PDA

View Full Version : How small a thread do you cut?



form_change
07-13-2011, 04:47 PM
Ace Ventura posted pictures of a lathe thread chart for JoeFin which went down to 224 tpi. My old lathe had screw cutting to 192tpi. I never even thought about using that - the lathe tool grind would be hard enough to get so the thread was meaningful. I'm guessing that these horribly fine threads are there because the gears can do them rather than they are used - some of the UN threads get down to 80tpi I think and I've seen threads for microscopes at 36tpi.
As a matter of interest, what are the smallest that members have cut on lathes (single point or chased)? (of course we want pictures...)

Michael

Alistair Hosie
07-13-2011, 05:04 PM
Well I suppose even if you use taps or dies they are made with machines. Alistair

Arcane
07-13-2011, 05:11 PM
I've never had need to cut more than 40 threads per inch but I have seen 180 tpi. When they built the Synchrotron here in Saskatoon a few years ago, the leveling blocks for the light beam were built by a local machine shop. One of the shop owners was a brother to a friend I worked with who had been given one as a souvenir and he brought it to work. AFAIR, the threaded block was roughly 1 1/4" thick by 2 1/4" square, threaded completely through, and the "weight jack" part of it was about an inch, maybe 1 1/8" in diameter. I do remember that the pitch was definitely 180 tpi and it turned as smooth as anything I have every seen.

gwilson
07-13-2011, 05:18 PM
Years ago when making a small working model of a flintlock pistol,I made a 224 thread tap of about a .020" diameter. 2 flute. Still have it somewhere.

John Stevenson
07-13-2011, 05:18 PM
I have a job waiting that is 5mm diameter by 0.35 or 0.4 pitch, can't get in with the pitch gauge at the moment and it's LH to boot.

It's a very neat bit of kit, it's a set of three knives / scraper that close under centrifugal force as they spin and you place a piece of enamelled wire in, start up and it closes and spins and scrapes the enamel off so you can solder onto the wire. I have to make a new driving shaft so it can be put on another motor.

Previous to this I had to do some 8mm threads at 0.3 pitch both internal and external. 0.3 is about 85 tpi.

Actually they had 8mm x 0.3 on one part and 8mm x 0.35 on the mating part. I rang up about it and was told just make it to the drawing. when I pointed out they wouldn't fit I was told not my problem if made to the drawing ?

tyrone shewlaces
07-13-2011, 05:26 PM
In my history I never went higher than 80. Did a 48 yesterday.

But on the other end of the extreme, I just noticed about a month ago that the large lathe at work will do 1/4 TPI (one thread every 4 inches). :eek:
That's just built into the quick-change gear box - no changing gears required. Just a standard pitch on the machine, as well as a progressive mix between that and more sane pitches. That's kinda cool.

No doubt others have use machines that are set up to do that and maybe even utilized it, but that's unique in my experience.

Funny that I told the other two guys there about it and they never knew the lathe would do that and they've both been there over 4 years.

rohart
07-13-2011, 05:41 PM
The thread I had to replicate when I made another tip for my Verdict DTI appeared to be 0.35mm pitch on a 1.6mm diameter, a sort of narrow 10BA - works out at about 72 tpi.

My biggest (attempts to divert original topic) pitch was 4mm on a 39mm diameter, giving a pitch factor of 11. Hmm... that's not as impressive as I thought it was going to be.

I once cut a 1/4" pitch in wood by filing freehand, as the retaining nut for a ceramic handle for an old kitchen table drawer - before I was tool-wise !

The smallest thread I used a tap and die for was No 1 UNF, 72 tpi, for a screw to retain the spring on the top/ring octave key on a saxophone.

38_Cal
07-13-2011, 05:55 PM
Finest I've ever single pointed was 6-48, but have used taps and dies down to 0-80.

David

gwilson
07-13-2011, 06:25 PM
The big lathe we had at work would actually do 1 thread in 10" !!!! It was a Promaster I bought from MSC. I liked the threading range,because we used it,among other things,to cut very large wooden threads with a router in the toolpost. I have no idea who would want to cut a 10" wide thread,though!

lane
07-13-2011, 06:52 PM
Smallest I have ever single pointed in lathe was 2-56 . Made 26 little hex head bolts for a model engine one time. Made them out of a 5-32 Allen wrench. Annealed the wrench with torch. Chucked it in 3 jaw chuck Turned it to size for 1/4 long and cut threads. After that I said never again I would buy them.

