Blind hole milling

[ROBERT BLACKSHAWParticipant @robertblackshaw40066]

I have just bored two 18mm x10mm deep in aluminium holes on my mill, the problem I have had was to get a good flat on the bottom of the hole. I used a end mill 12mm to get the depth, which started with a wobble, then used the adjustable boring tool, DRO the bottom and moved out at .5mm till I got the size,is this correct way of doing this.

Also cutting a recess on a fly wheel on the lathe, again done the job but not a good finish. Used a parting tool in 2.mm several times for the width of the boring bar, as the depth was 5mm, I went back to the beginning, in 2.5mm to make 4.5mm. Used the boring bar to cut what was required, then used left and right lathe tools to finish to 5mm depth. Starting with the parting tool it did not feel correct as the flat on the tool and the radius on the flywheel it started to bite in. What am I not doing correctly, I have checked on search forum but no results, sorry if this seems such a basic question.

[ROBERT BLACKSHAWParticipant @robertblackshaw40066]

[steveteeParticipant @stevetee]

You might do better with a slot drill which is designed to finish a flat hole , they usually have 2 flutes, which continue along the base to give a flat finished hole. End mills are usually multi-toothed and are more designed for taking material off , ie 'milling', than for finishing holes. As far as I'm concerned, as long as the questioner has looked at previous posts to see if his question has already been covered, then there is no such thing as a basic question, just a question that deserves an answer.

[JasonBModerator @jasonb]

A boring head will always give a bit of a stepped finish to the bottom of a hole as the clearance on the tool and not down feeding exactly the same amount will give slight steps to teh bottom of the hole. A boring & Facing head which advances the tool across the bottom of the hole will do it but they are expensive.

A slot drill will actually give a slightly conical bottom to the hole and also at 18mm dia you may get a bit of chatter when the whole cutting edge is trying to finish the bottom of the hole.

Having the work on a rotary table and using a milling cutter just over 1/2 the dia would give a nice flat bottom.

As for the flywheel its either too wide a cut giving rise to chatter or the edge of the tool not having enough clearance where it runs against the side of teh recees at the OD or at the hub

[Anonymous]

Depends how flat you want the bottom of the hole to be. Slot drills are slightly hollow ground, the same as endmills. The angle is only a degree or two but will not leave a truly flat bottom when plunge cutting. I've just machined some heatsinks for a client which required blind holes for screw head clearance. These were done by 'drilling' with an 8mm three flute centre cutting endmill. In this application it didn't matter if the bottom was truly flat. However, there was a distinct 'pip' at the centre. Presumably this is partly due to the hollow grinding and partly because the centre of the cutter has zero cutting speed and so isn't really cutting.

Andrew

[Tim StevensParticipant @timstevens64731]

Andrew and the centre pip:

It could also be that the centre cutting edge was not exactly on centre, whether because the grinding was not spot on, or the tool not held exactly central. I mean by this no criticism of the work of our contributors, I am trying to ensure that others can understand the various sources of error.

Tim

[Anonymous]

Pip was possibly the wrong word, more of a central small circular area with a slightly worse finish. I can't measure it as the heatsinks have been collected by the client.

The cutter was a Dormer slot drill, nearly new, held in a Clarkson style milling chuck on a Bridgeport mill. Each hole was 'drilled' using the quill. I can't remember the spindle speed, but each cut was fine, no chatter or noise, less fuss than real drilling.

As a matter of interest on the other side of the heatsink I had to 'drill' though 32mm of heatsink fins to provide flat areas for several screw heads. Where, and to what extent, the cutter was cutting heatsink fin or fresh air depended upon the location of each hole. That was done with a HSS 8mm long series three flute centre cutting cutter. Again no chatter or noise. The issue of a poor finish in the middle didn't arise as the cutter met up with a pre-drilled hole for the screw.

Andrew

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