How do I make threads?

[chalo]

[ QUOTE ]
RussH said:

The handbook then provides a table of tap drills and hole size limits and then several tables for different threads generally based on 75% of full thread depth. this formula is given for calculating the actual % of thread engagement:

Hole size= basic major diameter- (1.08253x%full thread / No. of threads per inch).


[/ QUOTE ]

Okay everybody! You are making this way too complicated.

I have been machining professionally for quite a few years now. The tap pilot hole size that I have always used for conventional fasteners and materials, which seems to result in the same sizes recommended on my tool-company wall chart (which says "for appoximately 75% thread"), is simply this:

Nominal thread size, minus one pitch length.

That's it. For 1/2"-20, it's 1/2" minus 1/20" or 450 thousandths. For 1"-8 it's 1" minus 1/8" or 7/8". Easy as pie.

For metric threads, which are specified by pitch length instead of TPI, it even easier. M6x1.0 uses a 5.0mm drill. M24x1.5 uses a 22.5mm drill.

It's worth noting, for cases like the C cell mag with its neither-one-nor-the-other thread diameter, that male thread measured diameter runs about 1% under its nominal diameter for most mass-produced threads. You can use that figure for matching an unknown but measurable male thread. In the example of the C mag, the measured 1.090" male thread becomes 1.101" for the purposes of calculating the pilot hole diameter. So the pilot hole size I'd use would be 1.051" to match its 20TPI.

Nominal thread diameter minus one pitch length. Got it?

Chalo Colina

 

[Greymage]

[ QUOTE ]
Nominal thread diameter minus one pitch length. Got it?


[/ QUOTE ]That makes it simple, thanks!

But... um... what does a 2-56 specify? 2mm, 56 threads per...inch? cm? how does that convert to pitch?

 

[McGizmo]

Cool Chalo!

So for #2-56 it would be the measured diameter of a #2 - 1/56" /ubbthreads/images/graemlins/thumbsup.gif

So what have you guys found with roll forming taps? Larger pilot hole?

 

[tvodrd]

Chalo,

That's a new one for me, and it makes a lot of sense! Thank you!

Don, I've only used roll-forming taps a couple times. They seem best for sheet metal and lube them well. I don't like them!

Larry

 

[PEU]

Speaking of threads, does this thread pitch gage is a good purchase?

and how about this tap set ?

I already purchased this Quick Change Tool Post

I'm building my shop, for my soon to be received lathe.

I'm looking for a knurling tool.

Any suggestions on tools are welcome!! /ubbthreads/images/graemlins/blush.gif

Pablo

 

[tvodrd]

PEU,

Thread pitch gages are a must. They are available by Starrett, Mitutoyo and others. A high quality one isn't necessary to do the job. Taps and dies are a must have too, but I would recommend purchasing name brand HSS ones individually as you need them. Carbon steel are fine with aluminum. Cutting fluid is a must when threading.

Larry

 

[gadget_lover]

When making threads, what's the demarcation point between cutting threads on a lathe rather than using a tap/die? If you have the tap do you always use it? Are there special times when a lathe is just better? Do you ever start with a tap or die and finish with the lathe?

And then there's the whole issue of duplicating existing threads. Is there a trick there other than using a thread guage and micrometer?

Daniel

 

[McGizmo]

GadgetLover,

Others will likely have differing opinions but I feel that if you have a tap or die, it is a quicker and easier way to go. However, in some cases, you need the thread to be either real short or the thred must be full to the "bottom". In this case, a bottom tap might work but sometimes you just need the full thread and can't get it with a tap or die that leaves partial threads.

For duplicating threads, I first uyse a thread guage to determin the pitch if unknown and them I measure the existing parts major OD (male thread) or bore (female thread) I usually shoot a bit smaller on OD or larger on bore and then cut the thread until the parts will mate. I then take a few thous more off to allow for anodize or variation in tolerance for other mates that I can't confirm.

 

[tvodrd]

/ubbthreads/images/graemlins/grin.gif Of course, there's always thread measuring wires and a micrometer. /ubbthreads/images/graemlins/grin.gif I fully concur with Don. Sometimes you need a ******* diameter and it is easiest to thread the mating part to fit.

Larry

 

[gadget_lover]

Thanks Guys. That helps.

The next question that comes to mind is diagnosing problems. There are a lot of ways to screw things up (no pun intended) and not know it.

How do you tell what's wrong when your threaded parts don't fit together?

Thanks

Daniel

 

[tvodrd]

Hi Daniel,

Asssuming both parts are threaded to the same pitch/tpi and your cutter was 60deg and square to the workpiece??? Small pitch errors in deeply tapped holes can cause things to wedge-up. This is usually the result of an out of spec tap or die. Geometry "V" errors usually don't bite you unless you're trying for 90+% engagement. If you can, cut a 45deg chamfer on both male and fem parts before threading- this reduces the everpresent burr. I also usually sandpaper the crests of a thread after cutting it- single-pointing leaves small burrs everywhere. If it is a lack of pitch diameter clearance, you're in deep doodoo. It is possible to "pick-up" on an existing thread- put it back in the lathe and cut it deeper. It's not too hard for a male part where you can cut in the air next to the part and carefully eyeball it in. If you are fighting a slightly too tight thread, the parts can be lapped together with an abrasive to fit. Hope some of that helps.

