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Shopgeezer
04-15-2015, 05:42 PM
Hi to the group. I have need to cut extremely fine threads for camera filter adapters and telescope accessories. These threads are both very fine and large in diameter, up to 3 inches across the filter holder. Looking at a typical camera filter the thread is vanishingly small. I can't begin to cut that fine with anything I have in the shop. Is there a guide somewhere to cutting such small threads over such large diameters? Can you get or make a die that could be used for this purpose?

Thanks

DonM

philbur
04-15-2015, 05:53 PM
Do you have a lathe, what is the thread pitch, a single point tool should work.

Phil:)

rohart
04-15-2015, 05:58 PM
Half millimetre pitch IIRC. Very low forces, so no real accuracy needed for the thread profile, and since you'll probably be doing it in ally, or brass at a pinch, the cutting forces will be low. This all means that a small HSS tool that you grind freehand will be more than adequate.

What is more difficult is setting the work up concentric again if you need a second operation. So it's important to plan the job for as few remounts as possible.

Bob Fisher
04-15-2015, 05:58 PM
You WILL need a lathe for that. Not a chance to find a tap in those ranges, if you could afford one. If you do not have a lathe or the knowledge it will be way cheaper to just buy what you need. Bob.

Fasttrack
04-15-2015, 06:03 PM
I've done a lot of these recently. 0.5mm pitch is common and usually has to be done without half-nuts (unless you have a metric leadscrew).

Hand grind the tool and use a jeweler's loupe to check it against a fishtail. Stone as necessary to get it touched up just so. Then it's off to the lathe where the loupe comes in handy again. Use it and the fishtail to get squared up on the workpiece or to pickup the existing thread if necessary. Hopefully the lathe has an electric brake to take away a little of the pucker factor if you're going towards a shoulder!

elf
04-15-2015, 06:39 PM
These threads are fairly easy to cut in aluminum. As mentioned above, a freshly ground and honed HSS tool works best. It's also easier to turn the lathe spindle by hand. You can make a handle that fits on the outboard side of the spindle, even better is a large wheel (If you accidentally turn the lathe on, the wheel will be a little safer). I use a ratcheting wrench as a handle as it is easy to remove. Once the lead screw is engaged, I think it is better to leave it engaged when reversing.

Shopgeezer
04-15-2015, 06:50 PM
I like the idea of hand turning the lathe. Haven't ground a tool that small before, but I guess there has to be a first time. One head scratcher is how to set the threading gears for something that fine but with a diameter that large. Have to slow down the lateral movement in relation to the circumference of the work otherwise i won't get accurate pitch across that large a diameter. A commercial telescope part I own is about a foot across with camera threads on the outside. No idea how you would do that. Can't buy the parts I need since I am experimenting with adapters for my own telescope design.

elf
04-15-2015, 07:15 PM
The diameter doesn't make any difference. The thread count is per revolution. If you post which lathe and gears you have, someone can tell you how to set up the gear train.

CalM
04-15-2015, 08:06 PM
Traditional methods would make use of a chasing apparatus.

It's still a very good way.

Those short thread length, fine pitch rings are a pain in the butt any other way. (numerical controls excepted)

Errol Groff
04-15-2015, 08:12 PM
Give thought to turning the tool up-side down and running (or hand cranking) the spindle in reverse. That way you don't have to worry so much about running into the shoulder.

CalM
04-15-2015, 08:20 PM
I don't believe you will ever be pleased with the surface finish of fine threads produced by "hand cranking".

I could be wrong, but typical filter rings are made from 6061 (or worse), Anodize color uniformity being of high importance.

To produce the fine finish expected by the optics community, cutting speeds (sfpm) needs to be adhered to.

Paul Alciatore
04-15-2015, 09:18 PM
Yes, cut them in a lathe.

You need to properly support the ring or tube you are cutting them in properly or it will distort. A six jaw chuck would be helpful. Or a heavier ring that clamps over it to keep it round.

A DEAD SHARP tool. HSS is probably best as carbide can be hard to fine tune. After sharpening it to a sharp vee, use a 10X magnifier to check and to guide you in putting a small radius on the tip. I just eyeball these radii. Be sure to continue that radius down the relief angle. I use a side to side motion across a fine stone, with the tool at the relief angle and rotating from one side of the vee to the other on each stroke. Or, at least more than half that angle on each half stroke. This only takes a few strokes on tools for fine threads, perhaps only one.

