Flat bottomed hole with a boring bar - technique?

You haven't actually got a 'blind' hole. You just need to nibble the drill point away in both directions & your smallest boring bar looks too big for this task.

Tony

Thinking about it, I don't actually have a 12.7 mm drill, only up to 10 mm.

I do have a 12 mm end mill though, which I suppose I could use once the 5/16" hole is drilled.

Then it would be just a case of taking another 0.35 mm off the radius, which I should be able to do with that boring bar?

Looks like that boring bar will fit down a 10mm hole so open up the 5/16" through hole with your 10mm drill. Then bore out to the required 1/2". Take say 0.5mm depth of cut for the first two passes and then measure the hole and take your final cut..

If you touch the tip of the tool against the stationary end of the work and zero the handwheel dial then it's easy to set the 0.3" depth

Final i/d is 0.6"

But what I'm asking is the specific technique for boring:

- from the open end down the side, then in, repeat until the set depth?

- from the bottom of the hole (at the set depth) radially outwards, then pull out through the bore?

Edited By Dr_GMJN on 17/03/2021 15:25:20

For that one I would make a series of passes along the 0.3" length starting at the end increasing dia each time and when at final dia you can take a clean up pass across the bottom by winding cross slide away from you.

Looking at the plan view of the boring bar insert, there must be a fairly low limit to how far I can feed into the base of the hole and then move across - It looks like it would be moving inwards into an increasingly thick triangle of material?

If the base of the hole is already flat to the correct depth, and I'm just increasing the i/d, by a small amount (as in this case) then its less of an issue I guess.

If you've got a blind hole that's not deep enough and not big enough diameter, is it a case of going down the bore, cutting into the base a bit, then going to the centre, and repeating this until the right depth and diameters are achieved?

As you have the through hole you won't end up with a cone in the middle. If I were doing it I would make the length ways passes 0.298" and then on the final pass the that all the way in to 0.300" so you are just skimming the "steps" left from the end of each pass

If it were a blind hole you would drill as much as possible and then use a boring bar that front to back is less than half the hole dia so you can in effect take facing cuts to clean up the bottom

OK, understood, thanks.

Sorry to disagree, but I don't see how the boring bar will produce a flat bottom, unless the bar and cutting edge is half the finished diameter of the hole, or less without the outer edge of the bar contacting the "far side"of the 0.6" hole. That is 0.3 ". The insert boring bar in the photo looks far wider than that to me. I would venture to suggest that using an end mill will give the best result and even then the bottom of the hole will be slightly convex due to the nature of the grind. That probably won't matter a great deal and I doubt it will be seen, either. I won't tell, if you don't.

I have a small Sandvik boring cutter which is about 6mm across the end which might just do the job without the cutter contacting the opposite side of the hole..

Not a very good photo, I'm afraid. The micrometer reads 5.943 mm. This would do the boring and produce a flat bottomed hole as well. I would drill to 10mm, the tip of the drill to required depth, then use the boring cutter to finish the hole to size and form the flat bottom at the same time.

John

Edit: Jason posted while I went out to take the photo!!

Edited By John Hinkley on 17/03/2021 16:22:06

Doc has said there is a 5/16" through hole so tool only needs to move inwards 0.144 from finished dia. Front to back of tool looks about 3/8" so 0.375 + 0.144 = 0.515 so back of tool will not hit the far side of the hole.

Hi, I’m not able to give any advice on how to do the job,but I think it would be best to practice what others have suggested on a piece of scrap .

Maurice

I missed the bit about the through hole, Jason. Must remember to engage eyes before typing!

John

We often make flat bottom drills, for the flat face of a hole. The only time it is bored, is if the bottom requirement for the flat face is to match another flat face that needs to shut out . Many years ago, you used to by flat bottomed 2 flute end mills. They also has straight sides. Different from the 2 flute slot drills that had a helix on the sides and a fishtail bottom geometry.

When taking a drill point or drill side to flat bottom, I normally touch on the hole edge at the top for the reference diameter. Then go down the wall at 0.1mm less than the diameter until it touches on the drill edge. Depending on the bar diameter to bore length ratio, will determine the speed, depth of cut and feedrate. When the bore length is deeper than 4 diameters of the bar, it will need to be running slower and smaller depth of cut. After the length is over 8 times the bar diameter, is when it needs to have a sharp edge and a small nose radius to reduce the cutting forces, and a slower feedrate.

I've got a couple of those Sandvik tools, and a holder (Coroturn XS CXS-A10-05). The inserts for it have a chamfered end - I could have sworn mine were for internal threading. Perhaps they are, but you can get also boring bar inserts for them. I wonder what the difference is? Do you happen to know what catalogue # your small Sandvik boring bar is?



I've had it years, and only tried internal threading with it last week (unsuccessfully). I ended up using an HSS tool, and eventually I had some success. I think the tool I have was too small for the thread, and it was in aluminium, which seemed to tear very easily. I couldn't get a clean cut with the Sandvik insert.

I can't find the original listing as I purchased the boring tool a number of years ago, but this, from the Cormorant range has the same tip shape.

The one on the left is the same profile as mine and will produce a flat-bottomed hole.

John

Regarding the direction for boring I usually do the direction of movement away from the centre line for diameter and in from the open end. This makes sure the forces from cutting material are always pushing the tool back against the leadscrews. I would do this part by leaving a small machining allowance on the bore and the depth then take a final pass starting with the tool outside the part, moving the tool from the centreline side out to the final bore setting, move in to final depth and then move toward the centre line. Withdraw the tool.

Martin C

+1 for Martin's finishing cut method.

Emgee

Thanks Martin.

pity you did not give the sizes and through hole a day earlier as I would have taken a few more pics of this one for you. 0.531" dia x 0.080" deep with a M4 x 0.5 hole in the middle done with an insert boring bar. You have a couple of these to do soon.

Princess Royal pistons?