Evan
07-13-2011, 07:25 PM
The smallest I have ever made that had to actually fit something is 2-56. As an experiment a while back when this subject came up before I turned some outside threads in stainless steel at 160 TPI just to see what they looked like. They look pretty good. Making a part to match would be a real challenge because the thread depth is less than most tolerances for most parts.

http://ixian.ca/pics7/160tpi2.jpg

http://ixian.ca/pics7/160tpi.jpg

jdunmyer
07-13-2011, 07:46 PM
Jerry Kieffer makes threads to at least 250 TPI and .020" diameter, I think even smaller. He wrote the article in HSM some time back on using optics for micro machining. He told me that the typical Chinese lathe won't do that fine work because the bearings, etc. simply aren't good enough. He uses Sherline equipment.

Weston Bye
07-13-2011, 09:39 PM
My Electronic Threading attachment for my Sherline lathe it theoretically capable of 249.5 tpi. Realistically, it would be limited to the sharpness of the tool bit. So far, I've only done down to 40 tpi.

Here is my most recent threading job.
http://img.photobucket.com/albums/0803/Weston/DSCN4044.jpg
It's a 1.140 - 32 tpi test plug for a lock repair I'm doing, a duplicate of the lock cylinder. I needed to have the plug on hand because the cylinder and existing lock housing needs to remain in service while I build the replacement housing. The plug will be used to gage internal threads when I thread the hole in the lock housing.

The plug was simple - the threaded hole in the lock case will be more interesting. I plan to thread it on my lathe with Electronic Threading the same way as one would line bore a hole. I will be making an adjustable between-centers boring tool to hold the threading cutter, and mount the lock housing on the cross slide. Stay tuned.

jkilroy
07-13-2011, 09:44 PM
My Rivett has a threading chart into the 600+ tpi range with change gears. NO IDEA what that would be fore. I don't think I could get a cutter sharp enough to cut that.

flathead4
07-13-2011, 10:59 PM
The gear box on my SB goes from 4 to 224 but someone on the PM forum speculated that it was only because SB felt obligated to put a TPI on the chart for each corresponding feed. And maybe for those few maniacs that would actually find a use for threads that fine.:D

Tom

DATo
07-14-2011, 02:27 AM
We have 000-120s and 00-90s at work but to my knowledge the only time they have ever been used in the 35 years I've worked there was for a model of a classical guitar I once made for an old girlfriend of mine. I clipped off the heads of six brass 00-90s and flattened the ends in a vise and used them for the tuning screws.

Off the top of my head, I don't really know, but I doubt that I've ever cut anything below a 2-56 in my career as a machinist. I imagine, based on my own experience, that extremely small screws would be quite useful to people who make scale miniatures. Other than that the only practical use I can think of would be for extremely small mechanical instruments such as timepieces.

If I had a project that required extremely fine threads I would probably buy the screw and modify it to be either soldered, epoxied, press fitted, Loctited, or otherwise artificially attached to the workpiece. I am a proponent of the doctrine that ... Discretion is the better part of valor. *LOL*

macona
07-14-2011, 02:38 AM
I think the smallest I have single pointed is 4-40. The smallest I have tapped is 0-80. I just drilled and tapped some holes the other day for a Nikon F mount. It uses 1.6mm x .35 threads. Local hardware store even stocked the taps!

John Stevenson
07-14-2011, 03:23 AM
As an experiment a while back when this subject came up before I turned some outside threads in stainless steel at 160 TPI just to see what they looked like. They look pretty good. Making a part to match would be a real challenge because the thread depth is less than most tolerances for most parts.



That last sentence says it all.

Evan
07-14-2011, 03:35 AM
The one place that you will routinely find very fine threads is on mirror cells for optics. This is especially the case on old telescopes with brass lens holders. The lens is held in place with a threaded ring that fits inside the barrel of the lens cell at the front.

It was considered a mark of high craftsmanship to make the threads as fine as possible since they are very visible. This is especially difficult to do because of the very large diameter compared to the thread pitch. Assembling the parts without cross threading is probably the most difficult part of the job and requires a special jig to align the ring with the barrel.

darryl
07-14-2011, 03:43 AM
I'm waiting for someone to come along and show an acme thread at 80 tpi, or finer. Or a square thread- even a 4-40 square thread would be a considerable achievement.

