Making a Chuck Back Plate

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Table of Contents

Calculating the Bolt Hole Positions

Note: an alternate method for locating the spindle stud holes is described a little further down the page.

Next we need to mark 3 (or 4, for a 4-jaw chuck) evenly spaced points on the spindle bolt circle. You can use a dividing head or rotary table if you have one, but if not, you can measure and mark the points using a dial caliper and divider.

There's a simple formula for determining the distance between holes of a bolt circle.

B  = D x sin( 360 / (2 x N))


B = distance between bolt holes
D = diameter of bolt circle
N = number of holes in circle

For the inner spindle plate bolt circle

D = 2 x 1.299 = 2.598, N = 3, B = 2.598 x sin (360 / 6)
B = 2.598 x sin 60, = 2.598 x 0.866, = 2.250

For a 4-jaw chuck (4 mounting bolts)

D = 3.307, N = 4, B = 3.307 x sin (360 / 8)
B = 3.307 x sin 45, = 3.307 x 0.707, = 2.338

For a 3-jaw chuck (3 mounting bolts)

D = 3.307, N = 3, B = 3.307 x sin (360 / 6 )
B = 3.307 x sin 60, = 3.307 x 0.866, = 2.864

Note: 3.307 is the bolt hole diameter specified in the data sheets that came with my chucks. If your chuck uses a different bolt circle diameter, plug that value into the equations above.

Marking the Bolt Hole Positions

Pick any point on the circle as a starting point and make a prick punch mark there. Make sure that the punch mark is right on the center of the line. Set your dial caliper to 2.250" and scribe an arc from that point to an   intersecting point the circle. Now scribe an arc at a point on the other side of the circle.

Circle_.jpg (49587 bytes)

As a double check, position the caliper points between the points where the two arcs intersect the circle. They should be exactly the same distance apart as the caliper points.  If they are off by more than a few thousandths of an inch, you will need to repeat this process until you get it very close. It is important for every step be done very accurately or the bolts may not line up properly with the spindle or the chuck.

Now you need to repeat this procedure for the outer bolt circle.  Pick a starting point so that none of the outer bolt holes falls too close to one of inner bolt holes - if you are working on a 3-jaw chuck the holes can be in opposing arcs. Now set your dial caliper to the proper distance from your calculations, 3-jaw versus 4-jaw, and scribe marks as before.

Bolt Hole Placement - Alternate Method

Due to the difficulty of precisely locating the bolt holes by measurement and layout, an alternative is to make some little 'transfer punches' out of steel on the lathe.  These little punches are then seated in the bolt holes of the spindle with the points protruding. On my lathe, at least, the spindle stud holes are slightly smaller in diameter at the back than at the front, so the little punches seat against the back edge of the hole. You could probably make them longer and have them seat against the headstock if necessary. In the photo below, the short punches are for the spindle plate holes, and the longer ones are for the chuck bolt holes. The second photo shows the punches in the spindle plate.

Punches_.jpg (45107 bytes) Punches2_.jpg (47546 bytes)

The dimensions are shown in the following table. Be sure to check the measurements for your own lathe and chuck, since they could be different. Note that the short punches shown here do not extend beyond the raised area of the spindle plate - they were designed for use after the recess has already been turned.  This presents a Catch-22 since you need the mounting holes drilled in order to mount the plate to turn the recess. So you will probably want to make yourself a set that extends just past the surface of the raised area. Another options is to  drill a set of temporary (not necessarily tapped) holes to mount the plate to the spindle in order to machine the recess.

  Short Punch Long Punch
Diameter .270 .258
Height to shoulder .285 .908
Height to tip .320 1.010

Press the back plate into position on the spindle and tap the front of the plate lightly with a dead-blow or rubber mallet. This will leave little punch marks at the locations of the punch points. Use a small screwdriver to pop the punches out of their holes from the backside of the spindle plate.  

For the chuck bolt holes, first machine the land that mates with the recess in the chuck, then insert the punches into the bolt holes on the back of the chuck. Place the plate on the chuck and tap lightly with a rubber mallet.

