Grinding flat, not curved, relief angles on HSS lathe tools

Hello

I have been having a go at making my own lathe tools using HSS blanks, but my grinder is a 6 inch jobby. That means the curvature is quite pronounced and as a result, I am ending up with curves to my relief angle faces rather than nice flat ones.

Is there a knack to grinding the faces so they end up flat when using a circular bench grinder wheel?

Cheers!

Hi Ross,

Starting my tool-making apprenticeship in 1950, I've never found any need to worry about cutting tools being 'hollow ground'.

It's what happens at the point of contact with the work-piece that is important. The angles are meant to provide clearance and chip control without compromising the strength of the cutter tip. No doubt you'll get more advice shortly.

A very old toolmaker who operated the 24" shaper, finished his cutters on the side of the grinding wheel. Personally, I'm not in favour of recommending that, especially if you are new to workshop practice.

Regards,

Sam

Finish off your toolbits by rubbing them on a bench oil stone, the type used for sharpening knives, chisels etc. It will bring the ends of the hollow-ground surface down flat at the very edges where it counts. Although, there is nothing wrong with hollow ground surfaces. It gives a little sharper tool that cuts well but wears a little faster, that's all.

It seems to be common practice in the model engineering world and its literature to use the sides of grinding wheels to finish off tool bits but I try to avoid that as eventually the sides of the wheels clog up and need dressing, which you can't really do safely. Wheel manufacturers and industrial training usually seem to recommend not using the side of the wheel.

Edited By Hopper on 11/06/2018 03:09:19

It's the angle where the the edge meets the top that matters not what happens further from the cutting edge and by further I mean 0.1mm away.

The slight concave surface also makes it far easier to stone the tool as you are only rubbing two small contact patches and not the whole area

All the above Comments apply so do not worry. However if you really do want flat faces fit a "Cup" wheel to the bench grinder I have one made for use on the "Deckle" style of tool and cutter grinder it is 100x50x20mm (20mm is the spindle bore size) I have two in 60 and 120 grit but mainly use them for small drill grinding.

A belt sander will flatten a slightly curved tool face, if you need to, or an angle grinder.

Nothing wrong with a bit of 'hollow ground' concur with other above comments. Normal grinding machine wheels are not designed for side grinding, only front loading, if you must use side, as Chris Evans 6 says, fit a cup wheel.

George.

From what I read many years ago about wood working tools ago hollow ground tools are normally sharper but less durable than flat ground tools. I expect the same is true about metal working tools. Unless you’re finding your tools don’t stay sharp very long I wouldn’t worry though. I flat grind all my tools on a belt sander set up for the purpose.

Exactly.

For the likes of us sharp, really sharp, always beats theoretically correct angles. With a curved face giving line contacts at each end of the ground area its very easy to hold the tool flat against the hone or stone without rocking as you finish up by bringing the cutting edge to a mirror surface. Only the part of the tool from the cutting edge to immediately below needs to be at a controlled angle. The rest is just there to support. A flat of 1 to 2 mm below the edge is fine. Once it gets much bigger its time to re-grind.

With a flat face its very easy to rock the tool slightly as you hone it so the edge is no longer really sharp.

Another advantage of a tool face is that moving the tool up and down with respect to the wheel centre alters the basic angle. Hence modifying the grinding rest to take a suitable set of spacers makes it very easy to get a consistent relief angle so your tool always behaves the same. If you are set up to quickly, easily and repeatably re-grind the tool to the same shape you won't be tempted to carry on too long with a tool well past its sell by date!

Clive.

Thanks all for the (as ever) amazing and comprehensive suggestions. I will try them out and see how I get on, but its good to know its not a huge problem. Especially good to know I dont need to go out and buy a bigger grinder

It's not a problem, full stop -)

If you worry about angles, measure then where the cutting edge is and tangential to the wheel.

Hi Ross, have look at this link, rough grind on the front of the wheel and finish on the side where necessary -- shock horror grinding on the side !

Well I have been doing this way - as shown as an apprentice - for close to 60 years and never had a problem and neither did anyone else to my knowledge.

Clearly you would not rough out on the side of the wheel but for finishing and getting a flat relief angle is fine IMO.

**LINK**

John

George,

I have seen many old texts on metal working that show grinding on the side of the wheel. I do it myself. I generally rough out my tools using a very coarse grit on the belt grinder, then finish on the bench grinder.

Exactly what I do, but didn't like to say so in case I got rusticated.

Andrew

When I did my apprenticeship back in the late 1970's, I was also taught to rough out on the diameter and finish on the side faces.

Britan used to make a 4 spindle grinder **LINK**

We used to change these wheels when the thickness was down to 5mm wide.

As you can see from the link, the tool rests are nice and solid and go around the side.

Dave

Ross, I sent you a short PM.

Sam

I use the side of the wheel all the time,to grind two angles at the same time.

It seems that this is simply NOT DONE !!!. [Not in the best shops,anyway],all sorts of things are supposed to happen,but so far,I have not noticed any of them. I tried using the front of the wheel but did not like the results,so I went back to using the side. OK,you do have to dress it from time to time,but it,s no big deal,and anyway,wheels are not that dear..

No doubt this will be shot down,so dont take my word for it. ! Let,s just say that for me,it works.!