Help with drilling a flat bottomed hole

Hello All

I need to drill a flat bottomed hole 10mm in diameter and 4mm in depth in the end of some bar stock. It will be used to locate a 10 mm outside diameter bearing.

I’ve put my bar stock in a collet chuck (in the lathe), I’ve centre drilled, then drilled to 4mm diameter and 20 mm depth with a twist drill, then drilled to 6mm diameter and 20 mm depth with a twist drill, and then drilled to 10 mm diameter and 4 mm depth with a 2 flute end mill. Unfortunately, I’ve ended up with a 10.35 mm diameter hole.

I’ve no idea why, can anyone offer any guidance ?

Thanks

David

Is the tailstock aligned with the headstock ? The end mill could be cutting on one side only and working like a boring tool.

Paul

The only sensible way of creating a bore of the correct diameter and depth that your bearing requires is to bore it (with a singe point tool).

Apart from the fact that an end mill is not designed to bore holes of a precise diameter it would have to be very accurately centered which is not really guaranteed when a chuck etc and tailstock are involved.

If you have not got a small boring bar and dont want to grind one, you can always clamp a small (say 5mm) endmill in the toolpost and use one of its teeth as a boring tool.

Ian P

I did wonder if that was the case, and it is a while since I checked tailstock alignment. I will do so tomorrow. I haven’t noticed any other problems ,and 0.3 mm error does seem quite bad.

I should add that the part is brass.

David

+1 on small boring bar. You can grind up something to do the job from a piece of 5/16" HSS or the like.

Thanks all. It sounds like the boring bar is the way to go!

David

If the worst comes to worst, you could grind up a little boring tool from an old file, old chain saw files are good for this, and as you are boring brass, carbon steel is ideal.

Ian S C

If you don't like boring small blind holes or your bar is long and you don't like fixed steadies then.

1. Turn a 10mm spigot on the end of the bar say 5mm long.

2. Turn up a collar 9mm long with a drilled and reamed hole up the middle. OD a tad bigger than the bar.

3. Loctite the collar to the spigot.

4. True up the bar in the four jaw and turn the outside of the collar to match the OD of the bar.

regards Martin

Why does it have to be flat bottomed ? Printing presses I worked on had several bearing positioned in blind holes none of which were flat bottomed, the cone shaped depression formed a sink for grease

I'm surprised here,

You can grind your own flat bottoming drill out of a regular twist drill. The geometry isn't that complicated to do on your own bench grinder.

Bear in mind that the drill is not suitable for anything other than bottoming at that point. But what's the cost of a 10mm twist drill? not much I wager.

Not the best image, one with angles would be better. But it demonstrates the basic cutting shape you're after..

So it's basically flat or 180 degree kind of shape, but with relief angles towards the edges, so that it cuts. Theres another couple of angles on the sides, that's just to make sure the cutting point goes all the way to the centre and doesn't leave a pip or dimple.

You can buy flat bottomed drills commercially in some places, but I think they cost considerably more than a regular drill, simply due to their uncommon usage.

So to use it, drill your normal 10mm hole, to depth, including the point of the drill. Then run this one down the hole, make a note of how deep the angles of the point are on the regular drill, then that's your number for your flat drills working depth. Drill to that depth, and you should have an accurate flat depth.

My experience for this comes from college, where I had a 15mm thick piece of steel where we had to drill a variety of tapped, reamed, spot facing and flat bottomed holes!

Michael W

Edited By Michael-w on 08/02/2017 10:22:34

+1 on small boring bar.

Michael there is not much difference between regrinding a drill and using a milling cutter like the OP did but getting the hole spot on dia is not easy with either method.

I'd also use a small boring bar, if you do't have one a slot drill of say 6mm dia held in the toolpost makes a reasonable boring tool.

Pfft, I never work to accuracy

Michael W

End mills are usually ground to nominal size with better accuracy than twist drills. The're also shorter/stiffer than your average twist drill.

If your hole came out 10.35mm suggests to me around 0.175mm misalignment in the tailstock/chuck combo ?

As others have said bore to size is best for accuracy.

If however you're lazy / don't like boring small holes ( like me) ~try drilling with a 9.8mm/9.9mm or 25/64" drill, then "ream" out & flatten bottom with 10mm EM.

Or maybe drill 3/8" if you think above drills will still result in oversize hole before "reaming" ?

just a suggestion.

I've had good results with EM's 'boring' holes in the tailstock, wasn't looking for super precision though.

A slot drill and end mills are ground slightly hollow ground so when plunged they do not produce a true flat bottom,,a slot drill held in the tailstock and plunging into brass will not hold a true or exact diameter,and certainly no good enough to locate a bearing accurately,always bore the hole with a single point tool.

Thanks Chaps

The idea of using an end mill as a boring bar worked perfectly!

Neat ! glad you got sorted & thanks for sharing video.

Very interesting!

How did you set the tool height?

Paul

I'll take a guess it's by using a QCTP holder, micrometre knurled screw head To set the height. Or he might have a verticle slider.

It might be a bored round homemade holder or the QCTP holders with a vee slot for holding round shanks.

Michael W

Edited By Michael-w on 14/02/2017 13:30:11

Um, should have been clearer, I meant how high was the cutting flute set... at centre height as usual? Not having a tool to hand I can't visualise the fall off/relief to think if it would cut properly set like that or whether it would need to be set lower and rotated a little.

EDIT But on reflection it's obviously no different to normal milling just in a different orientation and with the material curving in toward the cutting edge. I must try this later!

Edited By PaulR on 14/02/2017 14:19:37