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Disc Sander

Disc Sander as 5 page pdf

 

Introduction

I must confess I’ve always though Sanding Disc attachments on lathes were rather silly.  For starters I don’t use my disc sander much.  But when I was considering making one for an attempt at writing a book it occurred to me that if I made one, and got rid of my seldom used disc sander, I’d have room for half of another lathe.  Don’t tell my wife.

 

Turns out that wasn’t the only advantage.  Variable speed is very nice to have in a disc sander.  My old disc sander, with the worn disc currently on it, will burn cherry if you even barely touch the wood to the disc.  But on the lathe mounted disc I can dial down the speed where shoving it in as hard as I can doesn’t burn even end grain in cherry.

 

SPMS-SD12:  A scrap of cherry after aggressive sanding on end grain, with absolutely no burning.

 

Changing abrasives is a pain on my stand alone disc sander.  That’s one of the reasons why the disc is pretty worn.  On the lathe based disc sander, getting the table out of the way is a simple matter of sliding back the banjo.  And I can easily make as many discs as I want for different grits.

 

My lathe already has aggressive dust collection.

 

My stand alone disc sander has a not quite flat disc.  There’s not much I can do about it.  With my lathe based system I can shim or turn the disc flat because I’ve made the mount myself.

 

Making the Disc Sander is pretty simple.  For the disc all you need to do is mount a disc of some kind of sheet goods and mount it to your lathe somehow.  You could easily make it mount on your 4-jawed chuck but I suggest making wooden faceplates (see references).  Making a fixed table is equally simple, as the only potential complication is lack of room above your banjo and lathe center.  A small rectangle of sheet good mounted to a post is all you need.  Making a tilting table is a little more involved but quite doable.

 

A surprise benefit is a foam backed disk.  Make spoons?  It would be very handy for roughing out convex contours.

Disc

Mount

You could mount a disc directly to your 4-jawed chuck, but I don’t recommend that as it’s harder to true up and not a very strong mount as most sheet goods are fairly weak under tension perpendicular to the surface.  Better to use a hardwood disc and turn a tenon on it.  A more compact mount would be a wooden (or PVC rod) faceplate.

Turn the Mount round, then turn the face as flat as you can get it.  I’ve found that a large straight scraper works best for getting the Mount flat.  Check your results with a straight edge.  Time spent getting really flat now will save you time later.

Disc

You can pick any material you like for the disc.  Sheet goods of some kind are probably the cheapest and most likely to remain flat.  I used Melamine, both because I have some, and because I thought the PSA discs would release easily.  The size disc depends on the capacity of your lathe and ease of finding discs.  I used 12”. 

 

Start by drawing an oversize circle directly on your material.  Mark the center as well as the diameter.  Allow enough extra material to make up for possibly mounting off center initially, as well as an extra ¼” beyond the abrasive on the rim.  As building a guard isn’t too practical, having disc beyond the edge of abrasive will make it less likely to cut yourself.  Rough cut the disc round on your band saw.

 

Use at least three screws to attach the Disc to your Mount.  It’s probably stronger to have the screws go thru the sheet goods disc into the hardwood Mount.  Be sure to countersink the heads of the screws so that they’re below the surface.

 

Mount the Mount/Disc on your lathe and turn the rim round.

Adjust

After the Disc rim runs true, check and see how true the face of the disc turns.  If it runs true, congratulations, you did a good job of flattening the Mount, and selected flat material for the Disc.  If not, try to avoid turning the face flat.  Sheet goods dull turning tools very quickly.  Instead, figure out where the high spot is, then mark the sheet goods on the back 180° from it.  Loosen the mounting screws a bit, insert some paper or other shim material at the mark, retighten the screws and check trueness again.  You may have to do this several times, but you don’t need to get a dial indicator out, as there’s more variation than that in abrasive.

 

SPMS-SD01:  A front view of the Disc mounted on the lathe.

 

SPMS-SD02:  A back view of the Disc mounted on the lathe.

 

 

Abrasive

Single

Once your Disc runs flat and true on the lathe you can mount abrasive.  You may find it easier to mount the abrasive correctly if you fold back only part of the backing.  You can touch the abrasive to the Disk where the backing still is to align the abrasive, the press on the part of the abrasive where the backing has been removed.  Remove the rest of the backing and press all over the abrasive.  If you have trouble with PSA abrasives staying attached to your Disc, make sure the surface is clean and dry.  Then, remembering that the P in PSA stands for Pressure, try clamping a plywood disc over the abrasive to the Disc and leaving it there for a few minutes.  You can also double up on the adhesive by using 3M #77 spray adhesive on the disc before mounting the PSA disc.

