How Do You Efficiently Grind A Hss Boring Bar

[tomw]

Dear All,

This post is really two questions. The first is how do go about grinding a boring bar in a way that does not take a weekend. The second questions is what is the optimal shape for a boring bar.

For the first question:
I need to make a boring bar to enlarge a 3/8" diameter x 1 7/32 length core hole in a bronze casting.

I have 1/4" HSS tool bit stock.

I am grinding the tool bits on the wheels that originally came with my 6" Jet bench grinder. They have worked fine (as far as I know) for grinding regular lathe tool bits. One is coarse, one is less coarse. Both wheels were dressed before commencing the operation.

I spent 45 minutes removing 1/32" of material from 1-3/8" of said HSS stock, picking that side as the "bottom" of the bit. According to my math, I need to remove at least another 1/32" for clearance on just the bottom of the bar. Am I consigned to spending the next 4 hours making a boring bar?

Second question:

What is the optimal shape for a HSS boring bar ground from square stock to enhance rigidity while maintaining clearance?

So, where do you grind the clearance. Do you take some off the top, of just the leading side, or where?

I have done the Google and BoobTube thing, and have not found any good answers. I also can't find answers in Machinery's, though this may be to unfamiliarity with the index and table contents.

Thank you all for your help.

Cheers,

Tom

 

[Doubleeboy]

You could just buy a miniature boring bar. Micro 100 are my favorite. You can get them at most of the mail order houses.

cheers
michael

 

[John Hasler]

I rough out that sort of thing with a fiber cutoff wheel in a Dremel and then finish it on the grinder.

I don't think I understand why you are removing so much material though. Pictures?

 

[Tozguy]

Tom, just to make sure I understand, the hole to enlarge is 17/32 deep. Why do you need to grind for 1-3/8'' ?
The wheels that came with your grinder are likely grey and do not cut very quickly. There are better wheels for roughing out quickly.
Is there a way that you can hog some off quickly with an angle grinder?
Hopefully someone will come along to comment on the second question. That should be answered first so you know what the grinding requirement is. It might take a cut-off on a dremel to get in some corners.

 

[mikey]

Tom, do yourself a favor and buy a 1/4" boring bar. You can grind them from HSS and it will work fine but it is a lot of work to make them. The modulus of elasticity of HSS is the same, regardless of shape. Using a 1/4" HSS bit you will probably have an extension capability of 1" max. Since your hole depth is 7/32" beyond that you will have deflection and that will affect your accuracy.

If you insist, I would grind a positive lead bar to reduce radial forces and simply round the section under the head for clearance. As for shape, a slightly tapered shank reduces resonance on a solid bar but it will not make it more rigid.

If you need this hole to be accurate then the best option is a carbide boring bar, either solid carbide (I like Micro 100 bars for this, too) or a carbide bar with inserts. The best 1/4" bar I have used is a Circle Machine CCBI 250-4-5R. This solid carbide bar is a positive lead bar that beds the inserts with a positive axial rake. This geometry reduces both tangential and radial forces, making the bar very accurate. It also provides outstanding tactile feedback and will go up to 2.5" deep.

 

[Wreck™Wreck]

Start with a round blank, split one end on center, (at one time you could buy round blanks with split ends from McMaster) 1/4" back from the cutting edge, grind a few .000's clearance on the body so that it doesn't hit the finished bore, grind a small bit of clearance on the front then rotate the tool post so that the back side of the tool will clear the back side of the bore at depth.
Rotating the tool post lets only the cutting edge contact the work minimizing the clearance grinding, if a flat bottomed hole is required attention must be paid to the angle at the front of the tool.

 

[rex21]

I ground one a while back and if I recall it took less than 20 minutes. It has no rake on the top. End relief is whatever angle that was on the blank I started with. Cuts great in the aluminum and brass I have used it in.

 

[GarageGuy]

rex21 is spot on. It doesn't need to be any more complicated than that. Especially for cutting bronze.

GG

 

[tomw]

Doublee and Mikey, I will look into the boring bars you mentioned. I agree that HSS is going to flex, but I my current set of carbide bars are too short of the job.
Tozguy, the bore is 1 and 7/32" len x 3/8 dia. I need to enlarge the dia. along it's entire length to 0.500. I am sorry I made that so muddled in the original post.
Wreck^2, that is an interesting option. I will look into it.
Rex21, Thank you for the photos.

Thank you all for your help. I am going to pursue my options today. I will let everyone know the outcome.

Cheers,

Tom

 

[mzayd3]

What about drilling just slightly undersized and boring it out with a 5/16" endmill? I have used this setup for larger bores.


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