Center Drilling the Bolt Holes

Use a small (#0 or #1) center drill to make starting holes at each of the intersecting arcs and the center. Once again, it is important to locate these holes as precisely as possible. Next, enlarge the holes with a #2 center drill. I hold the plate very loosely as the larger drill enters the hole so that the plate can move and center itself with the drill.

Center drill_.jpg (31541 bytes) Center_drilled_.jpg (46452 bytes)

For maximum accuracy you could clamp the work down after centering the scribe mark and drill each hole out to its final size without repositioning it. This approach, however, requires many more operations to change the drills each time. I opted for convenience instead and still achieved a result that was accurate enough.

Tapping the Bolt Holes

On the stock 3" chuck, 6mm studs are used.  The holes in the spindle are large enough in diameter (.270) to accommodate 1/4" studs, however, unless you have a rotary table or are very experienced, I recommend using 6mm studs rather than 1/4". It is very difficult to get the studs precisely on center with the spindle holes. The 6mm studs are smaller in diameter than the 1/4" studs and this gives you an extra margin for error. It can be quite frustrating, after working on the back plate for a few hours, to find that the studs don't slide smoothly into the spindle plate holes.   Trust me, I know ;-)

Now drill the 3 holes on the inner circle with the proper size tap drill (5mm or #9 for 6mm). Use a cutting fluid such as Tap Magic Aluminum to get a nice clean hole.  Here again, accuracy is critical. You want to make very sure that the tap is square with the surface of the plate. If it is skewed, the studs will be also, and may not mate with the spindle holes even if they are properly spaced. I made a little tap guide on the lathe to ensure that the tap is square with the plate surface. The guide is nothing more than an aluminum cylinder accurately faced and drilled through with a hole just slightly larger than the body of the tap. When starting the tap, I hold down on the top of the guide to keep it in place.

tap guide_.jpg (41138 bytes)

Here's another trick to ensure that the holes are tapped square to the surface of the plate: Open the top of your drill press and disengage the drive belt.  Mount the tap in the drill chuck. Lower the tap down to the hole, apply pressure from the drill press handle and turn the drill press pulley by hand to start the tap into the hole.   Back off a quarter turn every so often to break the chip and make a nice clean thread. The main problem with this method, at least on my drill press, is that the return spring on the handle is fairly strong and has a tendency to lift the work up from the table, possibly defeating the purpose of this exercise.

Tap chuck_.jpg (43730 bytes)

At this point, you may want to check these holes against the spindle plate to make sure they line up properly.  If not, you may need to mark a new set of holes and try again.

Note that the bolt holes for the chuck do not need to be tapped. We will deal with them later.

Making the Mounting Studs

Next we will make 3  studs about 7/8" long by cutting the heads off 1/4" or 6mm bolts, depending on which you chose, using a parting tool. I got these stainless steel 6mm bolts and mating SS nuts from Small Parts, Inc., a good source for small quantities of parts.

It's a good idea to protect the threads by making a sleeve from brass or aluminum:

Bolt cover_.jpg (37600 bytes)      Bolt head_.jpg (30313 bytes)

After parting off the heads, make a smooth facing cut on the end of the stud and clean up the end of the threads by filing them lightly at low speed at about a 45 angle. Thread one of the studs into each of the tapped holes in the back plate and test mount it on the spindle using nuts.

Mounting and Centering the Back Plate

Screw the studs into the tapped holes a few turns and mount the workpiece on the spindle plate for facing. Later, we will bore the recess for the land on the spindle plate and, for that operation, it is essential that the recess be concentric with the mounting bolts. To ensure concentricity, carefully center the back plate on the lathe using a dial test indicator as follows:

Centering_.jpg (58516 bytes)

Making the Center Hole

Now we are ready to drill out the center hole. The drill size is not critical but should be large enough to provide a starting hole for your boring tool. I used a 3/8" drill. Mount the drill in the tailstock chuck and drill all the way through the backplate.  Use cutting fluid for best results.

