Precision taper/polishing job

Lloyd E. Sponenburgh wrote:


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
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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
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Lloyd E. Sponenburgh wrote:

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
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On Tue, 10 Apr 2007 12:12:27 -0400, "Lloyd E. Sponenburgh"
<snip>

<snip> Excessive wear is relative to your production requirements and cost of the tools.
The suggestion of another poster to use precision tubing for the mold form seems to have merit.
A quick web search indicates stainless steel tubes are available, but small quantities may be a problem.
See http://www.sandvik.com/sandvik/0140/Internet/se01595.nsf/GenerateFrameset1?readForm&url=http://www.sandvik.com/sandvik/0140/Internet/se01595.nsf /(DocumentsInternetWeb)/A3EEB94DA5D1A68DC1256B680035B18F http://www.mertl.com/anonymous/info2.pdf http://www.mertl.com/anonymous/info6.pdf
While the tubing may be hard to obtain in small lots, pneumatic/hydraulic cylinders could be disassembled. See http://www.cylindersonly.com/ncmseststboc.html
As an alternative, depending on the pressure/force used, it may be possible to use pvc pipe. The bores are generally very slick, and it is intrinsically corrosion resistant. Also, it won't produce sparks. This could be cut to suitable lengths, and movable plugs placed in one end, and a close fitting ram used to compress the form. The plugs and rams could also be nylon or pvc. The compressed form could the be extracted from the tube by either removing the support for the plug and pushing on through the tube, or in a second op, inverting the tube and pushing on the plug. One source for PVC tube/pipe is your local ace hardware. Enco has pvc/delrin/nylon rod for the plug and ram.
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).
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yup.
I should have been more "complete" in my description of the entire machine. It exists. I'm seeking a better, more durable, and easier-releasing cavity than what's in there. In the meanwhile, I'm building a new loader for more precise filling with clumpy mixtures, modifying the pressure elements to directly measure the height of the pellets, adding a finished product stacker/orienter (for the next operation), and modifying the press cycle for better 'self cleaning' of the mould and its components. And all of this is WAY less work than building a new machine with new principles from scratch.
So, I cannot arbitrarily use "split moulds", etc. It has to be a 1.125 x 4 bored rod.
PVC won't work -- the pressures _start_ at 1465psi and go up.
The original designer made it with Ni-Tuff coated cylindrical aluminum cavities. They wear out after about twenty thousand strokes -- not economical. PVC and Delrin liners were tried, and they wore out faster. Even _with_ a degree of taper, they don't seem to hold up to the abrasion at these pressures.
The pill press fellas use steel. We get hundreds of thousands of fill/press/eject cycles from each of their punch/die sets, so I figger they've got the method... but they won't let on about what alloy, etc, they use... only the geometry. About the only things I know about it are 1) it wears like iron (*ulp* _steel_) 2) it will rust if exposed to both composition and high humidity, but if clean, it won't and 3) it loses its initial polish almost immediately, but doesn't seem to change dimensionally for hundreds of K cycles.
Our production demands are growing, and this machine is a pivot point for several product lines. It will get a twin brother, as soon as these new features are added and tested. I have the prints for the existing version, and have built replacement wear parts for it on several occasions.
I think I hear myself saying, "Buy a tapered reamer." I've never had any fun reaming aluminum, but 41L40 probably will not gall and mark as badly; no?
This is a "small production lot" project, not a one-off. But the production will continue in dozens-lots for the forseeable future.
LLoyd
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On Tue, 10 Apr 2007 15:39:14 -0400, "Lloyd E. Sponenburgh"

==========Ah So
Given your ongoing production, you will almost certainly be better off getting a custom reamer ground. You won't need an entire custom reamer, but one that is as close as possible reground. Shouldn't be that expensive.
I would look at using drill rod for the mold. In many cases simple W1 [the cheap stuff] will be adequate, particurarly if you can make the molds quickly/cheaply. There is also precipation hardening A1 that is considerably more expensive but heat treats well with minimal distortion. Given your requirements, most likely a good vacuum or inert atmosphere heat treat would be the way to go.
Generally drill rod machines very nicely. We used this when we made the mounting post for the quick change tool holders. Its good that you have 1-1/8 diameter as this is the largest that can be chucked in a common C5 collet.
Hard chrome or TiN in the bore would greatly increase both the wear and the corrosion resistance, but with a custom ground reamer you should be able to chuck the mold in a collet and touch up the bore as required.
For examples of drill rod see the enco urls below. Lots of places have 1-1/8 drill rod. http://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2 they don't show 1-1/8 S7 high shock, but its available. FYI see http://www.use-enco.com/CGI/INSRIT?PMAKAB2-1296&PMPXNOx65918&PARTPG=INLMK3
Stress-Proof is also widely used in these applications
see https://www.flatground.com/catalog/item.jsp?DEPARTMENT_ID=3&DIAMETER=1-1%2F8
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).
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On Tue, 10 Apr 2007 15:39:14 -0400, "Lloyd E. Sponenburgh"

Greetings Lloyd, Aluminum can be polished to a very smooth surface. Then hard anodize to maximum coating thickness and do a final polish if needed. Another solution which may be better is to make the part from heat treated 4140 or 4340. Though a little tough to machine it's easy to get a very good finish. If the chrome moly is too tough then use 12L14, aka Ledloy. No matter what kind of steel you use have the surface coated with titanium nitride. There are all sorts of places doing this coating now and the costs have come way down. The coating has the multiple advantages of being corrosion resistant, really hard, and really slick. I'll bet that it may be the longest lasting part you could make. ERS
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wrote:

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
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wrote:

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: http://www.newmantools.com/tech/taper.htm
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Here is a link to the "time-saver" lapping compound. http://www.newmantools.com/lapping/time.htm
Roger
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On Tue, 10 Apr 2007 07:47:24 -0400, "Lloyd E. Sponenburgh"

P20 and a CNC lathe
Gunner
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