# Single point internal threading tool relief angle?



## Russel (Nov 7, 2011)

I'm playing around with some ideas for making a internal threading tool to thread 3/4-16 internal threads. I would like to make the threading tool from 0.500 W1 drill rod and a .125 x .125 HSS cutting bit. My first idea is to cut a slot in the end of the half inch drill rod and secure the HSS bit for cutting the threads. But, when I check the cutting profile with a 10 degree relief it just won't work. (Take a look at the diagram.) So, I figured maybe a 20 degree relief would do the trick, but that won't do either, 3/4-16 threads, if I'm not mistaken, are 0.0625 deep. 

So, my question is what relief angle should I use? Should I just increase the angle until it will clear? I'm relatively new to machining, is there better way to configure a solid internal threading tool?


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## rkepler (Nov 7, 2011)

Well, you could solve it as a portion of a circular segment with the circle having the minor diameter and the length of the sector being equal to twice your tool height, the height of the segment would be the amount of interference of the tool in the cut. So, for a "tap drill" of 11/16 and a tool height of .125" I come up with something like 11 degrees if the tool is dead on height. So something like 15 degrees would be OK.

Don't forget the helix angle.


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## Maryak (Nov 7, 2011)

Russel  said:
			
		

> Should I just increase the angle until it will clear?



IMHO Yes

Best Regards
Bob


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## kvom (Nov 7, 2011)

For a 75% thread fit the thread depth is .061 ( minor diameter = .689). I'm not sure why the diameter shown on your diagram is .779.

The tool just needs to fit inside the .689 diameter hole, meaning that with a .5" rod the cutter need project at most .189, which is plenty for a .061 thread depth. If you draw a circle with diameter .689, then a horizontal line through the center and another parallel and .125" lower, you'll see that the angle between the points that intersect the circle define the angle where rubbing would start. The angle is 11 degrees, so having the angle 12 or more will clear.

(looks like rkepler did the same calc as I).


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## mu38&Bg# (Nov 7, 2011)

The relief angle you are drawing is not the same as how you are likely grinding it. I have to assume you're grinding 10° away from the edge called side clearance angle, and not setting up to get 10° at the edge below the point where the two faces meet, called end clearance angle.

For sheets, I drew it knowing that the thread helix is small to see what would work. 7° side clearance will just clear. 10° side clearance is plenty and actually yields almost 20° under the tip.

There were many posts in between, I see.


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## MachineTom (Nov 7, 2011)

Remember when cutting thread the DOC is quite small .010 per pass, no need for a large cross section of tool. Myself I like the solid carbide threading tools like the Micro 100 type. They will last until you do something bad to them, likely to never wear out, you just break them. After you break them you can regrind them to do something else.


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## Russel (Nov 7, 2011)

kvom  said:
			
		

> For a 75% thread fit the thread depth is .061 ( minor diameter = .689). I'm not sure why the diameter shown on your diagram is .779.[...]



The .779 diameter is suppose to be .75, I neglected to correct that before posting.



			
				dieselpilot  said:
			
		

> The relief angle you are drawing is not the same as how you are likely grinding it. I have to assume you're grinding 10° away from the edge called side clearance angle, and not setting up to get 10° at the edge below the point where the two faces meet, called end clearance angle.
> 
> For sheets, I drew it knowing that the thread helix is small to see what would work. 7° side clearance will just clear. 10° side clearance is plenty and actually yields almost 20° under the tip.
> 
> There were many posts in between, I see.



When I say relief angle, I could be using the term incorrectly, I am referring to the vertical angle at the end of the 60 degree point. 

I plan to make a jig that will hold the HSS bit at a 30 degree angle from horizontal so that I can use a 2 grinding stone from my drill press to grind each side of the bit. The resulting point comes out at 60 degrees. So if you imagine looking at the jig from the side, the bit will be at 30 degrees from horizontal. OK, now imagine looking straight down at the cutting bit from on top, looking from this angle I add 10 degrees. The top side of the cutting bit faces toward the center of the jig when looking down from on top. To illustrate what I mean, I've uploaded a couple photographs of a jig I made to grind a 1/4 HSS bit to a 40 degree point with a 10 degree end relief. At least I think that is what it is doing. The jig in the photos holds the bit at 20 degrees from horizontal when viewed from the side and the pointed end of the bit is 10 degrees in when pointed from the top. 

I guess I could grind the top of the 1/8 by 1/8 bit at the cutting point so that it was maybe 1/16 tall. That would reduce the end relief needed.

Sorry, if some of this stuff doesn't sink in that fast. I just want to have it figured out before I make another grinding jig.


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## mu38&Bg# (Nov 7, 2011)

Edit


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## mu38&Bg# (Nov 7, 2011)

Russel  said:
			
		

> OK, now imagine looking straight down at the cutting bit from on top, looking from this angle I add 10 degrees. The top side of the cutting bit faces toward the center of the jig when looking down from on top.



