Taper turning with Boring Head in lathe

[Maryak]

CJ,

I follow your analysis and it looks correct to me. :bow: :bow:

All I can say is that using tan rather than sin to calculate the angle has not let me down over a good few years, don't ask me how cause I don't know.

On the other hand matching tapers M to F I use bearing blue and adjust for the final cuts.

Best Regards
Bob

 

[chipswarf]

Maryak, your technique of using tan instead of sin is right on for small angles. Recall that tan = sin/cos. For small angles, cos angle is nearly one.

Mark

 

[kvom]

Often a taper is defined not by the angle but by the different diameters at each end of the taper. If the drawing shows these, then it's just a matter of moving the boring head by the difference in the radii at each end (which is what the calculation is determining).

It's probably safe to say that most tapered parts on model engines are decorative, so the angle isn't critical but more an esthetic choice.

 

[black85vette]

Being a former US Marine, while in SE Asia I got a good Tan and learned to Sin along with the best of them. :big:

Sorry, lame humor. I'll try to make up for it:

Here is a handy triangle calculator that I use on-line. I am sure there are many more. I do have a basic understanding of Trig but don't trust my memory to recall every function. With this you can put in your known variables and it will spit out every thing else. I find it useful. Many problems can be reduced to a simple right triangle function anyway.

http://www.carbidedepot.com/formulas-trigright.asp

Here is another good link sent to me by a co-worker that has all sorts of tutorials on math and science. You can go as far as you want on this web site;

http://www.khanacademy.org/

 

[mklotz]

I think you folks have gotten a bit off track here. You're worrying about how to calculate the offset rather than worrying about how to cut a taper.

The real advantage of using the BH (boring head) is the fact that it's a quick and dirty way to offset the TS center without disturbing the TS itself. The fact that the BH offset is calibrated is nice but not key to the utility of the technique.

If you were turning a non-critical taper (e.g., the column on a beam engine) where a fraction of a degree or so didn't matter then, yes, you could do the setup using the BH calibration. You mount the workpiece, its length determining one side of the triangle, then crank in the offset on the BH, said offset determining the other side of the triangle. What really matters though is the distance between the two pivot points of the workpiece and that changes minutely when you introduce the offset. Thus you'll never turn *exactly* the taper you calculated.

However, if you're turning a critical taper (e.g., a Morse taper to fit a machine socket), you would never use the technique described above. It's just not accurate enough.

Mount a true cylindrical piece of stock between centers. Mount a straightedge of some sort on the compound. By pushing the straightedge against the stock, get it aligned parallel to the lathe centerline and clamp it in place.

Now, mount the BH and set up the workpiece in the lathe. With a sinebar set to the correct angle against the straightedge, twiddle the adjustment on the BH to bring the workpiece parallel to the sinebar, thus establishing the correct angle.

Aside: A precision angle block can sometimes be used in lieu of the sinebar.

If fiddling with a sinebar and such is too much for you, there is another less accurate way. Rough adjust the BH using the calculated offset. Now, with a DI mounted in the toolholder, measure the offset of the workpiece over an accurately measured (use another DI) interval of saddle motion. If that saddle motion is 'L' then the DI-measured offset should be:

os = L * tan(phi) (phi = taper half angle)

Continue fine tuning the BH offset until you get the calculated value for 'os'.

 

[Captain Jerry]

Marv

I think yoy are right about the sine bar being the only way to get an accurate taper. I think the real problem of using a calculated offset is knowing the effective length of the workpiece after center drilling and setting ball centers.

Goofy question: Why not use a tan bar? *club*

Jerry

 

[kvom]

Goofy question: Why not use a tan bar?

Sine bars have a precision length hypotenuse that doesn't change as the angle is changed. So when you set it with precision gauge blocks you need the sine of the angle.

 

[mklotz]

Some very interesting tangent bar designs have been proposed, e.g....

http://www.freepatentsonline.com/2669027.pdf

However, with all the moving parts in that design, I fail to see how it could be as accurate as a sine bar. Still, it rates a 10 on the coolness scale.

I've never seen a tan bar offered for sale. Does anyone market one?

 

[DavesWimshurst]

Marv,
Perhaps a sine bar made out of wood? :big:
Dave

 

[mklotz]

Marv,
Perhaps a sine bar made out of wood? :big:
Dave

Reminds me of an article I saw in Fine Woodworking.

Some woodbutcher had "invented" this super accurate way of setting up miter cuts on his table saw. Basically, he had built a 12" sine bar out of wood replete with wooden rollers.
He had even made up a set of wooden "gage blocks" with which to set the angle.

