Designing an M48 0.75 Tap

Have a unique application that requires a tap for photography threads, and nowhere on earth can I find this tap for sale, so I am going to try and make it. I have successfully had a helical boring function on my home made CNC that works, but I have always wanted to make a tap.

I will be starting with a 2" OD 1045 steel bar and harden it as best as I can. It will only be used on 6061 aluminum, so I am not worried too much about the hardness yet. After cutting I will take a propane torch to the teeth to add some hardness to see if the tap works before going through the hardening process for real. (meaning it taps a 1/8" sheet and my part threads on nicely.

The question I have is about tap geometry. Every generic tap I see has slots dropped dead center on the axis of the tap. I wonder why they are never offset to increase the rake geometry? for example:
Attachment 472806

Using a 1/2" ball end mill dropped in like normal I get something like a 76° rake angle, vs just offset I can get 73°. If I drop the end mill way closer to the edge, I have a 48° rake angle. I am genuinely curious why I have never seen a tap of this configuration before? I am guessing the center will give the teeth the most strength? Or is this to cut in both directions vs only in the threading direction? This will be the first tap I have made, so i am open to others thoughts.

I do plan on dropping the end mill along the axis, but I am curious if its to cut in both directions (cutting threading, chip breaking reversing) Keep in mind these threads are probably 0.5mm in height. SUPER find threads. I have that drill rod-kit thing to measure the pitch of the threads coming to make sure its done right, but to be honest so long as my part threads, I am happy. That is the go/no-go test.

Just look at the difference in geometry:
Attachment 472808
It looks like a much sharper tap. (blue cross is the axis of the part, and the U is the 1/2" end mill dropped along that axis.

hy :) what are photography threads ?

is not only hardness or angles, but more edge sharpening and resistance, thus what matters more is final tool condition

i have crafted reamers, centers, threading holders for indexable inserts, etc

once you start wish to diy, is all about the level of quality that you wish to achieve :
... something low : simply do whatever you wish, and what you do is what you get :)
... high quality : get material for tooling, rough, heat threat, then grind; contact a local tool manufaturer/sharpner, so to only quote the grinding after heat treat, and discuss with him geometrical conditions that your part must met before sending it to him

about rake angles, i can tell you that there are tools with non standard angles that still work; for your exact application, i could contact someone and find exact technology details, but until then, i will take a look inside a manufacturing catalogue for taps and dies, that, lucky for you/me, i have in house : voila, that u shape is not an u

the od of the tap is not a revolution shape, and that groove is an u with curved sides; from this consideration, you could simply deliver a revolution shape with a normal u groove to your tool sharpener, and he should take care of the details / kindly :)

You need to get a single point threader and thread mill it to the pitch you want.

Astronomy threads are large diameter objects with super fine threads. This tap I am looking to make is M48 x 0.75. So its practically 2" wide, with the teeth sticking out about 1/2 a mm on each side.

What do you mean the U shape is not a U?

https://cdn.mscdirect.com/global/med...te-diagram.png

good idea cbr_speedster :) like a threading tool for lathe

or you could get any 0.75 tap and grind/damage it, so to keep only 1 tooth from it, then use it helical ...

anyway, such an approach is simpler, with normal tools, so you could avoid the costs with a custom fine pitch big tap :) also, single point tools will work smooth, and won't put stress on the spindle like a tap / kindly :)

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Originally Posted by cbr_speedster

You need to get a single point threader and thread mill it to the pitch you want.

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I am currently doing that on my CNC, but its not as accurate as turning would be on the lathe. Plus if I need to touch up the threads, I will NEVER be able to do anything to those threads once it comes off the machine to drop it back down and re-indicated it properly. Just would be easier to run a tap through to clean up the threads. Even after anodizing the part if it gets messed up.

hey nateman, you are really into that tap, don't you ? :)

i was wondering, if you decide to do it yourself, you will use a lathe, right ? so why not thread you part directly on the lathe ? if it has complex shape, use a 4 jaw chuck if possible, or something, so to be able to dial it .... i don't know, just saying :)

I have been using this for helical boring on the CNC:
https://www.mcmaster.com/4425N21/

Yes I will be turning it on the lathe. The idea is to make the tap. for starters because I simply want to see if I can as a test of skill, and also to clean any threads on any parts I make in the future.

I just got a thread measuring wire set today in the mail. The 3 wires I need to be able to check the 0.75 pitch are 0.018" wires. I can barely hold them they are so tiny. This is going to be a pain, fun, and challenging.

yup, it's a nice chalenge :)

you could start with a smaller tap, like still 0.75, but not M48 ... you know, just in case, begin with something small

for example, try to craft a classical set of 1-2-3 taps :
... 1st must have a longer entrance cone and target a non-final depth
... 3rd should have a shorter entrance cone and target the final depth
... 2nd should be between them

it would help to use the metric iso shape as reference, so to know what geometry to target, what radius to use, etc

you should debur your tool very precise; when cutting threads inside a random part is not a must to debur, unless a tight tolerance is needed, but tools must be debured beyond that level; you shoud spring, recut, reverse, high rpm&low feed, etc, thus to make sure that final teeth is bur free, and a live cutting edge is there

at the outside, use a small facet, like <0.1, then grind somehow, so to be sure that your tap won't generate useles friction inside the hole; you know, you could begin with a short active lenght tool, thus focus more on the entrance cone and process steps, and try to achieve a shorter active length tap, like only a few calibration teeth; and among the way, if you feel that you get close to a nice thing, then start to target normal/standard lengths

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I just got a thread measuring wire set today in the mail

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after a certain point, for common stuff, you don't need wires to deliver tight tolerances :) there are tricks, and if you understand them, you can craft a thread however you wish

i just delivered these days a M30x2 threaded assamble 70mm length, with 0.04 axial play, for some 1600mm diameter missalignment coupling, etc ... no gauges, no wires; please, check attached image : i begin with a bit of math/cad, and i take into consideration iso shape :) last year i crafted M125*1.5 threaded assamble with <0.1 radial play : some stainless suport for a remote mini-submarine ?! whatever ... oh, i just remembered another one: i also crafted a hard role ( >50hrc) for a thread rolling machine, thus something similar to a tool; if i remember, you have to take into consideration the negative effect, like on a photo, for example, even if you thread od, the final purpose is to obtain an id thread, so you should take those into account

really, if you like it, i think you should go for it, see what you can do, and if you wish, we can discuss details / kindly :)

ps : don't forget the square at the other end / haha

M48 will put a lot of stress in the part when you force it in........ I have made a few dies in the past and the best ones had lands that were quite small in width so more narrow lands work better than wide ones but you need more of them......if the lands are too wide they will cause more friction and run tight.......6 narrow flutes would probably cut better than 4 wide ones.
Ian. .