Tag Archives: woodworking

Dog Weight Scale Part 13: Load Sensor Mounting and Final Assembly

In my previous post, I designed and printed a Centering Guide to line up the top and bottom pieces of the scale.  In this post, I finish assembling the scale.

Now that I have the Load Sensor Holders that I designed and printed, I drilled mounting holes in the blocks that will hold the Load Sensors.

Holding a Load Sensor over a support block that has mounting holes in it
Holding a Load Sensor over a support block that has mounting holes in it

(I used only the two front holes you see in the picture; not the other hole)

From there I lined up the support blocks on the bottom plywood circle, used a hand drill to extend the mounting holes through that part, then used a drill press and Forstner bit (on the bottom side of the plywood circle) to counterbore the holes that will hold the nuts that hold the bolts down.

Counterboring holes to hold the nuts
Counterboring holes to hold the nuts

Note in the above picture that I’m using a support stand to hold the large plywood disk while I drill using the drill press.  The support stand looks like a roller on a vertical bar.  It’s a safety thing, to keep the plywood from crashing to the ground at the wrong time.

Closeup of counterboring
Closeup of counterboring

Once that was done it was an easy matter to line up each Load Sensor with the support blocks below it, slip the bolts through, and fasten them with the nuts.

The assembled Load Sensor on its support blocks
The assembled Load Sensor on its support blocks

After mounting the Load Sensors, I mounted and connected all the other parts: the Arduino, Load Cell Amplifiers, Protoboard with resistors on it, and the plastic Centering Guide.

Mounted and Assembled Version 2 Scale electronics
Mounted and Assembled Version 2 Scale electronics

Finally, here is the long-awaited, assembled version 2 Dog Bed Weight Scale, ready to be calibrated.  It contains 4 Load Sensors, a pair of resistors per Load Sensor to change the Load Sensor into a Wheatstone Bridge, a Load Cell Amplifier per Load Sensor to measure the weight on each Load Sensor, and an Arduino to make sense of it all.

Top view of the assembled scale
Top view of the assembled scale

In my next post, I’ll describe the Raspberry Pi code to transfer data from the scale to the cloud.

 

Dog Weight Scale Part 11: Routing Counterbore holes

In my previous post, I 3D-printed parts to hold down the Load Sensors. In this post, I fix the counterbored holes that keep the nuts from protruding below the bottom of the bottom piece of plywood.

In the woodworking post, I used a router to cut counterbore holes on the bottom side of the bottom piece of plywood.  These holes hold the nuts that hold the circuit boards.

bad counterbore holes that don't hold the nuts
bad counterbore holes that don’t hold the nuts

Unfortunately, the router bit wouldn’t cut the center of the hole, so it couldn’t cut deeper than about 1/8″ – about half of what I needed.  Oh, and I probably burned the router bit too – don’t force the router, kids.

So I decided to try a Forstner bit.  Forstner bits are designed for drilling counterbore holes; they make a nice, flat cylindrical hole. But I didn’t know whether I could use a Forstner bit in a router. It turned out well – the bit fit in the router and fits great in the plunge router.

A Forstner bit in the router
A Forstner bit in the router
The bit fits fine in the plunge router stand
The bit fits fine in the plunge router stand

I was able to set the plunge depth to the 1/4″ I need for the circuit board nuts, and in no time had cut all the holes to the correct depth. I think I set the speed low enough to keep from burning the bit.

Setting the bit depth to 1/4"
Setting the bit depth to 1/4″

In my next post I do some more 3D printing, both good and bad.

Dog Weight Scale Part 5: Center of Gravity and a mounting Fail

In my previous post I described how to calibrate a load sensor. This post shows how to measure center of gravity, and shows a failed attempt to mount the load sensors to the scale.

Now that I’m using 4 load cell amplifiers rather than 1, I can calibrate each load sensor separately.  This in turn will let the Arduino calculate Pippa’s real weight accurately regardless of what part of her bed/scale she’s lying on.

