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[4]

First project-sheave

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JohnCT

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#1
Thanks to Dave, I have metal to make a replacement sheave for my Snipe sailboat.

Dimensions are 31mm dia, 7.8mm wide, 6.3mm hole and I don't have any way to measure the radius for the groove, it is rounded for wire rope.

The original is bronze and probably original from 1985, there was enough corrosion around the sheave to prevent from turning, and a deep grove got cut on one side.
Pictures to follow
 

westsailpat

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#2
Hi John , last time I dealt with a wire sheave it had a "V" type groove with the radius for the wire rope at the bottom . I would just measure the wire rope and then make the groove a little wider so that there is no binding . Also if you are not aware of this place , I find it very useful for making blocks and stuff . http://www.duckworksbbs.com/category-s/328.htm
 

JohnCT

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#3
Thanks for the info, I tried doing a little on it last night and I'm not happy with the results, I'll try to post pictures later, that's not working very either right now
 

kd4gij

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#5
What material are you using? What is the diameter of the wire rope?
 

JohnCT

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#6
aluminum, and it's closer to wire then rope, 1/8" maybe, didn't measure it.

I'm getting a lot of chatter marks and the finish sucks compared to last aluminum parts I turned
 

kd4gij

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#7
Ok. How are you holding the work piece?

Sent from my SM-G530T using Tapatalk
 

Dave Paine

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#9
There is a lot of contact area in the sheave profile. Your chatter may be due to the amount of contact area with the tool.

I would use a parting tool to make the groove in the middle, then a left and right hand tool to create the angle on the sides.
 

JohnCT

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#10
Thanks, I'll see what I can come up with. My overall finish is really poor, even on the facing and sizing cuts.

I'm also running a lot slower then I did when I made the spinnaker pole parts, I'm thinking that might have something to do with it also.

It's a new skill, will take some time to learn
 

JohnCT

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#11
Tried upping the speed a little, different cutter, different geometry, still getting a really ugly finish
 

francist

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#12
So what's changed? Obviously something has, you're getting poor results where you know you've done better before. What's different? Work through everything step by step.

Is the metal the same, is it held firmly in the chuck (sometimes bell mouth chuck jaws can be deceiving), is the tool post solid with no crud underneath, is the cutter on centre height, is the stick-out on the tool minimal, is the grind on the cutter good, is there a built-up edge on the tool tip, is your rpm/turning speed good (why did you change from faster before to slower now?), what is different that's not present now?

You got it once, and it could not have been all luck, so something has changed. Find it.

-frank
 

JohnCT

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#13
well pretty much everything has changed, the machine has been cleaned, oiled, mounted to a stand, belt replaced, other then looking at pictures, I don't know if my cutters are properly ground. I had it slowed way down to work some steel, that wasn't going as well as I would have liked either.

Last part I turned, the lathe was still on the transport dolly in the hallway, I ran a cord to it, stuck a piece in an went at it, so yeah, beginners luck.
 

Dave Paine

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#14
You have a few pieces, it is possible that the metal is contributing to the bad finish. Try mounting another piece in the lathe and take a light cut or face the end and compare.
 

JohnCT

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#15
Well it's functional, but 1.5 mm under what I wanted for finish thickness. I'll try another one IMG_20180429_130717.jpg IMG_20180429_130841.jpg
 

Dave Paine

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#16
Looks decent to me. Good practice and back-up if you make a better one on the next attempt.
 

BtoVin83

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#17
Like my dad always said when us kids screwed up "oh well you needed the practice"
 

whitmore

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#18
Thanks, I'll see what I can come up with. My overall finish is really poor, even on the facing and sizing cuts.
Actually, that's not too bad; the wire rope is supposed to turn the sheave, after all, it doesn't hurt
for the finish to have a little 'tooth'. It's the center pivot that has to run smooth.
A 1/8" wire rope has 1/8" circular diameter, but when bent around the sheave takes on
an oval (same area, different shape) as it slightly flattens against the sheave. So, the bottom
of the cut will be maybe 0.160" diamater, to allow the wire rope to not bind...
and a 0.160" wide plunge cut with a form tool is unlikely to leave a perfect finish, the chip
clearance is slightly problematic.
 

Dave Paine

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#20
Glad that round 2 worked. The weather may even warm up soon so you can get the boat on the water.
 

JohnCT

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#21
thanks
45 for a high today, 75 tomorrow, New England where the weather forecast changes every 10 minutes....
 
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