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- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Why so many different tools?
- Date: Fri, 18 Oct 2013 23:21:24 -0400
http://www.practicalmachinist.com/vb/general/why-so-many-different-tools-273710/
Why so many different tools?
Lets get straight to my doubts:
1) What is the difference between reaming, boring and drilling to increase hole size. The instruction sheets have different tools used for the same operation, how is a specific tool picked among these?
2) How do you spot center of a job in a machining center. And what tool do we use? Spot Drill or Center Drill?
Thanks in advance-
10-18-2013, 05:50 AM #2
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To merely increase hole size, drill it. But to increase it and be accurate, drill it, bore it and maybe ream it. That will put it in the center. I generally use a spot drill to start the hole. Center drills are used for, well...centers. Dead or alive.
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They do different jobs and with differing results.
Originally Posted by nradam123
1) What is the difference between reaming, boring and drilling to increase hole size. The instruction sheets have different tools used for the same operation, how is a specific tool picked among these?
Boring will generally give you a straight hole in the correct place with a good finish, and with care can get a really accurate size. This is a hole enlargement operation after (for example) drilling.
Reaming is intended to remove only small amounts of material. It should give a good finish, but is not intended to correct position. It is a relatively fast process compared to boring.
Drilling is a fast metal removal process that produces a hole roughly to size. For some applications its accuracy in diameter and position will be insufficient and another operation (like reaming or boring) will be needed to meet the required tolerances. Using drills to increase the size of an existing hole is not usually a good idea, unless the size of the drill is large with respect to the existing hole i.e. the existing hole should be not much more than the drill web size. If you use a drill to enlarge a hole that is close to the size of the drill, it will cut only on the extreme outside of the lips and this will cause the drill to move around a lot and possibly chip the drill edges.
If, and only if, you need to spot a centre, use a spotting drill with the correct angle. Originally Posted by nradam123
2) How do you spot center of a job in a machining center. And what tool do we use? Spot Drill or Center Drill?
Is this a homework question? -
@Ray. Thanks
@billmac. Thanks again. This is not a homework question, i am working in a company that manufactures automatic transmissions for offhighway vehicles. But i just finished my undergraduate studies in mechanical engineering and so this is my first industry experience. I am working on a project to improve performance of the machines, and i was looking into tooling now and had these doubts. -
10-18-2013, 07:52 AM #5
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A bit more detail:
Drilling, as noted, is a fast metal-removal method, an easy and common way to place a hole where there is none, and more than adequate if the drill itself is in good condition and the necessary accuracy of the hole is relatively low. However, if the center drilling or pilot hole was inaccurately placed, the drill will tend to "follow", and therefore produce an inaccurately placed hole.
Or, if the drill is poorly sharpened (such as the very common unequal length lips) the hole will not only be oversize, but may follow a crooked path, again producing an inaccurate hole.
Boring uses (generally) a single point tool, and will produce a hole very accurately around the axis of rotation of the machine- usually regardless of how badly offset the starting hole is. However, it needs that starting hole, meaning you have to do two operations- drill, then bore. Naturally, you wouldn't choose to bore the holes if not necessary, as it'd cost time and tool changes.
Boring is also used to produce a hole of a size not commonly available as a drill. If you need to produce a 2-1/56th hole, you could either have a huge and expensive drill custom-ground, or use an inexpensive boring head and produce the hole accurately.
Reaming is generally a finishing process, used to, well, finish a previously drilled or bored hole. You can get a smoother finish, faster, with a sharp reamer than you can with a boring tool, and/or finish a hole too small for a typical boring head. The reamer, however, like the drill, will tend to "follow" a hole- if the hole is offset slightly from the spindle axis, the reamer will flex slightly and follow the hole rather than "straightening" it.
Though again, it's a fast method if the hole needs to be a specific size, but not necessarily an exact location. (Bridge reamers are common for this, used to align two holes in structural ironwork- who cares where the holes are actually located, but they need to be accurately sized to each other for the secure placement of a bolt or rivet.)
The best use of a reamer is as a finish to a bored hole. So it's drilled first to remove the bulk of the metal, bored to get it close to size, straight sided, and perfectly aligned with the axis of the spindle, and then reamed to finish the walls smoothly and to size.
As for spot drills and center drills, two ideas that do the same thing. A spot drill is basically just a very short standard drill- short so it doesn't flex, so the spot it drills is accurately centered to the spindle. A center drill does the same thing, but it has a two-stage tip. The stepped hole it produces was originally intended (and still used) for lathe centers- the tapered sides of the hole give an accurate location for the cone nose of the center, and the clearance at the point makes sure the center doesn't ride only on the very tip of the point.
If you're using one to locate a spot to begin drilling, there's no real advantage between one or the other.
Doc. -
An insert drill is often used to drill a hole (even without spot drilling) then the hole is bored with the same tool.
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10-18-2013, 10:17 AM #7
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Also, there are drills and there are drills. You can buy plain cheap drills, or you can get fancy parabolic flute drills with special point grinds. And about 100 others. Sometimes, with the right drill, you may not need to ream. Pick the wrong drill and then you need an extra operation to clean up the mess. There are also a bunch of other finishing operations like honing and roller finishing you should know about.
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10-18-2013, 11:51 AM #8
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+1 Originally Posted by Conrad Hoffman
There are also a bunch of other finishing operations like honing and roller finishing you should know about.
Historically, gearcases and Sputniks - where improved precision of shaft support/locating holes meant longer operating life for the internals - were among the first uber-high-volume applications for some of these.
Shaft and case materials - CI, steels, light-alloys, bushings/NOT, lubrication/NOT, press or running fit, etc, are all factors/options as well in considering how best to generate and finish EACH of the holes.
Bill
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- [machinist] Why so many different tools?, Lawrence London, 10/18/2013
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