Straight flute slot drill

I have been given an assorted collection of end mills and slot drills from a closed engineering works.

Amongst them are some slot drills with straight flutes.

Can anyone tell me what they are used for and why.

See P 53 of This pdf Catalogue where it suggests

"For cutting even, parallel cuts often essential in holding tight tolerance slots or keyways. • Normally used in finishing milling slots and keyways. "

Always assuming they're not router cutters of course.

Bill

Probably are older ones, cheaper to make but not as smooth in use.

This was how they used to be made many years ago. The spiral flutes were a later innovation.

Michael W

As above posts that is the way things where years ago. I have a good selection from my apprentice days in the 60s. Useful when making form tools as it is easier to grind shapes/radius on the straight flute.

Thanks for the replies.

I was told that many of the tools I was given had been reground several times so it would explain why old style tools were in the collection.

I will probably touch them up on my new Acute Tool Grinder before testing them on some scrap.

I'd test them on some known material, which I accept your scrap bin my contain; that way there's only the one variable.

Regards for the New Year

Bill

I have spoken to the friend who gave me the tools and he confirmed what Bill posted. They were used for cutting very accurate keyways.

Apparently a conventional spiral fluted slot drill produces a slightly v shaped slot as the spiral pulls the tool sideways, removing a bit more metal near the top of the slot

I don't see how that works? In theory a two flute cutter has balanced cutting forces across the widrh of the slot, so it shouldn't deflect. Axially the cutting forces will be unbalanced, but that doesn't affect the width.

I have always assumed that cutter deflection is essentially that of a cantilever, ie, deflection is greatest at the tip and reduces to zero at the holder. To cut a slot wider at the top implies that the tip doesn't deflect, but a midpoint of the cutter does, seems a bit odd?

Andrew

When the cutting edge is at the front most point cutting the end of the slot it is moving sideways to the slot. Therefore the cutter will be bent towards the left hand side but with a straight flute there is no cutting edge at the left so it doesn't matter. For a spiral flute as the tip of the cutter is just starting at the edge some part of the flute will already be cutting sideways so there is a bit of side force so the bottom of the slot might be a little to the left.

On the other hand if the cutter is at all blunt the very start of each cut will not dead on where it should. It will rub a little until the pressure builds up so should be a little shy of full width. I can't decide how this will affect a spiral relative to a straight flute.

The main benefit of the spiral form flutes meant that you could elongate the overall length of the channel as opposed to having a shorter channel, this allows for more swarf to be carried away from the cutting bit more efficiently. The spiral motion combined with this makes the chips move up the length of the drill not dissimilar to an auger.

(Perhaps at the very slight expense of less rigidity, but this has more to do with how many flutes you have)

This traditional way was also applied to drills as well unsurprisingly.

The only benefit I can see it having today is making it easier to regrind any damage, and also a good idea for making your own custom cutters. But not really a commercial practice anymore, an interesting piece of tool history though.

Michael W

Edited By Michael-w on 28/12/2017 15:11:53

If the slot were a certain depth there is the possibility that the top of the slot could become wider even though the deflection is greater at the very end of the cutter.

Imagine that the cutter in the photo below were cutting towards you, as the corner of the leading flute gets to the ctr of the slot it will be removing the most metal and could deflect in the direction of the red arrow. This would cause the tool to move to the right as viewed but due to the helix angle the slot won't become wider at the bottom as there is noo cutting edge in contact but further up where the scriber point is the cutting edge would possible cut into the side of the slot thus making the top wider than the bottom.

I agree with Jason. Ignore my earlier post I had the spiral in reverse for some brain fade reason.

That makes sense, thanks. Intuitively I'd expect the effect to be dependent upon the depth of the slot and the helix angle. The Dormer slot drills I use have a helix angle somewhat less than the cutter illustrated. So I guess for normal depth slots the effect will be less.

From a practical point of view I can't say I've noticed the effect when measuring slot widths with slip gauges. I wouldn't be surprised if the keyways on the original SCC Burrell traction engines were cut with a hammer and chisel. So I'm not going to lose any sleep worrying about variable width slots on my models.

Andrew