How To Buy A Drill Press


by Grant Erwin


I. Introduction

This is a monograph which summarizes the lessons I have learned in buying and owning inexpensive drill presses. Many of the problems I describe can be entirely avoided by buying high quality machines, but these machines command a price few home shop enthusiasts can afford.

My first Taiwanese drill press was a bench model. I liked it right away from a usability perspective because you could crank the table up and down, the table had T-slots and a central hole, and the spindle was a standard 2 Morse taper. In addition it had 3 pulleys which gave 15 speeds, and an integral light.

I used that drill press for a long time but was frustrated by its three major failings. Its quill rattled and shook during extension, the entire machine vibrated excessively, and there was excessive runout in the spindle taper. It was possible to do good work with the tool, but it was frustrating. Here are some of the problems that manifested from the conditions described above.

To precisely locate a hole in a drill press, you can use a wiggler. (See http://www.loganact.com/mwn/howto/wiggler/wiggler.html for Mike Rehmus's excellent writeup on a wiggler). I learned about a wiggler, got excited, dug around my old tooling and found one, put it in my drill chuck and discovered that it would run true when the quill was retracted but as soon as the quill was lowered and the rattling would start, the wiggler would fly into its "helicopter" position. This was discouraging. In practice, it meant I could not use a wiggler. I will discuss this condition and how to diagnose it below.

This drill press, as I mentioned, was a bench model. It sat on a sheet metal tool stand, which sat on a wooden floor. If I put a cup of water on the drill press table with the machine turned on, you could easily see standing vibration waves on the surface of the liquid, at least until the cup "walked" off the edge of the table, it was that bad. Vibration to this degree is common in inexpensive machines. It reduces bearing life, increases power consumption, and makes it more difficult to get good finish.

Finally, when I measured the runout in the spindle (measured in the internal taper) I found about .007" TIR. Not bad, but not good either. This condition manifests itself in the point of the drill bit wavering around in a little circle when you are trying to get it to start in a center-punched hole. Once it starts it should run true, though.

When it came time to buy another drill press, I considered upgrading to a heavy precision US-made tool, but the cost was prohibitive for my needs. Since I like the usability of the import drill press design, I decided to analyze the source of the problem conditions I have found and to look for a machine on which these conditions either do not exist or can be corrected easily and within my overall budget.

II. Problem conditions and diagnostics

OK. Here is my list of problem conditions which are easy to spot:
  1. quill rattle
  2. excessive vibration
  3. more than .001" runout in the spindle taper
  4. more than .002" clearance between the quill and the head casting
The only way to find a machine which avoids these 4 problems is to go look at it and run simple diagnostics. Here are the diagnostic procedures I used for each condition.

QUILL RATTLE

If you haven't seen a drill press with this problem, you may not be familiar with the symptoms. It is my understanding that the problem noises and vibrations are the result of excessive spline wear. If the splines are worn it is my belief that a reconditioning job is beyond the capabilities of a home shop.

Anyway, with the quill fully retracted, and no power to the machine, if you grip the chuck and rotate it one way and then the other, you should not feel a loose "slap". It should feel solid and not loose. Then, holding the chuck with your left hand and the downfeed lever in your right hand, slowly lower the quill through its entire extension, repeating the back/forth rotation, looking for areas where there is excessive play. If there is, you will certainly notice it. The noise will sound different, and you will feel the difference. If you think you detect wear in the splines, then turn on the machine and lower the spindle slowly, listening to the sound. If you are looking at a machine with significant spline wear, you will absolutely hear it while lowering the spindle under power with no load. In my opinion spline wear is a showstopper.

