# FLAT THREADS



## riversidedan (Aug 24, 2021)

just got a new set of Irwin tap/dies and doing some threading projects using 6061, the parts threaded good but both the tap and die parts had flat threads " not pointed tips" but did fit together quite well........


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## matthewsx (Aug 24, 2021)

Hardware store stuff will make you sad, usually only suitable for cleaning up existing fasteners. Has been discussed often on here, one of the many places where paying more for quality is a good value....

John


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## riversidedan (Aug 24, 2021)

and here I thought they were above the norm.........didnt really make me sad but maybe thiers something else wrong I dont know about


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## Aukai (Aug 24, 2021)

Did you check the major diameter of the material, smaller than required will not form the peaks..


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## riversidedan (Aug 24, 2021)

major dia??  how Ive been working a piece is using a caliper from what the thread chart sez then getting the piece close to that size then cutting...


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## mmcmdl (Aug 24, 2021)

riversidedan said:


> major dia?? how Ive been working a piece is using a caliper from what the thread chart sez then getting the piece close to that size then cutting...


You really need to pick up a Machinery's Handbook and study it .  External threads have min and max major diameters while internal threads have min and max minor diameters . You miss them and your threads won't fit or they'll look strange . I wouldn't dare guess the class of threads you're making .


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## SLK001 (Aug 24, 2021)

A "Unified" class of thread should have a small "crest flat" and "root flat".  Show us GOOD pictures of what you are talking about.


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## Aukai (Aug 24, 2021)

Here are two charts....Take a look at 3/8 16 for examples...




__





						Coarse Thread UNC Series
					






					www.efunda.com
				











						Thread Pitch Chart - Portland Bolt
					

Standard thread pitch explanations and a chart showing number of threads per inch for National Coarse, National Fine and 8 Pitch threads.




					www.portlandbolt.com


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## MrWhoopee (Aug 24, 2021)

Pictures, we need pictures. Also specifics like thread size, stock size, tap drill size. Most of all, pictures.


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## riversidedan (Aug 24, 2021)

ok heres one I just cut as seen the threads are flat, also when looking inside the die the teeth are sharp where it contacts the work piece , so am curious why am getting flat threads


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## MrWhoopee (Aug 24, 2021)

The OD of the male part was definitely turned too small. What size thread is it and what size did you turn the stock?


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## riversidedan (Aug 24, 2021)

Aukai said:


> Here are two charts....Take a look at 3/8 16 for examples...
> 
> 
> 
> ...


so when it sez 0.375 is that the dia of the piece needs to be thats being cut?


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## riversidedan (Aug 24, 2021)

MrWhoopee said:


> The OD of the male part was definitely turned too small. What size thread is it and what size did you turn the stock?


if the male part was to small then how come it was so hard to thread?? and  is that what caused the flat threads??? the thread is 1/2-13


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## MrWhoopee (Aug 24, 2021)

riversidedan said:


> if the male part was to small then how come it was so hard to thread?? and  is that what caused the flat threads??? the thread is 1/2 16


If you're making a 1/2-16 (?) thread, the stock should be 1/2" diameter (a few thousandths under).
.4375 would be the drill size for the tapped hole.

1/2-16 is an unusual thread, 1/2-13 is standard coarse (UNC), 1/2-20 is standard fine (UNF).


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## riversidedan (Aug 24, 2021)

MrWhoopee said:


> This like pulling teeth. What size did you turn the stock?


dont rememeber.................will ask this>>>>>>>>> when turning a 1/2 piece down to 3/8 for threading  should the major dia be 0.375??


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## MrWhoopee (Aug 24, 2021)

riversidedan said:


> dont rememeber.................will ask this>>>>>>>>> when turning a 1/2 piece down to 3/8 for threading  should the major dia be 0.375??


If you're making a 3/8-(any pitch) thread, then yes the major diameter will be .375.
If you're making a 1/2-(any pitch) thread, the major diameter will be .500. The nominal thread size is the major diameter.


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## SLK001 (Aug 24, 2021)

Did you actually get a 1/2"-16 die in your set?  A 1/2"-16 is an "Imperial Unified National" standard that is not used all that much any more.  Your picture shows that you turned down your stock too much.  If it is 0.500" stock, you shouldn't have turned it down at all, except maybe a skim cut to remove the outer surface.


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## riversidedan (Aug 24, 2021)

SLK001 said:


> Did you actually get a 1/2"-16 die in your set?  A 1/2"-16 is an "Imperial Unified National" standard that is not used all that much any more.  Your picture shows that you turned down your stock too much.  If it is 0.500" stock, you shouldn't have turned it down at all, except maybe a skim cut to remove the outer surface.


if I  used the 1/2 stock w/o turning it down the die would in no way start to even fit..........even  if I chamferd the end/
>>>>>>>my mistake the die was 1/2 -13

I looked at the teeth on that die again and they are pointed not flat, still dont know why I got flat threads


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## riversidedan (Aug 24, 2021)

Aukai said:


> Did you check the major diameter of the material, smaller than required will not form the peaks..


so what your saying is I made the work piece to small right, thats why I got the flat threads right???


