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Slot Drills

Are all slot drills centre cutting

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Andy_C07/04/2019 19:08:47
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I am making a bearing pedestal for a Jan ridders engine which called for a 18mm hole to be drilled. Given that the centre of the hole was right on the edge of the work I thought I would use an 18mm slot drill from Tracey Tools to avoid issues of centreing the drill on an edge. I had though all 2 flute slot drills were centre cutting but quite clearly it cut a convex shape and then was in danger of jamming - so I stopped. Having looked it is clear that it is relieved towards the centre such that it couldn’t cut across the full diameter.

I thought all 2 flute slot drills were centre cutting - is it me or are ther variations on what a slot drill is and whether it is centre cutting.

JasonB07/04/2019 19:18:35
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That's quite a big ask to plunge down with an 18mm cutter even if for half a hole, I would have either started with smaller and worked up to 18mm or come in with a series of cuts from the side, in both cases the ctr would not need to do anything.

Can you post a photo of the end, most have one of the end cutting edges longer than the other so it goes past ctr but as it is barely moving may not plunge that well.

If there are two pedestals then a good method is to hold the two side by side and drill the two at once which will support a drill.

Andy_C07/04/2019 19:50:34
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bd33376b-603d-48f0-8129-63b4ac9e475e.jpeg4ab3a371-0953-4309-950e-e078ece28977.jpegPhotos of slot drill purchased from tracey tools. I can’t see that it would be centre cutting with angles as shown.

Edited By Andy_C on 07/04/2019 19:51:23

Edited By Andy_C on 07/04/2019 19:52:30

colin vercoe07/04/2019 19:53:29
25 forum posts

Slot drills are ground with one edge longer than the other past centre so that it cuts to the centre,They are also ground convex so only the outside edge cuts leaving a flat bottom to the machined slot.

Andrew Johnston07/04/2019 20:12:01
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Posted by colin vercoe on 07/04/2019 19:53:29:

Slot drills are ground with one edge longer than the other past centre so that it cuts to the centre,They are also ground convex so only the outside edge cuts leaving a flat bottom to the machined slot.

Correct. thumbs up

Theoretically if you plunge straight down the centre point has zero cutting speed, although it will cut if you have a positive feed rate. Likewise if you plunge down and then back up to create a blind hole the bottom of the hole will not be flat, but convex.

In my experience the bigger the slot drill the more likely it is to go walkies if you plunge straight down, and even more so if the cut is asymmetric. Even on a Bridgeport I wouldn't rely on an 18mm slot drill to cut a half slot accurately.

Andrew

Dave Halford07/04/2019 21:29:11
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That cutter looks off set to me, so it should plunge.

Does you machine have sufficient power at the speed you are using?

John Reese08/04/2019 04:39:11
701 forum posts

Standard slot drills and end mills are ground with the ends slightly concave, about 2* if memory serves. The intent is to provide relief so only the periphery of the tool engages the bottom of a slot. If you need a dead flat bottom the tool needs to be re-ground. When plunging I find the 3 or 4 flute end mills are more stable than the 2 flute.

Most 2 flute cutters will be center cutting. Not all cutters with 3 or more flutes are center cutting. The vendor should state if it is center cutting.

In re-grinding some of my own cutters I do not always try to re-establish the center cutting configuration.

The concave end of end mills does nothing to keep them on line. You depend on the rigidity of your machine and your part clamping to hold to keep the cutter on line. Starting with a smaller slot drill should help by reducing cutting forces.

Andy_C08/04/2019 17:32:08
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Very many thanks for replies. The tip from Jason regarding doing two side by side is worth a go. I will also remove more of the middle before getting up to the slot drill. Thanks again.

Neil Wyatt08/04/2019 18:01:30
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Posted by Andy_C on 07/04/2019 19:50:34:

4ab3a371-0953-4309-950e-e078ece28977.jpegPhotos of slot drill purchased from tracey tools. I can’t see that it would be centre cutting with angles as shown.

You can see the left edge is extended to cut to the centre in contrast to the right hand one.

Flat bottomed holes need a 'special' cutter.

In the old days not all slot drills were centre cutting.

Martin Connelly08/04/2019 20:33:41
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Broaching cutters can cope with half a hole as described as the centre slug effectively pilots the cutter. Too expensive for a one off job though.

Martin C

JasonB08/04/2019 20:52:22
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Posted by Andy_C on 08/04/2019 17:32:08:

Very many thanks for replies. The tip from Jason regarding doing two side by side is worth a go. I will also remove more of the middle before getting up to the slot drill. Thanks again.

That is the good thing about this method you can run a regular drills and if you don't have one big enough finish with a milling cutter or boring head, these were 16mm holes

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