Copying a taper - how close is close enough?

Hey all, rookie question about copying a taper. I have this crankshaft from a small motor, and the owner has found an identical one except on the new part, instead of a taper, it's a straight shaft. They'd like the taper cut into the straight-shanked one.



My basic question: how close is close enough? I have my compound indicated with a good DTI to within .01mm along the length of the 20mm taper. That end (where the live center is) is threaded, and a screw pulls this taper into the mating one. Is .01mm (4 tenths) deviation along taper that's almost an inch good enough to make a good mating taper? Since I have the part indicated on centers, my plan is just to swap the tapered part for the other and start cutting (with the apron locked).

Secondary question: how do you measure figure out the depth of the final cut? IOW, I have the taper set, but where do you put the calipers to know how far to push the cross slide for the last pass? Measuring either end of the taper seems like it could lead to error, because any lateral variation in the location you're measuring also changes the diameter you're measuring (ie, measuring the original right on the small end, but then measuring the copy just to the left of the small end will give an incorrectly large measurement). I suppose you could cut the mating taper in a scrap piece, and then measure how far it goes onto the original and new part - is there an easier way though?

Keen observers will note that my lathe has a taper attachment, so I know I shouldn't be using the compound to do this - but there's some slop in the taper attachment nut, so I'm not sure I can trust it, and this seemed faster than the testing to figure out if I really can trust the taper attachment's motion or not.

Since you are cutting between centers you can remove it and check easily enough. It is certainly one thing to get the taper angle right and another to get the size right.When you are close, and you will be closer than you think, check the fit. A little off the diameter will result in a lot more than you think in how far the taper goes in. Your taper angle is plenty good enough as the thread on the end will pull it tight and to shape. get close and finish with a file or abrasive. You will probably want a good finish anyway.

For one offs I check fit.
Almost all my work is one offs so I do lots of checking...

If you have the mating part, you can blue the new taper with magic marker, then give parts a twist to indicate fit. Creep up on final size.

Better to fix the taper attachment.

Then its relatively easy to set things up with a stop to limit the travel along the taper so it finishes in the right place.

If you have CAD it can be a great help to draw things out and apply dimensions so you can see what you are doing. I've always found that drawing and putting on dimensions is great for clarifying my thoughts on how to tackle a trick or not done it before job.

As your taper runs out at the tailstock end I'd be inclined to work towards the tailstock, especially if using the compound. I'd probably lock the apron with the tool at the crank end of the taper, use the cross-slide to bring the tool up so it touches the job and set the dial to zero. Then pull back the cross-slide and wind the top-slide forward until the tool is at the end of the job. Bring the tool up to the work using the cross slide and thats your starting point. Pull the top-slide back a bit to get a bit more than cut clearance, wind the cross-slide in by the depth of cut you plan to use and make the cut. Rinse and repeat until the cross-slide reads zero on the dial. Job done.

+1 for practice and test fitting using a suitable piece of spare material. That said when using the taper turning attachment on my lathes I just do it and expect to get entirely satisfactory results from anything short of a morse taper or similar high accuracy job. Even then its usually right.

Clive

Cutting it with the compound will work fine if you are within 0.01mm. Important: be sure that your dial indicator or test indicator touches at exactly the center height. It it is touching too high or too low then your taper will be off. A good way to line it up is to compare the ball at the end of the indicator with the tip of the center. Use a magnifying glass if needed to compare the heights.

If the axial location of the mating part is not very sensitive (say 1mm) then you can get the length right just by seeing where the taper cut ends on the cylindrical section (assuming that the cylinder section of the new and old crankshaft have the same diameters). Otherwise you are better off removing it from between centers and checking on the mating part.

After cutting it accurate to 0.01mm, a quick way to improve the fit, if you feel that that necessary, is with a bit of Timesaver fine lapping compound in a drop of oil. Just lap the taper to the matching part for a minute or two.

For rough fit, don't measure the diameter. Measure the distance where the taper transitions to cylindrical. When it comes to fit, the depth of the taper is the most important part. If you have an error in taper, you want the contact at the big end rather than the small end.

My way. Did good. No complaints from customer, but then '30 Packard parts are not laying around all over anymore

https://www.practicalmachinist.com/vb/general/setting-lathe-compound-tapers-174133/?highlight=ball



Yes, I got him to sign my usual hold harmless document

Good looking road locomotive

IMHO prussian blue is the only way to get it right

First time?
A practice piece is a smart idea

Until ya get close, chalk lines on the male taper will tell you where you are tight, then go to the bluing. Remember- changing the height of your tool tip will change the taper.

''Copying a taper - how close is close enough''

As I was taught - only blue is true.