Close fitting threads - how do I machine them?

Hi, all, i'm trying to machine a close fitting internally threaded sleeve for a 40mm X 1mm threaded spigot. Any tips on how I can do it? Current attempts (3) have all ended in loose fitting scrap. I'm cutting internally with a single point tool and repeating the final cut at the same setting until nothing more is removed. The sleeve will not screw on, a further miniscule cut results in the thing being loose. I really want a fit that produces a smooth movement with slight drag along a length of approx 20mm. Am I expecting too much? I'm begining to think that I will need to make the sleeve in two parts with some adjustment between to obtain the sort of feel I want.

Any tips, help would be much appreciated.

Best regards

Keith

Sounds like you're learning the hard way about spring cuts. If you let a tool cut and recut on the same setting it'll rub and stop removing any material. Add even a tiny amount to the setting and it'll dig in again and remove a big chunk of metal. You need to find a speed and feed that let you get the desired size on the first or second pass. Trying to 'creep up' on a dimension is one of the hardest things to do in engineering. You get to within a thou or so and then you're buggered because more cuts at the same setting do nothing and changing the setting goes straight past where you're trying to be. Machine rigidity is the key here. If there's no movement in the machine or the tool you can take tiny cuts with a reasonable amount of precision. Boring tools aren't rigid though by definition. External tools are always sturdier.

-- Dave Baker Puma Race Engines

Camp USA engineer minces about for high performance specialist (4,4,7)

For that reason, it may be a better game plan to machine the spigot thread to fit the female thread, not t'other way around.

Regards, Tony

Alternative approach - use a suitably thick & chewy grease on a relatively loose fitting thread to give you the right smoothness/drag charagteristics?

Regards, Tony

Lucas Red n' Tacky Grease - the stickiest, most-bugger-to-get-it-off-ish one that I've come across. Only applicable if you plan to go down said route of course.

Peter

I haven't tried it, but istm that you might be better off trying to grind/lap the internal thread. Maybe you could make a male thread that's a little on the small side and then slather on some grinding paste and just ease the fit using that ?

Hth,

In message , jontom snipped-for-privacy@hotmail.com writes

You need to make sure you are contacting on the thread flanks and not the crest or root. Bore it initially to a size that truncates the internal threads - 38.9 mm. Then cut the thread with a sharp pointed tool, no radius.

On or around Tue, 7 Nov 2006 13:41:54 -0000, "Dave Baker" enlightened us thusly:

making sure the tool is seriously sharp seems to help - I was cleaning out a bore the other day and getting erratic answers, then I reground the boring tool...

FWIW, and may not help in this case, it's easier IME to get close fitting threads by cutting the internal one first and then the external one.

Mind, had some annoying answers with that recently - I use some small ball joints which have M8x125 threads on control rods, which need to be adjustable length. Made a nice thread on the rods, which fitted the locknuts beautifully, but only screwed half-way down the ball joint thing. Running a tap down the ball joint thread didn't make it any better, had to tighten up the die a touch and recut the one on the rod, which naturally then did a crappy job (the die wasn't new). I've bought a new die now.

which reminds me, was going to order some bits and pieces...

Thanks guys' good advice as always. Unfortunately I can't cut the internal thread first on this occasion as I'm trying to make use of a plate and spigot I already have. If I can get it to work as I want then it will be an exposed thread in a dusty environment so I'm sorry to say the (old car MOT) trick with the thick grease is a non starter this time as well

I think Dave hit the nail on the head with my tendency to creep up on the finish size, I will try and be a bit more confident with my final cut and re-sharpen the tool as well to give it some sort of chance. In thinking about what I was doing I was also as Charles guessed trying to cut a full depth thread so I will truncate it and see if that eases the problem, the one on the spigot is only about 80% anyway now that I've had a proper look. I haven't cut a thread using a tap but will certainly give that a go as it must produce a better thread form than my poorly ground single point tool.

Altogether, a real team effort with something helpful from all of you, thanks very much it is appreciated.

Dave, on another subject altogether I enjoyed reading your article on what is available from the old Ford Crossflow. I have an old Westfield with one in and have been planning to replace it with a Zetec but reading your comments I think I will give it a bit more thought before jumping in to spend =A32K or so.

Best regards

Keith

If your machine and Internal tool are light, then you might try using the setover technique, which also works for internal threads. This will load you tool much less than a perpendicular cut, you can then take much lighter cuts.

The only difficulty with this technique is when screwcutting to a shoulder, but thats always a chore unless I'm doing it on one of the CNC machines.

Unless I'm in a hurry. I always use setover but mainly for external threads. It's much easier to keep track of what depth you at at, at any given time.

Wayne...

the finish size, I will try and be a bit more confident with my final cut and re-sharpen the tool as well to give it some sort of chance. In thinking about what I was doing I was also as Charles guessed trying to cut a full depth thread so I will truncate it and see if that eases the problem, the one on the spigot is only about 80% anyway now that I've had a proper look. I haven't cut a thread using a tap but will certainly give that a go as it must produce a better thread form than

I've never done any screwcutting myself as my mate with a CNC can do it for me but I don't think I'd even try with a hand ground tool for my first attempts. Those carbide inserts for threading will have the exact tooth form properly shaped on them and be very sharp. Trouble is that's a lot of expense for a one off job. If your tool isn't exactly the right shape and uniformly sharp in all areas I can't see you getting a good thread from it.

what is available from the old Ford Crossflow. I have an old Westfield with one in and have been planning to replace it with a Zetec but reading your comments I think I will give it a bit more thought before

I'd definitely go Zetec and it won't cost £2k. The late model (1998 onwards Focus and Mondeo) solid lifter 2.0 engines can be found for £250 or less these days. My mate gets them for peanuts because he has a nice relationship with his local scrappy. They have bigger valves than the earlier hydraulic tappet ones and despite being rated at only 130 bhp they crank out a solid

175/180 bhp every time with just 45mm DCOEs and a decent exhaust. The standard induction plenum and exhaust manifold are terribly restrictive. We found 15 bhp just from a tubular Focus ST170 exhaust manifold on both my 2.0 Focus and a friend's. DCOE's add the rest. Add a pair of cams and 200 bhp is not hard to get even without any head porting. The engines are bulletproof and don't show bore or crank wear because they all get run on the synthetic Ford 5-30 oil. They also have a brilliant baffled sump as standard which eliminates oil surge even in track use.

Even with fitting kits, clutch, exhaust etc £1k should cover everything and you won't get much out of a Crossflow by only spending that much. All Ford four pots have the same bellhousing bolt spacing so it's an easy fit to a Westy.

-- Dave Baker Puma Race Engines

Camp USA engineer minces about for high performance specialist (4,4,7)

Thanks Dave that's superb information. My concern was the ignition as a decent system seems to be lots of beer tokens. I will only really be looking for 150 bhp or so as my two kids currently drive it and I don't want to make it too "interesting" for them - I'll never get a go again. Pity about the 45's though as I have a pair of DCOE 40's in the garage doing nothing still I expect E-bay will come to the rescue.

Thanks again Dave that has made my mind up for me, I'll start collecting the kit of parts ready for the change.

Hadn't thought about the inserts either and I think I have one in some of the unused tools I have bought in the past as part of a set so I'll have a look in some of the darker corners of the garage.

Best regards

Keith