Getting the best from your BBT chamber reamer

Properly used, BBT reamers will give a finish at least as good as any other reamer manufacturer in the world - and better than most. But even the best tooling in the world will only give the performance of which it is capable when properly used. What follows is a few general principles which, if followed, will result in the superior performance and long life you should expect from a BBT chamber reamer.

What coolant or cutting fluid should be used with BBT chamber reamers?
Always use a neat cutting oil and never use a water based cutting fluid. Water based cutting fluids are fine for use on a lathe or milling machine when turning or milling, but have no place in reaming operations of any sort where a fine finish and good tool life is expected. The difference is not marginal - we have seen reamers effectively worn out and ruined after cutting just one chamber when using water based cutting fluids, whereas the same reamer will cut many barrels with virtually no observable wear when using a neat cutting oil. (Yes, we have done the test...)

Neat cutting oils having a high sulphur content are available and popular in the United States for reaming, but sulphur based cutting oils tend to be a breeding ground for sulphur loving bacteria and so such oils can go "off" pretty quickly and start smelling of rotten eggs. Sulphur based cutting oils are also not generally available in Europe. However, highly chlorinated 'extreme pressure' cutting oils such as used in deep hole drilling are offered by all the oil companies. I have found these to be very effective as cutting oils in reaming operations. In particular, I have found the Miller Oils "Millicut CAS 14" to be dermatologically friendly and this is the oil we recommend.

What speeds should be used with BBT chamber reamers?
If you want a good chamber with superior finish, don't be in a hurry! What distinguishes reaming from milling or turning operations is that you are removing relatively little material across a long cutting edge and if you try to push too hard then it will start tearing rather than cutting somewhere along that long edge. Speeds of around 90 or 100 rpm are not too slow at all, and much faster than that is definitely too fast.

This applied to carbide reamers too. It is often thought that with carbide reamers you should be able to use higher speeds and feeds. But the cutting mechanism is the same and so the limits are the same, and you have to use the same feeds and speeds as you would with 'steel' reamers if you want good results. The advantage of carbide reamers is not that they cut at higher speeds and feeds, but that they hold their edge much, much longer than steel reamers.

What feeds should be used with BBT reamers?
Feed should be of the order of 0.01 to 0.02mm (0.0005" to 0.001") per rev. Pushing the reamer at much greater feed that this risks a poor finish in the chamber - but much worse, it invites the reamer to 'grab' or 'take a bite' where it pulls itself into the chamber with a rapidly increasing feed. There is only one outcome and that is catastrophic disintegration of the reamer, which will happen quicker than you can open your eyes in astonishment! The danger of this is particularly high when the reamer is new and the cutting edges are sharp. You have been warned!

'Pecking', not using through coolant
Most gunsmiths will chamber chamber without through coolant and will 'peck' at the chamber, frequently removing the reamer to clear the swarf and renew the cutting oil. ('Through coolant' is feeding cutting oil through the barrel from the muzzle at relatively high pressure, so the swarf is continuously removed and the chambering operation is continuous rather than a 'pecked' operation.) When chambering without through coolant, the chamber depth should not be increased by more than 2mm (0.08") with each 'peck' before withdrawing the reamer to clean off the swarf and wash out the chamber. Increasing the depth of cut more than this risks clogging up the flutes with swarf - especially at the front of the reamer - so displacing the cutting oil which is necessary for a good finish. A squeezy bottle of cutting oil is handy to wash swarf out of the chamber. Too, having a coolant hose playing oil immediately behind the chamber ensures the reamer flutes are flooded with oil as the reamer is inserted and helps wash off the swarf as the reamer is withdrawn. It also ensures the flutes stay flooded with oil when cutting.

Using through coolant
Through coolant in a manual lathe setup is probably best - and simplest - done using the 'suds' pump or a relatively low pressure pump to pump cutting oil through the barrel from the muzzle end via some sort of rotary seal. This is not 'through coolant' in the strict sense as the pressure is too low to pump oil past the pilot at the front of the reamer while the reamer is cutting. However, clearing the swarf from the reamer flutes is a simple matter of withdrawing the reamer 20 or 30mm (an inch or so) so the pilot is disengaged from the barrel in front of the chamber and cutting oil can flood through the reamer flutes to remove the swarf. This only takes a few seconds and then the reamer can be pushed back into the chamber for the next 'peck'.

If the pump pressure is too high, say 100 psi (6 bar) it becomes difficult to actually push the reamer into the chamber against the cutting oil pressure. (In a production situation, where the reamer is mounted in a tool holder in a CNC machine, high coolant pump pressures when using through coolant are not a problem.) It needs this order of pressure for true 'through coolant' to work effective and for chambering to be done in one continuous cut, with swarf being flushed out as they are generated. In these circumstances, it is not sufficient to rely on enough oil passing over the pilot through the barrel grooves, it is also necessary for either the chamber reamer to be fluted through the pilot spigot (which can be done on request, and/or fluted pilots be used to ensure the additional oil flow required is achieved. True 'through coolant' is best done in an automated setting such as when chambering using a CNC lathe rather than a manual lathe.

Reamer chatter
All BBT chamber reamers are made to accept a floating pilot bushing on the front, which ensures the reamer cuts a chamber well centered on the bore axis of the barrel. The pilot also supports the reamer at the front and helps prevent chatter from starting and building up. Chambering without a suitable pilot is NOT recommended. Chatter is not usually a problem on BBT reamers as the flutes are not exactly 60 degrees apart, but are staggered at un-equal angular separation which prevents reamer chatter.

However, if your reamer does start to chatter, (you will feel the reamer starting to chatter on the tail-stock handle as you feed the reamer into the barrel), the cure is to reduce the lathe speed to the lowest rpm possible and feed in with short, gentle, "stabbing" steps. This will break up the non-circular lobes created in the chamber so to re-form a circular chamber once again. Once this is done and no more chattering can be felt, the speed can be increased cautiously.