Grinding a round nose tool bit

lensman57 · Aug 15, 2012

Hi,

I need advice as to how properly grind a 1/4" round nose lathe tool . I have a small belt- rotary snder and an 8" run of the mill grinder. I can grind left and right hand tools quite well now particularly using the belt sander but have no idea as how to grind a round nose properly. I will be using either an 8% or 5% cobalt hss stock.

Many thanks,

A.G

Tin Falcon · Aug 15, 2012

I spent 6 years as a pattern maker for a yacht builder. we basically made models at 12" to the foot scale. the pattern was used to make the mold. So sanding round shapes and making shapes by hand became second nature.
I do this in tool steel as well.
here is what I would do. set you tool rest for relief angle. then draw your half circle with a sharpie and a radius gauge or a circle template. take it in steps you are going from 1/2 square profile to 1/2 circle. so grind off the two sharp corners you now have 4 smaller sharp corners . grind them so you have 8 corners. then swing the tool back and forth in a circular arc. to get your radius. once it is close finish with a hand oil stone. just take your time and keep the tool cool. you can use a radius guage to check your progress as you proceed.
Tin

lensman57 · Aug 15, 2012

Good evening Tin,

Thank you for your reply and advice once again, I understand what you are saying but my problem is that my hands are not vey steady , so perhaps I should make a jig to sort of rotate the bit around and achieve the radius. Do you think that is a feasible option? My grinder has very a basic rest, the belt sander has an adjustable table but only in one plane.

Many thanks and regards,

A.G

MachineTom · Aug 15, 2012

Try adding this to Tins instructions, I have a 3 " steel disk, was a slitting saw but now its tool grinding helper. find a steel round like the slittting saw, the harder the better. rest your bit on the top of the disk, let the disk bear against the grinding belt of wheel, now when you bear down on the tool bit it will tend to carry the disk along, this will surprise you how much easier it is the sweep the bit across the grinding wheel with this technique. If your hands are not steady this should help more.

Rayanth · Aug 15, 2012

Tin Falcon said:
... we basically made models at 12" to the foot scale....

Tin

Forgive my sleep-addled brain, but ...isn't 12" to the foot the same as 1:1 ?

Bill Gruby · Aug 15, 2012

Yes, it is 1:1

"Billy G"

Omnimill · Aug 16, 2012

You haven't said how critical the grind is but do a search for "tangential tool" and have a look. You can (I do) fit a round piece of HSS in one of these tool holders and grinding is simple.

Vic.

Here's a really nice (round toolbit only) Tangential Toolholder, bottom of page:

http://concretelathe.wikispaces.com/How+To+Build+Part+8+-+Tooling+And+Coolant

Grinding a round nose tool bit

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