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## Grinderman (Aug 24, 2021)

The OD of your stock needs to be 1/2" or a few thou smaller. Put a good chamfer on the end. Are you doing this on a lathe or by hand?. The die has a larger end to start the thread. 1/2-13 is kind of tough to thread even on the lathe by hand.


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## riversidedan (Aug 24, 2021)

was on a lathe then by hand.........and yes am aware of the bigger die side to start

to start with I didnt have a hexagon die holder to use off the TS so had to put the die in the chuck, then the 1/2 6061 rod in the TS chuck and turned the chuck by hand till I got some threads. then turned the rest by hand in a vise with a die wrench
>>>>turning by hand with the wrench was hard to turn so maybe thats why the stock had to be smaller like everyone sez, but would the threads come out diffrent ??


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## Grinderman (Aug 24, 2021)

With a smaller OD than required  the thread doesn't get fully formed. I usually do it with a tailstock die holder, and even with 10" of handle it's kind of tough cutting 1/2-13 on steel. 6061 shouldn't be that bad though. Single pointing that size is easier.  I do have some Irwin stuff and it works but higher quality dies and taps work much better.
.500 Od for outside threads
27/64 drill for 1/2-13 tap


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## machPete99 (Aug 25, 2021)

Try single threading on the lathe to remove most of the material, then use the die to clean it up and get it to final dimension. The single pointing will be more concentric and make the die work a lot easier.


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## MrWhoopee (Aug 25, 2021)

First let's make this clear, the stock should be 1/2 in. diameter to produce a 1/2-13 male thread. You don't need a chart for this one. 

You mentioned that you are using a hex die. It is possible that it is intended for thread repair and not for cutting new threads, which might explain why it was hard to turn. Regardless, 1/2-13 is a relatively large, coarse thread and is going to be hard to turn. Getting the die started (and started straight) can be made easier by several things including a large chamfer, a short section of reduced diameter and using the tailstock on the lathe to push the die as you turn it. @machPete99 's suggestion to single point the thread before finishing with the die is a good one.


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## Larry$ (Aug 25, 2021)

I think most hex dies are for rethreading to cleanup an already made thread. Like everyone has said the work diameter should be the thread diameter. BUT since the points don't contribute much if anything to strength, turning the stock down a few thou makes life easier. If the die isn't started dead square it will try to cut ever deeper on one side as it advances. Same with taps. A set of high quality alloy tap & dies are worth their price. Get the adjustable round dies for making new threads. 

I finally broke down and got a tailstock die holder. Works great, much easier than trying to start a die any other way.


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## MrWhoopee (Aug 25, 2021)

Larry$ said:


> I finally broke down and got a tailstock die holder. Works great, much easier than trying to start a die any other way.


No need to buy one, this is an excellent beginners project. Get the tool and some experience in the bargain.


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## SLK001 (Aug 25, 2021)

MrWhoopee said:


> No need to buy one, this is an excellent beginners project. Get the tool and some experience in the bargain.


Or, learn to single point threads like this...


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## riversidedan (Aug 25, 2021)

MrWhoopee said:


> No need to buy one, this is an excellent beginners project. Get the tool and some experience in the bargain.



Im all for making my own TS die  holder just dont have the right size stock on hand


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## riversidedan (Aug 25, 2021)

speaking of thread info>>>>>>>>>.I have a 5/16 -18 tap and the chart sez   250     2570      2610     2656    2660
for the hole size to be tapped, which size drill would I use ???


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## mmcmdl (Aug 25, 2021)

Use an F drill . .257


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## riversidedan (Aug 25, 2021)

mmcmdl said:


> Use an F drill . .257


will give that a shot and respond shortly   thanx

you were correct the tap went in the 6061 with minimal amount of effort and the bolt fit fairly snug.   I see you chose the 257 which is after 250,  why would you chose 257?


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## mmcmdl (Aug 25, 2021)

riversidedan said:


> speaking of thread info>>>>>>>>>.I have a 5/16 -18 tap and the chart sez 250 2570 2610 2656 2660
> for the hole size to be tapped, am curious which size drill would I use ???





riversidedan said:


> will give that a shot and respond shortly thanx


No need to , I'm off to work !


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## hman (Aug 25, 2021)

riversidedan said:


> I see you chose the 257 which is after 250,  why would you chose 257?