The easiest way to calibrate all 4 load sensors is to do it all at once:

  1. Find the center of the scale
  2. Place a known weight at the center of the scale.  Each load sensor will support an equal amount of that weight.  That is, for weight W, each sensor sees W/4.
  3. Write down X = W/4
  4. read the values from each of the load sensors: Y1, Y2, Y3, Y4
  5. repeat steps 2 and 3 until you have a table of {X, Y} pairs for each load sensor and for a variety of weights.
  6. Using the calculations in the previous post, calculate the M and B values for each load sensor (M1, B1, M2, B2, M3, B3, M4, B4)
  7. Now the Sketch can load the M (SCALE) and B (OFFSET) for each of the four HX711 Load Cell Amplifiers.

For that calibration to work best, the weight must be placed at an equal distance from each load sensor. So one of the things we must do is to mark the Center of Gravity (not necessarily the center) of the plywood top of the scale.

Remember that the top circle of plywood rests on the 4 load sensors.  Ideally its center of gravity should be resting in the center of the square defined by the support point of each of the four load sensors. Two things can make the center of gravity of the top plywood circle different than the center of the circle: 1) errors cutting out the circle: if the saw blade drifted outside the circle for a while, that extra weight can move the center of gravity to one side, and 2) plywood doesn’t have uniform density: it can have voids, patches, and grain differences that can move the center of gravity.

So we need to measure the center of gravity of the top plywood circle.

One handy thing about the center of gravity is that if you suspend an object from a single point, the object’s center of gravity is somewhere directly below that point.  Do that twice for a flat object (like the top circle of plywood) and you can know exactly where the center of gravity is.

For a circle of plywood, the rough location of the center of gravity is obvious, but for a more complex shape, say a 1950’s kidney-shaped dog bed, it’s a little harder to guess the location correctly.

Of course, an easier way to find the center of gravity, if you have a few friends to help you, is to balance the top plywood circle on the point of a nail.  Once you have it balanced, tap the plywood so that the nail point marks the center of gravity.

Lacking the time of a few friends, to find the center of gravity of the top plywood circle, we need a hand drill and bits, a drill guide to drill nice straight holes, a weight on a string (i used a plumb bob, but some washers tied to the end of a string work fine), a nail, and (not pictured) a board to put the nail into.  This board will suspend the weight of the plywood.

What you need to find the center of gravity
What you need to find the center of gravity

Pick any place near the edge of the circle of plywood and drill a hole there.  The hole needs to be slightly larger diameter than that of the nail, so that the plywood can hang freely from the nail.

Drill one hole near the edge of the top plywood circle
Drill one hole near the edge of the top plywood circle

Hold the plywood circle next to the board (I used a 2×4, slightly taller than the diameter of the plywood circle) and drive the nail through the hole you just drilled, into the board.  In this way, the plywood hangs freely from the nail.  Now hang the weight on the string from the nail, and mark where the string falls, near the middle of the plywood circle.

Hanging a weight from the nail
Hanging a weight from the nail
Mark the line near the middle
Mark the line near the middle

(Note that the center of gravity isn’t quite the center of the plywood circle – the intersection of the two lines I used to make the circle)

Remove the nail and repeat for another point about a quarter of the way (90 degrees) around the circle from the first hole.  The intersection of those two marks is the center of gravity.

The center of gravity has been marked
The center of gravity has been marked

The load sensor mounting Fail

I’ve tried a few ideas for mounting the Load Sensors so they won’t slide off their mounting blocks, but nothing so far has worked out.  What follows is another example of a fail.

I’m getting close to breaking down and designing a little 3D-printed part to hold the Load Sensor in place, but I’m not there yet.

So I said to myself “Hey, I should be able to hold down the Load Sensor with a pair of bolts and washers, so that friction keeps it from sliding.”

The concept looked good.

test layout of two washers to hold down the Load Sensor
test layout of two washers to hold down the Load Sensor

I drilled a test hole to see how things would line up vertically: the load sensor, the washer angled on top of that, and the bolt holding the washer down.

Sensor base plus washer plus bolt head equals FAIL
Sensor base plus washer plus bolt head equals FAIL

To my dismay, the bolt head was very close to the top of the load sensor.  The top plywood circle is supposed to rest only on the four Load Sensors – the contact point is that little bump in the middle of the Load Sensor.  If the bolt head is nearly the same height as that bump, the plywood might rest on it as well, causing the weight seen by the Load Sensor to be completely wrong.

