What diameter rod to start with, for a given sized thread die?

cazclocker

cazclocker

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I can tell already this is going to be one of those obvious newbie questions, but here goes.
I need to make a very short piece (under an inch) of 360 brass with #8-48 threads.
How am I supposed to know what diameter brass rod to start with? Is there a chart I could refer to?
I have four 8-48 dies already, in "very used" condition. I want to make threads with each one of them, to see which one is in the best shape. All four of them are "adjustable".
Thanks in advance.
Doug H.
 
If you do not have a copy of this book, you should buy one. I have 2 copies and I used to use it all the time. It's out in the shop now and I'm in the house. If I had it in here I would answer your question. I would suggest you cut a shallow cut at the end of the thread with a cut off tool a little deeper then the tread diameter so when you stop treading the tool bit stops in an opening. https://www.amazon.com/Shop-Theory-Henry-Trade-School/dp/1163156329
 
I got off my duff and walked (15 feet) into the shop and measured a No. 8 tap. It is .165 across the threads. I would imagine that a rod of that diameter would be accept an 8-48 die.

I would expect all No 8 screws to have the same OD.

Edit, correct typo.
 
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Hi Doug,

Yes there are charts..... however #8-48 is not a standard fine(UNF #8-36) or course(UNC #8-32) thread so it's a little more difficult to find.

In fact, I don't even see it in my Machinery's Handbook under extra-fine (UNEF):

1669386304347.png

1669385557860.png
(the screen shots above intentionally shows the book version and page for reference)

However, if you notice above the number for 8-32 and 8-36 have nearly identical major diameters.
Based on these I'd start with the same, so 0.163" to 0.164" for your #8-48.
If too tight and I can't start the die, I'd reduce it from there.
If the thread come out with flat tops, I'd increase it slightly.

Notice too that the chart talks about "thread class". This has to do with the "slop" of the fit.
So some of it depends on the use of your final parts.

Don't forget a generous chamfer to help in starting the die.
Also, if you need to thread to a shoulder or head, use the die the normal way first (ie. with the "Start this side" on the die) and then take it off and turn it around to better from the threads near the shoulder.

Are the dies high-carbon steel or high-speed steel (HSS)?
HSS will stay sharp much longer.

Brian
 
Richard King 2 said:
If you do not have a copy of this book, you should buy one. I have 2 copies and I used to use it all the time. It's out in the shop now and I'm in the house. If I had it in here I would answer your question. I would suggest you cut a shallow cut at the end of the thread with a cut off tool a little deeper then the tread diameter so when you stop treading the tool bit stops in an opening. https://www.amazon.com/Shop-Theory-Henry-Trade-School/dp/1163156329
Click to expand...
Richard, thanks for that. The idea of first making a shallow cut at the end of the threaded area is brilliant. I would likely not have thought of that! I've bookmarked the Shop Theory book you mentioned, put it in my "to buy soon" list. Thanks again.
Eddyde said:
#80.164"

Fastener Guides - Measuring Screws And Bolts | Fastener SuperStore | Bulk Industrial Fasteners

Calculating diameter and length is a critical part of identifying screws and bolts. Check out Fastener SuperStore's guide to the rules of measurement.
www.fastenersuperstore.com www.fastenersuperstore.com
Click to expand...

T Bredehoft said:
I got off my duff and walked (15 feet) into the shop and measured a No. 8 tap. It is .165 across the threads. I would imagine that a rod of that diameter would be accept an 8-48 die.

I would expect all No 8 screws to have the same OD.

Edit, correct typo.
Click to expand...
The size of 0.165" is just a smidge under 4.2mm. Hmm, not a common size for brass rod, and certainly none in my metal rod drawer. Next available. Ditto for 11/64". But I have plenty of 3/16" brass rod... turning it down to 0.165" is easy enough! Thank you!

brino said:
Hi Doug,

Yes there are charts..... however #8-48 is not a standard fine(UNF #8-36) or course(UNC #8-32) thread so it's a little more difficult to find.

In fact, I don't even see it in my Machinery's Handbook under extra-fine (UNEF):

View attachment 427809

View attachment 427808
(the screen shots above intentionally shows the book version and page for reference)

However, if you notice above the number for 8-32 and 8-36 have nearly identical major diameters.
Based on these I'd start with the same, so 0.163" to 0.164" for your #8-48.
If too tight and I can't start the die, I'd reduce it from there.
If the thread come out with flat tops, I'd increase it slightly.

Notice too that the chart talks about "thread class". This has to do with the "slop" of the fit.
So some of it depends on the use of your final parts.

