Drilling and Tapping MachinesDrilling machines are the most common machines in a metalworking shop. Not surprising, since drilling holes is the most common operation performed in metalworking shops. Drills come in all sizes, from tiny bench-top machines used by jewelers and electronics manufacturers to huge radial drills found in fabricating shops and companies manufacturing offshore drilling rigs. Tapping can be performed by a special machine designed to do nothing other than tap holes, or a drill with the ability to tap, either with a separate attachment, or with the capability built into the machine.
|
|
Cylindrical grinders are constructed similar to an engine lathe in that the part is held between centers, or, in the case of short parts, in a chuck or collet. The metal cutting is done with a rotating grinding wheel instead of a single point tool (on a lathe). The principle difference is the precision achievable with a grinding machine is several orders of magnitude better than what can be achieved with a lathe. Also, surface finish is dramatically improved. While lathes leave cutter marks on the work piece, grinders are capable of erasing those marks and leaving behind either a mirror finish or a brushed look. The difference between mirror finish and brushed look is the choice of the grit in the grinding wheel selected for the job. A fine grit wheel will produce a mirror finish and a coarse grit wheel will give a brushed look to the metal.
The size of a drill press is generally given as the twice the measurement from the spindle center line to the column support of the drill head. In other words, the maximum diameter of a part you can drill a hole directly in the center of. Therefore, a 20" drill press can drill a hole in the center of a 20" diameter disk. Radial drills are likewise measured usually in feet 4 ', 5', 6' etc and, again, it is the measure from the spindle centerline to the column, but this time you can double that size to find the diameter of a part it can drill into the center of. Other size related specs are the spindle horse power, the taper in the spindle and the stroke of the spindle all relating to the diameter hole that can be drilled and the depth of the hole that can be drilled. The spindle taper determines the tooling required big drills generally have Morse Tapers 4MT, 5MT & 6MT are not unusual size tapers for radial drills, smaller drill presses usually are 2MT or 3MT or Jarno #33 tapers.
There are two types of feed on drill presses one is called sensitive and the other is power feed. The sensitive type is fed down by hand by the operator who can feel the drill cut into the metal (hence the name sensitive). The power feed drill press uses air or mechanical means (a gear box) to push the drill into the metal. Power feed allows the machine to drill a much larger diameter hole than is possible with a sensitive drill.
The types of drilling machines made are numerous. There is the turret drill that has 6 or 8 different spindles that can be indexed to drill, ream, countersink and tap different size holes. There is the multi-spindle drill that can come down and drill an entire whole pattern in one motion. There is the radial drill that has the largest reach some can drill into the center of a part as large as 24' in diameter. There is the Babin circle drill, which indexes and drills perfectly symmetrical hole patterns in pipe flanges, all automatically. There is the inline drill that drills multiple holes in a straight line in pipe to make industrial gas burners. There are bench top high speed precision drills for jewelry and electronics. Horizontal drills are used in production applications, because the chips fall away from the parts and clear themselves. There are gun drills for drilling gun barrels and deep hole drills that use high pressure coolant to drill far deeper holes than are possible with a regular drill press. There are drills made for every conceivable purpose.
Tapping as a feature on a drill press usually means that as the spindle goes down into the part the machine will hit a stop and reverse the rotation at the correct time and the tap reverses and backs out of the part. If there is a feed matching exactly the correct TPI (threads per inch) you can tap that thread with a solid tap holder. For example, if you have a feed of .125" per revolution (IPR) you can tap 8 threads per inch without a special tap holder. If you need to tap a TPI that you don't have the right feed for, you can use a floating tap holder which lets the tap pull itself down into the hole as it rotates and the holder allows for a certain degree of movement (the float) so the tap doesn't break (because the feed is wrong). Tapping machines generally use leadscrews of different TPI to adjust their feed to match the tap being used. These machines can cut any thread with the proper leadscrew without a special holder. Drill presses without a tapping feature can tap holes using an attachment called a Tapmatic (made in Switzerland) or similar attachments. It lets the tap float and handles the reversing function as well.
When looking for a drill press, know what kind of work you want to do the physical size of the parts and the diameter and depth of the holes are key, as are the features you'd like for example, tapping or coolant or power feeds. In a used drill, condition is important is the spindle quiet and vibration free? Is the table chewed up? Does the dealer stand behind the machines he sells? Will he take it back if it doesn't work for you, or for any reason? These are the kinds of things you should be looking for.
|
|
|