A little desperate, I tested whether the bolt alone, without the washer, might be low enough.  Nope.

The bolt without the washer is still worryingly high
The bolt without the washer is still worryingly high

So I’m going to (once again) set aside the issue of how to keep the Load Sensors from slipping, and in my next post turn my attention to the math to find the weight, W, on the scale and the center of gravity {X,Y}, of that weight.

Dog Weight Scale part 3: the woodworking and assembly

In my previous post I described the electronics of the Dog Bed Weight Scale. In this post, I’m doing the final woodworking and assembly – at least enough assembly to test the thing.

First I needed to design some sort of support for the load sensors.  Because of the design of the sensor – a “T” bar surrounded by a “C” shaped bar – I needed to make blocks that were 1) tall enough to keep the top piece of plywood from resting on or crushing the electronics and 2) cut out to allow the “T bar to bend below the “C” shaped part as weight was added.  You can find plenty of videos of people trying to use load sensors by mounting them on a flat surface; that won’t work.

So, to design the blocks, I first measured the dimensions of the load sensor, using a Caliper, then drew up a simple design from that.

Measuring a load sensor using a caliper
Measuring a load sensor using a caliper

Meanwhile, I drilled the mounting holes for the boards.  Because the plywood base is so large (~41″ in diameter), I couldn’t use the drill press.  So to make nicely perpendicular holes, I used a Drill Guide. I really like the one I use because it’s metal and it has a guide for each drill bit I use, creating nicely straight holes.

Because I was feeling a bit lazy, I didn’t measure and mark the holes for the electronics boards.  Instead I used the old “mark and drill” method.

Step 1: holding the board in place, drill just enough to mark the first hole – don’t drill deep.

Marking the first hole to drill
Marking the first hole to drill

Step 2: remove the board (so the drilling doesn’t damage it) and use a Drill Guide to complete the marked hole.

Drilling a vertical hole, using a drill guide
Drilling a vertical hole, using a drill guide

Step 3: Place the board back, drop the first bolt into the new hole, then mark the second hole as in step 1.  Repeat for all the board’s holes.

Marking the 3rd hole
Marking the 3rd hole

To prevent the bolts from sticking out from the bottom of the plywood, I chose 3/4″ bolts for 3/4″ plywood.  Because the bolts don’t stick out, I needed to counterbore the bottom of these holes so I can attach the nuts.  Again I couldn’t do this on the drill press, so I used a plunge router, set to bore just a little into the plywood.

Setting the router to counterbore holes
Setting the router to counterbore holes
Counterboring the mounting holes
Counterboring the mounting holes

Because the router bit I used has a space in the middle, the counterbore holes leave a little disk of wood, I used a chisel to clean out the remaining little disk of wood to make the counterbore flat.

Chiseling out the little disk of leftover wood
Chiseling out the little disk of leftover wood

Here’s what the 3 boards look like, fastened to the plywood base.

The boards mounted onto the plywood base
The boards mounted onto the plywood base
The back of the plywood base, showing the counterbored holes
The back of the plywood base, showing the counterbored holes

I then cut the blocks that will support the load sensors.  These are just temporary blocks, to let me test the circuit. The real blocks will (somehow) hold the load sensors in place and keep them from slipping from side to side.

The load sensor support blocks
The load sensor support blocks
Cutting slots with a scrollsaw
Cutting slots with a scrollsaw
The slot lets the sensor's "T" bar move
The slot lets the sensor’s “T” bar move

I chose the thickness of the blocks to make sure that the plywood top wouldn’t rest against or crush the electronics. See how the cross-section of the load sensor support block is taller than the circuitry.

The load sensor support block keeps the top plywood circle from crushing the electronics
The load sensor support block keeps the top plywood circle from crushing the electronics

I then cut out the top plywood circle. This circle will lay on top of the four load sensors.

The top circle of the scale
The top circle of the scale

For good measure, I placed Pippa’s bed on the top piece of plywood.  It’s a good fit.  Yes, that’s an Encyclopedia Britannica and a VCR in the cabinet…Pippa’s a Retro Girl (seriously, what can you do with old encyclopedias? I can’t bring myself to cut them up for papier mache).

