I would play about with a bit of scrap to get it right then not alter the topslide until you have made the actual part. With a bit of blueing you should be able to get contact just about right.

Any polishing near the open end would risk making the taper bellmouthed.

Tangent of 8° is 0.1405, sine is 0.1392. Set up a decent dial gauge in the tool post running against a really straight bar and it should be no problem to get sufficiently close that the direction of error is moot with an inch or, better, 2 inches, of travel to generate the appropriate movement on the gauge. Tangent for moving along the bed as with a proper taper turning attachment. Sine if swivelling the topside.

Metric mavens could use 25 mm travel. Look for 3.51 mm gauge deflection with a taper turning attachment and 3.48 mm for swivelling the topside.

As I have proper taper turning attachment and a bed stop I use a gauge block from an old, no longer wrings together, set. Not too hard to make a suitable block of your own. Using the dials to measure travel works but its easy to make mistake and have to start over.

If swivelling the topside make use some sort of opposing screws pusher to give controlled movement. Use nice fine threads. 40 tip ME is good if you have it. Pushing and tapping tends to deteriorate into a long sequence of "nearly there, "one more… rats too far….. back a bit….. rats too far" very naughty words.

Clive.

With patience you should be able to get a result close enough that you won't know if its over or under. (If I can anyone can…)

As suggested use a collet with some blue/felt pen as a gauge. Don't push it in as you don't want to close it up at all.

Neil

Trial and error on a scrap piece worked for me. If you use a collet as a gauge it is a good idea to insert a drill bit of the appropriate size to prevent the splits closing and distorting.

Here is my GHT pusher:

Also in the photo is a Martin Cleeve-inspired bushing type steady in use with an ML7 cross slide co-opted as a top slide for its extra travel.

I set my top slide to the graduations and unbelievably it was spot on, wouldn't like to bet on getting it right first time again though.

Mike

One way to set the taper exactly: Chuck up a piece of bar. Turn the OD so a collet will just slip on (a little force required). Slip the collet over the bar. Set an indicator in the toolpost. Adjust the angle of the topslide so the indicator stays on zero as you sweep the length of the taper. The indicator point needs to be at the same height as the spindle center or error will result.

So far, I've been lucky, and got it pretty accurate. But I finish off by wrapping a strip of emery round a collet, oiling it, and then GENTLY pushing it into the slowly rotating workpiece.

Push hard and the emery and collet will be snatched out of your hand!

You can determine the depth to which you bore the taper by measuring an unloaded collet at the mouth of a known good taper bore. If you are going to lap using a collet, boring should stop a thou (0.025mm) or so undersize, and let the emery do the rest.

DON'T forget to clean everything afterwards, to remove the abrasive dust.

Howard

A good way to use a collet as a gauge is to find a bar of silver steel, or a cutter, which is a snug fit in a collet. Not tight, just firm enough not to slide out when you tip the axis 20 degrees. Offer up the collet with the steel/tool in place and you can be sure it won't compress to give you a false reading.

Hope this helps.

Tim