Precision taper/polishing job

Is that a NASA-job? :-) I mean, can the customer verify those 0.001" and see any difference if you made it 0.003"? Anyhow ...

Free machining steel.

Case hardened.

That is a narrow hole for a boring bar.

You need to make a boring bar with the biggest possible diameter and the shortest stick-out. Remember the 1:4 rule?

You need to grind your HSS-bits by yourself. Aggressive angles, nice honing job on the edges. Low RPM (this will reduce cutting-forces and thus deflection). Cutting oil. Try different brands, they *do* make a difference. I'm using Jokisch (but this might be a local brand). It is a light oil with a bright color. The other thick and dark one I have doesn't work that good for this job.

A wooden stick with some fine grinding slurry will work. Or a diamond-charged brass-stick. Or Time-Saver.

BTW: Anybody wanting to sell me the Time-Saver slurry? I can't get something like that here.

HTH, Nick

Do you need me to mail you a piece of 12L14?

Wes

I'm the customer. More taper would make the pressed component, um, "unacceptable". Although, to be true, it probably wouldn't matter so much at 3 mils per inch vs. 1 mil per inch. The important thing is that they all be alike -- I'll be making a few dozen of these cavities. The extraction force for the finished part needs to be balanced among all the cavities, so the absolute taper isn't as important as repeatable, monotonic taper.

Won't the part distort some during hardening, forcing me to grind to final dimensions? Remember that absolute hardness isn't so important as abrasion resistance. There are small metal particles (50 u) in the composition being pressed. Most of the metals are softer than the steel, but when we use titanium, no.

I'm using a 5/8" diameter bar with a stickout of about 50-thousanths. It must be rigidity or tool angles.

I'll work on that end.

I've not seen that method. Are you saying to work the stick like a boring bar, on the same taper setup, but so it makes contact on one side with the full length of the bore? I could do that... never saw it, though.

LLoyd

Oops. I read too fast. What if you did use something like 12L14 which does take a nice finish then have it plated?

Wes

Nick,

What is Time-Saver?

Wes

That's nice of you... but if that's the alloy I need, I can have a hunk here by tomorrow a.m. from McMaster. It appears not to be expensive.

LLoyd

It looks like 41L40 might be a compromise -- it's listed as "good" for abrasion resistance in its annealed state.

LLoyd

"Time saving lapping/polishing powder": It first coarse-grinds, then laps, then polishes in one step. I assume it breaks down in grit size as it's used.

McMaster carries it under the above text. I've used it with a lapping plate, but never on a stick, in a bore.

LLoyd

Hopefully someone else will pipe up if it isn't. Chrome plating would satisfy both corrosion and abrasion.

Wes

With the stick-out, I meant the stickout of the boring bar.

No, I meant holding it in your hand, letting the part rotate in the lathe.

Nick

I always keep it to the bare minimum to reach the end of the bore. Maybe

4-1/2" from tool holder to bit. That's considerably more than the 4:1 ratio.

Oh! never thought of that. I have a "split stick" for carbo-paper I use sometimes like that.

LLoyd

This appears to be the definition of a "binding taper". Once you ram the stuff in there, there is NO WAY it will come out with a pull, and I believe no way with a push, either. Unless you will change temperature to use differential thermal expansion to release the part from the mold, it IS NOT coming out! This is the kind of taper you use on a steel mandrel for making gears, etc. and the part will bind REALLY tight.

The corrosion resistance, if you are serious, sounds like stainless steel.

Internal honing with a single stick sounds like the right scheme for finishing. If this is a one-off, I'd lap the ID. Make a mating part out of something soft, like copper or brass, with a matching taper. You can apply spotting dye to make sure the tapers really match up. Then, apply some fine lapping abrasive and gently put it in the bore. Don't ram it in or thermal expansion might bind it tightly. Keep the lapping compound dilute with lots of oil. You won't need a lot of this treatment to remove the high spots and produce a polished surface. Depending on the stickiness of the material you will be pressing, you may not want a mirror finish.

Anyway, my limited experience with molds is the draft angle is WAY, WAY to small to make this work. 5 - 7 degrees is more common for mold draft.

Jon

The problem with a stick is, that you want a taper. So I would try to hold it in the hand. Maybe with a bit of wobbling or pushing to one side to get the taper. If the work is hardened, it is easier to get a nice surface (sounds a bit odd, but it is true. At least to my observation :-) ).

I once wanted a smooth surface in some brass-tube and didn't get it, no matter how I tried (brass can be tricky if it is cold-drawn). Then I turned some stick out of hard-wood with a slightly lesser diameter (0.1mm) and put some valve grinding paste (that's what I meant with slurry) on it. That worked *like a charm*. The wood expanded a bit (getting whet) and I got a perfect surface in seconds. Measured it, and it was round and straight. But you want it conical, I know.