Test-fitting the dog bed to the top plywood circle
Test-fitting the dog bed to the top plywood circle

I then assembled the whole thing and started testing.  Pippa helped.

The assembled base of the weight scale
The assembled base of the weight scale
Pippa resting on the assembled scale
Pippa resting on the assembled scale

In my next post, I get to try out the circuit and try to weigh some standard weights.

Dog Weight Scale, part 1: cutting the circular base

I want to learn how to use Load Sensors to continuously weigh stuff with an Arduino, so I thought it would be fun to continuously weigh our dog, Pippa, while she sleeps in her bed each night.  The project is a little like Nate Seidle’s Beehive scale, but simpler.

The idea is to turn Pippa’s bed into a scale. Pippa’s in fine shape right now, but it’s always good to keep an eye on your dog’s weight, and a custom-made scale is a great way to do it.

My plan is to wire 4 Load Sensors into a Sparkfun Load Sensor Combinator board and Load Cell Amplifier board, then to an Arduino 101, and from there, send the results via the 101’s built in BLE (Bluetooth Low Energy) radio.  From there I’ll need to build a gateway to relay the data to a server, but that’s another project.

The ongoing project files, including a project diary and Bill of Materials (parts list) are on my CurieBLEWeightMonitor Github repository.

It Begins

I measured Pippa’s bed and bought a pair of 4’x4’x3/4″ plywood sheets, to cut into circles: one for the bottom of the scale and one for the top.

Cutting a plywood circle is a lot of fun, because you can use geometry (I admit it: I’m a geometry nerd).

First, (assuming the plywood sheet is roughly square), draw two lines, each one connecting opposite corners.  Their intersection will be close to the center of the plywood sheet.

Finding the center of the plywood
Finding the center of the plywood

(Ignore the circle on the plywood for now: I reenacted drawing the cross-lines after I’d drawn the circle)

Next (I love this part), draw a circle centered on that intersection, using a beam compass.  A beam compass is a lovely thing for drawing very large circles and arcs: instead of having two arms like a normal pair of compasses, a beam compass has a point at one end, and a pencil that slides along the beam.  I bought a beam compass kit that attaches to a yardstick to form a beam compass, and I love it.

A beam compass kit
A beam compass kit

Here’s me drawing a 41″ diameter circle centered on the plywood sheet.

Drawing a circle using a beam compass
Drawing a circle using a beam compass

Once the circle is drawn, you can cut it out with a jig saw.

Cutting the circle with a jig saw
Cutting the circle with a jig saw

Ta Da!  Now I have a nice, circular base for my Pippa-Weight scale.

What a lovely circle of plywood
What a lovely circle of plywood

In the next blog, I’ll be soldering the electronic parts together.

How to Make a Drill Press Fence

After completing my drill press table, I decided my next step would be to make a fence for it – so I can drill vertical holes in the sides of short boards.

First I cut a 3 1/2″ board of 3/4″ MDF, of a width to match the drill press table.  This first piece will be the face of the fence.

Ready to cut the fence face
Ready to cut the fence face

You’ll notice in the above photo that there’s no support for the cut piece: that when I cut the fence face, it fell onto the concrete.  I should have at least put a mat down to cushion the blow, which resulted in two chipped corners on the face. I placed those on the inside of the fence, so the fence has a flat face to clamp work to.

the fence face dropped to the concrete, chipping the corners
the fence face dropped to the concrete, chipping the corners

I chose the height of the fence face keeping two constraints in mind: the drill press handles should clear the fence, and the chuck should clear the fence.  Some fence designs have cut a bit of the center of their fence to allow for the chuck, but I decided against it.

The handles clear the fence height
The handles clear the fence height
The chuck clears the fence height
The chuck clears the fence height

I then cut a piece for the base of the fence, deep enough to make space for the knobs that will hold the fence to the table.  Since I’ll need to reach over the fence to tighten and loosen these knobs, I did a quick depth check.

Checking the space between the knobs and fence face
Checking the desired space between the knobs and fence face

Lastly, I cut 4 blocks to support the fence face and help keep it perpendicular to the table.  Some designs use triangles for these supports. I used rectangles so I’d have a back surface to clamp to.

