Correcting spindle taper

Hello,

It seems my Hardinge Splitbed spindle taper is a bit barrelled 2/3rds from the end. I'll hard turn it in place but am unsure how best to set the top slide to the correct angle. The obvious fixation is to try to mimic the best bits of what's there as made by Hardinge, but surely 4 degrees is 4 degrees and bringing back to a specific measurement is better. It seems I can't accurately judge which part of the taper is correct beyond trying to stick a vernier protractor on it.

I have tried setting it to make a backplate and found the spindle taper is getting on for half a thou high in that area. I'm not sure how much error is allowable but know it's way less than this. To set the top slide to true it up I'm thinking that a DTI can be used either..

1) directly on the end of the spindle (ignoring the middle reading),

2) I could turn a straight bar to indicate off (with some messing to remove any taper in the top slide when turning the bar) in brass so it's an easy smooth surface, let everything even in temperature, then set the slide by counting the travel of the DTI along a set travel with the top slide (I don't presently have a 4 degree angle gauge but wouldn't that just add more room for error?).

While the 2nd option seems to have the most potential accuracy in reaching the official 4 degrees I'm concerned that there's more to mess up / accurately measuring top slide travel etc.

Just looking to get some feedback and reassurance to go for option 2 - and I'm not certain what the standard tolerance is on this rate of taper from the factory.

Thank you,

Jonathan

Its a job for a pro, you have only 1 chance to get it right, Have any real good machinest friends?...Phil

Johnathan,
To set your top slide to exactly 4 degrees will be the easy part. Hard turning the taper may prove to be more difficult. To set an exact angle using a Dial Indicator true a smooth bar between centers and set the top slide(compound) at close to 4 degrees. Place the DTI tip on the stock and run the cross slide in .100 or so and set zero. Set zero on top slide as well making sure you have 1 inch of travel. Now when you move 1.0000 on the slide the DTI will move the SINE of the angle. Sine of 4 is .06976. Tap the slide, reset the zeros and do it again. If you lack that much travel you can move .5000 and read .03488.
When you got the top slide set exact now make the same movement on the spindle and take some readings at .05 intervals rotate the spindle. That should tell the whole story. Your slides must be good here or your the results could be not so good.
spaeth

You really need to be sure the taper needs fixing before you mess with it ! And you need to know you can actually improve it by machining.

Hard turning will require considerable cutting force (Hardinge spindles are very hard) and could easily force the top-slide out of alignment - risking ****ing up the whole lathe. A tool post grinder would be a far better option IMHO.

Billtodd said:

You really need to be sure the taper needs fixing before you mess with it ! And you need to know you can actually improve it by machining.

Hard turning will require considerable cutting force (Hardinge spindles are very hard) and could easily force the top-slide out of alignment - risking ****ing up the whole lathe. A tool post grinder would be a far better option IMHO.

Click to expand...

And even grinding needs a pro or some talent..., it is tough to grind anything you that have only one shot at if one is not a grinder hand... had some time grinding simple things.

I've had some time to think about it, and to have another look at the spindle. Thank you for the points and suggestions.

It's not really an ideal situation but the taper is out enough that I can see and feel a straightedge rocking on it when in line. Realistically I can swap the entire headstock if needed - chose this one as the brake mechanism is intact and I didn't really feel like swapping it over, didnt inspect the nose closely. This spindle has the worst nose.

The compound slide looks a bit rough but the ways are alright. Billtodd's comment about the slide moving from the cutting force is true, I'm going to check the mounting to look for play on the bed and room for any shimming / adjustment to lessen the sources of bad stuff happening. I'll keep a test bar in the headstock (in a collet) as a reference in case I need to check the angle of the slide. Shall also try to get a bit of practice on some hardened drill rod to get a feel for the inserts.

I'm going to stick with a test bar in just the headstock because I don't trust the tailstock alignment (to that extent) on a 1930's lathe. Will take a couple of light passes on some 7/8" brass, making sure there's no taper in the process - brass should take any skill out of getting a nice surface to indicate off.

A toolpost grinder would be great but I really dislike abrasives around machines. It's not an ideal situation but should be alright.. or I'll have swap the spindle / headstock.

Jonathan