I assume you are working with aluminum. Use a good cutting fluid, like WD-40 and keep it wet.

Check the thread depth often and use a fine feed for each cut.

Get a fine, brass wire brush to clean the burrs off the threads after each cut and before each check. Emphasis on FINE and BRASS.

Oh, and I also like the idea of a hand crank on the lathe spindle.

http://img.photobucket.com/albums/v55/EPAIII/P1010001-1.jpg (http://smg.photobucket.com/user/EPAIII/media/P1010001-1.jpg.html)

boslab
04-15-2015, 10:35 PM
I've only ever cut one thread for a filter, it's a bit of a pig, 1st go imploded, it was a peice of tube, second go out of solid bar, cut the thread then gently bored it out with a tiny homemade file tang boring tool, it fitted but not too good so it was adjusted with silicon grease!
I'm sticking to big course nuts and bolts, btw a microscope is handy, these days a usb one and tablet would fit the bill, I need glasses to find my glasses these days.
Mark

Forrest Addy
04-16-2015, 04:32 AM
I've made a good many camera lens fittings of one kind or another in the '70's and 80's. Most hag very short threads with maybe a turn or three full threads and a relief so they shoulder freely. I found the simplest way is to cut the thread without power; turning the chuck by hand and reversing back. The cutting force is very low. Usually it only takes three or four passes with maybe a spring or low indeed pass. I used WD40 on the aluminum, brass dry, and the one stainless 303 steel adapter I made I cut with bacon grease.

One thing: aluminum camera adapters absolutely MUST be anodized or they are sure to gall at the worst possible time.

One more thing: practice the thread to prove set-up and tools and make gages as you do.

Shopgeezer
04-16-2015, 05:05 PM
Thanks everybody for all the good advice. Got a handle planned for the lathe to try some hand turning. I will waste a lot of aluminum and see if I can get the thread size I need. The consensus is that the usual thread is .5mm? I have a King lathe, 10X22, with the usual change gear assembly for threading. Given that it does not have a metric lead screw, how would I go about setting this lathe up for a 0.5mm thread? Anybody have a similar lathe?

elf
04-16-2015, 06:43 PM
Little Machine Shop has a gear calculator (http://littlemachineshop.com/Reference/change_gears.php) for the mini-lathe. You may be able to use it for your gears.

p.s. They may also be 0.75mm. It would be best to measure them to make sure.

Jim Hubbell
04-16-2015, 07:03 PM
I like to turn a wooden disc to fit snugly into the aluminum tube at each end. The tail stock end is fitted with a bore to ride on a shaft held by the tail shaft chuck. Plus one on above tips.

Don Young
04-16-2015, 10:11 PM
To gear your lathe for a 0.5 or 0.75 MM thread it is very helpful to have your existing thread chart. At a minimum, you will need to know if the "stud" gear turns at the same rate as the spindle and the pitch of the leadscrew. Gearing for a metric thread that fine will involve several pretty large gears and probably two compound gears. If you can provide the gear chart and a list of what gears you have, that will be a good start.

CalM
04-16-2015, 11:21 PM
fortunately, with only the few threads that engage on typical filter rings, almost any near thread pitch will work just fine. (calculate the error to make yourself happy)

I was active in the production and sales of photometric filters for astronomy (all the "usual sets" Bessel, Johnson, etc etc...) The 28 and 32 mm ring sizes were very popular. Just TRY to get a "standard" dimension on those threads ;-) It comes down to "if it screws together, It's good!" Lots of "tolerance".

The filter's and rings from tiffen were always first class, and "could" be used as a standard. Although times change, and they may be sourcing "off shore" to the usual crap shoot of size, material and tolerance. I used to purchase the "haze" filters and "lens protectors" just to get the rings ;-)

