Reaming holes

[sanddancer]

Hi everyone,

I am about to start on a Stuart triple expansion engine, & have gathered info from articles in the Model Engineer magazine. The accuracy required demands the reaming of gunmetal blocks for main bearings etc. but I have no experience of reaming or reamers. Obviously, adjustable reamers give a better range of sizes, and can be useful for other project, so can anyone tell me whether it is advisable to have specific sizes, or will a set of adjustables do the job. Also, is hand reaming more accurate than machine reaming. A full set of reamers will be expensive, so now is the time to make sensible decisions. All advice will be gratefully received,

regards,

George

 

[mnewsholme]

on a related topic how do you adjust an adjustable reamer? I'm assuming you just measure across widest point with verniers? Is this right or is there a better method.

Matt

 

[Bogstandard]

Hi George,

These are my own personal views on reaming, and may be incorrect in other peoples eyes.

I only do hand reaming as a last resort, I almost always try to ream using my machines. Hand reaming can sometimes give spurious results as you are relying on being able to keep steady pressures in the same axis all the time. Sometimes hand reaming is the only way you can achieve what you are after, so if that is the case, I drill the holes so that I only have a very small amount of material to remove, and hope that the reamer follows the drilled hole, as it should.

With regards to adjustable reamers. Unless you can set them very accurately using a micrometer, and then treat them as a hand reamer, it is very easy to make oversize holes. I have used them and 'crept' up on a size, to obtain acceptable results. But I personally don't like them. You will also find, that because of the way they are made, you can not get the very small sizes usually associated with model engineering.
Normally 1/4" is about the smallest they go. I would like to add, that adjustable reamers can sometimes get you out of sticky situations, as to wait for a specific size to be made for you, could be both time consuming and costly

I suspect you are from the UK, if so, you might like to give these people a try.

http://www.tracytools.com/index.htm

They don't show everything they have on their site. I always call them up and ask them to make up a set for me. They are very reasonably priced for their full basic sets.

John

 

[Bogstandard]

Matt,

To answer your question, I have attached one of my famous rough sketches.

As you can see, the size is adjusted by moving the blades up a tapered ramp (exaggerated on sketch). The reamer has to be kept spotlessly clean in the ramp area, as one bit of swarf under any of the blades could cause havoc on your sizing. They have to be treated as a precision instrument, not as people usually treat them, thrown in the bottom of the tool box waiting for the next time you need a sized hole. I have a few that were purchased in a job lot, and would be ashamed to show them to anyone who was visiting.

You should always do your measuring at the drive end of the reamer (the big nut end on my sketch). This is because, if it is a good quality reamer, there will be a slight tapered lead in on the smaller end. I would recommend a micrometer rather than a vernier. You very gently measure across the tips of opposite blades.

They are like any other precision device, you learn from experience just how much to tweak each nut to get to where you want to be.

Hope this has helped

John

 

[Bernd]

I'd like to add to Bog's explination. Never run a reamer backwards as this will damge the cutting edge. Also use plenty of oil while reaming. Try to get the drilled hole to with in a -.010" to -.015" (-.25mm - -.38mm) of the finished reamed hole. Example: 1/4" reamed hole, use a "C" (.242") drill. Use low RPM.

Any additions or corrections to the above guy's?

Bernd

 

[LCT]

Reaming is as much an art as it is a science. There are many things that can affect hole size. Coolant type and amount (water soluables give larger holes, no coolant is larger than that, most oils give tighter holes, and tap fluides even tighter) The rpm will affect your hole size (typically a reamer is run at 1/3rd to 1/2 the speed that a drill bit would be run at.) Your feed rate can also affect the hole size. The faster you feed, the larger the hole. Also, the amount of stock left in your hole will affect the size. If you leave too much stock, the reamer has trouble clearing it's chips, and will cut larger. Too little stock, and it has trouble getting a good "bite" which will cause it to dull and cut smaller.

For most sizes, using cobalt reamers, I'll drill about 1/64th undersize, turn the reamer at about 30 sfpm and feed the sucker like I'm trying to get some work done. On very small holes (under .125") I'll leave less stock.

Adjustable reamers are a very last result. Usually, I'd rather lap a hole than use them. It's difficult to get a good size with them, and often you're hole won't be round. They are also pretty fragile.

 

[LCT]

on a related topic how do you adjust an adjustable reamer? I'm assuming you just measure across widest point with verniers? Is this right or is there a better method.

Matt

The blades in an adjustable reamer ride up and down an angle to cause them to expand. They are held in place, only on the ends of the blades. Because of this, the blades may flex, or lift from the bottom of their grove slightly making one blade touch before the others. The only way I've had sucess in using or adjusting them is to get the diameter close, then turn them by hand through the hole. Adjust to take as small a cut as you can, then measure the hole. Repeat until you have the size you want.

 

[kvom]

My shop instructor states:

drill 1/64" under holes up to 1/2", and 1/32" inch under holes larger than 1/2" for machine reaming. For hand reaming allow no more than .005" and preferably .002.

Reaming speed is normally 1/2 the drilling speed, feeding .0015-.004"/flute/revolution, always using cutting oil.

 

[Loose nut]

Fixed sizes should always be a first choice. That being said adjustable reamers are handy to open out a hole, you can't put a 3/8" rod into a 3/8" reamed hole, so an adjustable reamer can open it out a half thou or so, instead of having a set of over/under reamers. They take a bit of practice and patience to use.

 

[sanddancer]

Thanks to all for the advice on reaming gunmetal castings. It now becomes clear that machine reaming is more accurate than hand reaming, but there are still a few grey areas on this subject. I assume that chucking reamers are for machine reaming, but can someone confirm this. I am also confused about straight or tapered reamers, but assume that a straight reamer is best if only a few thou needs to be removed. The final difficulty is knowing whether to get a straight bladed or spirally configured reamer. Once that is clear I can dig up the family jewels from the back garden & invest in the best reamer that I can afford. Once again any help will be gratefully received,

regards,

George

 

[Bogstandard]

Hi George,

There has been another post on here about reamers not too long ago

http://www.homemodelenginemachinist.com/index.php?topic=2290.0

It might give you some insight into what to look out for. I posted a few piccies on generally available ones.

Just to answer a couple of your questions.

Chucking reamers are used for machine reaming, but they would most probably be no use for line reaming (crankshaft supports) because they are just not long enough.

Straight and tapered reamers. Both can be either machine or hand.

Straight reamers are for cutting holes with parallel sides (like a very accurate twist drill). Hand straight reamers usually have a fairly long tapered lead in, so you have to make sure that the reamer has to pass a long way thru the hole, so it has reached the parallel part of the reamer, otherwise you will end up with a tapered hole. They are no use for a bottomed hole where the hole doesn't go all the way thru the part.
Machine straight reamers have a much smaller lead in, but again they should go all the way thru the job. You can buy special hole bottoming reamers that are parallel all along the length. Usually of the chucking variety.

Tapered reamers are just that, they make a tapered hole for fitting taper pins, or in my case I use them for cleaning out or making morse taper fitting holes. If you want to go overboard, you can also buy tapered drills for drilling the initial hole.

I hope this answered most of your questions.

John

 

[sanddancer]

Cheers John, it becoming clearer now, but still a lot to learn!

regards,

George