Surface finish banding

[DKGrimm]

Just a curious response from a drop-in to this string: has anyone though about the possibility of a moiré-like pattern -- a beat frequency -- caused by a nonintegral ratio between number of steps in each axis?

 

[homemachinist]

Straightening long slender shafts is a tricky business - I spent years in the shockabsorber business as well as using the rod straightening facilities for straightening (lightly) bent motorcycle fork legs bent in racing mishaps. (don't try fixing badly bent fork legs.)
Using a hydraulic bench or arbor press can only get you so far - you reach a point where it will flex and accomplish nothing and then going just a couple of thou further will bend it the other way - often worse than it was.
Ultimately you have to hit it with a soft hammer or lead thumper - use aluminum supports or covers to strike points if using a steel hammer.
Start over the full length - then half and half - go back to full length - repeat until you get 0.001" TIR
Easy if you have the skill - practice on a similar diameter round barstock first just to get a feel for how hard you need to smite it.

Regards, Ken

Thanks Ken. I'll ask around, but in all likelihood (if it's an hour or more of work) the cost to straighten the leadscrew will be more than what I've paid for a replacement ($100 AUD). It's funny 0.2mm run out doesn't sound like a lot, I expected the saddle gibs to be rigid enough to prevent any lateral movement.

 

[homemachinist]

A quick update for posterity:

1. The whiplash or error is due to a combination of bent leadscrew and incorrect alignment of the bearing block at the far right side. I've adjusted the bearing block to improve the whiplash, also still waiting on the leadscrew replacement. Meanwhile I've made some modifications to make make this much better.
2. I've replaced the cast iron saddle gibs on the front with brass gibs that snug up the saddle in 2 directions, from bottom to top and from front to back. This has helped greatly in reducing the errors and have nearly eliminated the banding issue.

Here's an example part turned using canned radius turning cycles on my controller, not perfect but I'm happy with it for the moment.

 

[Steamchick]

Looks pretty, good! Have you turned a long Calibration bar yet? Check for circularity at 4 or 5 steps along the length, and for taper.
K2

 

[homemachinist]

It's a good suggestion, will do it once I've installed the replacement leadscrew.

Cheers

 

[awake]

Just a curious response from a drop-in to this string: has anyone though about the possibility of a moiré-like pattern -- a beat frequency -- caused by a nonintegral ratio between number of steps in each axis?

I suppose that is moiré-or-less what he has been working with.

 

[homemachinist]

An update on this, got a replacement leadscrew from the manufacturer and it's got a bit of run-out or straightness issue as well. I was surprised to find out that rolled or cut leadscrews have straightness tolerance of 0.015"/ft or ~1mm/metre. Machined ones fare better at 0.2-0.4/metre but are far more expensive.

How do people normally deal with this when turning long parts with half nuts engaged ?

 

[homemachinist]

Hopefully the final update on this. I've installed the new leadscrew, the run-out or sag at the middle is about 0.07mm which is better than the original. I adjusted the half nuts to make sure there is no change or deflection when the nuts are closed. This along with the new adjustable brass gibs on the back and front of the carriage basically fixes the issue. Nice shiny round parts and no visible banding anymore

 

[ajoeiam]

Hopefully the final update on this. I've installed the new leadscrew, the run-out or sag at the middle is about 0.07mm which is better than the original. I adjusted the half nuts to make sure there is no change or deflection when the nuts are closed. This along with the new adjustable brass gibs on the back and front of the carriage basically fixes the issue. Nice shiny round parts and no visible banding anymore

Good on you!!!
Yuh stuck with it until you gotter licked!!!
Its funny how you doing this is encouragement for me to keep on keeping on!!!!

Thank you very very much for sharing!!!