Filleting on a Lathe

[rexcsmith]

I'm back with my next question. I'm making axles for my steam loco. The plans call for the axles to be necked down (reduced diameter) between the ends (merely a cosmetic touch). Each end of the necked down area has a .125" fillet. I've tried making a filleting tool out of a 1/4" HSS blank, but it does a pretty bad job. Before I spend much more time on this, can someone point me to some instructions for properly grinding a filleting tool for a lathe, or somewhere I can buy one? thanks, rex

[Carm]

Questions
1. Large diameter of axle?
2. End diameter and length?
3. Workholding? Chuck or collet, soft jaws etc.
4. Material?

[SteveHGraham]

I will probably be strapped to the tracks for this, but I saw a guy hold a radius gage against a blank and spray it with Dykem. It leaves a picture of the radius you want to grind. Not all that accurate, I suppose, but it works.

[tornitore45]

What is filleting?
From the contest looks like some radius form tool but is it concave or convex.

[rexcsmith]

More info...

The fillet is concave, so the tool needs to be convex.
Fillet radius .125"
Large diameter is 3/4"
Small diameter is 5/8"
Length of small diameter is about 6" between the wheels
3 jaw chuck with live center
4140 steel

[Harold_V]

You'd be well served to lose the use of a ¼" toolbit. If your setup permits the use, go larger, for added rigidity. I'd recommend nothing smaller than 3/8", but ½" is better. Keep hang-over as short as you can, to aid in rigidity.

Hand grinding radius tools is quite easy. You should be able to rough the tool without the use of a radius gauge, but use one for fine tuning. Don't look down on the gauge/toolbit ----hold them towards a light source and read the light leakage where the tool doesn't touch. Grind away the highs. This is best done totally free hand (no tool rest), so the required relief can be created. Keep the wheel well dressed, running dead smooth, to prevent bounce when grinding. Mount this tool perfectly horizontal (don't introduce rake, as that alters the resulting radius). You should stone the ground tool after grinding, which will help create the desired radius, and improve the tool's performance. Be careful to NOT round the cutting edge. If the tool is properly ground, simply follow the top and bottom profile with your stone, honing side to side, not top to bottom.

Harold

[Steggy]

What is filleting?
From the contest looks like some radius form tool but is it concave or convex.

The term fillet refers to the transition between two adjacent planes of a machined piece. See attached for an example.

[SteveHGraham]

Harold agreed with me, so I'm on my way to buy some lottery tickets.

[rexcsmith]

Harold, do I read you right? No rake? That would be a lot easier than what I've been trying to do.

[rexcsmith]

Never mind, I just realized what you meant by rake.

[GlennW]

When I grind such a tool bit I never leave the end and side cutting edges of the tool at 90 degrees. I prefer about 85 degrees so you end up with the correct radius and a couple of degrees or so of negative lead if you orient the tool accordingly. That way only the radius does the cutting as you feed toward the shoulder and then feed the tool out along the shoulder. If you use 90 degrees you may end up with more chatter and finish issues as the "flats" will be in contact and may drag against the part, which isn't needed. It also can make the tool orientation much more critical to keep the "front end of the tool from dragging on the cut surface.

[Harold_V]

Never mind, I just realized what you meant by rake.

Rake is angle at the top of the tool. It can manifest itself in two ways, side rake and back rake. It can also be positive or negative.

When creating form tools, any alteration in rake creates a minor change in the resulting configuration, and can often lead to less than acceptable performance because the tool can't be on center except at one point. However, if one is adept at grinding tricky chip breakers, that may not be true, as rake can be created by the chip breaker, with the cutting edge on center over the entire profile. Negative rake would not be a good choice, as it creates greater tool pressure and performs poorly.

Rake provides considerable benefit, but it can also be troublesome. Too much positive rake can encourage chatter, although there's a large number of conditions that dictate if it might be acceptable. Light duty machines? Most likely not. They're generally too eager to chatter.

Glenn's comments reflect my grinding practice. I make such tools such that they can both turn and face, with the radius in contact only in the corner. I use such a tool for finish cuts only, as they do not serve well for stock removal (too much tool in contact with the work). Rule of thumb is one should present no more tool to the work than is necessary, to limit chatter. You also learn to "cheat" with form tools, stepping slightly away from full cut when chatter is an issue. Takes a quick hand and good judgment, but it's a great way to improve performance when nothing else does.

Harold