 

SPMS-SD03:  The Sanding Disc with 12” Blue Zirconium Abrasive mounted.

 

Donut

Sometimes you’ve just got to change abrasive grits.  It’s easy enough to make extra discs with different grades of abrasive, but if you want to switch abrasives quickly and constantly, and your disc is large enough try making donuts.  Select two different grades of abrasive discs.  Using either masking tape, or a tailstock pinned piece of scrap wood, fasten them centered on your Disc.  Make sure the backing of both discs faces out.  Mark the circle halfway between the center and the rim with a pencil.  Bring up your tool rest close to the disc and start the lathe at a moderate speed.  Then using a thin cutting tool,  cut thru the abrasives on the line.  You won’t have to cut much abrasive if the backing faces out, but still, I wouldn’t suggest using your best 1/16” parting tool.  I use a reciprocating saw blade with the teeth ground off, the tip sharpened, and a “handle” of wrapped duct tape.

 

Mount the outer donut of one abrasive on the sanding disc, and the inner portion of the other abrasive.  Save the other two pieces for when the first ones wear out.

 

SPMS-SD10:  The set-up for cutting abrasive donuts.  I’ve taped the edges of the abrasive to the disc, abrasive side towards the disc.  The tool rest is close to the disc, and I’ve traced a cutting line.

 

SPMS-SD11:  After mounting the cut abrasive donuts to the disc.

Fixed Table

I guess that a table with a fixed angle of 90° to the disc would handle 95% of my disc sanding.  It’s easy to build, solid, and doesn’t require too much thickness above the banjo.

 

Any stable material will do.  I used melamine again.  The size is pretty arbitrary, other than depending on the size objects you plan to sand, as you can easily move the table where ever you need it to suit.  Mine is about 9” by 12”.  Cut it on the table saw so that you’ve got a true rectangle with straight edges you can reference from.

Miter Gauge Slot

While you might not use a miter gauge slot much unless you’re a jig-head like me, it’s easy enough to add one.  Just mount a dado head in your table saw, set the height to match the thickness of your miter gauge, and cut a dado lengthwise on the table.  I set mine back 5” from the edge, but as long as it leaves room for the sides of your gauge the set-back is arbitrary.  Check the fit of the miter gauge.  You want it to be easy to move but not sloppy.  If the fit is too tight or too shallow, just make another cut.

 

If you don’t have a dado head, just make multiple passes with a regular blade set to the height of your miter gauge.  Might take less time than swapping out the blade anyway.

Mount

There are many viable methods you could use to fasten a post to the table for mounting in your tool rest banjo.  The primary design constraints are height, solidness, and adjustability.  While you can, of course, use any method you like, I’ll describe using a block of wood, a bolt, and a locking nut, which fulfills all the design requirements.

 

Select a block of hardwood—5” by 6” is a reasonable size.  If you don’t have much room over the banjo use ½” thick wood, but if you’re mounting it on a big lathe ¾” or 1” stock will do.  Find a bolt that matches the diameter your banjo accepts for a mounting post.  Look for one with an unthreaded shoulder.  You’ll also need a nut to lock it in place.  If you have lots of room over your banjo a standard nut will do.  If you don’t, you can get thin nuts from Industrial Suppliers—McMaster-Carr, for instance, has 3/4x10 nuts as thin as ¼”.

 

Mark the center of your hardwood block and clamp it on your drill press.  Select a tap drill for the size bolt you’re using (ordinarily 1/8” less than the bolt at these sizes) and drill through the block.  Without unclamping the block, remove the drill bit and mount a mini-tap guide in the drill press chuck.  Insert the tap with handle attached into the hole and lower the drill press quill until the mini-tap guide engages the center dimple of the tap or handle and the plunger is depressed.  Lock the quill.  Now turn the tap until the threads are well started.  You can remove the block from the drill press to finish tapping the threads.  Drill and countersink holes for four mounting screws.

 

Assuming your bolt has a substantial unthreaded shoulder, measure the combined height of your hardwood mounting block and locking nut, add 1/8”, and cut the bolt so that that much thread is retained.  Measure how deeply you can insert the bolt into your tool rest banjo, subtract 1/8”, and cut the shoulder area to that length.

