Poor Finish in Reamed Holes

[crueby]

Thanks guys!

[jadge]

I have the same question - doesn't a boring head/bar give a round, located hole, is reaming after that neccessary?

The theory is that a drilled hole may not be straight. So boring after drilling ensures that the hole is straight and round. But accurate measurement of holes is non-trivial and/or expensive. So using a reamer after boring gives an accurately sized hole with a good finish without the need to faff about.

Having said that I rarely bore then ream. I don't have boring bars for small holes, and I find that quality (Dormer) drills in the lathe seem to stay pretty straight, with no tendency to go walkies. So no need to drill, bore and then ream. While I have larger reamers (up to 1¼") I rarely use them. For holes much above ½" I bore to size after drilling against either a sample of the cold drawn bar that will be used as the shaft, or for machined shafts a ground test piece.

I've never heard the idea that you should withdraw the reamer with the work stopped. I've always withdrawn under power. However, advice is to never run the reamer backwards as it damages the cutting edges.

Andrew

PS: Ah well, much to slow on the typing, but having labriously done so I'm darn well going to post it.

[steamer]

What Ian said...and leave less stock for reaming...I usually drill, bore to .005 under, then ream.  The boring operation straightens the hole and makes it round, and the reamer then follows the straight hole.

Please forgive my ignorance, but why ream AFTER boring? I don't understand what reaming achieves that the boring didn't?

(Apologies to the OP for hijacking the thread)

Far easier to get consistent size with a reamer

[steamer]

I have the same question - doesn't a boring head/bar give a round, located hole, is reaming after that neccessary?

The theory is that a drilled hole may not be straight. So boring after drilling ensures that the hole is straight and round. But accurate measurement of holes is non-trivial and/or expensive. So using a reamer after boring gives an accurately sized hole with a good finish without the need to faff about.

Having said that I rarely bore then ream. I don't have boring bars for small holes, and I find that quality (Dormer) drills in the lathe seem to stay pretty straight, with no tendency to go walkies. So no need to drill, bore and then ream. While I have larger reamers (up to 1¼") I rarely use them. For holes much above ½" I bore to size after drilling against either a sample of the cold drawn bar that will be used as the shaft, or for machined shafts a ground test piece.

I've never heard the idea that you should withdraw the reamer with the work stopped. I've always withdrawn under power. However, advice is to never run the reamer backwards as it damages the cutting edges.

Andrew

PS: Ah well, much to slow on the typing, but having labriously done so I'm darn well going to post it.

I guess we'll have to agree to disagree....

[steamer]

Typically the reamer would give you a smoother finish to the hole. The boring prior to reaming assures it is round and straight. Depending on how much care is taken with the boring operation the bored hole and the reamed hole could be almost identical as far as smoothness though. The definition I always used for reaming when teaching the machine shop class was that it gives you an on size and smooth hole. As others have said though, it won't correct for a wandering hole if the drill doesn't drill straight and thus the need for boring to true the hole up. For most of us though, reaming 3/8" holes or smaller, boring becomes a more tedious thing. The boring bar must be smaller and therefore less rigid. Despite the theory, for holes say 5/16" or smaller, like for shafts to run in for example, I just drill and ream, but do try as much a possible to make sure the drill is sharp and properly ground so it wanders less, and giving it a good start with a center drill or spotting drill. For me this has worked successfully almost all of the time. Just my take on it.

I do disagree with Steamer on one point though. I think the reamer should go in and out both under power, but withdrawing it should be even faster than the reaming cut going in to minimize any rubbing of the flutes.

Bill

Understood.....I only have boring bars down to .060"....they can be tedious to make...but they do work.

A D bit is a good substitute for a Boring/reaming operation.

Dave

[Kim]

Thanks for the excellent discussion everyone! This was all very fascinating and educational for me, and I hope for others too.

Thank you for all the suggestions and help!
Kim

[Ian S C]

My reason for reaming after boring is that it usually takes me quite a time running the boring bar back and forth to take out the spring in the bar, and get the bore dead on parallel, the reamer make sure of that.
Ian S C

[DTR]

Thanks for the replies, ladies and gents. Apologies again to the OP

[Zephyrin]

I generally use the reamers by sliding the tailstock on the lathe bench, not by turning the handwheel, one go forward and a little bit faster backward. And a reamer removes a tenth of a mm on the diameter, ie, drilled hole is 7.9 mm for a 8.00 reamer.
It gives round, clean and perfectly sized hole through, but that the point with reamer, it is good for through hole only, dead sized blind holes are much more difficult to get on small size !!

[Gas_mantle]

Being fairly new to this game I only have a few fairly cheap reamers, when I bought them I'm pretty sure they were advertised as hand reamers (the flutes run straight on the shaft rather than having a slight spiral). However after a bit of experimenting I find they seem to work better in the lathe or mill under power at a slow speed of say 70 - 80 rpm.

What actually is the difference between machine and hand reamers ?

Thanks
Peter

[Bluechip]

Hand reamer has a tapered lead and usually a square on the end.

Machine reamer is parallel all the way to the end except for a very short 45^o chamfer and has a MT or parallel shank and no square. Parallel shank is a chucking reamer.

Machine reamers can be specified for RH or LH helix.

No doubt there are exceptions but the above is mostly right ... 

http://www.bing.com/images/search?q=machine+reamer&qs=n&form=QBILPG&sp=-1&pq=machine+reamer&sc=2-14&sk=&cvid=3D262B63551E403E8506ED15447EBA25

Dave.

[gerritv]

A good overview of Reaming, including a table with allowances.
http://collections.infocollections.org/ukedu/en/d/Jgtz102be/2.html

Gerrit