4 blocks will keep the face perpendicular to the table
4 blocks will keep the face perpendicular to the table

Next I test-laid-out the blocks and knobs, to make sure everything will fit properly.

testing the layout of the pieces
testing the layout of the pieces

Next I drilled the holes for the bolts that will hold the fence base to the T-rails, then tested to make sure they were the correct distance apart.

Testing the T-rail holes
Testing the T-rail holes

I then glued and clamped the parts together.

glued and clamped the fence together
glued and clamped the fence together

Once the glue dried (a day later) I double-checked and found the face of the fence was a little out of square.  Note the gap between the top of the fence and the square that I held flat against the drill press table.

The fence face is a touch out of square
The fence face is a touch out of square

I noticed the bottom of the face of the fence was a little lower than the base piece of the fence, so on a whim I used a trim router to make the base of the fence flat.

using a trim router to flatten the fence base
using a trim router to flatten the fence base
the bottom is now nicely flat
the bottom is now nicely flat

That did the trick: the face of the fence is now nicely square with the table.

with the flattened base, the face is now square
with the flattened base, the face is now square

Since unprotected MDF will absorb moisture and warp, I put two coats of paint on the fence.  In ages past I would have laid down newspapers to protect the floor and sawhorses, but in the 21st century who has newspapers?

painting the fence to protect it
painting the fence to protect it

Once the paint is fully dried (a day later), you can start using your fence.

using the new fence
using the new fence

CNC machine Good News, Very Bad News

I started cutting the X-axis pieces for the Build Your Own CNC Machine. At first things went well, then they went downhill quickly.

First, I clamped together the two roughly 2’x4′ pieces of 3/4″ MDF that would become the X-axis table, clamping together the best square corner of each.

I clamped the square sides of the two pieces
I clamped the square sides of the two pieces

Next I marked the 6 points where the bolts will go that will permanently bolt those two pieces of wood together.

marking the bolt hole
marking the bolt hole

I first drilled 1/8″ pilot holes, as the book suggests.

drilling the pilot holes
drilling the pilot holes

Once the pilot holes were done, I counterbored holes that will make the nuts sit below the surface of the X-axis table.

counterboring the nut holes
counterboring the nut holes

I then flipped the two boards over (still clamped together) and drilled the countersink holes that will put the top of the flat-head bolts just below the surface of the X-axis table.

drilling the countersinks
drilling the countersinks

Next came the 1/4″ holes for the bolts, drilled through the pilot holes.

drilling the bolt holes in the pilot holes
drilling the bolt holes in the pilot holes

Lastly, I bolted the two pieces together, with a bolt, washer, and nut for each hole. Note: I used a 1 1/2″ bolt rather than the 1″ bolt the book suggested, because I felt a 1″ bolt left too little wood on the nut side.

Bolt, washer, and nut for holding the X-axis together
Bolt, washer, and nut for holding the X-axis together

It all looked like a great start! I then turned my attention to the edges.  I started with one square, straight edge, and used a trim router to make the second piece’s edge match the first, to make a straight, square edge for the end of the X-axis table.

trim router to make the two pieces match exactly
trim router to make the two pieces match exactly

You know the old joke about the woodworker who tries to level a table by shortening the long leg, takes off just a little too much, and eventually lowers the dining table to the height of a coffee table?  I started looking at the long edge, and noticed that it was just a tiny bit out of square.  That error will translate into an error in the CNC machine’s output, so I decided to clean it up with the trim router, using a makeshift fence.

setting up the fence to fix a tiny error
setting up the fence to fix a tiny error

I made several mistakes at this point: 1) an aluminum angle bar isn’t stiff enough to use as a fence – it gives in the center, causing a curved cut 2) like the woodworker with the table, I should have been content with the slight error rather than trying to fix it 3) a trim router isn’t a good tool for cutting straight lines. 4) I don’t know what went wrong after that, but I wound up digging a diagonal line into the wood (!), then cut even more off in trying to fix that mistake.

The result was a completely ruined X-axis table.

what a mess!
what a mess!

