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Deep hole "D" bit advice needed.

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mark costello 124/02/2020 16:45:33
578 forum posts
12 photos

I have a job that comes up every year or two for a Customer. He needs a 1/2" hole 10"-12" deep in Aluminum. He wants no mismatch in the middle. I get a thousand or two when drilling from both ends. He notices it and does not like it, even for His application it does not hurt anything.

Would a "D" bit stay straight over that length?

Withdrawing very frequently would slow things down a lot.

Could I drill a 1/32nd or 1/64 of an inch smaller from both ends and expect the "D" bit to straighten out the hole?

Steviegtr24/02/2020 16:50:53
840 forum posts
191 photos

What's the longest 1/2" reamer available. Meaning to drill undersize & ream to 1/2".


Andrew Johnston24/02/2020 17:09:39
5204 forum posts
599 photos

I've just bought a long series 1/2" reamer for a part that is about 7" long. Longer than that and I suspect you're into specialised (equals expensive) tooling. Best bet would be a gun drill, but ideally they need thru drill coolant at considerable pressure and are not simple to set up. A D-bit will probably sort of stay straight, but there's going to be a very long length of D-bit that is a close fit in the final hole. That's a recipe for something seizing.

Personally I'd explain to the client that it's going to cost him x pounds more per part and does he really want to cough up the extra readies?


not done it yet24/02/2020 17:34:49
4170 forum posts
15 photos

These might just do the job from one end. Not expensive.


Andrew Tinsley24/02/2020 17:43:34
989 forum posts

Long series drills will certainly give you a long hole, but it wont be straight. As Andrew Johnson has already said, gun drills are the only real way to produce long, straight, round holes and that set up is NOT cheap.


Martin Connelly24/02/2020 17:48:37
1026 forum posts
122 photos

Try drilling 12mm from both ends and then a d bit. See if it works.

Martin C

old mart24/02/2020 18:18:53
1252 forum posts
116 photos

I had to ream out a deep hole and only had a standard hand reamer in the correct size. I turned down the end and bored a slightly smaller rod and silver soldered them together. I held the reamer in aluminium soft jaws in a vise to make sure the cutting edges did not overheat and it worked. Starting with a drill 31/64" would probably work fine.

Bazyle24/02/2020 20:00:11
5013 forum posts
198 photos

Drill undersize from both ends as before. Make a D bit with a pilot section up front that is the width of the initial hole and of course a lead in taper. After a bit for alignment behind the cutting point relieve the diameter to reduce chance of seizure.
You mentioned not wanting to withdraw the tool for chip clearance. No need as you have a hole already put a pipe down from the back end with a flow of coolant, lube etc.
The tricky point is the meeting place of the two holes. You know exactly where it is so you might want to make a drill for just this bit with a modified pilot that copes with the transition.

As you are a professional you might want to try asking on the PM forum. Just don't mention you are using an Atlas lathe for the job.

Edited By Bazyle on 24/02/2020 20:02:10

David George 124/02/2020 20:14:27
1121 forum posts
369 photos

In toolmaking when a mould tool is heated you have to fit heaters and they were 1/2 inch diameter and they could be up to 3 feet long. We just reamed them with a 1/2 inch ream welded on to a 12mm diamiter extension bar.


Chris Evans 624/02/2020 21:01:45
1592 forum posts

When I worked in the mould making industry we had our own gun drill set up. Prior to getting this I sub contracted to a company in Coventry who where reasonable cost.

vintage engineer24/02/2020 23:53:37
235 forum posts
1 photos

Shaped charge explosive?

Hopper25/02/2020 00:02:19
4166 forum posts
89 photos

Can you use a chucking reamer where shank is smaller diameter thsn the cutting flutes. Slver solder an extension on the shank. Drill from both ends and ream from one.

Or cheat and do it the way you have been then get rid of the small ridge in the middle by using a length of wooden dowel with a hacksaw cut in the end to hold a strip of emery cloth etc as a flapper. Spin with electric drill and viola! ridge gone.

ega25/02/2020 00:05:12
1512 forum posts
123 photos

Would a core drill do the trick?

mark costello 125/02/2020 15:40:34
578 forum posts
12 photos

It takes around an hour of buffing to get the ridge out. Will try a D bit next time He orders.

John Baron25/02/2020 16:38:18
172 forum posts
69 photos

Hi Guys,

A "D" bit, like a drill, will try to follow an existing hole, so wont straighten a curved one. A properly made "D" bit will create a straight hole that is accurately sized. But it takes a lot of pecking even with a reduced shank.

I think that Hoppers idea of extending a reamer might be the way to go !

John Reese27/02/2020 05:15:23
824 forum posts
Posted by vintage engineer on 24/02/2020 23:53:37:

Shaped charge explosive?

The hole would be bell mouthed.

JohnF27/02/2020 11:02:37
934 forum posts
126 photos

Mark, Plus another 1 for extending a reamer, however it does depend on how accurate you need the hole with regard to straightness and location at either end. If you go this route I would use tin solder to join the reamer to the extension thus less chance of affecting the reamer hardness.

For a D bit of that length, although I have never used one that long I would consider either make a flat on the top of the bit or maybe better a V groove along the full working length of the D bit to facilitate lubrication.

Another method that you could use is spill boring - used in the past to bore gun barrels, shotgun in particular. It achieves an very straight hole but its an old fashioned method and very slow really superseded by modern tooling but its cheap to produce and does work.

Lastly you could make a draw reamer from silver steel [drill rod] as the name implies you pull this through the hole rather than push it through the pull rod being smaller than the hole.


mark costello 127/02/2020 15:05:39
578 forum posts
12 photos

Might try a draw reamer the next time, or doing it in the lathe which has the needed length. Don't know how accurate it would end up.

Mark Slatter04/03/2020 19:27:13
49 forum posts
7 photos

I was going to suggest line boring on the lathe, if the part will fit.

mechman4804/03/2020 20:24:51
2592 forum posts
395 photos

If he's that pedantic; & you don't need the job, which clearly gives you some stress, politely tell him where to go ..thinking


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