I need some help from the wise and experienced.

[hwingo]

Hi guys,

I need a little help from the wise and experienced. I have a metal part that’s 0.187” dia at the base X 0.144” dia at the top and from base to top is 0.154” long. As you can tell, it’s a very small tapered piece of metal. Using a drawing program, I have determined the angle from the base to the top to be 7.07 degrees.

I do not have a taper attachment on my lathe. I want to duplicate this tiny part as closely as possible. I am beginning with a piece of round stock that’s 0.187” in dia. If I use a square nose tool and turning my compound so that the square face of the tool is parallel to the long axis of my work piece, then turn the compound 7 degrees where the tool is beginning the cut at the edge of the stock (where the face meets the long axis of the stock) and advance the cross feed in by 0.043”, would this provide me with the taper I need as well as a correct diameter at the small end?

Thanks,
hw

[TGTool]

hw,

That's one way to do it, but remember a couple things with that setup. You're cutting the full length of the part (.154") so you're more likely to have chatter. You should only advance the tool .0215" to get the small diameter unless you have a direct reading dial on the crosslide.

I'd be more inclined to turn the compound close to parallel to the ways and then set your angle. You can check this by putting a piece of stock in the lathe that you can verify is straight with the ways. In other words, put an indicator on the carriage and make sure it reads zero as you move up and down the ways. Then set the indicator up on the compound making sure that it's plunger or point travel is exactly square with the ways. Then crank the compound .15549" (as close as you can set) and see if the indicator has moved by .0215". It may be easier in practice to use a multiple of the numbers. If this checks out the compound should be set at the correct angle to make your part.

Just for the record, I figured the angle to be 7.95 degrees (Tan-1((.187-.144)/2)/.154) if you've given the right numbers for large end (.187) small end (.144) and length (.154)

[J Tiers]

Aside from the chatter aspects, it is probably easier to set the compound to move at the angle than it is to try to set the tool parallel and then set the compound at the appropriate angle.

For several parts, if you ground the tool to be a 'form tool" at the exact angle, then you could aline the long side of the tool perpendicular to the workpiece axis. Then you could plunge cut, or move the carriage , whichever worked better.

The angle would be as smooth as the tool was ground. using a shorter cutting edge will create some marking of the tapered part, so a one-piece cutter is good. But you would still have the potential for chatter.

If you needed to make a lot of them, best of all would probably be a "box tool". That is a one-piece cutter is set to the angle precisely, and held in a fixture which also has a bushing or rollers to hold and support the workpiece. That would be something that could be left set up, so that it could be removed and re-installed without losing the angle. The support helps prevent chatter.

If you need a very smooth taper, grinding would be best. All turning tends to leave marks.

[hwingo]

hw,

Just for the record, I figured the angle to be 7.95 degrees (Tan-1((.187-.144)/2)/.154) if you've given the right numbers for large end (.187) small end (.144) and length (.154)

I followed your math to a point then got lost. Starting with the inner most parenthesis I cleared those to get .043. Next I cleared the second set of parenthesis .043/2= .0215. Now the equation is Tan-1 X .0215/.154 = ?

So how do I find the Tan so I can subtract 1 and multiply by .0215/.154??

Hwingo

[dp]

tan-1 is also known as arctan, or the inverse tangent function.

http://en.wikipedia.org/wiki/Arctangent#Practical_usage

In your case the side opposite is 0.154 and the side adjacent is the one half the difference between the larger diameter and the smaller diameter, or (0.187 - 0.144)/2 = 0.0215. The arctan (tan-1) of 0.0215/0.154 is 7.9477 degrees.

[Evan]

It should really be written (tan^-1) or (tan E-1)instead of (tan-1).

[Your Old Dog]

I don't know anything about math but let me offer this for S&G's

If I only needed one: I'd set the compound at 7 degrees and taper a much larger piece then needed. Then I'd set my calipers at the desired smaller measurement and slide it down the taperd form until it stopped and that would be my first cut. Then I'd to the same for the larger measurement. If I needed many of these I'd go back to school and learn enough math to do it the way you guys are trying to do it now as it would be much cheaper.

So, how many of these parts do you need?

[hwingo]

Super!!!!!!!

Ok fellows, I got out the calculator and did the math. I now know how to mathematically figure the degree taper. This is very helpful.

Your different approaches to completing the job are also beneficial as options are on the table and I thank each of you for your responses.

Old Dog: To answer your question, at this time I need only one as this will be a prototype. However, later I may have a need to "mass produce" (in limited quantities) these parts. My project involves working with living tissue. The various parts I am fabricating will be used as instruments in a surgical procedure but these instruments do not remain in situ, hence, limited contact with tissue. The working parts will be fabricated from 440C SS and hardened to RC60. It's been an interesting project as I seem to envision a "better mouse trap" and I am following through with my idea.

Thanks again for your help.
hw

[dp]

It should really be written (tan^-1) or (tan E-1)instead of (tan-1).

It's a shame that the HTML MATH addition to the HTML draft was dropped. It would be nice to have some basic math symbology available. I do think though, that the abuse of the inverse function n^-1 to describe other inverse functions such as tan^-1 (or more cleanly, arctan), is a tortured shorthand. It is somewhat akin to using i or j in identifying the imaginary component of complex numbers which is another annoyance for me.

[Carld]

The part is very small and it's hard to know if you have the correct angle or not. A small variation in the diameter of each end on that short a length would cause a big variation in the taper.

Can you get the specs for the part or a drawing?

I assume you are intending to part the piece off the bar stock. Your compound may not be accurately marked for angles. Assume it is or check it out with a known taper long enough to use a dial indicator over a known distance measured with another indicator.

Set the taper and turn the bar to the small diameter using a dial indicator on the crossfeed starting on the faced end of the rod. Now you will have to switch to a cutoff tool and measure the width of the tool, add that to the length of the part and move the carriage that far and part if off.

It sounds like you need a CNC lathe, unless you are getting paid for what ever time it takes to make each part. Charge them time and material at top shop rate. The surgical company will charge an outrageous price, so should you.

[hwingo]

Can you get the specs for the part or a drawing?

It sounds like you need a CNC lathe, unless you are getting paid for what ever time it takes to make each part.

Charge them time and material at top shop rate. The surgical company will charge an outrageous price, so should you.

I have an actual part but it's unlikely that I can obtain a drawing from the company.

The part I am making will serve as an analog and the small diameter of the piece will intentionally be made oversize by several thousandths. Initially, it will be I who benefits from the variant currently sold on the market. This particular piece is a "better mouse trap" in that it will cut surgery time more than half. I will fabricate this prototype and when I am satisfied with its performance, I will have these fabricated in "production mode" and sell my product to surgical suppliers. Since this is only a "tool", and not something that will be retained in the body, FDA acceptance is not necessary.

I am confident that I can command a reasonable price for my invention.

Harold

[BadDog]

Why not just use "atan"? Fewer characters and perfectly clear.

[dp]

Why not just use "atan"? Fewer characters and perfectly clear.

Wasn't that the name of the dwarf in Conan the Barbarian?

[Your Old Dog]

hwingo, for all the longer the part is, could you make a lathe bit to form it in one cut? Don't know if your lathe is big enough to do it but it would make a snap out of making them with next to no scrap compared to the other suggestion I had.

Good luck on your project, it sounds as if it might be worth your time.

[Evan]

I will fabricate this prototype and when I am satisfied with its performance,

Umm, dare I ask just who or what you are "practicing" upon?