EXCESSIVE VIBRATIO

Causes of vibration:
  1. Motor-related problems
    bent shaft
    worn bearings
    static or couple armature imbalance
    electrical imbalance (eccentric air gap)
    bad motor mount (misaligned motor bearings)
    bad motor assembly (improperly torqued end caps)
  2. Pulley-related problems
    misaligned pulleys
    imbalanced pulleys
    eccentric pulleys
    worn, low quality, or improperly tensioned belts
  3. Machine construction-related problems
    machine inherently lacks damping
    machine insufficiently rigid
    thin sheet metal
To check for vibration, simply run the machine. Listen for sheet metal rattle. Place an object on the table and see if it "walks". Feel the machine in various places. Lower the spindle while listening. Put your hand on the motor. Look at the belts. If they are flopping, try tightening them under power (carefully!).

If you hear sheet metal rattle under power, try opening and closing the lid of the belt guard slowly, feeling the vibration. I have seen a case where there was little vibration in a machine but it happened to be at the resonant frequency of the belt guard. Wide open, it was fairly quiet. Fully closed, it rattled slightly. But open 1", it sounded like an amplified hornet. The condition of resonance can only be cured by adding or removing mass or stiffness. For the above case, the rattle was greatly reduced with some magnets. For many imported drill presses, the simplest way to deal with resonant vibration in the sheet metal belt guards is to put a chunk of lead on top when they are closed.

1. MOTOR-RELATED VIBRATION

To eliminate pulley and belt problems, try running the machine without belts. If the motor by itself vibrates excessively, try shutting off the power, then turning it back on, then back off again. The point is to see if the vibration stops when the motor is at speed but not powered. If so, there is likely an eccentric air gap in the motor and it probably needs replacing. If not, then try removing the motor pulley. If that stops the vibration then that pulley is the problem. The next step is to remove the motor and examine its mount while loosening the bolts. If the sheet metal "ears" aren't parallel to the motor base, it is possible that the motor mount is causing the problem. The diagnostic for that is to run the motor without having it bolted down to the mount. If it then runs smoothly, the problem is what is called a "soft foot" which means that the motor lacks a flat mount, which causes the shell of the motor to "wrinkle" when torqued down. This can be corrected either by shimming or by fixing the motor mounts directly. If the motor vibrates excessively with no pulley and not bolted down, then it may have a condition of static or couple imbalance. Your choices are to pay to have the motor balanced, to buy a new motor, to try to balance it yourself, or to live with it. I have included some tips on home balancing in Appendix A.

If you really want to try to get to the root of the problem, and if the tool vendor will let you, you can put the motor on a bench and run it. Loosen the end cap screws slightly to allow the bearings to align themselves, then slowly and carefully retorque them. I have done this with excellent results, which proves that assembly technique can be related to motor vibration, especially for inexpensive motors with sheet metal end caps.

If the motor is not the source of excessive vibration, it is time to examine the pulleys.

2. PULLEY-RELATED VIBRATION

Belts and pulleys can be made to run vibration-free to the extent that the pulleys can be balanced and aligned, and were not machined eccentric. The belts need to be high quality and tensioned correctly. It is not always possible on these drill presses to align the pulleys. The spindle pulley is taper-mounted on many import drill presses, and as such is not adjustable in the vertical direction. The motor pulley can always be aligned precisely with the spindle pulley. The center pulley must therefore be able to be aligned with the spindle pulley. One condition on imported drill presses that is of concern, therefore, is misalignment of the center pulley.

With the belts removed, grasp the middle pulley on the top of the machine. This is the one that acts as an idler between the motor pulley and the spindle pulley. Pull up to see if there is excessive play between its mounting pin and the corresponding hole in the head. I have seen machines where there is over .025" of play. This condition manifests itself as pulley misalignment, and causes vibration. The hole in the casting should be bored parallel to the spindle, and the mounting pin for the center pulley should fit solidly in the hole with little "rock". The pulley should swing smoothly about the mounting pin, and should always remain aligned with the spindle pulley.

If the plane of movement of the center pulley is not perfectly horizontal, it will not be possible to have it in alignment in all positions. A bad drill press will have over 1/4" of misalignment of these two pulleys. Look for one that is well within 1/16". If the drill press you are examining has severely misaligned pulleys, it will require correction to run true.