So I’ve realized that all of the cuts in the CNC machine will need to be just as accurate as the X-axis ones, and that I don’t have the tools (table saw) required to make those cuts.

So I’m putting the project on indefinite pause.  You can see the parts list (which the book lacks) I put together from reading the book, and my project diary with details, at my github project page for this project.

How to Make a Drill Press Table

As multiple woodworkers have mentioned, a drill press as-is is poorly suited to doing woodworking: the cast iron table can transfer grease to the wood; the table is small; the table has limited places to fasten clamps to hold the wood down.

So here I am making a drill press table.

I started with a sketch based on this drill press plan, with an offset waste plug circle from this nice design.  I’ll narrate the project as a “how to” from here on.

First, cut two rectangles of 3/4″ MDF, a tiny bit over18″ deep and about 24″ wide. They don’t have to be exactly alike, because you’ll use a trim router to make them match exactly later.  Note: I mistakenly made my table 19″ deep, then later (see below) found my T-rails are only 18″ deep.

rough cut 24"x19" 3/4" MDF
rough cut 24″x19″ 3/4″ MDF

Here’s an old woodworker’s trick: once you’ve decided how the two pieces will line up, scratch a couple diagonal marks along one edge.  Later these lines will let you put those two pieces back together again with the right faces against each other.  You can see how if the proper faces aren’t matched later, the two parts of the diagonal line won’t line up.

draw a few diagonal lines on one face
draw a few diagonal lines on only one edge

To maximize the space between the fence and the drill bit, we want the table to wrap around the drill press column a little.   Measure how far back you want the table to go. I measured 3/4″ (vs. 1″) to keep the table from running into the rack on the side of the column. This is a good time to double-check that your drill press crank will clear your new table (see Step 10 Houston we have a problem).

measuring the desired edge of the table
measuring the desired edge of the table

Then measure the diameter of the column. I measured about 2.9″, which I rounded up to 3″ so my cut won’t have to be very accurate.

measuring the column diameter
measuring the column diameter

Next, use a pair of compasses to draw a partial circle of the measured diameter (3″) that intrudes into the table by the measured amount (3/4″).  You’ll need a scrap of the same wood as the table is made of, because the center of the arc is off the edge of the table.

Drawing the cutout for the column
Drawing the cutout for the column

The front corners of the table will be rounded so you won’t bump into them as you use the table.  While you have the compasses out, set them to the radius of the rounded corners you’re going to create (I used 1 1/2″) and draw the two rounded corners.

Drawing the two rounded corners
Drawing the two rounded corners

Cut out the column semicircle and the rounded corners using either a scroll saw or a band saw, or even a router.

Cutting the rounded edges
Cutting the rounded edges

Even slightly sloppy results look ok.

the cutout for the column fits well
the cutout for the column fits well
The rounded corner is a little sloppy
The rounded corner is a little sloppy

Position the one cut table piece on the drill press. Once you’re happy with how it’s placed, clamp it to the drill press iron table and use a pencil to draw the outlines of the iron table slots.  You’re later going to countersink some bolts with knobs to clamp the finished wooden table to the metal drill press table through these slots.

Drawing the outlines of the table slots
Drawing the outlines of the table slots

Now flip the wooden table over, mark some likely places for 4 bolts to hold down the table, and drill 1/8″ pilot holes. We drill pilot holes because we will countersink the bolts, and will need pilot holes to make everything line up.  I clamped the two parts of the table together and drilled the pilot holes through both, which wasn’t the best idea (see shifting during gluing, below).

Drilling pilot holes for the bolt holes
Drilling pilot holes for the bolt holes

Next we’ll make the waste plug in the center.  This waste plug will act as a protection to keep from tearing out the wood you’re drilling and to lessen the chance that you’ll accidentally drill a hole in your table. Set the compasses to 1 1/2″ radius and draw a circle off-center of where the drill will be (but encompassing the largest drill bit size) – this will allow you to rotate the waste plug, using one plug for many cuts.  You could cut this hole with a router or maybe a hole saw.

By the way, save the disk you cut out.  That disk is your first waste plug.