Nick

It would be, if the whole of the cavity were filled with pressed material... but it's not.

This happens to be the taper that pharma folks recommend for dry-pressing tablets. The cavity length is so long, because the compo is very compressible, but when it dead-presses, it'll only be from 0.5" to 1.1" high. It will release upon being pressed toward the wide end, although it takes a bit of muscle to get it to move at first. This, I've verified with minature equivalents of the mould that were made-to-purpose for medical pill presses.

My real problem is that I will need to make these on a continual basis, as they wear out. Our pill press vendor can make them custom, but they'll be hundreds of dollars each because they don't have any designs in that size range -- it'll be full-custom tooling and setup for only a few dozens of parts a year. We'll probably wear out one to three a week. The economics of the job make that too high a variable cost, unless we can do them in-house with semi-skilled people.

My problem with that method is unless the lap is also precision machined and initially polished, it won't maintain the monotonic taper we need. I think a reciprocating polisher is more like what we need. The stick.

This is the recommended angle in the pharma industry. We use pill presses, and all of our cavities (dies) are made to that divergence angle. Straight dies wear out much, much too fast with abrasive compositions.

LLoyd

If you need multiples of a specified taper, your best bet is to make/ buy a reamer with the desired taper and go from there. That said, I have my doubts as to whether your specified taper is going to do what you want. I really think what you need is a split mold, two blocks dowel pinned with your desired hole size. Ram your composition, remove the mold from the clamps, divide length-wise and dump your finished grain. You're probably going to want some sort of mold release, too, if the stuff is as sticky as you say. No taper needed at all that way.

Stan

Ah! Now I understand the problem!

You certainly have more experience (I have none), but I have a different idea:

Make the form in 3 parts: One straight tube that takes most of the powder and is just some precision-drawn tube (cylindrical) that can be easily replaced when worn. The form itself that takes the finished tablet. It is just one cavity split in half (lengthwise) to get the tablet out (D-shaped). Put the tube over the form, fill with powder, press, split the tablet form (it was sitting in a ring while pressing) and voila!

But maybe you already have a working setup and don't want to change it.

Nick

Hmm. So far we've talked about machining the taper from scratch, but I'm going to suggest a different tactic. Your taper seems along the lines of a commercially available Morse or B&S taper. My first choice would be to find a precision ground taper socket and adapt it to your needs. This would give you pretty good finish, hardness, etc, and with someone else responsible for making sure it's right. Failing a premade socket in the correct configuration, select a suitable taper and bore the workpiece to appropriate dimensions. Then acquire a taper reamer for the taper desired. Reaming, perhaps combined with a bit of final lapping, should produce a reasonably good surface finish, likely better than boring. This would also increase your accuracy significantly. Maybe I'm completely wrong, but this was the first idea that jumped into my mind when I read about your project. ww88 BTW, here's a good site with dimensions for various commercial tapers:

from first post

================ The additional production specifications indicate that this is more of a production than prototype job. This justifies additional tooling, and possibly some design changes, for example a through hole to allow the finished form to ejected rather than attempting to pull it out with the possibly of fracture and the need to dig out the remaining form.

You may also want to check your numbers. a taper of 1 mil per inch, even on the side, over 4 inches is only 0.008 on the diameter.

(1.125 - 0.780)/4 = 0.345/4.000 or 0.086 inches per inch on the diameter or 0.043 inches per side

You may want to consider a Morse taper which is about 0.050 per inch on the diameter. There are also other standard tapers but the reamers can be harder [more expensive] to find. see

and many others -- google on

The advantage is that there are standard reamers available at [relatively] reasonable prices, and with a little care these will produce identical cavities. for example see

The little end through hole can also be reamed, and if you select a standard size, a dowel pin would be a good choice. Oversize reamers are available to provide plating clearence. shown, most likely HSS will be adequate. or Most mill supply stores and catalog tool sales will have these.

Consider hard chrome plating or TiN coating the cavities and through hole and punch. Most pyrotechnic mixtures tend to be both abrasive and corrosive.

Good luck on an interesting project -- I am sure you will get a bang out of it. :-)

Let us know how things turn out.

Unka' George [George McDuffee]

------------------------------ Watch out w'en you'er gittin all you want. Fattenin' hogs ain't in luck.

Joel Chandler Harris (1848-1908), U.S. journalist. Uncle Remus: His Songs and His Sayings, "Plantation Proverbs" (1880).

You should check your glasses. :-) He was talking about O.D. 1.125 and I.D.0.780 and a taper of 0.001" And not small diameter 0.78", big diameter 1.125"

Sorry, Nick