Author Topic: Pennsylvania A3 Switcher (Kozo)  (Read 227102 times)

Offline cnr6400

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3045 on: May 25, 2023, 06:50:54 PM »
Hi Kim, forces on the stock can be very high when using form tools. Couple of things I have found help with shapes like you are making - relieve as much stock as you can before using the form tool, and take the lightest possible cuts with the form tool. A 90 deg point chamfer tool can easily knock corners off and dig out the stock between ball and base, to rough out the contour. The second thing is to start with longer stock and leave a nub past the ball at the tailstock end with a centredrill hole for use with a tailstock centre. By using a tailstock centre for the roughing and form tool work, leaving a small diameter of stock between ball and tailstock support, forces are spread between the support and the work, reducing the likelihood of a bend / break / launch event. It does waste a bit of stock, and means sawing off the waste end and filing the end of the ball, but overall it will make like easier. If you get all the handrail supports you need without any breakages or incidents, you are better off than you would be if you had to use more stock and do all the ops to completely remake a bent or broken one.

On form tools, I usually try to make them narrower than the feature to reduce the forces. For different balls and semi spherical shapes I have made the tools about 60-70 degrees of the ball curvature, not 150-160 degrees, and 1 made 1 left tool and a right tool. Two tools, with the ball cut same distance from the end and centres at same distance to the side means you can make one part of the ball with tool 1, making note of the feed depth. Then change to tool 2 for the other half without moving the carriage along the bed, and feed to same depth as tool 1. If you didn't move anything there should be no mismatch line or a very small mismatch line on the ball. Just food for thought.  :cheers:
"I've cut that stock three times, and it's still too short!"

Online Kim

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3046 on: May 25, 2023, 10:19:57 PM »
Thanks Michael!

That looks pretty similar to what I have here.  Unfortunately, I don't think it's the tool geometry that's the limiting factor here.  I think it's the little 0.099" connecting pin.  This becomes more clear after my next post (the one I'm about to make :)).

Kim

Online Kim

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3047 on: May 25, 2023, 10:23:21 PM »
Well, turns out I did get a little shop time today! So I started by inspecting the form tool to see if resharpening might indeed help.  As it turns out, yes, I thought it might!  There had clearly been some rubbing happening on the front part of the tool where it sticks out I now realize I cut that straight with NO relief.  So I took it to the grinder and made some relief there, and made sure I had relief in all the other awkward places being very careful not to change the actual tool profile (much anyway).  Then I repositioned things so I could grind nice flat across the top of the tool to make sure it was very sharp!

After this, I used a fine diamond stone to clean it up and put a nice hone on it.

Then I mounted it back in the lathe, verified the center height, and gave it a try.

I was initially optimistic, I could see little shavings coming off where the tool was touching.  But then I heard a thunk and the part stopped turning. The chuck and mandrel were still turning, but the part wasnt. :(


Guess it was just a little much for that 3-48 threaded portion to handle.  Id considered ways to try and do the form cutting while still on the parent stock, but then I couldnt figure out how to hold it to turn down the threaded portion.

So now Im thinking on how to go about this again.  I may do a little step-off table to approximate the sphere then just use files & sandpaper to clean it up.  Ive done this on a large number of parts that had to be similar and it worked out pretty well.  I turned 28 identical cannons for a model ship one time!  Now that was an interesting exercise in wishing I had a CNC lathe! :)

Ill try another approach tomorrow.
Kim

Offline Firebird

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3048 on: May 26, 2023, 08:06:39 AM »
Hi Kim

I have used this method. They can be held in a collet, drilled and tapped etc. Then silver soldered on





Cheers

Rich

Offline Zephyrin

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3049 on: May 26, 2023, 08:07:40 AM »
I did my own handrail supports for a gauge 1 loco exactly as you proceed, but from a 3 mm brass rod, much softer as stainless steel !
the form tool clearance can be easily made with these small coned diamond tool bits for the Dremel. It seemed that the tool required front and side (peripheral ?)clearance to prevent the head from being ripped off at the end of the cut! 

https://photos.app.goo.gl/ShVffCBoyKH7R5D29


Offline Admiral_dk

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3050 on: May 26, 2023, 09:58:12 AM »
I think that I would start with the 'New Tool' you just finished and use it as the first opperation, followed by drilling (Centredrill/Endmill), threading and then the rest of the finishing details.
I know that this requires that you change back and forth between the Lathe and the Mill.
In the Mill, I would use a Collet holder + and Endstop, to ensure placing the holes @ the same distance from the Top.

Sorry for not commenting much (here and elsewhere) but I do enjoy your Build more and more Kim  :praise2:     :cheers:

Per

Offline Florian Eberhard

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3051 on: May 26, 2023, 10:03:58 AM »
Hi Kim

Form tools can be quite tricky to work with...
Why dont you machine the ball first, second the threaded side and in the end the section between ball head and threads?
This way you have the part well supported for the operation with the highest cutting forces.

