Machining filter threads

[ADG]

Are there any machinists here who could comment on the feasability of cutting filter size metric threads (58, 67, 77mm) on a Myford Super 7 centre lathe? I had thought I might have a go at making stepping rings, system filter adaptors, etc in brass or 6082 aluminium. To be anodised or painted black obviously.

[Dan Jolicoeur]

Not familiar with that lathe; that being said I single point threads on a lathe very often. These threads are very small and much better suited for a cnc lathe.

Yes you can make them, but hard to measure without the proper equipment. I myself do, and would buy them. My thought is that if the price is not way out of reason why make something you can buy.

I myself need a couple of series VII filter holder covers. That is the cover that goes onto a series VII filter adapter. Before I take the time to make them I will look for a couple of other adapters or covers before I make them and I have been doing this for just about 29 years. The last 9 have not been in a high production mode, but still make and design many things on a daily basis. Unless you want to have the experience of building it; do your self a favor and buy them.
Have fun,

[Emmanuel BIGLER]

To ADG. A few hints.
Independantly of the problem of actually milling the thread, one of the problem is how to hold a thin ring of metal without bending it.
If your start from a cylindrical block, the back un-milled side is easy to hold, you simply need to know how to make a thread on the front side without breaking the threading tool ;-)
Then you'll have to cut the ring off from the block, so probably you don't want to bore the hole before milling the other thread so that you can actually hold it firmly without bending it.
A friend of mine once modified the outer diameter of an existing filter ring to my specs, it took him more time to fabricate a threaded block suitable to hold the filter than machining the filter itself.
So once you get a disk with two threads, ready to bore the hole, my understanding, from what I discussed with my friend, a skilled professional precision machinist is that you'll have to fabricate a special threaded mounting block to hold the adaptor and finish the work without damaging the threaded ring.

[Ben Diss]

ADG- You might want to ask this question here: http://www.practicalmachinist.com/

[Conrad Hoffman]

I make stuff like this pretty often. It's tedious because you really need to make up some plug gages to check the female threads, and some threaded holding fixtures as said above. You'll spend more time making gages and fixtures than actual parts. If you can do the threads backwards, that will avoid the problem of crashing the tool on the short threads. Or, use something with an automatic thread stop, like a Hardinge toolroom lathe. Maybe the Myford has such a thing. Or, as I do, just go slowly using the back gear. I often make such things out of Delrin. It's plenty strong for filter rings, will never seize, and is easy to machine. Since most people buy black, it eliminates the problem of anodizing aluminum. I'm not up on aluminum types, but if you use aluminum, use something that machines well, like 6061 with the appropriate heat treat (T6?).

[Donald Qualls]

First, assuming your Myford has a metric lead screw -- if it doesn't, cutting a metric thread is a major pain in the anatomy.

Cutting the outside thread is easy, and as suggested above, you'll want to cut that before you part the ring off from the parent stock. To cut the inside thread in the actual ring, you'll first want to cut the same inside thread in a larger piece of the stock. The sequence to be sure everything stays nicely concentric would be to cut the outside thread, bore the inside to size, and turn the exterior, then part off. Bore and thread inside the remaining stock without rechucking; reverse the ring and thread it into the threaded stock in the chuck (use a lubricant, since turning will tighten the ring -- don't reverse the lathe spindle, because that will unscrew the ring, which would be *very* bad). Bore and thread the inside of the ring, and you're done. Oh, if the ring is to be knurled, you'll want to knurl the exterior before boring the interior or parting off, when the stock is strongest.

The hardest part is single point cutting a fine, inside thread (which requires cutting from the inside out if the lathe is running in normal rotation). If you can do that when you start, you'll find it's not a big deal. If you can't, don't start, you'll just waste a bunch of stock and frustrate yourself... Hint: consider making or buying a crank handle for the spindle, so you can turn the machine over by hand for the inside thread. It'll be worth either the time or the money...