Turning thin rod with accuracy

[tkitta]

Hello,

I have been making a small part for the hinges of the garage door - the hinge nails are 12cm long with s 0.5 cm "head" and 11.5cm "tail". The head is 11mm in diameter and the "tail" is 7mm in diameter.

I started with a 1/2 inch rod and quickly machined off the excess material. No matter how hard I tried the "tail" next to the live centre is 7mm but the "tail" near the "head" is around 7.1mm.

I assume the rod is simply bending and producing such a shape? I also got it "fatter" in the middle.

My lathe is accurate to 1/1000 of an inch which is 0.025mm - but my error is 4x that.

My next attempt to get it more accurate is to machine only 6cm of the rod to 7mm, then flip it the other way and do the remaining 5.5 with the 7mm in the chuck -- my first attempt was with the stock 1/2 inch rod that was over 10inches long with all the excess in the chuck. I tried to take light cuts of just around 0.005 inch - around 0.1mm.

I am not sure whatever I could fit any steady rest somewhere in there - the part is only 12cm - less than 5inch.

Suggestions? Or this is the best I can get? I have 12 x 24 gear-head lathe.

 

[ricsmall]

If you have access to a follow rest that would do the trick.

Richard

 

[tkitta]

OK, I try it out with the follow rest as well - see whatever that will work.

 

[Karl_T]

learn to think in diameters. divide the length by the diameter. Over 6 diameters you're going to have significant part flex. Light finish cuts help. If you got the lathe to do it, very high rpm and fine feed with a honed HSS tool makes a huge difference. Nothing wrong with using a file to true it up.

You're in the region where the work needs to be supported by a follow rest or tailstock.

karl

 

[mikey]

The fatter mid-section is a dead give away that the part is deflecting away from the tool so I agree that you need a follow rest. A sharp knife tool with a very small nose radius will also help to take whisper-thin cuts off of thin stock.

 

[f350ca]

Hi, if the dia at the tailstock is smaller than at the headstock your tailstock is probably offset toward the front of the lathe. The larger dia in the middle is due to flex.
To help with the flex you could try a vertical shear tool for your final cut, I've had good luck machining long slender parts with them and as an added bonus you get a great finish.
A quick search should show what they look like
Good luck
Greg

 

[Karl_T]

...
To help with the flex you could try a vertical shear tool for your final cut, I've had good luck machining long slender parts with them and as an added bonus you get a great finish.
A quick search should show what they look like
Good luck
Greg

HEY, you just taught the old dog a new trick. Don't happen often. I had to watch a you tube of a shear tool in action. I can see that would work great for shaving a thou or two off for finishing.

Thanx

Karl

 

[f350ca]

Karl, they seam to cut very efficiently, as in they don't push the work away from the cutter.
Had to machine this long piece of 2 inch shed 80 pipe, it measured only 1 thou over in the middle, can't imagine getting close to that with a standard cutting tool.



And the finish is great, I was pushing the feed here as I was getting bored on the long cut.



Found out about them on a post about shaper cutters some where.

Greg

 

[savarin]

My longest so far is 6" of 5mm.
I used the fastest speed the lathe would go and got very acceptable results and finish.
I second the shear bit, amazing finish.

 

[epanzella]

Do half at a time like you already mentioned. Cut the span in half and rigidity goes up by a factor of 8X.