finning ????

[d-m]

Ok so it is going on 2 (or is that 3 ) years since I started my sterling fan I do have a box of parts made. But I decided on a redo for the displacer cylinder (5th one) this time I went a different direction.I decided to make it from one billet like Bill Lindsey did see his build here. http://www.homemodelenginemachinist.com/index.php?topic=3366.0
I made the first one put it on a mandrel to cut the fins the first few were fine but as I progressed the corner edges bent toward's the chuck not a big deal as it was easy to gently pry them back into shape but in the end I wasn't happy so I scraped it and made a new blank. I used a parting blade ground to correct with and set it on center squared it to the chuck and proceed slowly and gently with each fin. I am not sure why I experienced the bending but I am willing to bet someone can help me out I am going to use a 3/8 blank HHS and regrind a cutter for the task any tips on grinding ?
Thanks Dave

 

[Blogwitch]

Dave,

I don't know how narrow your fins are, but it definitely sounds as though you are not quite square with your tool to the workpiece.

I am lucky in that I have a small surface grinder that I can knock my tooling up on. One thing I don't do for finning tools is to put any side relief on the tools at all, they are totally parallel and only cut on the tip, with top rake to suit the material being cut, as in no rake for brass, and rake for almost everything else.

Once you have spent the time getting everything spot on square and exactly on centre height, no problems should oocur.

This shows the setup I use for getting finning done.

http://madmodder.net/index.php?topic=1539.msg15474#msg15474


Bogs

 

[Longboy]

Your fins should be as wide as the parting tool cut. Your tool isn't 90 degrees to the cyl. stock as you proceed into the cut. Either the tool post is rotating clockwise to the stock or the tool bit is flexing the cyl stock. The clue is the bending of the fins toward the chuck. Start the finning farthest away from the chuck and finish closest to the chuck.

 

[d-m]

. Thanks John and Longboy
I grabbed a blank of HHS from the shop this morning before work and spent my brake time today on are bench grinder at work. I was just going to ruff it out close and take it to a friends shop after work and use his surface grinder to finish the job. As I got closer to the final size (within .010 ) I started to feel good about the way it was shaping up. I located the edge of the inside of the wheel and marked the grinder rest so I could be square, moved the line over and eyeballed a bit more. I then clamped a strait edge to the rest and slowly started removing material. In a few passes I had worked out a slight angle to the strait edge so I was just working the edge of the stone. This worked really well and was able to control the cut vary well. Working my mic up and down the blank I was able to find high and low spots and work them out nicely. I ground the cutter long so I would have enough stock to grind the cutting tip and not fall short, this worked well and as I miked I was not off more than 1/2 thou from one end to the other woohoo1 After all not every one has a surface grinder!!! some were someone has had to do this before!!!
Thanks
Dave


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