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Flat bottom holes

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Tony Martyr23/10/2011 16:15:18
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226 forum posts
45 photos
I have never been able to get a decent flat bottom to a hole in a bronze valve casting using a home-made D-bit, chatter or an undulation has resulted. The next job I have to do is not a blind hole (more difficult) but a 9/322 flat bottom hole with a 5/32 through drilling into which the 45 deg valve spindle head screws. Having given up on D bits I think I will buy a 9/32 slot milling cutter to do the job in an expensive bronze casting.
Is this sensible?
Tony
David Littlewood23/10/2011 16:44:00
533 forum posts
Tony,
 
I have used slot drills before to make flat bottomed holes; it works quite well provided the component itself is pretty solid and cannot bend. If the component has a through hole, you could use a spot-face cutter instead. For flimsy components such as O gauge coupling rods (in which I needed to make a recess for a retaining bush) they have the advantage that the component will not shift and cause a spoiled job. With a substantial item, the slot drill should be fine.
 
David
JohnF23/10/2011 21:04:29
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1243 forum posts
202 photos
Hi Tony,
Its worth buying a 3 flute slot drill they are stiffer and more stable when cutting so less likely to chatter.
 
An FC3 cutter would probably fill the bill--try J & L Tools they someitmes have them on offer http://www.mscjlindustrial.co.uk/cgi/insrhm
 
John
Tel23/10/2011 21:19:31
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157 forum posts
28 photos
 
I've always had much better results with an ordinary 'D' bit rather than end mills/slot drills tho'
Andrew Johnston23/10/2011 22:31:45
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7061 forum posts
719 photos
Conversely, I've never had a problem using slot drills in the lathe and vertical mill to make flat bottomed holes.
 
The only caveat I would add is that in bronze a positive rake slot drill may have a tendency to get dragged into the work, in similar fashion to a twist drill. This can be ameliorated by ensuring that the setup is rigid and, more importantly, keep the feedrate up. Use a fairly aggressive feedrate, stop at depth and immediately back out.
 
Regards,
 
Andrew
colin hawes24/10/2011 16:14:06
570 forum posts
18 photos
Tony please note that slot drills are not ftat on the end but relieved to their centre to aid their guidance.To get a flat bottomed hole means regrinding a slot drill's end.

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