The "magic rule of thumb" for tap drills (metric or inch) is to subtract the thread spacing from the nominal diameter.  For 5/16-18, the thread spacing is 1/18", or 0.0556.  5/16-0.0556 = 0.3125-0.0556 = 0.25694 ~0.257


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## rwm (Aug 25, 2021)

hman said:


> The "magic rule of thumb" for tap drills (metric or inch) is to subtract the thread spacing from the nominal diameter.  For 5/16-18, the thread spacing is 1/18", or 0.0556.  5/16-0.0556 = 0.3125-0.0556 = 0.25694 ~0.257


That's brilliant. First time I have ever heard that. My chart is now in the garbage.
Robert


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## wildcatter (Aug 26, 2021)

the die you are using is a thread repair die. They are a six sided die. The one you should use will be round on the outside.


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## riversidedan (Aug 26, 2021)

have a round  Metric set from HF that weirdly enuff work fairly well for what they are and didnt know the ones im posting about were for repair..........anyone else wanna chime in on that>?


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## savarin (Aug 26, 2021)

hman said:


> The "magic rule of thumb" for tap drills (metric or inch) is to subtract the thread spacing from the nominal diameter.  For 5/16-18, the thread spacing is 1/18", or 0.0556.  5/16-0.0556 = 0.3125-0.0556 = 0.25694 ~0.257


No wonder I love metric, so much easier.


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## wildcatter (Aug 26, 2021)

do a search on dies for repair or cutting new threads. Im notsaying this is why you have flat threads, just suggesting it may be part of the problem you are having cutting threads.


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## riversidedan (Aug 26, 2021)

I FOUND THIS, 

>>>>>>*Round dies* are usually the choice for cutting new threads whereas hex die nuts are commonly used as a chaser to repair existing threads. Round dies also require a holder/handle to use them, whereas hex die nuts do not. Proper Hand Thread Cutting Techniques Taps Are Brittle – Handle With Care


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## riversidedan (Aug 26, 2021)

savarin said:


> No wonder I love metric, so much easier.


curious why you said that?


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## hman (Aug 26, 2021)

Metric threads are specified by diameter and distance between threads, so it's a simple matter of subtracting one from the other.  Inch threads are specified by diameter and pitch.  You need to take the inverse of the pitch to get the distance between threads ... an added mathematical step, and one more opportunity for error.  Not to mention having to convert fractions to decimals, etc.


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## Larry$ (Aug 27, 2021)

M6x1, Tap drill 5mm,   6-1 =5 Now we are talking simple!


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## riversidedan (Aug 27, 2021)

Larry$ said:


> M6x1, Tap drill 5mm,   6-1 =5 Now we are talking simple!


M6x1 die??    tap drill  is 5mm???   6 minus 1 =5     that what you mean???


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## savarin (Aug 27, 2021)

For metric dies the rod dia is the same as its designation, ie, M10x1.25 mm. will cut a thread on a 10mm rod with a pitch of 1.25mm
A M4x0.75 die will cut a thread on a 4mm dia rod with a pitch of 0.75mm

With the taps the same designation is used ie M8x1 mm to cut a 1mm pitch thread for an 8mm bolt
The dia of the hole has to be 8 minus the pitch ie 8-1 to give 7, therefore drill a 7mm dia hole, simples.
None of the faffing about with fractions or obscure decimals


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## MrWhoopee (Aug 27, 2021)

riversidedan said:


> M6x1 die??    tap drill  is 5mm???   6 minus 1 =5     that what you mean???


M6x1 TAP, tap drill is 5mm
M6x1 DIE, stock is 6mm .


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## riversidedan (Aug 27, 2021)

MrWhoopee said:


> M6x1 TAP, tap drill is 5mm
> M6x1 DIE, stock is 6mm .


so your saying the stock is the same size as the part??  such as 6 dia rod uses a 6 mm die?  if thats the case the chamfer needs to be pretty steep for the die to get started


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## MrWhoopee (Aug 27, 2021)

riversidedan said:


> so your saying the stock is the same size as the part??  such as 6 dia rod uses a 6 mm die?  if thats the case the chamfer needs to be pretty steep for the die to get started


YES!!


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## riversidedan (Aug 27, 2021)

YES!!


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## SLK001 (Aug 27, 2021)

riversidedan said:


> so your saying the stock is the same size as the part??



Yes, just as with an SAE (or Imperial) thread.  

1/2"-13 thread starts with 1/2" stock.

3/8"-16 thread starts with 3/8" stock.


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## Larry$ (Aug 27, 2021)

riversidedan said:


> if thats the case the chamfer needs to be pretty steep for the die to get started


True, sort of. If you take a look in your Machinists Handbook you can find the tolerances allowed  for diameter and everything else about threads. The *maximum* diameter is the full, in this case 6mm, but the diameter can be less according to the MH. Lots of things about threads in that book if you want to get all the information. You can reduce the diameter to make for easier threading, up to a point w/o reducing the strength of the connection. Typical thread engagement is 75%. Even if you go for pointed threads it is normal practice to give a quick swipe or two with a file to knock the points off. (Assumes you are using a lathe.)


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## aliva (Aug 27, 2021)

Looks like the major diameter is too small.


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