I need your help to machine a taper on the lathe. Please....

rodjava · Mar 1, 2019

Hi,
I only turn my metal lathe a few times a year. So I'm really not a machinist. I was asked to turn a slight taper on metal rod. I've seen a few Youtube vids on how it can be done by turning the compound to a certain degree.

Attached is a picture of the part.

The diameter of the rod is .400 and the total length is 10 inches.
There is a 4.00 inch taper at the end.

The end is .320 and tapers to .400.

What degree would I turn the compound to machine this taper?

Thank you all in advance for you help.

Rod in SF

jspivey · Mar 1, 2019

Use some right triangle trig. Inverse tangent function of the (side opposite the angle divided by the side adjacent the angle) will give you the angle. Adjust your compound that degree off of parallel to the spindle axis.

johnoder · Mar 1, 2019

What degree would I turn the compound to machine this taper?

A little over half a degree - .57294 degrees to be close

That is the result of .040" / 4.000" = .010000 = tangent of the above angle

rodjava · Mar 1, 2019

Thank you to the membership that chimed in.
I'm old and with bad eye sight. I hope that I can dial in less than 1/2 degree on the compound.

I will give it a try and let you know how it turns out.

Rod in SF

johnoder · Mar 1, 2019

Progress checks - say when you got the little end down to .360 the cut ended at 1.94 - you would know you need a little less of an angle. At .360 little end, cut should be 2" from little end

rodjava · Mar 1, 2019

johnoder said:
Progress checks - say when you got the little end down to .360 the cut ended at 1.94 - you would know you need a little less of an angle. At .360 little end, cut should be 2" from little end

John,
Thanks,
I will keep those referenced in mind while machining.
Rod

Derek Smalls · Mar 1, 2019

Maybe I'm missing something, but the drawing you provided and the explanation of the part you gave don't seem to jive to me. You say the shaft is to be .400" diameter and 10" long with only the last 4" tapering down to .320". I take this to mean that you are saying a 6" section will be .400" diameter??? BUT, the drawing shows a 10" long shaft, .320" diameter at the small end, tapering at .020"/in. (.080" over 4") for it's entire length, making the large end approx. .520".

Either way, I think I would much rather use a taper attachment(If I have access to one) OR, turn it between centers using tail stock offset to cut such a long, shallow taper. Finnish would be much better using power feed over hand cranking the compound, and unless your compound can cut the full length in one pass, you will have a blend point of two or more passes.
Assuming you don't have a taper attachment, tail stock offset calculations are easy. Offset will be 1/2 of the taper/in. X the length of the part. So in this case, the taper is .020/in. and the length is 10".
1/2 X .020 = .010
.010 X 10 = .100
Simply adjust the tail stock .100" toward the operator and you will get the taper you want.

john.k · Mar 1, 2019

DS is quite correct......the other point is the tool must be set to cut at centre height ,or you will not get the desired result.

johnoder · Mar 2, 2019

My apologies Rod, for not noticing drawing until mentioned by DS. Adjust my comments about length of cut accordingly. Apparently it won't be .400 out 4" until you make it that way. Clearly, the 4" merely gives you a GAUGE LINE on a full length taper

GregSY · Mar 2, 2019

If the taper is along a 10" length, there's a good chance the compound won't travel that far anyway.

Terry Keeley · Mar 2, 2019

You'll never be able to accurately set your compound to a certain angle by eyeballing it, the scale is there to get you close.

Your taper is 0.010" per inch per side: .400 - .320 = 0.080 / 4 = 0.020 / 2 = 0.010 per side.

Set up a dial indicator on the side of your compound and move your carriage an inch, see how much the indicator moves. Adjust accordingly.

ps: If you have an indicator holder with long reach you can move the carriage longer and get more accurate results. If you want to be super accurate set a carriage stop and use gauge blocks to measure the distance travelled.

Hopefuldave · Mar 3, 2019

As the taper extends over 4", as far as 10", I can't see setting the compound to an angle and covering the full length, unless you have a sensibly large lathe...

Old School:
drill centres in each end of a longer (12"?) piece of material, before drilling and tapping for the 1/4-20;
lathe dog hooked in faceplate, centres in head and tail stocks;
run DTI along material to ensure it's centred, adjusting tailstock as required;
turn down to required diameter (for a continuous taper over the full 10" length = 0.520"?) over just more than 10" length (probably with light cuts and a travelling steady to prevent springing);
adjust tailstock so that over 4" you get the required taper reading on the DTI/DI (half the 80 thou" difference, so a reading of 40 thou" on the indicator or to be more accurate 100 thou" over 10") bringing the tailstock towards the tool;
perform a series of cuts (light, sharp tool), starting at the tailstock end of the work, until you have the 10" taper. A toolpost grinder would be my weapon of choice, if I had one, but there would be a lot of passes to take it down from 520 thou" to 320...

My taper attachment would do it in one go (with a couple of inches to spare), but you don't say you have one, so...

Dave H. (the other one)

Terry Keeley · Mar 3, 2019

GregSY said:
If the taper is along a 10" length, there's a good chance the compound won't travel that far anyway.

Missed that. He needs a taper attachment...

iwananew10K · Mar 3, 2019

and there is the issue of supporting a slender 10" piece...gotta work that out too.

Hopefuldave · Mar 3, 2019

iwananew10K said:
and there is the issue of supporting a slender 10" piece...gotta work that out too.

Hence my suggestion of a toolpost grinder when cutting the taper (after getting it uniform with a travelling steady first) - much lower forces involved, but going to take a week at a thou" or so per pass...

Dave H. (the other one)

Derek Smalls · Mar 3, 2019

Problem is, a follower rest does NOT work on tapering work pieces. A stationary steady rest will need to be used, probably with a lot of fiddling about to keep it near enough the tool to do any good.

Hopefuldave · Mar 3, 2019

Yep. I was suggesting the travelling steady for roughing it down to overall diameter, not cutting the taper - 10" of half-inch rod is going to bend/climb the tool with anything like a roughing cut

Dave H. (the other one)

Nmbmxer · Mar 8, 2019

I'd put it in a collet with a half inch sticking out and turn it to .330 and tap the end. Then pull out 1/2" inch, turn that to .340, supporting it with the tailstock as necessary, and repeat. Once it is all turned, use a lathe file and emory to remove the tool marks. I bet if it's shiny the overall diameter tolerance will eat any steps that might be present. You'd be done faster than setting up a taper attachment and dealing with deflection and bad finish on a ~3/8 diameter 10in long.

It's that or start with 7/16" material with a follow rest on the unturned diameter and cut it in one shot feeding the crosslide out as you move toward the headstock.

DDoug · Mar 8, 2019

What is the lathe that is intended to be used ?

jim rozen · Mar 8, 2019

Terry Keeley said:
Missed that. He needs a taper attachment...

Nope. Set the job up betwen centers, and offset the tailstock. This is the way it was done for
ages, still works fine.

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