Help With Threads Please

[Mot3s]

I am new to machining and this is for a senior design project. I have designed a new valve spring height micrometer. The threads are 3/4-20. Given that this needs to be a precision instrument, so to speak, I need to shoot for a pretty tight fit, preferably class 3. The problem is that I think my only option is to tap the internal threads with the class 2B tap available and cut the external threads because I have no way to gage internals with what I'm given to work with. Given that I don't know the pitch diameter for the internals, what am I to shoot for when cutting the externals to give me the best fit? Do I simply take small thread cuts and check the fit with the nut until I think it's good? Do I make the starter hole for the tap as small as possible, and make the pitch diameter for the external larger than a class 2A fit? Am I overthinking this? If I had a GO NOGO gage I believe I'd just cut both and check both for size. It seems unless I buy a way to measure internals, I'm stuck attempting to produce a 2B-3A hybrid (or something?).

What would you all do? I could use all the advice I can get. It's hard to find information online for specifics like this and it's getting to be overwhelming attempting to learn machining in a couple weeks to pull this off. Thanks a lot!

 

[johansen]

Regarding the internal threads, no one that I know of makes micrometers without a tapered thread and tightening collar. Just tap the internal threads or cut them on your lathe, cut a 3/4" pipe thread on the outside, slit it in 4 to 8 places, and turn down any standard 3/4" pipe fitting to get the tapered collar. now obviously you might want to use something other than a pipe fitting to do this but if you don't have the time to cut a custom tapered thread and matching nut, this is the fastest way to get what you need.

use mild lapping compound to get the threads fitted nicely over the whole length of the threads, tightening the collar as you go to get a good fit.

 

[rons]

The threads are 3/4-20.

The problem is that I think my only option is to tap the internal threads with the class 2B tap available and cut the external threads because I have no way to gage internals with what I'm given to work with. Given that I don't know the pitch diameter for the internals, what am I to shoot for when cutting the externals to give me the best fit? Do I simply take small thread cuts and check the fit with the nut until I think it's good? Do I make the starter hole for the tap as small as possible, and make the pitch diameter for the external larger than a class 2A fit? Am I overthinking this? If I had a GO NOGO gage I believe I'd just cut both and check both for size. It seems unless I buy a way to measure internals, I'm stuck attempting to produce a 2B-3A hybrid (or something?).

Internal thread diameters:
From valley to valley = 3/4"
From crest to crest = 3/4" - 2 * (0.625 * Pitch cos(30)).

Look at the Machinery Handbook to double check. The equation above gives the tightest fit possible. I drew a picture for myself and confirmed what the Machinery Handbook said.

I often get a nut and use that as a gauge when cutting externals. When I want a tight fit the increments I move at are a few thousands at a time when cutting the external thread.

You can get the fit really tight and then coat the threads with some lapping compound and run the nut back and forth, If you use a compound with carbide particles like Clover Brand then the carbide will embed into the metal and the fit will loosen up over time. At least that is my theory.

 

[Mot3s]

I'm still confused. Even if I find formulas for optimal internal sizes, how do I know what the tap produced? Doesn't the tap cut the crests? Or are the crests controlled on my end?

But ultimately, you are suggesting to make small cuts and check the nut on it correct?

Lapping, that's a good idea. I'm gonna look into that. Thank you!

 

[9100]

You can get the fit really tight and then coat the threads with some lapping compound and run the nut back and forth, If you use a compound with carbide particles like Clover Brand then the carbide will embed into the metal and the fit will loosen up over time. At least that is my theory.

Hans Fischer, who was the honing guru at Sunnen did tests on grit embedment on something like steel, it had to almost be deliberate, applying pressure straight down. Using a piece of sandpaper on cast iron will not do it. In the case of binding threads, there may be enough pressure to cause it, but not much and the grit will soon dull, anyway. For something like that, I would use 1 micron synthetic sapphire from Microabrasives.

Bill

 

[Forrest Addy]

Mike the tap over wires being careful to measure over the cutting diameter and not some part of the radial relief. Use this figure to cut the external thread.

Don't fall into the trap of going undersize with the tap drill. The tiny amount of extra thread attained has practically zero effect on thread fit or strength but greatly increases the tap's driving torque. There are words in Machinery's Hanbook to this effect.

A zero clearance measuring thread may be partly approximated by carefull fitting and providing an adjustment feature to reduce the internal thread pitch diameter. A collet type adjustment is probably the most practical. Dismantle a micrometer head and reverse engineer the adjustment. There is no simple solution to this problem; careful design and workmanship are required.

OTH, you may be able to finesse the situation. If spring measurement is involved, the spring tension will hold the internal and external threads solidly mated to their loaded flanks. Backlash may be present in the unloaded condition but once a small load is applied the flanks mate up and bi-directional readings from a dial collar will be reliable and consistant as the pitch error and dial graduations allow.

 

[Mot3s]

Ok, so stay within the recommended drill or hole sizes(I'll probably bore) for 3/4-20 tap? In my case is .702-.707. Well, as far as pressure on the threads, this device will emulate where a valve spring would be under running conditions, and will be turned counterclockwise until it presses the spring retainer and spring seat and the valve is seated and everything is tight, so there will be pressure applied to these threads before reading the measurement dials, yes. Not only that, once tight the valve locks and spring retainer don't really allow movement for the threaded systems, so maybe I'm worrying about precise fit when I don't need to. Thanks

 

[Mot3s]

With the little reading I've done, there are people who don't suggest lapping actually if you are to keep tight tolerances, as you can't control the evenness of the lapping compound applied, is what I understand. It sort of makes sense, and now I'm a little hesitant to run on my threads given that a 7 degree rotation of this device is suppose to lift the screw 0.001 inches. I'm not sure I like the possibility of inconsistency there. Then again, I'm saying this with knowing nothing about it and maybe micro abrasives do take this factor into account

 

[kenton]

I would make your "nut" with the available tap and then use that as a gauge to measure the external thread you cut on the lathe. I wouldn't screw around trying to make the female thread minor as small as possible, In my mind you want the engagement on the flanks of the thread, not on the radius on the minor dia of the male thread.

 

[awake]

Doesn't the tap cut the crests? Or are the crests controlled on my end?

No, the tap does not cut the crests - that would require an inordinate amount of torque. The size of the hole you drill or bore before cutting is what determines the crest.

 

[Illinoyance]

OK, so this is a design project. In the real world we would probably design around a standard micrometer head or a dial indicator.

Follow the advise given by the other contributors. It is all good.

 

[Mot3s]

If I can control parallelism of the critical surfaces, IT WILL WORK, in the REAL WORLD.

 

[Mot3s]

Yeah, and Grizzly Adams had a beard

Please gentlemen, search "valve spring height micrometer". Most practical engine building and racing teams need install height only to about 0.005" accuracy. Mine is merely a slight customization to a new cylinder head design. It's already been done in the real world.

Anyways, thank you all who gave me useful advice. I appreciate it. Take care

 

[rons]

Hans Fischer, who was the honing guru at Sunnen did tests on grit embedment on something like steel, it had to almost be deliberate, applying pressure straight down. Using a piece of sandpaper on cast iron will not do it. In the case of binding threads, there may be enough pressure to cause it, but not much and the grit will soon dull, anyway. For something like that, I would use 1 micron synthetic sapphire from Microabrasives.

Bill

The person that told me about carbide particles embedding into surfaces is a guy I trust. He had a warehouse of machinery that he bought and sold as a hobby. He was a ME during the day.

Anyway, he told that what I should use is a compound that steadily degrades over a short time and then just add more as needed.