I should have clipped the article and put it in my "highly polished turds" file.

 

[websterz]

Some very interesting tangent bar designs have been proposed, e.g....

http://www.freepatentsonline.com/2669027.pdf

However, with all the moving parts in that design, I fail to see how it could be as accurate as a sine bar. Still, it rates a 10 on the coolness scale.

I've never seen a tan bar offered for sale. Does anyone market one?

Found this one Marv...

http://cgi.ebay.com/NEW-6PC-TANGENT...emQQptZLH_DefaultDomain_0?hash=item48392dfa8c

 

[Captain Jerry]

Marv, Bob, Websterz, Kvom, and others that I may have offended,

I was really being facetious when I asked about a tan bar. I had no idea such things existed. Of course the sine is the proper function to use when the hypotenuse is known and constant. Thats the point I was trying to make.

I live on a horse farm and occasionally horses die of natural causes. Not from being beaten. And we certainly don't beat them after they are dead. But once in a while I ........ Please forgive me.

Jerry

 

[chipswarf]

Marv

I think you are right about the sine bar being the only way to get an accurate taper. I think the real problem of using a calculated offset is knowing the effective length of the workpiece after center drilling and setting ball centers.

Jerry

CJ, the effective length of a center-drilled workpiece can be measured as follows. Seat a pair of balls in the drilled centers, ones that match the diameters of the lathe center balls. Measure length over these balls and subtract the radius of each ball. This is the workpiece length (hypotenuse) to use for the offset calculation. It remains constant as offset is cranked in. Offset (short leg of the triangle) increase results in a decrease in the longitudinal separation between the lathe centers (long leg of the triangle) - you have to feed the tailstock in a bit when offsetting the center, to keep the balls seated in the holes. We don't care about this long leg, as long as the centers are well seated.

One advantage of using ball centers is that we can so easily measure this length, and it remains unchanged, unless the balls brinell into the center holes (you could give each a little rap with a hammer). Two of the corners, or vertices, those at the ends of the workpiece, are always at the centers of the balls, which removes head scratching about what happens with an offset on conical centers. This workpiece length is analogous to the length of a sine bar, but not known to the same accuracy. The offset replaces the gage blocks. Same calculation. Measurement errors would make this less accurate than the sine bar for setting an angle, but much faster in roughing in a taper, and you still have to cut and try to get it nailed. Note that my other post uses taper per foot directly in the formula - no calculation of an angle, no sine tables, no more intermediate steps.

I don't yet have ball centers (propose to use tooling balls, see McMaster), so I can't try this yet. I'm sure curious to see what real accuracy is inherent in these calculations. Fun stuff.

Mark

 

[Maryak]

Jerry,

I for one am not offended in any way. I can only say thank you for thinking out and pointing out something I have taken as read for many years. :bow: :bow:

Now it's time to get out of the bar, go get a tan and find a cosine(r) whilst I'm still young enough to remember what I should do. :

Best Regards
Bob

 

[websterz]

Offended? Pish...you haven't enough time in the day to find a way to offend me. :big:

 

[Metal Butcher]

Metal Butcher:
See this link for tips.

http://books.google.com/books?id=vd...Q6AEwBQ#v=onepage&q=tailstock setover&f=false

for a morse 3, the taper is 1/20 per inch or 0.050. If you have a workpiece 9 inches long, then you'd set over 9 x 0.50 or .45 inch.

Thanks for the link.

I started to read it and it looks like a lot of good info.

-MB

 

[fgleich]

I think I had the setover wrong . It shows in the article what it should be, .025 for a total of .05. So for 9 inches, it would be 1/2 of what I figured or 0.45/2. Duh........lol ;D

 

[chipswarf]

Well, fui, or good for me, depending. I started thinking this Lt/24 thing must have been familiar to the old shop hands and just found it on page 481 of the 4th edition, 1991, of "Machine Tool Practices" by Kibbe, Neely, Meyer and White. Probably in every comprehensive shop text since 1871. So much for my sniffing about how I value my library. Gots to read the stuff, chippy.

They didn't derive it though, the buggers, just laid it out. So, I can still feel good about myself. :

Mark

ps - note that the 'setover factors' in the chart shown in the above link are all t/24, where t = taper per foot. So, Offset = L x setover factor = Lt/24

As Basil Fawlty might have said - "Here we have Mr Chipswarf, special subject, the bleeding obvious."

 

[fgleich]

Here's a link with a picture of one set up, with a level setting jig attached. Cool !

http://home.iprimus.com.au/stevor/BoringHead.htm

 

[Blogwitch]

That really is an easy way of getting the boring head level.

Thanks for the link


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