Larry

 

[chalo]

[ QUOTE ]
Greymage said:
But... um... what does a 2-56 specify? 2mm, 56 threads per...inch? cm? how does that convert to pitch?

[/ QUOTE ]

The "56" part is threads per inch, and the #2 part is arbitrarily .086 inch. Here are the diametral values for number screws as given by Machinery's Handbook:

#0000 - .0210"
#000 - .0340"
#00 - .0470"
#0 - .0600"
#1 - .0730"
#2 - .0860"
#3 - .0990"
#4 - .1120"
#5 - .1250"
#6 - .1380"
#8 - .1640"
#10 - .1900"
#12 - .2160"

Or, as McGizmo suggests, you can just measure a screw. That should be good to within a percent, and the error will fall on the side of deeper threads.

Chalo Colina

 

[KC2IXE]

RE Threads on a lathe vs tap and die

For ME, on INTERNAL threads, under 3/4", I'll almost always use a tap, and I can't remember the last time I cut an internal thread smaller than 1/2 on the lathe

As for EXTERNAL threads - as someone said, it depends. If your going to the bottom of a shaft, the lathe works better - again, over 1/2" I'll always use the lathe if I can - you ever price a big die? Smaller than that? If the part is in the lathe, these days I'll almost always cut the thread. It will guarntee that the thread is straight (yes, taps can start crooked - some rare times I will start single pointing a thread, and finish up with a die to make sure the thread forms are right

The exception for the above for me is metric threads - I don't have a set of metric transpostion gears for my lathe, so I almost always use a die

 

[LoneRanger4404]

Hello folks,

On the subject of threads, I am just in the process of sorting my first "homebrewed mod" and have been trying to find out the thread specs needed to fit a kroll "clickie" tail switch. Can anybody help please?

Thanks in advance & regards.

Lawrence W.

 

[McGizmo]

5/8" X 28

 

[Chop]

You can order the tap from MSC-direct.com among other places.

 

[LoneRanger4404]

What can I say? Rapid response or what? /ubbthreads/images/graemlins/grin.gif

Many thanks McGizmo, much appreciated.

Regards

Lawrence W

 

[LoneRanger4404]

My thanks to you too Chop. Much appreciated.
(I think we were posting at about the same time)

Lawrence W

 

[gadget_lover]

I sucessfully cut the .75 mm threads needed to mate a new body to a Dorcy AA head. Thanks for your help, guys. Now I need to do internal threads. Don't ask me why. Well since you asked... /ubbthreads/images/graemlins/smile.gif

I want to make a cr123a body to work with the electronics of the Dorcy 2AA - 4 LED light. I've modified a Dorcy AA head to use a TV1J LuxIII. It puts out over 30 lumens when run from 3 volts. I can use that head with a pair of AA or 1/2 of a CRV3 or a single AA alkaline or lithium. I guess it's a poor man's UBH.

I want to do internal threads because after boring out the body to fit a CR123A there is not enough meat left to use the original tailcap. I could build a custom tailcap, but that would require internal threads too.

I've found many references that told how to set up the lathe to do eaternal threads. Do the same things apply when doing internal?

Do I run the bit from right to left? Do I cut the inside wall furthest from me or closest? Do I set the compound for a 29 degree angle? I'm working with a .650 inch bore, so the tools have to be small. Do you make your own tools? If so, what do you start with?

I still don't have any of my Harbor Freight accesories, like the tailstock drill chuck or the QC tool post with holders. I did get some 1/4 inch indexibe and some 3/8 inch indexible tools to go with my HSS set (hand ground).

As I write this, I remembered that I do have alot of taps and matching dies, so I could go that route, though it's never been my forte.

Thanks for any help.

Danniel

 

[Rothrandir]

if you've got a tap and die, that's a much easier way to do it, especially internal threading.

internal threading requires a different bit, because you need it to be like this: v------- (you know what i mean /ubbthreads/images/graemlins/tongue.gif ). an internal threading tool is much like a boring bar.

like i said, taps are better for internal threading, because with the internal threading tool, it will flex a bit, like a boring bar does. this means that your threads aren't 100% the same diameter at the entire length. this can be a big problem when you are trying to get really tight tolerances, because you can start screwing the piece on, but screw it on too far and it's permanent /ubbthreads/images/graemlins/icon15.gif
of course it's still possible, by running several times at the same setting, but internal threading is just a lot more complicated than tapping, and you have to reset the gears each time you change the pitch.

basically, what i do:
if i'm internally threading for something which i have a tap for, i use a tap.
if i'm externally threading, and the gears are already set for that threading, i lathe thread. if the gears aren't already set (setting them is such a pita!), and i have a die for it, i'll use the die.

lathe threading can result in a more precise fit, but is sometimes more difficult to do and set up.

something i found that makes tapping a lot easier though, is that if you've got an endchuck (you said you ordered yours from hf right? (it'll be weeks before it gets there! /ubbthreads/images/graemlins/icon15.gif)), is to chuck the tap into the tailstock chuck. this ensures that you're tapping straight.
the same principle applies for a die, but you need to machine something to hold the die in place, and chuck the thingy into the tailstock chuck.