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Reamers - what are they and why?

A reamer is a type of finishing tool. When we drill holes in lumps of metal, we typically use a fairly standard drill bit in either our mini-lathe or micro-mill. Drill bits do a pretty good job. If you only need a hole to pass a bolt through, or to tap with a thread, then a standard drill bit will create a suitable hole.

Sometimes our requirements are a little more exacting. What if we wanted a hole which was a tight fit for a ground silver steel bar which we happened to have? Or we wanted to use the hole to hold a rotating axis without too much play?

Normally in the home metalworking workshop when we want a hole in a part which is truely round and of a known inner diameters, we use our mini-lathes to bore out the hole to the required diameter. Now consider the case where the hole has a very small inner diameter - say less than 10mm. It would be difficult to bore out such a narrow hole in a part using a standard boring tool. As the required inner diameter reduces, so the difficulty increases until it becomes impossible.

We need a different tool.

Enter the reamer. The reamer is a sort of cross between a drill bit and a side mill. They are ground out to be a very exact size. A reamer will take a rough hole cut by a drill bit and turn it into a truely round hole of a known inner diameter.

The flutes of a reamer are quite delicate and you should take great care with reamers - they are a known "fixed" precision item - and this helps us to build parts that fit properly - abuse such tools and their precision will be lost.

Why don't we all have a nice wide range complete set of reamer to go with our nice sets of drill bits? Reamers are extremely expensive for what they are. You should only buy one when you really need one. Purchasing a whole set just for its own sake is wasteful of money. I can buy a cheap 5mm drill bit for about 40p in the UK. A 5mm reamer will cost me 2.75 in the same shop.
a typical reamer used in the home workshop on a mini-lathe or mini-mill

You should only use a reamer if you can't get the part bored out on the lathe because it is too awkward, or it the required hole is too damn small to bore. In the sort of work I do, I rarely find a need to use a reamer. I think I only own a couple, and they don't see the light of day very often.

Try not to let the reamer do too much work. Make sure you use a drill bit which is at most 0.5mm smaller than the reamer, otherwise the reamer will struggle and you'll probably end up with a mess and not a nicely reamed hole.

Hand and Machine reamers

You will hear reference to both "Hand Reamers" and "Machine Reamers". The Hand reamer will have a longer "lead-in" or taper at the business end - this is to aid the hand user in starting a hole by hand. Machine reamers are used in machines which are supposed to be rigid and pre-aligned. As we know, some of our home metalworking workshop machine tools are not as rigid and well aligned as we might like, so it is always better to use hand reamers, even if you are using them on a mini-lathe.

You can get completely tapered reamers. These are handy for finishing a sleeve with an internal Morse taper - however it is very rare that you will need one of these in the home metalworking workshop - which is a good thing to, as they cost a bloody fortune.

Just do not forget that a reamer is a precision tool that is only supposed to make a minor enlargement to an existing hole - don't use a reamer to cut a hole from scratch.

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