Note that there are different designs of the center pulley shafts. On one model I looked at, the pulley was pressed onto a bearing so adjustment in the vertical direction was easily done in a bench vise by just lightly pressing the pulley to move it in the desired direction. If the entire pulley sits slightly too low, it should be possible to slip a washer under the part of the center shaft that inserts into the head casting, thus raising the pulley slightly.

It is also possible that the pulleys are imbalanced. It is possible to correct this, however, shop time at a precision balancing shop is expensive. It is also possible to balance pulleys using the knife-edge method, in a home shop. However, the spindle pulley, being mounted on a taper, will be difficult to balance inasmuch as it will be difficult to put it on a shaft to balance it.

Finally, even if the machine has high quality balanced pulleys, and if the center pulley seems tight and is aligned with the spindle pulley, there may still be vibration due to the pulleys being eccentric. I found a couple of real bad pulleys by just lightly touching the edge of them with a finger while the machine is running. If the pulley isn't round or if its hole isn't centered, you will feel a distinct "buzzing" on your finger. Also, the belts will look like they are "hopping" instead of appearing motionless as they should. Of course, it is possible to check this condition with a dial indicator. If a pulley is only slightly off it can probably be trued up in a lathe, or even be replaced.

3. MACHINE CONSTRUCTION-RELATED VIBRATION

Expensive drill presses are heavy and have massive castings and large thick columns that are much more rigid than imports. A column that is insufficiently rigid acts as a spring which can resonate at certain frequencies. I have heard the suggestion that the column be filled with oil-soaked sand, pea gravel, or concrete to dampen vibration. In addition, the motor can be rubber-mounted. This added dampening may help, or it may be necessary to add rigidity to the column itself. I have seen suggestions that a clamp be fabricated which clamps around the top of the drill press column and bolts to a wall. This is said to significantly reduce overall vibration.

I found an article on a mechanism called a "dynamic absorber" which looked like it might work for a drill press. I am not going to describe this in mathematical detail but I will briefly describe the technique and give the reference and hopefully it will be easy to find the reference if needed. This information is below, in Appendix B.

Vibration causes excessive noise and shortens bearing life. I also believe it detracts from the finish of the hole being drilled, and can make it more difficult to align the bit with the workpiece. I am of the opinion that it is well worthwhile to go through a machine and remove the vibration. It makes the tool much more pleasant to be around.

SPINDLE RUNOUT

To check spindle runout, you will need some measuring equipment, plus some tooling to remove the chuck. I use a 2MT drift pin and a chunk of type metal to remove the chuck. I use a magnetic base and a dial test indicator to make the runout measurement.

Remove the drill chuck, insert the test indicator, and rotate the spindle by hand. Note the extreme values. Their difference is total indicated runout, or TIR. A good spindle will have less than .0005" runout. A bad one can have over 1/32" runout. My old one had .007" which isn't that bad but I wanted better for metal working. Look for a machine with less than .001" TIR on the internal spindle taper.

PLAY BETWEEN QUILL AND HEAD CASTING

To check play between the quill and the head casting, I use a set of feeler gages. With the quill fully lowered, push it sideways and test clearance with the feeler gages. I would consider over .002" play to be a showstopper.

It is possible to fix this condition by removing the quill completely, entirely disassembling the head, mounting it on some machine tool and boring out the spindle cavity, and then sleeving it. Of course, it is also possible to build a drill press from scratch. In my opinion I would recommend choosing a drill press without excess play over taking on such a restoration project.

