View Full Version : boring heads and differential threads

01-26-2015, 10:43 PM
Have been doing some thinking about my boring tool dilemna (wanted some tools to bore bearing fits to exact size) and have had several ideas. I've looked at hundreds of pictures of various adjustable boring heads, and now I've dropped the idea of the fixed size tool. I will be making another boring head to suit, and in the mental design stage I'm at the point where the adjustment screw needs to be figured out.

Today I remembered about differential threads. It occurs to me that I could set up such that one full turn of the screw would advance the head some fixed amount, say 10 thou, making it easy to dial in a movement of a tenth or two. That corresponds to a lead screw with 100 tpi- or a differential thread with one section at 10 tpi and the other at 11 tpi. My math could be wrong here, but at any rate the desired action is obtained.

Do any existing boring heads use the differential thread idea- anybody know?

It would seem the major problem is that the lead screw with the two threads on it now has to travel within the boring head and not just turn in place. With my example it would have to move one inch to make the head move 100 thou- perhaps not very workable.

If I stick with a single thread on the lead screw, perhaps it should be 50 tpi- meaning that one revolution adjusts the head 20 thou. 20 marks around the dial should be easy enough to work with and interpolate between. Problem being the dial is quite a small diameter, so actually being able to see space between the marks is important.

I'm rambling on with a few ideas, but maybe somebody knows what thread is usually used in a boring head, and might be an optimum-

A final thought for the moment- keep in mind that I can turn external threads of a very fine pitch, but the matching internal threads would probably have to be made with a tap. Since I'll be building this myself, I am not likely to want to buy an expensive tap-ah, but perhaps I could use the Evanut idea to make the internal threaded part- hmm.

01-26-2015, 11:07 PM
How about making your tool bit travel at an angle almost parallel to the axis? It would travel 0.001" upwards or downwards and only 0.0001" on radius.

01-27-2015, 12:50 AM
Yes, I thought of that too- might still do that. Was thinking of using a 1/4 inch square HSS tool bit set to some exact angle for a nice and easy 'so many thou down = so many tenths out- pretty sure it was mentioned on this forum at one time what angle is needed. It would be good to be able to set the tool by a known amount.

What I have in mind for this project is a 'holder' which would mount to the mill spindle, then one or more attachments which would mount to it. Each attachment would have its own adjustment, so whenever you mounted it it's already set up for a certain diameter. You'd be close right away, then you tweak it to final diameter. Or if I use the garyhlucas method, each attachment would simply mount in a fixed position, then the fine adjustments are made by raising and lowering the cutter.

Nothing stops me from using either type of attachment with the same holder. Re and re would be by loosening one side of the dovetail- the other side would be fixed. The way the design is worked out so far in my head, there are no critical alignment issues. The calibrated dial would be mounted on the allen wrench that's used to adjust, and the holder would have a series of marks set up like a vernier. I should be able to read it to a tenth- and if the dovetails are kept clean it should repeat well.

If the design passes the next day or so of thinking about it, I'll try to come up with a drawing.

So far I've found a 3/8-24 tap and a 1/4-28 tap. Neither one gives a good division of an inch so dial markings can be worked out. I'd have to use either 20 tpi, 25 tpi, 40, or 50 tpi. The only standard there is 20, and that's not fine enough. I don't want to go too small in diameter for the adjustment screw, otherwise a 4-40 would work.

01-27-2015, 01:24 AM
How about something totally different? Like more like an adjustable reamer, Use a tapered tip of a screw to jack apart two chunks of metal, One of those chunks holding your boring bar/cutting tip. No backlash and very rigid without needing a locking mechanism since it will be spring loaded against the jack screw by the body of the boring head.

I don't think you really need exact calibration marks either. you are going for one exact size so its going to start within a few thou of the correct size, its not going to be 'oh I need to turn it 1 grad and I'll be done', its going to be 'oh I need to turn it one fly fart, recut and remeasure as I need to sneak up on the dimension ever so carefuly'

01-27-2015, 05:26 AM
I often wonder when looking at boring heads what would it work like if the tool holder was on an inclined dovetail so the set screw that moves the tool would advance the tool as a resultant of the "triangle" if you see what I mean, the radial movements should then be smaller to get more control.

01-27-2015, 06:04 AM
It's all well and good being able to advance the tool with incredible accuracy and / or in very small increments, but unless the tool tip is sharp enough to shave off these tiny amounts, then it's all academic isn't it? Just a thought.


01-27-2015, 07:40 AM
The mill must also be able to remove .0001" or less as well. The average mill will not hold those tolerances.

01-27-2015, 10:05 AM
The mill must also be able to remove .0001" or less as well. The average mill will not hold those tolerances.

Mill spindle bearings are not that accurate, that is. You can lock the X/Y axis for the job.

Zahnrad Kopf
01-27-2015, 11:56 AM

Yes, there very much are some that use Diff Threads. I have a few that allow .0001" increments of adjustment as well as a few that allow .00005". Do it. :)

01-27-2015, 11:56 AM
At a previous job when I needed to be really fussy on a bore (most work was just steel fabs) I had the programmer make a paper disc that was divided on the circumference into 100 divisions (the boring head had a 1/2-20 thread) then I had them laser out a 6" disc of .020" aluminum onto which the paper disc was glued with a 3/8" hex in the center which was a tight fit on the wrench that adjusted the head each division was .0005", it worked great.

