Models for grinding HSS Lathe Tools

[DavidR8]

Never turned Delrin before but I have turned some nylon. Super sharp tool just like Mikey shows worked a treat.

 

[fitterman1]

Plus one for the super sharp tool.
Positive rake is a must from my experience.
Your clearance will be determined by shaft diameter and how much the delrin swells once submerged in water.
If i remember right, nylon swells quite a bit. Vesconite is good for potable water, it has a very low absorption rate. Couldn't tell you how it performs in saltwater.
A thou per inch of diameter is the general rule of clearance. Adjust it for swell and you can't go wrong.
Set your feed to your speed to achieve the smoothest finish.

 

[ddickey]

Usually you'd want to finish the ID then finish the OD as the OD can move when the ID is done. This is in general not Delrin specific.

 

[mcdanlj]

Going to be machining some bearings out of delrin for a friend. The bearing are around 2.375 OD and 1.911 ID. 1.25" long. I will be rechecking all of my measurements this weekend. The bearings are for a rudder shaft on a Santa Cruz 27 sailboat. I have never worked with delrin before.

I will be starting with 2.5 OD 4" long rod held in a 3 jaw chuck. Plan to turn the OD first and then bore the ID. I could do it the other way too. ID first then OD. Makes no difference to me. I would like to bore all the way through. That way I can test fit the bearing on the shaft without removing the bearings from the chuck

Looking for suggestions on clearance and rake angles and speeds and feeds.

I haven't done precision work in delrin, so my limited experience that it's been forgiving of rake angle may not be highly applicable. I've used a sharp flat turning tool (for brass) and polished aluminum inserts both; even with a sharp, high rake edge it didn't seem to grab and pull, but it also turned nicely with the flat tool. Given that it will swell in water, precision probably is not super relevant for you either, but you'll need clearance. What's the diameter of the rudder shaft?

Might want to ask your friend about instead using UHMWPE (swells in hydrocarbons but not water) or Vesconite (especially if the sailboat has a reserve engine and might have gas/diesel/oil near the stern) instead. @fitterman1 wasn't sure about saltwater, but it's OK: https://www.vesconite.com/marine/

 

[mickri]

This sailboat is over 40 years old. The Santa Cruz 27 was the first pocket rocket downwind flyer. The marketing slogan was "Fast is Fun." The original ULDB. Light weight, no engine and minimal accommodation down below. It was the fastest 27' sailboat for decades.

I measured the rudder shaft at 1.911" I am going to measure it again this weekend because 1.911" is not close any standard size stainless steel round bar. And the shaft has not been machined. I think that my measurement is off by .025 which would make the shaft close to 1 15/16." The shaft is polished smooth where the bearings rode against the shaft. You want absolutely no play in the rudder. I plan to machine the ID to a tight slip fit. Same for the OD.

We looked into UHMWPE. The consensus was that it is difficult to machine to close tolerances because of thermal expansion. My friend wants delrin. That is what has been recommended to him by shipyards that have replaced these bearings.

Thanks for the suggestion to do the ID first.

 

[mickri]

My friend measured the rudder shaft this afternoon and came up with the same basic numbers that I did. He measured the shaft at 1.910."

 

[fitterman1]

Is that the diameter of the unworn shaft or the worn part where the bushes run?

 

[mickri]

1.911" is the diameter of the unworn shaft. I measured the shaft at the bottom bearing at 1.911 fore and aft and 1.906 athwartships. The shaft at the top bearing had uneven wear. From memory the shaft at the top bearing was 1.905/1.903 or maybe 1.901. The unworn shaft has to fit through the bottom bearing. The rudder is inserted into the boat from the bottom. The top bearing can be made to fit where it goes on the shaft. I will have the rudder so I can test fit the top bearing on the top of the shaft. Part it off and then fit the bottom bearing to the unworn shaft.

 

[fitterman1]

Some pictures would help.
Is the shaft removable from the rudder?
If so, you can replace it with a larger one and clean up the old bushes to suit.
Can you skim the whole shaft and then make new bushes?
It sounds like the top shaft journal can be machined down to clean up and a new bush made easily.
I'm trying to come to grips with the bottom journal, as long as you can make a new bush and slide it down to its journal with minimal interference and the correct clearance you should be fine.
You state it is 6-8 thou undersize in the middle of the journal but ok at each end which I find odd.
In this situation I would expect the journal to wear eccentrically so the wear is greater on one side. but I could be wrong.

 

[mickri]

The old bearing were destroyed in removing them. Had to cut them apart. The rudder shaft is not removable from the rudder. There is no way to machine the shaft.

You have misunderstood my nomenclature. Fore and aft is boat talk for front to back or bow to stern. Athwartships is side to side. I don't know how else to explain it. The wear is eccentric. There is more wear side to side then front to back.