 

Attach the mounting block to the sanding table with wood screws.  Where you attach it is fairly arbitrary.  I mounted mine to the left side of the table so that I wouldn’t have to push the banjo past center.  Be sure to pick a location where you miss screwing into the miter gauge slot, and pick screws that won’t break through the top of your table.

 

Thread the locking nut all the way on to the mounting post and screw the post into the mounting block.  Make it snug, but don’t crank on it.  Now tighten the locking nut.  You’ll need to keep the mounting post from moving with a pair of locking pliers while you tighten the nut.  Tighten aggressively.

Adjust Tilt

Mount the sanding table on your lathe and bring it up close to your sanding disc.  Lock the banjo in place.  With the lathe off, check to make sure the table is at 90° to the disc with a square.  If it’s not, loosen the mounting screws, insert shim material (I used a piece of folded over junk mail) and retighten the screws.  Recheck squareness.  You may have to do this several times until you’re satisfied.

 

SPMS-SD05:  Checking to make sure the table is perpendicular to the Disc.

 

SPMS-SD04:  The underside of the fixed table.  You can see the paper shim I used to adjust the tilt sticking out from under the mounting block.

Set/Check Parallel

To set the table parallel to the disc find a thin flat sheet of any material to use as a stand off gauge.  I used plastic because I had some, but cardboard, plywood, or thin hardwood would work.  What’s important is that it be uniform in thickness, and that the thickness be an acceptable amount for the table to stand off from the disc.  Hold the stand off gauge against the edge of the sanding table and push the table into the disc.  While keeping the table against the disc, lock the banjo securely, and then remove the stand off gauge.

 

SPMS-SD06:  Aligning the table.  I’ve clamped the banjo in place while pinning a piece of plastic between the disc and table.

 

 

Your miter gauge slot should be parallel to the disc if it was cut parallel to the table edge, but you can double check by clamping a scrap block to the miter gauge so that it just touches the abrasive and moving the gauge back and forth along the disc.

 

SPMS-SD07:  The set-up to check that the miter slot is parallel to the disc.  I’ve clamped a scrap piece of wood to a miter gauge which can be slid back and forth in the slot. 

Tilting Table

Making a table that tilts requires a bit more work, and, given the limited space above the banjo, won’t be quite as solid.  Thankfully it can be a little simpler than the table mount on commercial disc sanders, because height adjustment is built into the tool rest banjo mount.  The table for the tilting table is much the same as the fixed table except for a 45° bevel on one edge.  The mounting mechanism again uses a Post mounted in the tool rest banjo.  A Rod fits into a hole drilled in the post, and is held at the required angle by set screws.  If you’re using a mini-lathe and are really cramped for space or limited to a ¾” tool post you may have to resort to building a table that mounts in the bed ways.

Bevel Edge

Start by cutting out a table of some stable material.  I used melamine, about 9” by 12” again.  However this time cut one long edge at a 45° to allow the table surface to get close to disc when tilted.

 

SPMS-SD13:  The bottom of the tilting table with the front edge bevel cut at 45°.

Miter Slot

If desired, cut a miter gauge slot.  Use a ¾” dado head set to the height of your miter gauge, or make multiple passes with an ordinary blade on your table saw.  Do check the fit of your miter gauge before moving on.

 

SPMS-SD14:  The tilting top after cutting the miter slot.

Post

This mechanism assumes your tool rest banjo accepts at least a 1” post.  Cut the post from steel rod of the diameter your banjo accepts.  The length should be ¾” longer than the maximum length of rod that will fit into the banjo.  Check to make sure the rod actually fits at this point.  If your banjo was drilled with a 1” drill, and you use 1” rod, it will likely be too tight.  If so, mount the rod on your lathe, using a 3 or 4 jaw chuck, turn the lathe on at a slow speed and reduce the diameter with a file held against the turning rod.

 

Use a center punch to mark locations for drilling holes for the set screws and Rod.  The locations should be at right angles to each other ½” below the top of the post.  Move to your drill press and drill thru the Post with a ½” drill bit.  Turn the post 90° to the other mark.  Drill thru the Post with a #7 drill bit.  Tap the smaller hole all the way thru with a 1/4x20 tap.

Rod

To make the Rod, cut ½” steel rod to 7” long.  Debur or bevel the ends.