Scroll sawing the waste plug hole
Scroll sawing the waste plug hole

Choose a circle slightly smaller than the hole you just cut.  I chose 1.65″ radius.  Cut the disk you just made so it’s a circle of that radius (use the compasses). Note: I just noticed that 1.65″ is unlikely, when I cut a 3″ diameter circle – I think I mis-measured either the hole radius or the plug radius.

About to cut the plug down to size
About to cut the plug down to size

Note the size of this plug, because you’re going to need to cut several of these in the years you use the drill press.  I plan to write the radius on the table or plug.  By the way, this disk is a little smaller than the hole you made so that you can pull the plug out with your fingernail.

Make sure you can pull the waste plug out with your fingers
Make sure you can pull the waste plug out with your fingers

Next we’ll glue the two table pieces together.  Remember the diagonal lines we drew to keep the pieces lined up?  Re-align the proper faces of the two pieces by lining up those marks again.

Once that’s done, open the two pieces like a book.  Sand the inner face of each piece with very rough sandpaper, to make the glue sink in better.  Clean the resulting sawdust off with a damp paper towel.  Then put a generous amount of glue on one of the two pieces.

Gluing the two table pieces together
Gluing the two table pieces together

Then fold the pieces back together, align them, and clamp them.  I put small rods (pieces of coathanger) into the pilot holes to keep the pieces aligned, but then I made the mistake of removing those rods before I put all the clamps on – the pieces slid a little bit. That sliding will make the pieces a little misaligned, but I can fix that with a router later.  I probably should have bolted the pieces as I glued them, but the bolts hadn’t arrived yet.

Clamping the two boards together
Clamping the two boards together

You can clearly see how the two pieces have slipped during gluing. I’ll need to fix that with a router.

The pieces slipped a little during gluing
The pieces slipped a little during gluing

Once the glue has dried, use a trim router with a flush trim bit. With the rectangular table piece on the bottom, the flush trim bit cleans up the slip that happened during gluing, so the top and bottom have the same outline.

a trim flush router bit cleans up the edges
a trim flush router bit cleans up the edges

Next, flip the glued table over so that the piece with the cutout for the drill press column and the rounded corners is on the bottom.  Run the trim router around the edge (clamping the table down so it doesn’t slip).  A little sanding wouldn’t hurt here.  The result is a 1 1/2″ thick table with smooth sides.

The two pieces now have the same outline
The two pieces now have the same outline
The table so far
The table so far

Use a countersink bit to sink the bolts that will hold the table to the iron drill press table.

The countersink bit
The countersink bit

Now the pilot holes come in handy: they give you a center for drilling the 1/4″ bolt holes after the countersink bit has removed your original marks.

The pilot hole shows us where to drill the bolt holes
The pilot hole shows us where to drill the bolt holes

Now that the holes are drilled and countersunk, it’s time to test-bolt the table to the drill press.  Collect your parts: Four sets of a 1/4″ flat-head bolt, a fender washer to put underneath the drill press table, and a threaded knob to tighten it all down.

The parts for bolting the table down
The parts for bolting the table down

Here’s what the top of the table looks like at this point.

The table bolted to the drill press
The table bolted to the drill press

…and the bottom.

The knobs bolting the table down
The knobs bolting the table down

Next you’ll mark the center-lines of the two T-rails that will hold hold-downs when you use the table.

First you need to decide where (left-right) the rails will be positioned.  There are two constraints: you don’t want the T-rail any closer to the center than the hold-down can reach; on the other hand, you don’t want the right hold-down’s knob to interfere with the drill press handles.

The two left-right constraints: the chuck and the handles
The two left-right constraints: the chuck and the handles

For my hold-downs and chuck, the center of the T-rail is at 4 1/2″ from the centerline.  Draw lines for the T-rail centerlines on the left and right of the center.

Next clamp a straight piece of wood to the table, that will be the fence for the router.  My router has left and right marks in the base, which let me line up the fence for cutting the T-rail channels.  Adjust the fence to make the marks line up, then clamp it to the table.

Set the fence so the router is centered on the T-rail channel line
Set the fence so the router is centered on the T-rail channel line

I made a mistake here: in using a router with a fence, always cut with the fence to the left of the router.  If you move the router the other way, the bit tends to push the router away from the fence rather than pulling toward it.  That is, if you move the router the wrong direction, your channel cuts could wobble some (as mine did). Those wobbly channels look pretty bad later, because the straight T-rails emphasize the wobbly channels.

