D bits

[SignalFailure]

A few questions if anyone has answers....

What is the recommended speed for using D-bits compared to drilling

Can they be safely used when the hole is deeper than the flat (on the D-bit)

How much undersize should the initial hole be i.e. roughly how much should the D-bit be removing?

I've not had problems in the past but had two 'seizures' today, one in cast iron which I've never tried with a D-bit before and one in a deep hole in brass.

TIA

Paul

 

[peatoluser]

Hi Paul,

i've only ever used homemade D bits from silversteel to flatbottom holes, not to ream to size.
as it's a carbon steel as opposed to HSS, I use a speed of about 1/3rd of that for HSS and plenty of cutting oil.
when I make them, I always reduce the dia. that enters the hole by about .020" ,not for swarf clearance, but to stop any chance of binding.
but the 'business end' (the part with the flat) I leave the same size as the hole.

hope this is of some help
peter

 

[SignalFailure]

Thanks Peter, reducing the shaft seems sensible with hindsight! Oil on brass and cast iron...I never use it normally, maybe I should!

Thanks again

Paul

 

[oops again]

Hi Paul
They can be tricky to use as you have found. Only aim to take a few thou out with them if you want them to cut to an accurate size, otherwise as much as you like provided they are started ( putting in a square sholder ect). Peter is right on the money with reducing the shank and lubricant, just leave about one diameter at the finished size behind the D. The finished size and speed and feed are conected. The slower and finer the speed and feed the better the finish and the closer the finished size will be. I make mine about 1 thou under the finished size.
I figured it out with the suck it and see routine. Managed to stall the lathe in top gear ounce, it has a 2hp motor and stopped in a split second, the part looked a bit secondhand after that. I overliiked the fact that Alloy expands when heated and didnt think to slow the machine down. We have our moments. They cut tru provided they are started right and are happy cutting deep holes provided the chips are cleared and the machinest dosen't get bored and try to hurry things with a more agressive feed (poor finish). Hope this helps Dave

 

[SignalFailure]

Thanks Dave, very helpful. I've probably being trying to take out a bit too much e.g. starting from a 2.8mm drilled hole to get a 3mm finished hole. Once the current project is finished I'm going to make a proper set of D-bits in the sizes I most often use (3,4,5 and 6mm).

Paul