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[Fixed] Need help with doorknob capacitors bolts

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PhilGood

Wed May 09 2012, 03:23PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hello,

I bought on Ebay some Sprague 650pF 18kV capacitors.

Imag0116

Now I'm trying to find suitable bolts or threaded rod

The metric 4x0.7mm bolts almost fit in, but I can screw only one turn, then it blocks because the thread pitch is different.

I found a french online shop selling all US sizes bolts, but I don't know how the US system works for bolts... I'd be glad if someone knowing these capacitors could tell me what is the correct US size for these bolts.

Thanks for your help :)

Steve Conner

Wed May 09 2012, 03:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

They're probably 8-32, that's the nearest common US size to M4.

PhilGood

Wed May 09 2012, 04:36PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks Steve for your answer,

in 8-32, is 32 the number of threads per inch ? and what means 8 ?

(still trying to figure out how th US system works... lol)

EDIT :

I found a bolt that perfectly fits in the capacitors. It's diameter is 4.15mm and thread spacing is very close to 0.8mm, which gives 32 threads per inch.

I also found this table: http://www.abms.ca/Technique/Filetages_des_boulons.pdf
It helps a lot ^^

8-32 is the right size... Thanks again Steve

EDIT 2

I found on Ebay 8-32 brass threaded rod and nice 8-32 brass nuts. This will look very nice and allow to easily connect caps in series if needed

klugesmith

Wed May 09 2012, 11:23PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Good work!

In case you didn't notice, the US number-size thread standards (unlike number-size drills and wires

)
have equally spaced basic diameters.
That makes the diameters easy to construct from memory, with or without the formula d = 0.060 + 0.013 * N.

The reference you pointed to does not indicate the relative popularity of the small inch threads.
Only rarely do designers (when restricted to inch-size preferred parts lists) depart from:
#0 0.060 -80
#2 0.086 -56
#4 0.112 -40
#6 0.138 -32
#8 0.164 -32
#10 0.190 -24 (UNC) and -32 (UNF)
1/4 -20 (UNC) and -28 (UNF).
Except Harley-Davidson motorcycle company uses 1/4-24.
And inch micrometers use 1/4-40.

When it comes to screw-cutting lathes, the gears for N threads per inch
are no more complicated than those for N/10 millimeters per thread.
One amusing standard used in billions of parts is the US garden-hose thread:
Basic OD = 1 + 1/16 inches, threads per inch = 11 + 1/2.

Review question:
How many threads are on a 12-inch length of 8-32 all-thread rod?
Answer: One.

Ash Small

Thu May 10 2012, 01:55PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

As an aside, if you have an old imperial screw cutting lathe with change wheels, and you have a change wheel with 127 teeth, you can cut metric threads on an imperial lathe (127/5=25.4, so you can cut 25.4 threads per inch, and multiples thereof, etc.).

PhilGood

Tue May 22 2012, 12:05AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

@klugesmith: very interesting explanation, thanks for it

What bothers me is that some sizes are designated by their number, while others are identified by their size in fractions of inches, but I'll get used to it.

@Ash Small: I don't have this material, but it's good to know the trick

I found them on Ebay, they look nice :) I'll just have to cut a few mm, they're a bit too long for these capacitors.

klugesmith

Tue May 22 2012, 06:00PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

PhilGood wrote ...
... I found them on Ebay, they look nice :) I'll just have to cut a few mm, they're a bit too long for these capacitors.

Nice bag of screws you got there, Phil.
For making them shorter, are you familiar with the screw-cutter feature of many wire strippers and connector crimpers?

You open the pliers until the threaded holes line up with unthreaded clearance holes on the other side.
Screw in the screw as far as you like.
When you close the pliers, it shears the screw (crushing some threads on the far section).
The sheared end of the near section gets cleaned up when you unscrew the shortened part.

I would be surprised if there aren't similar tools for cutting metric screws.

Oh, about the sizes chosen by U.S. designers, and stocked in hardware stores.
Below 1/4 inch it's almost always even-numbered sizes. #10 is practically the same as 3/16", and #12 is uncommon.
For 1/4 inch and larger screws, we use the familiar binary fractions: 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 1 inch.

My middle employer (of 3 U.S. start-ups and IPO's) adopted a preferred-metric policy in the beginning.
The mechanical engineers and purchasing staff got used to it. I didn't hear of any problems.

PhilGood

Wed May 23 2012, 05:42PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

I remember this feature on older crimpers and strippers like the one you posted.

But I never used it earlier, and it's a good thing you mentioned it, because I just forgot it. I'll get one of these pliers

! (Or even two: one metric and one imperial)

I usually use a Dremel with a cutting disk to cut screws, the result is perfect and it does not damage the threads, but it is more complicated.

It's funny and quite uncommon to see an US company using the metric system, but hey, why not

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