Next, choose a Straight (flat-bottom) router bit that is the width of the T-rail (3/4″ for mine), and set its depth to just barely deeper than the T-rail height (1/2″ for mine).

Setting the router depth to the T-rail depth
Setting the router depth to the T-rail depth
My clamped makeshift router fence
My clamped makeshift router fence

If all goes well, you’ll have lovely channels for the T-rails that will hold the hold-downs.

A T-rail resting in the newly-cut channel
A T-rail resting in the newly-cut channel

Next you’ll be drilling the holes for the bolts and nuts that will hold the T-rails.  You want the bottom of the table to be uninterrupted by nuts and bolts, so choose a bolt length that doesn’t protrude, and you’ll be boring counterbore holes for the nuts.

The bolt is shorter than the table thickness
The bolt is shorter than the table thickness

Line up the T-rails in their slots, then drill holes for the first two bolts.  These bolts will eventually hold the rails in place while you drill the rest of the holes.

One bolt per rail will hold down each rail for a moment
One bolt per rail will hold down each rail for a moment

Choose a Forstner bit wide enough to give you space to tighten the nuts on the bolts. I chose a 3/4″ Forster bit because I had plans to add 3/4″ washers (that didn’t work out – see below).

A Forstner bit large enough for a wrench
A Forstner bit large enough for a wrench

Once you’ve fastened one bolt for each T-rail, you can drill the rest of the T-rail holes, and then bore the counterbore holes for the corresponding nuts. Make those counterbore holes deep enough so that the nuts are below the surface, and so there is enough bolt for the nut to hang on to (allowing for a washer, if you want to use a washer as well).

All the bolt holes and nut counterbores are cut
All the bolt holes and nut counterbores are cut

One note: The Incra T-rails I bought are 18″ long, so it doesn’t help much to have a deeper table.  I should have made my table 18″ deep instead of 19″ deep.

Gap between the rail length and table depth (oops)
Gap between the rail length and table depth (oops)

Now that all the drilling and cutting is done.  It’s time to put a coat or two of paint on the table, to keep it from absorbing water.  MDF is notorious for absorbing water and warping, so a sealant is important.

Painting the table
Painting the table
The painted top
The painted top

Next, do the final assembly of the table: put the rails in, fastened down by the bolts and nuts, and bolt the table to the drill press table with the countersunk (“countersinked”?) bolts, fender washers, and knobs.  I would have liked to have put 3/4″ washers under the nuts that hold the T-rails down – to diffuse the pressure a bit –  but found the counterbore holes I’d made weren’t deep enough.

the T-rail bolts and nuts are in place
the T-rail bolts and nuts are in place

Finally, mark the radius of the waste plug in the waste plug hole with a permanent marker.  You might prefer writing that radius on the inside edge of the waste plug hole, so it doesn’t get as much wear over time.  I’ll see how well my hole-bottom note will hold up with use.

Write the waste plug radius on the inside of the waste plug hole
Write the waste plug radius on the inside of the waste plug hole

Finally, you can start using your drill press!

Hold downs keep the wood from slipping
Hold downs keep the wood from slipping

For further work, you might add a Fence as in this design, or build a vertical drilling jig for end-drilling long pieces of wood, along the lines of this other design.

My first real machinery arrives

The drill press arrived this evening!

I’ve never assembled a machine tool before, so I was put off by the rust-preventive coating that needs to be cleaned off, through mysterious and inadequately-explained means.

The drill press being assembled
The drill press being assembled

The manual says to remove the gunk with kerosene or some other solvent, and assumes that the reader is a seasoned machinist who knows what in the world they’re talking about.

After much searching, I found that they’re saying basically “it’s covered in grease; use a grease-cutting goo to clean it, then protect it with something”.  The most benign thing I found recommended online was oven cleaner or 409. Many folks swore by kerosene (which is highly flammable) or mineral spirits.

So my plan is to clean the parts with 409 then, for the parts that need protection, use the (non-silicone!) wax I bought for the scrollsaw table.