, and generally reserved for when there's a really good reason...

finished threads will always have a dimension that is below the nominal size, as the thread peaks are NOT the theoretical sharp V which the cutting and nominal dimensions are based on. So whatever your nominal size is, it's going to have to be larger than what you measure. If you really need to know the actual nominal diameter, thread wires would be the most approachable solution to find a quantitative answer.

You've got the book, so I'm not digging mine out, but a good rule of thumb for actual outer diameter is to reduce the diameter by 10 percent of the thread pitch. 7/8 and 15/16 don't really fit the mold, plus you say it's fairly certainly a metric thread of 1.50mm. So the "generalized" reduction in this case would be 0.15mm. If these were "standard" threads, that's not really enough room to come out to anything nominal in any system. If these are "heavily rounded" on the top, it could be 24mm. It might be that simple, but that's heavily rounded. If this is a chip plug to keep a fixturing hole clean when it's not occupied by another accessory, those threads might have been made "loose" on purpose, for quick and easy engagement. Hence why I say that measuring the pitch diameter (what the wires do), is really the only way to find out for sure what the threads really are, modified or standard.

I guess it doesn’t really matter if I’m cutting threads on the lathe. But it’ll be my first threading project and it would be nice to find an actual standard with tolerances. Just want to be sure I’m not overlooking something obvious…

And there's that.... Make the part to fit, and it doesn't matter what size it is. That would be an excellent excercise in all of the numbers that make up a thread to come together. The hardest part of that is going to be figuring out how to do your test fits, as you won't be able to remove the part from the chuck until you're done. (Well, you can, but picking up an existing thread to chase it requires pretty good runout control on the chuck you're using, and keeping track of where you are in time and space is kind of the 201 class lesson. Very doable, but you really want B's and better in the 101 class before you jump into that...

There's a thousand ways to skin this cat... but I really think thread wires would be your best solution.

This advice comes from a guy who has and continues to dig his heels in, and has made literally barrels and barrels of chips trying to duplicate stuff that would have been easy if he'd have gotten off of his duff and just picked up some thread wires..... So my experience is first hand when I tell you that doing it the hard way without the right measuring tools, is VERY educational as far how stuff works, and making you proficient, but very inefficient as far as getting stuff done.

 

[OCJohn]

The 70s vintage Taiwanese 4x6 bandsaw that I converted to vertical was an interesting mix. Threads were imperial, the bars that supported the blade guides were imperial, but the heads of the fasteners were metric.

So glad you mentioned this. I thought I was imagining things!

 

[OCJohn]

@Jake M good post. I will definitely whip out the wires before I start turning. Thanks.