Making a Chuck Back Plate

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If you have not already done so, please read the Disclaimer (last updated 10/18/09)


Update 4/4/02
LMS now offers a ready-made backing plate, item 1692. I have not evaluated it, but it looks like a much simpler alternative than making one from scratch.
End of update

The stock chuck and certain optional 3" chucks mount directly to the spindle plate of the 7x10 and 7x12 mini-lathe. Four-inch chucks, the largest I recommend using on the minilathe (Update 04/07: I later tried a 5" 3-jaw chuck. Although it is a little large, I have been using it now as my main chuck for several years) , require an adaptor back plate to mate with the spindle plate. In this article, I will describe how to make a back plate to adapt a 4-inch chuck to the 7x10 or 7x12 mini-lathe. Specifics are included for both 3-jaw and 4-jaw chucks. Here's the finished plate for the 4-inch 3-jaw chuck:

Back plate_.jpg (33868 bytes)

Check out Ty Hoeffer's site for additional info on chucks and back plates 1 & 2; I recommend reading Ty's comments before starting out on this project.  Some more relevant reading can be found on Peter Harrison's web site.

Spindle Plate Mounting Holes

The Harbor Freight SKU 33684 7x10 lathe and the Grizzly 7x12 lathe have 6 holes in the spindle plate - 3 for the stock 3-jaw chuck and 3 more which, together with one of the other 3 holes, mate directly with the J&L 3-inch 4-jaw chuck. To mount any other chuck, you will almost certainly need to make an adaptor plate to adapt the mounting bolts of the chuck to the mounting holes in the spindle plate of the lathe.

Note: the HF SKU 33916 7x10 lathe, some older versions of the HF SKU 33684 and the Grizzly Outlet 7x10 lathe have only 3 mounting holes in the spindle plate.

Spindle_.jpg (54455 bytes)

Time, Skills and Tools Required

Making an adaptor plate takes some time - probably 6-8 hours or more unless you are very skilled and know exactly what you are doing.  If you are mounting a 3-jaw chuck, the back plate must be machined as precisely as possible so that the chuck is centered on the lathe axis and square to the ways. Centering is not quite as demanding if you are mounting a 4-jaw chuck since the chuck jaws can be offset slightly to compensate.

For either chuck, you will need to lay out, drill and tap a number of   mounting holes as well as a machine a carefully fitted recess for the lathe spindle and, most likely, a raised area to mate with a recess in the back of the chuck.  This combination of operations, plus the required precision, make the fabrication of a back plate a challenging project and one that is sure to make you think and work with care.   Properly done, though, it will produce a finely finished and very useful tool that you can admire for the life of the lathe.

If you are fairly new to machining, you may want to leave this project till a later time when you have gained some experience or you may find it very frustrating. Meanwhile, if you need a 4-jaw chuck, the J&L 3-inch 4-jaw mounts directly to the spindle plate without requiring a back plate.

For the benefit of those readers who have limited shop capabilities, I was hoping to make the back plate entirely on the lathe but I ended up using a drill press as well. The drill press was used only for drilling the mounting holes. This could be probably done on the lathe if you have or make a faceplate so that you can offset the back plate to drill the holes, or mount the back plate to the cross slide on an angle plate. Some operations, such as counterboring the mounting holes for the chuck, could have been done more easily using my Grizzly mini-mill, but I wanted to present a plan that would work for lathe owners who have only the stock lathe and a drill press.

You will also need a few other tools:

Cutting the Rough Stock

I cut a piece of 1/2" thick aluminum plate down to 4 1/4" square on my 4x6 bandsaw. If your saw is accurate and you measure carefully you can save yourself some turning down to size later on by starting with a 4" square, but 4 1/4 gives you some margin for error. Some time ago I added a vertical guide plate to my bandsaw to facilitate cutting plate stock. This could be done with a hacksaw, of course, but the bandsaw makes a much easier job of it. I have also cut aluminum plate on my radial arm saw using a carbide tipped blade and making multiple passes increasing the cut depth about 1/16" on each pass.

Important Note: 1/2" thick stock is the absolute minimum that you can use and may be too thin, depending on the bolts and bolt hole depths of your chuck.  5/8" would be better, but 1/2" is more likely to be on hand in your shop. Another good alternative would be to start with a piece of 4"  round stock about 9/16" or 5/8" thick.  A good solution would be to purchase about a 2" length of 4" diameter 6061 Aluminum round stock (assuming that you have a bandsaw to cut it down to 5/8" thickness) You can buy it for about $8 per inch from  I used the 1/2" plate because I had a good supply of it on hand.

After cutting out the 4 1/4" square, mark the center and mark out the corners at 4 1/4".  Then saw off the corners to make an octagon.

Find_center_.jpg (29507 bytes) Octagon_.jpg (33274 bytes)

Laying Out the Bolt Hole Circles

Once you have your workpiece, locate the center point and make a small indentation there using a punch or a #0 center drill. Next, we lay out a bolt circle to match the 3 mounting holes in the spindle plate used by the stock chuck. Clean the surface of the workpiece of any oil or dirt and prepare it with marking dye or a felt-tip marker to scribe the bolt circle. Using dividers or the points of a dial caliper, accurately mark out a circle of 1.299" radius.

It is easier to use dividers to scribe the circle than to use the points of the dial caliper. Also, I don't like to use the points of the caliper for this since they aren't really intended to be used in this way. Since it is difficult to set the dividers to an exact setting such as 1.299, I first use the dial caliper to mark a small arc, then set the dividers to match this arc exactly.

Note: on the diagram on Ty's site, the radius is specified as 1.293".  The value specified in the manuals that came with all three of my chucks is 1.299 and careful measurements on my lathe showed it to be 1.305. Ty and I have exchanged email about this and his viewpoint, clearly stated on his template instructions, is that you must measure your own lathe, since there are variations from machine to machine. Unfortunately, this is a difficult dimension to measure accurately. 1.300 is probably a safe value to use if you are unsure.

Next, scribe a circle 1.654" in radius for the chuck bolt circle. This corresponds to a diameter of 3.307" as specified by the manual that came with my 4" chucks but may be different for your chuck.

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Mini-Lathe   Mini-Mill   Bandsaw   Grinder    Anodizing   Lapping    Links   Projects    Resources   Safety   Premium Content

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