Planning a new project: How to machine threads on a narrow aluminum ring?

D

dansawyer

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The project is to create a thread 6061 adaptor for a telescope focuser. The telescope in place back thread is m68x1 metric male external thread and the focuser is m90x1 metric female. I envision a disk 1/4 to 3/8 thick with external threads and internal threads to screw onto the scope and allow the focuser to screw onto.
The question is: given a blank disk with appropriate starting diameters; what is the best way to maching the threads?
What I have thought of is to machine the inner threads into a larger blank, sufficient in size to support being mounted for threading. Then to create a threaded jig to hold the blank by the inner threads, then to finish machine the outer diameter and then the outer threads.
I would appreciate any flaws to avoid in the above method and / or any alternative methods.
I have a lathe with 3 and 4 jaw chucks of sufficient size and a face plate. Accuracy for the project is important.
(Would mounting the disk blank on offsets on the face plate be a better alternative? )
This project is a couple of weeks out so based on the advice I will start to order material .
Thank is advance, Dan
 
Probably several dozen ways to do this. I'd start with a piece of tube like this, that was at least 2" long, chuck it up in a three jaw using parallels to get at least 1" sticking out beyond the jaws, then turn and bore to appropriate ID/OD, and thread, then part it off.

As long as you turn and thread the ID and OD in the same setup, you'll get accuracy to the tolerance (runout) of your lathe,
 
rabler said:
Probably several dozen ways to do this. I'd start with a piece of tube like this, that was at least 2" long, chuck it up in a three jaw using parallels to get at least 1" sticking out beyond the jaws, then turn and bore to appropriate ID/OD, and thread, then part it off.

As long as you turn and thread the ID and OD in the same setup, you'll get accuracy to the tolerance (runout) of your lathe,
Click to expand...
Thank you. Yes. One setup is the criteria to design around. Thank you. I will make sure that is part of the design. Dan
 
I think you are on the right track, as per the procedure you outlined in post #1, doing the external threads last with the part screwed onto and held by the jig/fixture is one way, but doing both threads in the same setup is probably the most accurate and probably easier- no jig required
-M
 
Good morning all. I decided to go with the single setup model. This was a two step process. I started with a 2"x4"x4" block. Using a CNC mill I bored out the center and the first inch of the outer diameter, I left 100 thou clearance on both the inner and the outer diameters. This created the 'blank'.
I then loaded the blank in a 4 jaw chuck, centered it to a couple of thou, and aligned the fhe block square to the lathe. Given the 100 thou extra material this insured the final part could be cut from the blank. I then cut the outer diameter to 90mm and the inner diameter to 78.75 mm. I have turned the outher threads, faced the mating end, and tapered the fit.
So far so good. The mating part fits very well to the outer threads. It mates to the surface and threads cleanly and tightly on and ingages about 10 full threads.
Questions:
First: What is the best way to clean the threads? As shown they contain some residual cutting residue.
Second: Both inner and outer threads are right handed. Do I turn the inner threads with the same machine setup as the outher? That is the carrage is moving toward the head and the spindle is turning in forward.

threads_1.jpg
 
A little maroon scotchbrite will take the fuzz off the threads.

Joe
 
The easiest way to thread the inside is to run the spindle in reverse so you are threading away from the headstock, and thread on the back side. As long as the spindle and apron leadscrew are both turning in the same direction you'll get RH threads. Of course you'll need an appropriate threading tool for that set up.
 
rabler said:
The easiest way to thread the inside is to run the spindle in reverse so you are threading away from the headstock, and thread on the back side. As long as the spindle and apron leadscrew are both turning in the same direction you'll get RH threads. Of course you'll need an appropriate threading tool for that set up.
Click to expand...
Thank you for that connection. I had not heard it before, but it makes sense.
 
I have cut fine thread on aluminium tubes with very thin wall ( 1 mm ) and my experience is that several passes need be done at the same cutting depth setting before there is no more chips coming out. I guess this is called "spring pass" ?
 
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