 

SPMS-SD15:  The Post and Rod during test fitting.

Mount

Cut a 3-1/2” square out of ¾-1”hardwood.  Bevel one end grain side at 45°, leaving about 3/8” unbeveled.   Clamp the square side grain face up, and drill a ½” hole through the block 1” from the beveled edge.  Remove the Mount from the drill press clamp and tap the Rod into the Block.

 

Drill and countersink holes for mounting screws in each corner of the Block.  Place the Mount on the bottom of your Table  with the edge the Rod sticks out of is flush with the edge of the Table and so that the bevel on the Table and the bevel on the Mount line up.  That’s a long ungainly sentence for a simple concept—The Mount should be located as close to the Disc side of the table without interfering with tilting.  It should be at what will be towards the front side of your lathe when mounted so you can adjust and lock the tilt.  You could round over the Post, place the Mount nearer the center of the Table and carve a divot in the table over the Post to allow it to tilt.  It would be more solid that way, but it would also be a pain to adjust the tilt.

 

Measure the diameter of a finishing nail and select a drill that is the same diameter.  Drill through the Mount into the Table, so that it goes through the middle of the Rod.  Cut the nail so that it’s shorter than the combined thickness of the Mount and Table.  Tap the nail into the drilled hole to lock the Rod in position.

 

Insert the Rod into the ½” hole drilled through the Post.  Insert a ¼” long 1/4x20 set screw into each side of the Post.  Mount the Table in your tool rest banjo and try it out.

 

SPMS-SD16:  The underside of the Tilting Table with the Mounting Block and Rod attached.

 

SPMS-SD17:  The underside of the Tilting Table viewed from the front.

 

SPMS-SD18:  A detail view of the underside of the Tilting Table from the side.

 

SPMS-SD19:  The Tilting Table mounted on the Lathe.

 

SPMS-SD20:  A front view of the Tilting Table mounted on the lathe.

 

Variations

Accessory Table

You can use the sanding table as an accessory table when you’re not using the tool rest banjo, or when using another lathe.  Serves admirably for holding coffee cups.

 

SPMS-SD22:  The Fixed Table used to hold an essential accessory.

Padded

Since you can easily (certainly as compared to a standard disc sander) get the table out of the way, and can easily switch plates and abrasives, you can try some rather non-standard things.  It occurred to me that a big foam backed disc might make a poor man’s substitute for a pneumatic sander.

 

I cut out a disc of some foam I had, and sprayed both the foam and the melamine with spray adhesive.  After the adhesive had dried, I pressed the foam onto the melamine.  Then I sprayed the other side of the foam.  After that dried I pressed on a PSA disc.

 

It worked pretty well on convex surfaces.  Certainly good enough that any facets could be cleaned up easily with a 220 pneumatic drum sander.  It certainly seemed prudent to keep the speed down, just in case the disc and foam decided to part company.  I kept the table out of the way and sanded on the lower part of the disc, directing the dust towards my dust collector pick-up.  Deeper and softer foam, coupled with a more flexible backed abrasive might make it even more useful.  Just the thing to use after the bandsaw on freeform spoons.

 

SPMS-SD21:  Large foam padded disc.

V-Block for Bottoms

Reverse chucking a turning to turn the bottom is the nicest way to finish it.  But that isn’t always possible nor cost-effective.  If all you want is a flat smooth surface, for say the bottoms of a batch of Christmas Ornaments, miniatures, or desk accessories, try the disc sander.  You can sand the bottoms freehand, of course, but a little more trouble yields a nicer look.  Cut a V-notch into a short (8” or so) board.  Clamp the turning into the V-notch, then place the notched board against the miter gauge so that the V faces sideways.  Adjust the angle of the miter gauge so that the bottom is parallel to the disc.  Then turn on the lathe (a slow speed feels more comfortable to me) and sand the bottom, holding the board against the miter gauge.  Now do one freehand and compare the two.  I think you’ll find the dead-flatness and absence of subtle rounding of the edges to be worth it.

 

SPMS-SD23:  Sanding the bottom of a small item.

 

SPMS-SD24:  Sanding the bottom of a Christmas Ornament sized item.  The snowman is clamped to the V-Block.  The V-Block is held against the miter gauge to maintain the correct angle.

 

References

Wooden faceplates (also consider using PVC rod instead of wood):  http://www.davidreedsmith.com/Articles/WoodenFaceplate/WoodenFaceplates.htm