Occurs to me that the early audio recordings on tubes would be a fairly fine thread, yet not as fine as that on a typical LP- remember those- What are we looking at there- LP plays for say 20 minutes per side, say about 4 inches of groove or 5 minutes per inch of groove so about 160 or so grooves per inch. Not just straight grooves either- 2 separate patterns on the flanks of the thread. How much variation from the desired pattern was allowed- not much. Try to 'play' the thread you make on your lathe and see how much deviation (noise) there is- seems the recording industry could routinely make intricate patterns at 160 tpi. Makes me wonder how the cutting tool was prepared.

jkilroy
07-14-2011, 10:32 AM
"' Local hardware store even stocked the taps!"

You are F'n me!?!? They stocked that tap!!! That is amazing, I am lucky to get a 3/8x24 around here, I need to move.

macona
07-14-2011, 11:22 AM
"' Local hardware store even stocked the taps!"

You are F'n me!?!? They stocked that tap!!! That is amazing, I am lucky to get a 3/8x24 around here, I need to move.


Yeah, they had it in plug, taper, and bottoming even. HSS too, and cheaper than McMaster. We have a few hardware stores totally out of the norm around here, Suburban Ace, where I got the tap, and a couple others. They even have odd ball screws like #5 and #12 individually and stocked in boxes of a hundred. Taper pins, you name it.

SGW
07-14-2011, 02:20 PM
macona, you are Blessed with a real hardware store! Not many of them left.

The fiinest thread I've had occasion to cut was a 10-56. I've used finer threads, down to #0, but I've always been able to buy the bolts I needed.

Hopefuldave
07-14-2011, 02:24 PM
Try to 'play' the thread you make on your lathe and see how much deviation (noise) there is- seems the recording industry could routinely make intricate patterns at 160 tpi. Makes me wonder how the cutting tool was prepared.

The cutting tool is usually a micropolished diamond very much like a lathe threading tool with a 45-degree point and a tip radius of about 2 or 3 um (0.002 to 0.003 mm), the interesting bit is the turntable it goes on - known as a "cutting lathe" and fitted with a platter in the 100-pound-plus weight range running on air or hydraulic bearings, it has a variable-speed, extremely high-precision-surface leadscrew, and *accomplished* cutting engineers would have a couple of run-throughs making notes of when the loud and quiet passages came along, so they could speed up or slow down the leadscrew to allow for the groove width changing...

Material for the master is usually cellulose acetate, which is electroless plated and then overplated with copper which is then peeled off to make the stampers for the actual vinyl records.

The cutter heads are driven by hefty-magnet, ultralight-moving-coil solenoids with pretty flat and linear (or at least predicatable and compensatable) response from around 10Hz to 30 KHz or so, usually with peak power inputs of around 45W in the two orthogonal directions for the two channels, the amplifiers are *real* hifi and older valve units are quite sought after! They also have equalisers to reduce the low-frequency levels and boost the highs to control the amount of groove excursion (low) and improve signal to noise ratios (high) - this is one of the critical bits of the replay chain (reversing the pre-equalisation) for "proper" hifi, and can have a pretty profound effect on the way music sounds through the speakers :)

Just my ha'pennorth,

Dave H. (who used to be in that business, in the days before CDs came along)

Weston Bye
07-16-2011, 01:41 PM
I recently posted a picture of a 1.140 - 32 threaded test plug.

And now, the rest of the story......

I needed to build a new lock housing for a door, using as much of the existing external hardware as possible, including the lock cylinder.
http://img.photobucket.com/albums/0803/Weston/DSCN4053.jpg

I first built a between-centers boring bar and ground a broken tap shank to 60 to use as a cutter.
http://img.photobucket.com/albums/0803/Weston/DSCN4052.jpg

Having previously bored out most of the hole on the mill, I transferred the workpiece to the lathe, but the hole had to be accurately centered, so I turned a temporary plug to fit in the bored hole of the workpiece, and left it in the chuck.
http://img.photobucket.com/albums/0803/Weston/DSCN4054.jpg

It was then a simple matter to fit the plug in the workpiece while clamping it to a piece of aluminum angle bolted to the cross slide supported the workpiece.
http://img.photobucket.com/albums/0803/Weston/DSCN4051.jpg

The threading operation was straightforward and unremarkable except that the boring bar needed to be removed when returning the workpiece to the start position, and then reinstalled, returning the lathe dog on the bar to the proper slot in the faceplate. While I used my Electronic Threading attachment to the Sherline lathe, the proceedure can be used with any threading arrangement on any lathe.

jkilroy
07-16-2011, 03:21 PM
Mr. Bye, I am pretty sure that for the cost of that block of brass (bronze?) you could have just about purchased a new door!