Drilling_.jpg (49742 bytes)

Use your boring tool to open up the center hole to about .875".  The diameter is not critical, but don't make it any larger than .900 or you will start cutting into the side of the spindle bore which is about .920" in diameter at the face of the spindle.

Boring5_.jpg (35361 bytes)

Making a Spindle Plate Template

We're going to digress for a while here to make a spindle template. We will need this a little later to machine the spindle recess on the back plate to precisely the right diameter. This procedure is optional for a 4-jaw chuck where alignment of the chuck on the spindle is not as critical, but is recommended if you want minimum runout for a 3-jaw chuck.  Here's what the finished template looks like:

Template4_x.jpg (17234 bytes)

Basically, the spindle template simulates the raised land on the spindle plate. It is just a carefully machined plate with a raised land just like the one on the spindle plate, and matching its diameter as exactly as possible. The template is used as a gauge for testing the diameter when you machine the spindle recess on the back plate.  When you are turning the recess, the back plate is mounted on the spindle so you can't use the spindle itself to test fit the recess. Instead, you use the spindle template.

It is difficult to accurately measure the inside diameter of the recess to less than .001", but it is easier to measure an outside diameter accurately. Additionally, when you turn the diameter of the template, you can test fit it into the back of the stock 3" chuck until you get a snug press fit.  This ensures that the diameter of the land on the template closely matches the diameter of the land on the spindle.

On his web site, Ty Hoeffer describes how to make a spindle template. My version is simpler. Since I use it only as a gauge for the diameter of the spindle recess.

Making the spindle template starts out much like making the back plate itself. It can be thinner, though, and a 3" diameter piece of 1/4" aluminum plate is all you need in the way of materials. If  you start with a square piece of plate stock, cut off the corners just like you did on the back plate to form an octagon. Then mark the center and layout a bolt circle 2.199" in diameter, just as you did for the back plate. Scribe another circle 2.165" in diameter to mark the edge of the land area.

Template1_.jpg (35593 bytes)

Use #0, #1, and #2 center drills in sequence to center drill the center hole and the three bolt holes. Then drill and tap the holes 6mm or 1/4". Mount the template on the spindle plate using 6mm or 1/4" studs, then carefully center it using a dial test indicator as described for the back plate. The studs should only be screwed in about half the thickness of the plate or you will cut through them in the following facing operation.

Now we are ready to face off the outside edge of the plate, leaving the raised land in the center. The land should be 2.165" (check your spindle to be sure) and about 1/8" high. Since the final diameter of the land is critical, first take a light facing cut and stop just outside the inner circle that you scribed. This marks out a rough guideline that you can work to without risk of overshooting and making the land too small a diameter.

Template2_.jpg (37518 bytes)

Note the extra holes are not necessary - they were for a technique I was testing.

Continue turning down the outside of the template stopping at the reference point until the land is about 1/8" higher than the surrounding plate. Make a nice smooth finishing cut. Now, with the compound set at 0 degrees and the carriage locked in place, start removing a little metal at a time from the diameter of the land. Use a sharp pointed tool as you would to turn a shoulder, since that's what we're actually doing here.

Template3_.jpg (37644 bytes)

As you approach the target diameter of 2.165", take very light cuts leaving a smooth finish, Use the recess on the back of the stock 3" chuck to test for a snug press fit.  Use a file to champfer the sharp edge of the land just enough to remove any burrs. When the chuck will just barely fit over the land you are done with this operation.

Mount a right-hand cutting tool outboard in the tool holder as shown below and use it to turn down and clean up the outside edge of the template. Use a rounded facing tool at about 1500 RPM to put a nice facing finish on the raised surface of the land. Then you are done.

Edge turning_.jpg (34566 bytes) Template4_.jpg (37561 bytes)

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Mini-lathe:  Accessories   Adjustments   Capabilities    Chucks    Dial Indicators   Features   Getting Started   Glossary     Introduction   Materials    Modifications   My Shop   Operation    Reviews    Sieg Factory    Tool Grinding    Troubleshooting   Tuning     Versions