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How to make an accurate internal hole

Whether to drill, bore or ream?

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John Coates05/06/2011 09:09:25
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558 forum posts
28 photos
I need to make an adapter spindle to get my Chester Champion mill/drill back into active working condition
 
Basically the story to this point is that I converted it to 3 phase using an inverter. The spindle on the new 3 phase motor was narrower and shorter than the original single phase so I had to make an adapter to fit the old four cone drive pulley onto the new motor spindle. This was a sleeve with a slot for the keyway that sat inside the cone's bore and was fixed by a hardened 5mm allen socket bolt into the threaded end of the spindle. All worked well until I was cutting a 6mm slot with the machine at 1000rpm (as per Harold Hall's WPS 35) when the cone dropped and I found the bolt had sheared off, leaving part of it in the threaded hole so that method can't be used again!
 
So now I am making adapter mk2. This will have the upper half (30mm OD) surrounding the motor spindle (14mm OD), being secured into the keyway. The lower half will be the new spindle (15mm OD) having a single drive pulley secured by a taper boss. This will sit on a bearing resting in a housing seated on the base of the motor plate so the whole is more rigid than version mk1. Having changed from cone drive pulley to single pulley I now have a lot more clearance to play with and can make a more substantial adapter. The single pulley will be able to be secured in one of two positions to match up with two of the pulleys on the driven cone to give me the greatest speed range.
 
My main concern is to make sure the hole for the motor spindle is an accurate tight fit to reduce potential vibration. Last time I drilled it out sequentially to 13mm then used a boring head until the bore fitted the motor spindle snugly but I would like to use a reamer.
 
So does anyone have any advice or guidance on methods for drilling, reaming and boring, particularly the correct speeds to use by size of tool (I mainly use FCS) as I can't seem to find this info in any of my GHT or WPS books.
 
Also why is the advice always to drill out sequentially? I have tried to drill a 13mm hole straight from a centre drilled "cone" but the drill tip seems to waggle before starting to cut and I am convinced this is truly central and wonder if this is the reason for the advice?
 
Right, plenty of questions there so I had better stop now.
Chris Trice05/06/2011 09:49:43
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1376 forum posts
10 photos


Drills tend to follow existing holes so if you drilled a small diameter pilot hole in the right place, the larger drill will follow it. Generally, one drills a hole a few thou smaller if you intend to finish with a reamer.
Keith Long05/06/2011 10:17:47
883 forum posts
11 photos
Isn't this a classic case for using a "D" bit?
Andrew Johnston05/06/2011 10:37:37
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7061 forum posts
719 photos
The reason for sequential drilling is all to do with the chisel point on the drill. This is very inefficient at cutting, it has a large negative rake. Consequently it requires high axial forces to drive the drill into the work. (See the table in Appendix A of WPS12). With relatively low power machine tools it is much better to use a small drill first, so that the chisel edge on the larger drill is in the clear, and the drill only cuts on the lips, with positive rake.
 
Personally I wouldn't bother with a reamer; I'd rough drill and then bore to size. You're not looking for extreme accuracy on hole size (+/-0.001" should be fine) and by boring you will ensure concentricity.
 
Regards,
 
Andrew
Ramon Wilson05/06/2011 10:39:27
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1655 forum posts
617 photos
Hi John,
 
To achieve a hole such as you intend it is best to drill a pilot hole that is slightly larger than the land between the flutes of the larger drill. This will relieve the pressure on using the larger drill and make it less likely to wander. Sequential drilling will help reduce wandering to a small extent but on this size only one drill between the pilot and largest drill would suffice. However, as already said a drill will tend to follow the previous drilled hole so before reaming it is always best to take the last few thou out to reaming size by taking a light skim with a boring tool. This will true the bore to give the reamer the best possible circumstance to cut true. Leave sufficient for the reamer to cut but not too much - .12 - .15mm on a 13mm bore should be ideal. Lots of cutting fluid and very low revs.
Unless its a very deep bore keep the reamer cutting right through in one hit ie don't retract it half way unless the flutes begin to crowd.
All the above refers to a standard fluted machine reamer.
 
FWIW a drill, used as a 'reamer' by the same method - boring before the final cut will give a good size and a reasonably true hole. The surface finish can be improved by stoning a radius on the corners of the drill - it's a 'get you by' but a reamer is much better.
 
Hope this helps - Ramon
Tony Pratt 105/06/2011 11:21:30
2319 forum posts
13 photos
John, boring to size is the way to go, more control and concentricity guaranteed, reamers seem attractive but can do strange things.
Tony
Andrew Johnston05/06/2011 11:34:36
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7061 forum posts
719 photos
Basic rule of thumb for reamers is half the speed and twice the feedrate that would be used for an equivalent size drill.
 
Regards,
 
Andrew
John Coates09/06/2011 12:24:49
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558 forum posts
28 photos
UPDATE
Just to let you know the adapter has been finished. I bored the hole in the end having found a homemade boring bar in some tools a fellow ME gave me at the Harrogate Show. This was longer and thicker than the one I have so I chose it. I just took lesser and lesser cuts as I approached the finished ID and tested the fit with the motor spindle. Once I was within 5thou I upped the speed to improve the finish and took 1 thou off until the fit was nice and tight. Should take the 1 phase off and put the 3 phase back on tonight so hopefully it will run quieter now due to less vibration as the new adapter is more substantial.
 
John
 
jomac09/06/2011 14:10:38
113 forum posts

I find that if the drills are not sharpened properly, or the part in the drill chuck is worn that the drill will try to cut with one flute only, thus throwing the drill off line, hence a good sharpening device is really a must for truing up a drill. Dave Clarke some where in ME or MEW, said that rounding the sharp outer edges of a drill, makes it into a good cheap reamer????. although I have not tried that method yet, as generally I can find a reamer or make a D bit to fit the job, its worth some thought.

John H

Tony Pratt 109/06/2011 17:40:33
2319 forum posts
13 photos
I have used new drills with the corners rounded [stoned] off many times as a form of reamer, leaving a few thou in the hole and finishing with said doctored drill will produce a tight hole as long as the drill is running true and care is taken in the drilling, cutting oil is also a good idea.
In my humble opinion drills won't produce a precision hole on their own and hand sharpened ones can be a nightmare, as has been said you really need some form of mechanical sharpening device to achieve decent results.
Tony

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