Weston Bye
07-16-2011, 03:46 PM
There's only about $60 worth of brass there. Still a lot, but the lock will last forever. The original casing was zinc and had broken from ordinary use. I had originally suggested to the customer that a locksmith be called - but there would be none of that - would rather pay me to do it to last. The rest of the lock is rather ornate and the door is custom, so changing the style of the lock was out of the question.

By the way, this is the first time I ever attempted internal threading with a boring bar. Seemed like it should have worked, so I did it.

Black_Moons
07-16-2011, 04:32 PM
One small note is the lock has a keyway. You could of put a keyway (or a notch) Into the threads you turned so that you could rotate the lathe spindle to align with it, then move the carriage back to starting position after a pass, allowing the tool to pass through the notch.

Quite the impressive job! Must of been figity trying to advance the cutter a few mils in the boring bar every pass.

John Stevenson
07-16-2011, 05:00 PM
The threading operation was straightforward and unremarkable except that the boring bar needed to be removed when returning the workpiece to the start position, and then reinstalled, returning the lathe dog on the bar to the proper slot in the faceplate.

No ??

Just stop the lathe with the tool pointing to say the rear, then wind on say 100 thou on the cross slide and return the tool. Then wind the cross slide back to where it was on centre for the next pass.
Boring bar stays set up all the while.

Weston Bye
07-16-2011, 05:30 PM
I didn't want to trust myself to get it back in the right position. Maybe I'll get more reckless with experience.:D

DATo
07-16-2011, 05:39 PM
The threading operation was straightforward and unremarkable except that the boring bar needed to be removed when returning the workpiece to the start position, and then reinstalled, returning the lathe dog on the bar to the proper slot in the faceplate. While I used my Electronic Threading attachment to the Sherline lathe, the proceedure can be used with any threading arrangement on any lathe.
__________________
Weston Bye

I'm a bit confused. How did you advance the cutter for each cut? It appears that your only available movement is with regard to the part itself which, if moved, would cause you to cut more from one side than the other.

form_change
07-16-2011, 06:02 PM
Being brass I suspect that Weston had the bar set up for full DOC (on 32tpi, not much) and then ran through a couple of times, relying on the spring of the boring bar to indicate when the thread was finally to size (that is, no more metal being cut off).
For a small thread in brass it would work but not a trick that you would use often...

Michael

Weston Bye
07-16-2011, 08:04 PM
Full marks, Michael.

Actually, I did the first pass just timidly grazing the surface, then moved the cutter up to full depth and ran it through a couple of times. Tried the aforementioned aluminum test plug to satisfaction, then went and borrowed the cylinder long enough to be sure it fit, then returned it.

form_change
07-17-2011, 04:46 AM
Inspired partly by this thread and partly by Mototed's "lesson in life and internal threads" thread (and the advice given by Forrest in that thread), I decided to have a go at one of those back burner projects.
My lathe comes with a bracket that attaches to the bed that holds a micrometer head. The spindle bears against a dial indicator attached to the carriage. The idea is that you can get depth adjustment to the thou (tenth if you like), adjusting with the micrometer. However, the lathe was missing the micrometer head. A normal one would do, but the graduations would be upside down.
Today I decided to have a go at making up a micrometer head. The hardest part would be the 40 tpi internal thread. The photo below shows my efforts -

http://i1140.photobucket.com/albums/n574/form_change/P1010195Small.jpg

The barrel is a lump of stainless I had around the place and the threaded hole is around 8mm (major diameter). The bar I used to tap the thread was around 3/16 diameter, so that I was getting something like 100 thou deflection trying to cut the stainless. I think next time I'll be a bit more selective about what I pull out of the scrap box...

The resultant head is here, along with the bracket -

http://i1140.photobucket.com/albums/n574/form_change/P1010194Small.jpg

Graduating it should be a simple dividing head job. The catalogue of things that could be done better is long, but as a trial piece/ prototype I'm pleased that it worked so well. (Messers Starrett, Mitutoyo, Brown. Sharpe et al have nothing to fear however - it will be a while before I can challenge their products)

Michael