01-27-2015, 01:39 PM
I have a small head with a differential screw. It moves in .0001 on the dia. per mark. I can't say without digging it out and checking the threads but between the two threads the cutting tool is moves in small increments. It has been very useful where it is needed.

01-27-2015, 03:36 PM
have a look at how the Devlieg Tenthset boring tips work.

01-27-2015, 08:44 PM
Differential screws were discussed here some years ago. Interesting thread (ptp).


01-27-2015, 09:43 PM
I once did a job on a Fadal 4020 machining center where we tried to use a regular Criterion boring head to sneak up on a very close tolerance bore. The metal work hardened and we finally sent it out to be ground to size. When a second job came up with about ten holes we convinced the boss to buy us a Sandvik Coromant boring bar for $600 to do a very small range of sizes. Bored once, made one adjustment, bored all ten holes right on size, done. Well worth the money!

Paul Alciatore
01-27-2015, 10:29 PM
Perhaps a two screw approach. One coarse adjust with a large range and one fine adjust with perhaps 0.001" or 0.002" range. The fine adjust could be a differential. Perhaps this could be accomplished with a floating nut between the fixed shank and the moveable tip. One screw would be in the fixed base and the other one in the tip which moves and holds the cutting tool. Both screws would go into the nut with their own threads. And two calibrated dials, one in thousandths and the other in tenths; one on the base and the other one on the tip.

01-27-2015, 10:29 PM
I have an Ericson Tenthset boring head. Every mark is .0001 on diameter. It works very well. I got it on eBay for around $100. Mostly I use a Nova boring head that is .001 on diameter per mark, but it has been really good to have the Tenthset one a few times.

01-27-2015, 10:35 PM
Would one potential problem with placing the tool bit on an angle be boring to a particular depth? In other words, if one had a blind hole to bore and wanted to use a depth stop at the end of the blind hole every time you advanced the cutter for the next cut the depth stop would be in the wrong place.
I'm not saying it's a bad idea, it just may be something that causes an issue on a future project. If you are going to go to all the work of making a boring head make it so it can be used in as many ways and as easily as possible.

01-28-2015, 12:13 AM
I have thought about the problem of losing your height reference if adjusting the cutter by sliding it up or down- I agree that it would probably end up being a bit of a nuisance, though it is potentially an elegant way to get fine adjustment.

One of my main concerns with this project is to be able to make a fine adjustment without having to maintain a 'gnats hair' finesse while turning the adjusting screw. The ultimate distance the tool moves doesn't have to be exactly what the dial says it should be- what's important is that when you're sneaking up on the last couple thou and splitting that into tenths, is that you can actually make the head move by those small amounts. If my dial is 36 divisions and the screw is 28 tpi, each dial marking is about a thou- not exactly, but then I need to be able to see enough light between markings so I can visually break that space up into about four parts. My best resolution then becomes a half thou in the final result. That can make the difference between a loose fit and a too-tight fit. I can make the tooling skim off a few tenths without much fuss, but interpolating between the dial markings is the real issue for me. That's the part of this project that I want to make suitable before I start cutting metal.

It looks to me like even with only 36 divisions on the dial, the markings will still be too close together to allow me to resolve several discreet steps between them. I will definitely be limited by the diameter of the dial, so either a differential thread or a fine thread lead screw will be required. For the sake of simplicity I'd like to stay with a simple lead screw, albeit a very fine threaded one. Having looked it all over now, I'd like to use 40 tpi and a dial with 25 markings.

I will have to make the lead screws (one for each attachment). Not a problem turning them out, as I can use my travelling steady and will want to have at least 5/16 or so diameter and the depth of that thread is so little.

Here's an idea now- I could turn the thread with a rounded bottom, then turn a coil from .020 music wire and thread about an inch of that onto the lead screw. Then mix an epoxy putty and knead that around the spring. When the buildup is enough, let cure then turn the OD to a finish size. That becomes the nut, and gets epoxied into the attachment that carries the cutting tool. If I time it right, I can break this nut free from the threads before the epoxy putty is fully cured. It will become a zero play nut. Part of helping the nut not adhere to the threads is to coat the threads with a release agent, like a polish-on dry lube.

Zahnrad Kopf
01-28-2015, 12:49 AM
Darryl, my head that does .00005" uses an auxiliary vernier scale that allows further division of the standard, single hash mark. So, for "gross" adjustment one uses the normal hash mark and aligns the dial's mark to that. For "fine" adjustment, one then uses the vernier to further "divide" that scale. Clear as mud? I can take a pic tomorrow if need be.

01-28-2015, 01:02 AM
Clear as glass, actually. All I need to figure out now is how to make the dial an integral part of the head, while being able to solidly couple the lead screw into it each time I swap out an attachment. The plan is coming together.

01-28-2015, 01:24 AM
Hmm- I can make the dial and differential thread a fixed part of the head, then each attachment has it's own coarse adjustment with a lock nut. Did somebody mention this already?:) Good idea, I think I can make that work. Best of both worlds.