You could even split the form tool into two separate tools, one for the left part of the ball and one for the right part if it still does not work.

A verd big influnce is also material choice - is that free cutting steel you are using? Regular mild steel really tends to climb on a tool if the rigidity is too small for a cut.

Good luck on your next try 😉

Cheers Florian



Online Kim

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3052 on: May 26, 2023, 06:13:14 PM »
Thanks Rich, Zephyrin, Per, and Florian,
Lots of really good creative ideas.  Great food for thought as I consider how to move forward on this.

I think I may give what Per and Florian suggest here a try.  I may see if I can use my form tool on the stainless rod first, and see if it will even cut the shape.   If it does, then I will come up with an order of ops that will allow me to machine the rest of the part AFTER the ball is formed.

BTW, Florian, I'm using 303 Stainless for these parts.

Kim

Offline Dave Otto

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3053 on: May 26, 2023, 11:12:28 PM »
Hi Kim

Sorry I missed the part where you said that you heat treated the tool, I thought that I had read through your post carefully but I guess not careful enough. :facepalm:
I was thinking along the same lines as Florian and Per, but wasn't able to get a post sent last night. It has been a busy couple of days.

Attached is a photo of little form tool that I made to machine the detail on the spreader bars of a PMR arbor press stand, the stock is 1/8" in diameter. Thought you might enjoy seeing it.

Dave

Online Kim

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3054 on: May 27, 2023, 04:57:31 AM »
Thanks Dave!  That's pretty nice looking.

I think I've got it to work now, thanks to everyone's help! (spoiler alert)
Kim

Online Kim

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3055 on: May 27, 2023, 05:11:07 AM »
So first thing today, I did a quick test just to see if the form tool could cut without digging in too badly. I tried it on the end of the 1/4 stock and it seemed to work fine.  I didnt go all the way, just got it cutting some to prove to myself that it was possible.

Then I took the end of the rod down to 0.200" for the largest diameter, then 3/16 for the ball end part of the bracket.  I was going to try forming the part in the other direction this time so I had more meat there while using the form tool.


Now, to apply the form tool to the end where I want the ball.  However, I didn't get very far with the form tool before this happened.  It is actually just barely hanging there it bent and snapped.


I decided to give it one more try before I gave up on the form tool. I wanted to try with the bare minimum of stock sticking out of the collet (rather than almost 1 as above).  This actually worked!  :cartwheel: I was running the lathe at 480 RPM and that seemed to do well.  I advanced the form tool VERY slowly, but it seemed to work just fine.


With the ball formed, I pulled out the rest of the required length for the 3-48 stud, turned the top part down to 3/16 to the base, and 0.200 for the rest of the length.  I also put a quick mark in to indicate the bottom of the base though I dont think Ill do that for the rest of them.  I thought it would help me as a registration mark, but I didn't end up needing it.


Then I changed to a 3/16 collet and put the ball end in the collet all the way up to the base ridge.  I hoped that the ball plus the rest of the 3/16 section would provide enough to hold the part firmly, and it certainly seemed to.


Held in this orientation, I turned the stud down to 0.099 and cut the 3-48 threads.


Now, going back to the 3-48 mandrel that Id made previously, I set the compound slide to 6 degrees, and using a round-nosed tool I tapered the stand.  I had to use a little file to get the top edge of the taper right next to the ball, but this seemed to work pretty well.


And here are all my attempts at the handrail bracket. The three on the left are the blanks for my original process that I have now abandoned. The top right is the one that broke, the next one down on the right I broke today, and the bottom one Im claiming is a victory!


In this close up I can see that I need a little more file work at the base of the bracket, but it looks pretty good.  And I still need to drill the hole. But Ive got a plan for that :)

I think this will work. Just have to make two more of them!
Kim

Offline Firebird

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3056 on: May 27, 2023, 08:06:57 AM »
 :ThumbsUp: :ThumbsUp: :ThumbsUp:

Offline Florian Eberhard

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3057 on: May 27, 2023, 08:31:26 AM »
Hey Kim

Very Nice!

Now for drilling the balls, i found it useful to mill a very tiny flat on top of the ball - this shows the center of the ball pretty well.

Florian

Offline cnr6400

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3058 on: May 27, 2023, 12:01:09 PM »
 :ThumbsUp: :ThumbsUp: :ThumbsUp: :popcorn: :popcorn: :popcorn:

The bottom one's looking great Kim. Well done!  :cheers:
"I've cut that stock three times, and it's still too short!"

Offline Admiral_dk

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Re: Pennsylvania A3 Switcher (Kozo)
« Reply #3059 on: May 27, 2023, 01:00:48 PM »
Good to see that you 'Cracked this one too' found the solution  :ThumbsUp:

Per        :cheers:

 

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