Don't forget a generous chamfer to help in starting the die.
Also, if you need to thread to a shoulder or head, use the die the normal way first (ie. with the "Start this side" on the die) and then take it off and turn it around to better from the threads near the shoulder.

Are the dies high-carbon steel or high-speed steel (HSS)?
HSS will stay sharp much longer.

Brian
Click to expand...
Brian, your reply reminded me that I have a 15th edition of Machinery's Handbook from 1954, which I should have thought of to begin with. I found the section dealing with the #8 threads, and it mentions 0.164" major diameter. I don't see any mention of 8-32, 8-36, or 8-48. I'm assuming all that information is elsewhere in the book. I've never learned how to get around in that amazing book. But, in any case, it appears the measurement 0.164" is universal to the #8 screw size.

What I'm intending to make is a series of brass "shellac chucks" to fit my Rivett watchmaker's lathe. This photo shows my shellac chucks, with the #8-48 threads exposed:
IMG_7933_cropped.JPG

What I want to end up with will be a set of 4 of these, so that I can affix various grits of diamond foil, for graver & cutter finishing. By the way, all four of my dies are HSS.
IMG_7934.JPG
IMG_7936.JPG
 
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To start with, depending on a "chart" is a PITA. My shop/charts are next door and I'm in a wheelchair. I do have a copy of Machinery's Handbook accessable from my desk, but I much prefer to have a good understanding of how any number is derived. That saves me a trip and I never need to depend on anything outside my mind.

In the case of machine screws, the basic concept is to start with 0.060 inch. That is size "0" or naught (zero). Each machine screw number adds 0.013 inch to the outside diameter. As in a #8 screw, a size I work with a great deal in electrical systems. The basic formula is: 0.060+(8 X .013), or .060+(.104), or .164 inch.

Machine screws go below 0.060 as well, down to the (metric) optical screws. Such as size 000, 3/0, a size that shows up on my brass models. 0.060 minus (013 per step) or 0.060-(2 X .013) or 0.060-(.026) or 0.034 inch. (3/0-120 TPI) Screw sizes get smaller, but I don't use them. 4/0 and 5/0 screws (and taps) are available from specialty suppliers. Metric screws are more common for the smaller sizes, a Nr4/0, at 0.021 inch is more commonly replaced today with a 0.5mm screw.

The depth of thread for 60 degree threads, from the metric system, is to subtract the thread "pitch" from the outer diameter. For a #8 screw, the "tap drill" size is the major diameter (0.164') minus the pitch, 1/32, 0.03125" or 0.13275". Lacking that exact size, using a 0.133" drill will suffice. That's where the "wierd" steps for number sized drills comes from. An 0-80 screw is (theoreticaly) 0.060 minus the thread pitch of 1/80, 0.0125", or 0.0475 (practicaly 0.048)" inch then becomes the tap drill for that size.

Thread pitch is a convenient source of calibration screws. Many lathes have 10TPI lead screws, giving 0.100 per revolution. A 20 pitch lead screw gives 0.050 per rev. A 40 TPI, such as a micrometer, 0.025 per rev. The next step down, 80 TPI, is seldom useful to most machinists. But does show up on some calirations. The same concept is often applied to fractional adjustments. Such as an 8-32 giving 1/32 inch per rev or a (#3)-64 TPI with a 1/64 per rev.

Some screw sizes are obsolete by today's standards. A #10-32 versus a 3/16-32 cannot be easily distingushed one from the other. #10 is 60+(10X.013) or 0.192. A 3/16 is 0.1875. If one goes to a hardware store for supplies, asking for one may well yield either. There are other curious numbers, a 1/4 inch screw is 0.250 inch. A #14 machine screw (now obsolete) is 0.242 inch. And metric falls so close it must be remarked on, a 6mm screw is 0.232 inch.

.
 
I have very good results by the method of using nominal diameter less 10% of the thread pitch. Easy to calculate (or look up), and it covers everything you're likely to do. Standard threads, non standard threads, obsolete threads, and threads that you just made up on the spot. Obviously special threads (Acme, Buttress, etc) are different, but any conventional V thread that you can come up with, it works well. Essentially you "pre-cut" the peaks of the threads to finish size, before you run the die (or the single point cutter) along it. This is what they are doing on the Little Machine Shop link that ChazzC posted. You just have to remember (if you're single point cutting) that you're cutting the threads that are already "truncated", so you won't go fully to a sharp point, since it's already "trimmed". It also makes a die run a LOT smoother, and with a lot less cutting force when making threads that way.
 
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