APPENDIX A: BALANCING IN THE HOME SHOP

I was recently able to largely balance a motor which had a bad static imbalance. I used a knife-edge setup as follows:

I started by leveling my surface plate carefully. If you don't have a surface plate, you can use the top of a table saw or some other flat surface you can level. I bought a piece of 1/8 by 1" O1 ground flat stock and machined a 45 degree bevel on one end. (I did this with a 12" disc sander with a tilting table - a milling machine may be much faster.) I cut the 18" beveled stock in half, giving me 2 roughly 9" pieces. I used 4 3/8-16 screws and washers as rough "clamps" on the sides of 1-2-3 blocks. I then clamped a block to the end of each knife. I lined up the knives so they were flat and parallel and level.

I then put the motor's armature (aka "rotor") on the knives. I saw that the two ends of the shafts were of different diameters. On one such motor there was a narrow area close to the rotor body where there were equal-sized shaft diameters. I used those. On another such motor I had to turn a suitable bushing in the lathe. Anyway, once on the knives the rotor with the static imbalance immediately tumbled to its position with the heavy side down. I drilled round the rim on both ends of the rotor to bring it as closely back into balance as I could, frequently checking the balance on the knives. Reassembled, that particular motor ran far more smoothly.

One more note on home balancing - it is possible to remove weight from the heavy side by drilling as just described. In a couple of cases I have not been able to remove enough weight this way and I have found a method that worked well for me, to add weight to the light side. Start with an old wheel weight. Then, on the light side, make a cone-shaped indentation with the drill (i.e. start a hole but don't penetrate through). Then take the whole thing to your kitchen, and pick your rattiest kitchen spoon. Cut off a chunk of the wheel weight about 1/4" cubic, and melt it in the old spoon, then pour it into the hole you drilled. It will not bond, but it will sit there and bead up on top and will exactly fit the indentation. Then, after everything is cool, put a drop of superglue into the indentation and then put the just-cast weight in, just as you poured it. Hold it there for 30 seconds or so. This glue worked just fine for me. I was able to make the light side just slightly heavy in this manner, and then hit the raised part of the weights with the belt sander a "kiss" at a time until the part came balanced on the knives.

APPENDIX B: BUILDING A DYNAMIC ABSORBER

The idea is to add a piece to the machine that behaves like a cantilevered leaf spring with a weight on one end. A flat spring with one end fixed and a pendulum-like weight on the other end has a definite resonant frequency. It is possible to "tune" this frequency by moving the weight in and out slightly. Anyway, if a machine is being excited by a vibration source and it is vibrating excessively, if at the point of excitation such a leaf spring-weight assembly is attached and tuned to the driving frequency, the idea is that most of the energy is transmitted to the pendulum (the "dynamic damper") and not to the machine. I designed a small damper for my drill press, and it does seem to help.

Here is my design. It can be used as is for drill presses with NEMA 143 frame motors (3/4" shaft, 4" bolt hole separation in the direction parallel to the shaft, 5 1/2" bolt hole separation in the direction perpendicular to the shaft). I made mine all from 1/8 by 1" mild steel flat bar. I cut two pieces of flat bar so that they bolted onto the motor mounting bolts, and had 7" sticking straight down below the motor. This is the cantilevered spring, albeit split into 2 parts. I then cut 4 pieces 8 1/2" long, and drilled 4 3/8" holes to bolt them. Starting from an end, the holes are in 1/2" and 2 1/2", so they straddle the springs hanging down. I cut 1 1/2" pieces of 5/16-18 allthread, and used nuts and washers to bolt the 4 pieces together, clamping over the springs. All together, the 4 pieces of flat bar, the allthread, the nuts and washers form a weight. A weight suspended from a spring forms a system which has a natural frequency.

You can "tune" the frequency by adding washers, or by moving the weight up or down. When I get mine within 1/16" of the right spot, it really takes off. When it is operating, soaking up the vibration, I can balance a nickel on the table of my drill press.

The design equations were published in a book titled, "Practical Solution Of Machinery & Maintenance Vibration Problems" by Ralph Buscarello, pub. 1979. I found this book in the Seattle Public Library.

March 1998
Grant Erwin
Seattle, Washington


Email: Grant Erwin

Return to Home Page

Rev: 04/06/98