I think what you're doing is close. Without seeing a drawing of your jig I can't be sure, due to the angles distorting the views. It get tricky when a part is inclined in two directions. The inclination to get the side clearance slightly changes the inclination required to get the 60° point.

Before my first post I had to look up the terminology myself to be sure I could convey what I wanted to before posting. I see it clearly, but never had to describe it. I've been doing 3D design and drafting for a long time now.

A 1/8" square tool with 10° side clearance will cut even a 3/4"-10 thread, but it would be close. I actually checked in a 3D drawing.

http://web.mit.edu/2.670/www/Tutorials/Machining/lathe/Description.html

Greg


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## mu38&Bg# (Nov 7, 2011)

After all that, I just buy my threading tools.


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## robcas631 (Nov 8, 2011)

What material do you intend on working with?


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## Russel (Nov 8, 2011)

dieselpilot  said:
			
		

> I think what you're doing is close. Without seeing a drawing of your jig I can't be sure, due to the angles distorting the views. It get tricky when a part is inclined in two directions. The inclination to get the side clearance slightly changes the inclination required to get the 60° point.
> 
> []
> 
> A 1/8" square tool with 10° side clearance will cut even a 3/4"-10 thread, but it would be close. I actually checked in a 3D drawing.



I'm pretty sure that I understand what you mean when you say a 10 degree side clearance. I think that I can incorporate that into a jig. But, I didn't explain the jig that I am planning correctly.

Looking at the photograph with the grinding stone in it, the HSS cutting bit is angled up 20 degrees (this photo is of a bit ground to a 40 degree point) from horizontal with the bottom of the cutting bit facing toward you. Looking at the photograph posted with this message, you can see that the HSS bit is angled toward the center of the jig by 10 degrees, with the point of the bit farther away from the side than the square end. But, the ten degree angle pivoted on an axis that is 90 degrees from the length of the cutting bit.

In my plan of a jig for a 60 degree point, if I'm measuring the angles correctly, the point would be 60 degrees and the end of the bit comes out to 10 degrees with the side clearance (view AA) comes out to somewhat less than 7 degrees [edit] 8.5 degrees. I'll have to see if I can work out a jib for a 10 degree side clearance (view AA)

I appreciate your help, I sometimes have a hard time wrapping my head around this stuff. One thing that I've learned is to use my head for more than a hat rack whenever I can.



			
				robcas631  said:
			
		

> What material do you intend on working with?



I would like to cut 3/4 - 16 internal thread in a steel part. That's why I was thinking of a 0.500 drill rod boring bar style internal threading tool.


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## mu38&Bg# (Nov 8, 2011)

Look again at my last drawing, the views on the right. That is the required position of the tool in your jig. To be clear, side clearance angle comes from rotating the square tool on it's longitudinal axis. I wasn't quite happy with the fact that the 10° angle came out to a nice round 10°. It's actually 10.05° and changes depending on the point angle. If you set up your jig on angle vises or with angle blocks to 30° and 10°, the resulting tool will not be what you expect. I'd have to really dig to sort the math for this, but I would normally draw in CAD to get what I want.

The end clearance is a result of the side clearance on the two edges that create your cutting edges, you don't actually define end clearance on a threading tool. Side clearance is important, end clearance is just what you end up with and don't worry about it. If the tool interferes you can just remove the offending portion below the cutting edge.

One of these drawings shows the resulting tool errors using simple 30° and 10° angles. The other shows the required angles to get a proper tool. I'd have to think the that to find this is not too bad and out there is some simplified or tabulated form.

Greg


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## Russel (Nov 8, 2011)

Almost all of my lathe tooling uses carbide inserts. I haven't learned much about HSS single point cutting bit geometry as a result of that. Consequently, I've misinterpreted the importance of "end" relief as opposed to side relief. Thank you for pointing this out to me Greg. Your illustration clearly points out how I can make a jig for grinding the HSS cutting bit that I want. I will try a 60 degree point with a 10 degree side clearance and see how that works. 

Ironically, the mill configuration to machine a jig configured like your latest illustration should be much easier than the one I used to make the 40 degree point jig. When I machined the jig in the photographs above I placed the part in a vise mounted on a rotary table on top of an angle plate. I rotated the table so that the base of the jig part was at 20 degrees (that bit has a 40 degree point) with respect to the X axis and raised the right hand side of the rotary table 10 degrees. Then machined the slot for holding the bit along the X axis. To machine a slot such as in your illustration, I just need to mount the mill vise on the rotary table and rotate it so that the base of the jig is at 30 degrees with respect to the X axis, then tilt the Z axis back 10 degrees and machine the slot along the X axis.

Well, one thing is definite. I need to spend some time reading about HSS single point cutting bits. Thanks for the help.

Russ


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