Lathe turning unwanted taper

[Hackman]

I was trying to turn some 1/2" 6061 aluminum rod down to 12mm on a 7x10 mini lathe, but it's not coming out the way I expected. i have about 1" of the rod in a 3 jaw chuck and 2" of the rod sticking out which I am turning down to 12mm.

I started turning and measuring the resulting diameter with a micrometer and noticed that the diameter closest to the chuck is narrower then the diameter of the end furthest from the chuck by about 0.001". I'm using the power feed and not touching the compound so the angle of the compound doesn't come into play.

Anyone have and ideas on what could be causing the problem and how to check for it? I don't have much experience with the lathe so I'm kind of at a loss.

Thanks!

[Acme Man]

Is the tool on center and what is your spindle speed?

[Oldmanandhistoy]

Are you doing another/other cut at the same depth to remove the effect of deflection caused by the tool?

John

[Geof]

Originally Posted by Hackman ... 2" of the rod sticking out which I am turning down to 12mm.....the diameter closest to the chuck is narrower then the diameter of the end furthest from the chuck by about 0.001".....

0.001" on the diameter is 0.0005" on the radius. Over 2 inches of length on 12mm diameter 0.0005" deflection is to be expected. I would be surprised if you get much better than this even with the sharpest tool you can find. It is possible to reduce the deflection my going to a tool with a very small nose radius and taking the feed very slow to maintain a decent surface finish.

[Hackman]

Is the tool on center and what is your spindle speed?

I'm turning at around 750 to 800 RPM, the speed knob is at the 12 o'clock position on the mini-lathe in low gear. I'm using a 5/16" tool bit which is on center.

Are you doing another/other cut at the same depth to remove the effect of deflection caused by the tool?

I don't think i did. I got kind of discouraged when I figured out the taper was happening and decided to call it quits for the day.

It is possible to reduce the deflection by going to a tool with a very small nose radius and taking the feed very slow to maintain a decent surface finish.

I will try a tool with a smaller nose radius. Good to know about the feed setting in the future. Right now I'm already running the slowest gear combination, but I'm trying to CNC this lathe so I can run slower in the future.

Can i compensate for the deflection by turning the piece twice without changing the cutting tool's position? I'm trying to cut a 2" in length 12mm shaft with -0" + 0.0005" tolerance. (I know, metric + english combination = bad). I'm not sure what the mini lathe is capable of with the stock 3 jaw chuck. Is this pushing it with the mini lathe?

Is the deflection caused by the tool bending or the workpiece bending? I'm trying to picture in my head what is going on.

[Geof]

Originally Posted by Hackman .... Is this pushing it with the mini lathe? Is the deflection caused by the tool bending or the workpiece bending? I'm trying to picture in my head what is going on.

It is not so much pushing the mini lathe but pushing the workpiece, quite literally .

The tool has to exert a force on the work in order to cut; it is impossible to do otherwise. If you want to cut something you have to exert a force. The force needed to cause the tool to cut is enough to deflect the small diameter, not very stiff metal, you are cutting especially over the length you are dealing with.

If it is imperative you have it parallel to within better tha .0005 on the radius turn it between centers. But then of course you may need to align your tailstock center to get this precision.

A perfectly legimiate alternative is to carefully polish the taper out with some very fine emery paper and delicate fingers.

[acondit]

Originally Posted by Hackman I was trying to turn some 1/2" 6061 aluminum rod down to 12mm on a 7x10 mini lathe, but it's not coming out the way I expected. i have about 1" of the rod in a 3 jaw chuck and 2" of the rod sticking out which I am turning down to 12mm. I started turning and measuring the resulting diameter with a micrometer and noticed that the diameter closest to the chuck is narrower then the diameter of the end furthest from the chuck by about 0.001". I'm using the power feed and not touching the compound so the angle of the compound doesn't come into play. Anyone have and ideas on what could be causing the problem and how to check for it? I don't have much experience with the lathe so I'm kind of at a loss. Thanks!

It doesn't sound like deflection to me. It is smaller close to the chuck than out at the end. Wouldn't it be the other way around if it was deflection?

I would chuck of a piece of drill rod and mount a dial indicator on the tool post and run it along the length by moving the carriage to see if the headstock is aligned to the ways (for a start).

Alan

[Geof]

Originally Posted by acondit It doesn't sound like deflection to me. It is smaller close to the chuck than out at the end. Wouldn't it be the other way around if it was deflection?.... Alan

No, the end deflects away from the tool; in other words the centerline is further away from the cutter so the size is larger further away from the chuck.

[acondit]

Geof,

OK, I understand now. I just wasn't thinking clearly.

Thanks,
Alan

[Splint]

If the lathe has a gap bed make sure the gap peice is seated properly. There may be swarf or a bur on the casting which is preventing the bed from sitting correcly in position, as the saddle of the lathe moves across the section it may be moving away from a level motion resulting in the a taper from the tool rising or lowering as it travels.

Cheers
Splint

[Hackman]

Thanks Geof. I think I understand what the deflection is now. I think I will try turning between two centers, but first I need to try to align the tailstock. I will do some reading on how to align the tailstock and give it a go.

If the lathe has a gap bed make sure the gap peice is seated properly.

Nope, the 7x10 isn't a gap bed lathe so no problem there. Haha, but after reading about gap bed lathes to find out what they were, I've got crazy ideas in my head about bandsawing parts of the 7x10 off to make a gap bed lathe.

[Geof]

Originally Posted by Hackman .....I've got crazy ideas in my head about bandsawing parts of the 7x10 off to make a gap bed lathe.

I hope you are joking. No insult intended but this does qualify as a crazy idea.

When you cut significant chunks out of something that is cast you are going to relieve stresses in the metal. There is a chance you would finish up with the ways and the surface the head is bolted to no longer co-planar or parallel.

Gap bed machines have the bed reinforced around the gap and as far as I know the final finishing on the critical surfaces is done with the gap installed so that everything is true.