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Thinking about trying to make another taper attachment

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mickri

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This is not going to be a long drawn out thread. I have been looking at lots of plans for what I call your traditional taper attachment. The kind where you have a flat bar mounted behind the lathe that controls the taper. My question is how precise does that flat bar have to be? I know that the front and back edges need to be parallel. And that the edges need to be flat and straight. But how straight and how parallel? Are we talking about a tolerance of .010, .001 or .0001.

I am thinking about having 3 bars. A short one no more than 12" long for steep tapers. One as long as the travel on my mill/drill which would be around 16" long. And a third bar as long what will fit behind my 12 x 36 lathe.

The short and medium bars I don't think will be a problem for me to make. I won't have to move the bar to machine both side in one setup. The long bar is another story. The first 16" will be just like making the medium bar. After that I will have to move the bar and somehow dial it in several times. I think that I can do this as long as the tolerance can be within a thousand or two.

I also don't know how much load will be applied against the bar. Can anybody help me figure this out. Once I know the load I can put the load into my bending stress spreadsheet to get an idea of the dimensions for the bar. I could just go way over size. Would rather have bars sized for the anticipated loads.
 

Dabbler

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#2
since the bar determines the accuracy of the taper (technically the taper attachment is a tracing attachment, the profile it follows is straight)...

So if your bar is .010 out, then your taper you cut will be out also. 1:1; if you know a guy who has a surface grinder and can grind your bar and follower, you will get very accurate tapesx indeed....
 

Bob Korves

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#3
You can buy Starrett or other brands of precision ground O-1 tool steel flat bar in various sizes and lengths. It will be quite parallel and straight, tough, too.
 

mickri

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Thanks. What about how much force is applied to the bar? How would I calculate that?
 

WoodBee

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since the bar determines the accuracy of the taper (technically the taper attachment is a tracing attachment, the profile it follows is straight)...

So if your bar is .010 out, then your taper you cut will be out also. 1:1; if you know a guy who has a surface grinder and can grind your bar and follower, you will get very accurate tapesx indeed....
I agree 100%. In addition the rigidity and slop in the coupling between bar and cross slide will influence your accuracy.
I don't know how to quantify the forces applied to the bar, but I guess they will vary with the cutting forces (material, depth of cut, feed, you get the idea). Personally I would inspire my dimensions from an attachment on a comparable reputable lathe.
Peter

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Dabbler

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#6
Here's a quick back-of-the-envelope calculation:

During the cut, the forces on the cross slide lead screw are considerable, but for an Acme thread withe a leade of .100 per turn, you are getting roughly a 14 times mechanical advantage, and the difference in diameter of the lead screw and the handle multiplies with this. Assuming a .750 lead screw, and a 4" diameter cross feed, it makes it 14*6 or 84 times the pressure at the cutting tool than on the handle. plunging in .040 requires , say 4 pounds of pressure on the handle, which is almost 350 lbs force on the cutting tool.

What you need to primarily worry about for accuracy is the strain, not the stress on the taper attachment. Th strain is the amount of flex in the affected member. This is why all the quality taper attachments use a dovetailed plate with a very large cross section in the direction of force. The leblond 15" for instance uses 3/4" high by 4 inches in cross section. it's run is also very short, it is about 11" long. This is because the strain in a member is proportional to x squared when it is supported on both ends. So a taper attachment 4" long wiil' have half the deflection under pressure (roughly) than a 5" one, and so on. Unfortunately, the resistance to strain by cross section is only in X so to reduce the longer one to the same deflection would require twice the width of the dovetail slide.

So having a taper attachment that is really long makes a certain kind of functional sense, but only if you don't mind it tracing a curved line!

The real value in a taper attachment is making tapers like MTx , Jacobs and the like. If you set your sights on a 6" taper attachment with a wide cross section of rigid (read heat-treated) material, you would do very well indeed.

Sorry if this all sounds very complex. I've simplified it as much as I can. (for the Mechanical Engineers out there - I know that I've made some generalizations and shortcuts, please be easy on me)

For a 12 inch lathe, I'd choose a design that uses a pre-ground piece of O1 tool steel 3/4 X 3" with a run of 6" between supports. I'd have it heat treated, and then dress it with 800 grit wet/dry. With a beefy follower, you should get very accurate tapers doing .005 cuts. (Remember your roughing cuts can be curved, you are interested in accurate finishing cuts).

I hope this long-winded answer helps your design!
 

mickri

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#7
Thanks Dabbler that helps. I am not an engineer. However I have been very interested in free standing masts for sailboats for decades. In the sizes of boats that interest me a free standing mast is prohibitively expensive both to have professionally designed and made. So I embarked on a self study many years ago to learn how to design free standing masts. Eric Sponberg one of the leading experts in the design of free standing masts over the years graciously shared his knowledge with me and kept me pointed in the right direction. I now have a massive spreadsheet that takes into consideration both the design and construction of free standing masts. As part of that spreadsheet I included stress and strain calculations for a variety of common shapes including flat and round shapes. Input the dimensions and the load and the spreadsheet kicks out the bending stress and the deflection.
I put in some trial numbers and it was readily apparent that a flat solid bar has far less deflection than a round tube. In thinking about the direction of the forces involved it seems to me that the cutting force is not pressing on the bar but rather primarily pulling the bar towards the lathe. If I am correct in my thinking than the back edge of the bar is the most critical edge and the front edge is mostly just along for the ride.
The long bar would be for turning non critical tapers. Probably would not matter if the taper was either a little convex or concave. The short bar would be for critical tapers like you mentioned. I inputted 350 psi load on a 12" x 3 x .375 flat bar and my spreadsheet kicked out a deflection of .0005. Changing to a 6" x 3 x .75 flat bar gave a deflection of .00003. A 6 x 3 x .375 gave a deflection of .00006. I could not even measure deflections this small.

I want to thank you guys. I think that my questions have been answered.
 
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