Spin Doctor
04-17-2015, 09:26 PM
Let's see, an M.5 is very close to a 50TPI in terms of its lead. 0.01968 vs 0.0200. Over three or four threads I really don't see a big deal. M.75 is 0.02953. The closest TPI is 34. Now I am going to make an assumption here. I think your leadscrew is a 3/4-12TPI. This based on the King 10x22 being basically the same as a Grizzly G0602. http://www.grizzly.com/products/10-x-22-Bench-Top-Metal-Lathe/G0602. This means if the leadscrew is turning at the same speed as the spindle it will cut a 12TPI thread. This is what I was taught to think of as the Base Ratio on the machine*. This means that for a 50TPI thread yourchange gears would equal a ratio of 12/50 or .24. Gearing of a compound set-up of 30/60/24/50 where the 60 and 50 tooth gears are the drivers should work. For 34TPI on a 12TPI leadscrew the gear ratio is .352941176. For this a set-up of 24/48/24/34 is very close. As stated dead sharp high speed tooling. In this instance this is where it really pays to have access to either a surface grinder and threading tool grinding fixtures or a tool grinding set-up with a table and protractor. Grinding one of these off hand is of course possible but let's face it. Most of us our eyes ain't what they used to be.

*Another type of machine that can be thought of as having a base ratio is a Gear Hobbing Machine. The Barber-Coleman type I used to use was a 30-1 base ratio. If one had the same number of teeth on both the input and the output shafts it would cut a 30 tooth gear. If one needed to cut say a 49 tooth gear it was simply a matter of using a 30 toothgear on the input shaft an a 49 tooth on the output shaft (30-idler-49) to get the correct ratio. Of course we were cheating in that we had gears from 22T up to over 80T for both the index and feed gears

Spin Doctor
04-17-2015, 09:34 PM
One other thing. If one is going to do this sort of thing on a regular basis then IMO this is where a Single Tooth Dog Clutch for threading really pays off. With a kick out rod to control the end of the cut it allows you to stop the thread to with in .001 per pass. The STDC should also be of the reversing type. No need to stop the spindle and reverse if you are keeping the halfnut closed.

Hopefuldave
04-18-2015, 06:52 AM
I don't believe you will ever be pleased with the surface finish of fine threads produced by "hand cranking".

I could be wrong, but typical filter rings are made from 6061 (or worse), Anodize color uniformity being of high importance.

To produce the fine finish expected by the optics community, cutting speeds (sfpm) needs to be adhered to.


I agree with Cal, hand-cranked speeds won't give much of a finish.

Industrially this would be a good candidate for thread milling, where the SFM is generated by the tool, not the work - then you could turn the spindle slow as you liked! Die grinder on the toolpost with a 60-degree as-many-teeth-as-you-can-make DIY milling cutter on the end, think gear cutter, and the carriage under leadscrew control?

Excuse the very rough drawing...

http://i979.photobucket.com/albums/ae279/hopefuldave/Daft%20Ideas/Die%20Grinder%20Thread%20Milling_zpshs6ufsab.png (http://s979.photobucket.com/user/hopefuldave/media/Daft%20Ideas/Die%20Grinder%20Thread%20Milling_zpshs6ufsab.png.h tml)

edit: Thinking about it, the tilt to the helix angle would probably be essential with a homebrew cutter.

Another edit: I think I'll try one of those setups...

Hopefuldave
04-18-2015, 06:58 AM
One other thing. If one is going to do this sort of thing on a regular basis then IMO this is where a Single Tooth Dog Clutch for threading really pays off. With a kick out rod to control the end of the cut it allows you to stop the thread to with in .001 per pass. The STDC should also be of the reversing type. No need to stop the spindle and reverse if you are keeping the halfnut closed.


I agree, I'm working on one for my lathe (its bigger brothers had one, there appears to be room (deliberately?) for the mechanisms although one will be pretty intricate!). Having used bigger Holbrooks, Hardinges, it makes threading a lot faster and is less stressful than having to reverse the whole machine on "foreign" threads!

Spin Doctor
04-18-2015, 03:24 PM
It just so happems HSM is running a series on a Threading Clutch for the Grizzly model lathe that looks for all intents and purposes identical to the King 10x22

epanzella
04-18-2015, 08:10 PM
This probably won't help anybody that is dead set on cutting the camera filter threads themselves, but when I needed those fine threads and started researching the grinding of a tiny tool I could hardly see and the anodizing I took a different tac. I was making adaptors for my riflescopes so I could use clear camera filters as lens caps for hunting in rain & snow. Instead of threading the part I bored it out and epoxied in a camera adaptor ring that already had the thread I needed. It was already threaded and anodized and cost all of three bucks.