Question about 180 degree tap drill "point"

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Bill C
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Location: Pensacola, FL

Question about 180 degree tap drill "point"

Post by Bill C »

Hi all,

I just bought some globe valve castings from PM Research and want to try making them soon. In studying the plans, there is a footnote that reads:

"1. Tap Drills - start with regular drill. Finish with drill ground to 180 degree point."

Best I can figure they mean for me to grind the bottom of the drill bit flat (similar to an end mill) so it makes a flat bottomed hole at the depth called for in the prints. That would make sense as the drill would be forming a flat surface for contact with the valve seat on the spindle. Guess the word "point" is throwing me off, seems they could have said "flat" or square bottom....

Just wanted to be sure before I grind up a few bits. As always, you thoughts are much appreciated.

Thanks,

Bill C.
Andy Rafferty
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Re: Question about 180 degree tap drill "point"

Post by Andy Rafferty »

I'll jump in here Bill since I bet we are all scratching our head in unison... I bet it could have been written easier for us simple folk, but I'm thinking the same way you are but when I think flat drill bit I'm seeing a "D" bit stronger and less grab in bronze/brass. I guess the objective is not wrecking the seat nor loosing any material for a proper model taper tap. Just my two cents. Let me know how It works out their valves are on my radar for a piping task coming up.
Bill C
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Re: Question about 180 degree tap drill "point"

Post by Bill C »

Andy,

Thanks, I agree and also thought of homemade D bit as the solution. One of my first projects was the Geo. Thomas rotary table. In his instruction book, Guy Lautard explains how to make special tooling and that is fun also.

Happily, I do have several sets of drill bits (lucked out and bought the tools of a retired machinist several years ago) so I could grind one of those if needed.

I'm sure many people have made valves from the PM Research castings, someone here will be able to let us know if we're on the right path.

Thanks!

Bill C.
Jtrain
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Re: Question about 180 degree tap drill "point"

Post by Jtrain »

I am contemplating to order these valve kits also from PM Research. I would have thought to use a end mill also at first, but now I'll wait and see how it should be done.
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kenrinc
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Re: Question about 180 degree tap drill "point"

Post by kenrinc »

I used a D-bit but I've got a large assortment of off size end mills and they will work as well. I was a bit perturbed by those instructions because the typical HSM is not going to have a flat bottom drill nor are they going to take the time to make one. I urge you to take a look at a modern valve from the hardware store or even look at PM's 1/8" NPT valves to see how they did them. The instructions are a bit archaic in that regard; doing an undercut in the valve body after the threads. Forget that. Also, for packing they provide old school packing, I'd put a chamfer at the valve neck for an 0-ring.

Ken-
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Harold_V
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Re: Question about 180 degree tap drill "point"

Post by Harold_V »

kenrinc wrote: I've got a large assortment of off size end mills and they will work as well.
A few things to think about. They may help save a part from being scrapped.
End mills, properly ground, are not flat. There should be a slight taper towards center, so the end mill cuts only at the tips. That's to ensure that when one takes cuts, the resulting surface is flat. If a truly flat surface is desired, it won't create one when used for a plunge cut. That's true, even when the end mill in question is center cutting. That may or may not be an issue, but is information that one should know.

End mills are intended to cut on the periphery. They make poor drills for that reason. If, when the tool is applied, there is any reason for the tool to wing, it will cut accordingly. Something as simple as one flute loading with chips can be the cause of winging. Fine feed and regular clearing of chips will help limit the problem, but there's no guarantee the resulting hole will be on size. The less rigid the machine on which the tool is applied, the greater the chance for cutting oversized. Holding an end mill in a drill chuck is an invitation to problems.

Rake created by the helix of the common twist drill can lead to hogging (self feeding), especially if the material being machined is a copper alloy. That problem can be overcome by simply grinding a tiny flat on the cutting face (on the helix face) of the drill, once it has been ground flat. So long as the flat doesn't extend past the margin of the drill, it will still cut size.

Keep the drill as short as you can, to prevent the tendency for the drill to wander, as that, too, can result in an oversized hole. Assuming the resulting hole has depth, the best results can be achieved by using a pointed drill to create a full diameter, then switching cutting tools to cleaning out the hole, making it flat bottomed. By following this procedure, the margin of the flat bottomed drill will pilot on the established bore, limiting, or even eliminating, the flat bottomed drill from winging.

Flat bottomed twist drills cut best when there is a center hole the size of the web, or greater. Unless the web is split, and properly ground, they don't cut well in that area.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Bill C
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Re: Question about 180 degree tap drill "point"

Post by Bill C »

Thanks for the replies, everyone. At least it sounds like I was reading the instructions correctly. I also emailed PM Research about this, will let you all know what I hear.

Harold, thanks for the detailed info. I was aware of the "fish tail" on the bottom of the normal end mill and how that would not be a good choice for an application like this. I understand your comment about thinning the web in a drill bit if I go the route of cutting or grinding one to a flat bottom.

Tonight I will make a jig for holding the valve (found several threads describing jigs over on the HMEM website). Will post photos or video as I make some progress.

Thanks again,

Bill C.
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Fred_V
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Re: Question about 180 degree tap drill "point"

Post by Fred_V »

I have made a small boring bit from a larger lathe tool by grinding it to a 1/8 to 3/16" square about an inch long. This works great to go into a valve and square off the bottom of the drilled hole for the valve seat. You can also touch up the drilled hole for the outlet and make the seat nice and round.
Fred V
Pensacola, Fl.
Bill C
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Re: Question about 180 degree tap drill "point"

Post by Bill C »

Fred,

Thanks, I like the idea of a boring bit as well. I found a YouTube video describing how to make a flat bottom drill bit. He also shows how to put the clearance angle on the flat drill bit. Here's the link to it:

http://youtu.be/qwuSBJFa5uM


I started making the jig tonight. Went pretty well, except the largest ball end mill I have is not big enough to make a proper sized hole for the globe. May try grinding my own...tomorrow...

Bill C.
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Fred_V
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Re: Question about 180 degree tap drill "point"

Post by Fred_V »

What size do you need? I have several ball end mills.
Fred V
Pensacola, Fl.
Bill C
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Re: Question about 180 degree tap drill "point"

Post by Bill C »

Fred,

Thanks, the body of the globe is an odd size, about 0.645". I was using a 9/16" ball mill (biggest one I have) thinking I just needed a small round, hollow spot. Turns out it needs to be a bigger hole. Guess 5/8" might work. If they even make 11/16" ball mills, that would be oversize.

Text me if you like and I can come over....

Thanks!

Bill C.
Bill C
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Joined: Fri Aug 08, 2008 9:14 pm
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Re: Question about 180 degree tap drill "point"

Post by Bill C »

Update. The nice folks at PM Research emailed me back to confirm that the "180 degree point" meant a flat bottom, just as we had figured out. They also sent me a couple photos of a jig they use for machining. I finished my jig last night and will start a new thread here on machining the PM Reasearch globe valve, with links to the videos.

Thanks again for all the help and advice.

Bill C.
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