LumberJocks

Angle Grinder Concrete Planing Jig

  • Advertise with us
Blog entry by KTNC posted 07-27-2021 12:40 AM 661 reads 1 time favorited 4 comments Add to Favorites Watch

Angle Grinder Concrete Planing Jig

Quick Summary:
This blog documents how to make and use a device for flattening a concrete surface.

Disclaimer:
I consider myself to be an intermediate level woodworker and have very little experience with concrete or angle grinders. If anyone chooses to make this please learn all you can about how to use an angle grinder safely.


Background:

About a year ago, I built an outdoor all-masonry workbench with a 2’ x 5’ top. I use it for woodworking projects. When I made it, I was hopeful that if I screeded the concrete within forms that formed a plane, I would get a flat surface. I learned that due to shrinkage that is not the case. After a few months, the surface was warped into a “smile”. It was high on the ends and low in the middle. There was about a 1/8” gap between the surface and a straight edge (6’aluminum level) laid along the long dimension. I learned that angle grinders with diamond cup grinding wheels are used for tasks such as removing humps in concrete sidewalks and floors. I borrowed an angle grinder and bought a diamond cup grinding wheel and found that it did indeed easily remove high spots in the concrete workbench. I have made and used router planing sleds to flatten wooden slabs, so I started looking for something similar for an angle grinder. I didn’t find any, so I designed one.

Results:
This jig worked very well. I was able to flatten my workbench so that the maximum gap between a very accurate 50” long straight edge and the table is 0.015” (about 1/64”). The surface is flat but not smooth: there are pits in many places. I think the pits are because of small stones in the concrete mix. Sometimes the diamond cup wheel cuts them smoothly and sometimes they get pulled out leaving a pit.

Time to design/build: One to two man weeks over one calendar month

Expense: Grinder $80, Diamond cup wheel $45, lumber and misc about $50

Terms:
The picture below shows the
- X-axis
- Y-axis
- Carriage
- Glide Rail
It’s not labeled, but the supporting structure is what the X-Axis rails are clamped to: what I call the horizontal and vertical members. The angle grinder and diamond wheel cup are hidden on the underside of the carriage. You can see the electrical cord sticking out on the right.

Materials used for Angle Grinder Concrete Planing Jig

Angle Grinder – Makita 9557NB
Diamond Cup Wheel – RIDGID AWD40

Carriage
5/8” thick 9 1/2” x 11” plywood – for grinder base plate

#10 machine screw 1” long qty 2

#10 flat washer qty 4

#10 lock washer qty 2

#10 hex nut, qty 2

simpson strong tie bracket, black, 1 1/2” by 2” qty 2 – I think this is model APA21

M8-1.25×12mm hex bolt qty 2

5/16 flat washer qty 2

5/16 lock washer qty 2

2” pipe strap

1/4-20×1 1/2” long hex bolt qty 2

1/4 flat washer qty 2

1/4-20 hex nut qty 4

Handle
1 1/8” dia dowel 10 1/2” long – I used an old wooden rake handle

5/8” thick plywood 2 1/2” x 2 1/4” qty 2

#6×1 5/8 screw qty 2

#6×1 1/4 screw qty 1

Glide Rails
16” long 2×2 qty 2 – for glide rails Note: I cut strips from an old maple cutting board
1/4-20×1 1/4” long hex bolt qty 4
1/4-20 threaded inserts qty 4
1/4 flat washer, qty 50 – for setting depth of cut: buy a box of 100

Y-Axis
1×4 x 43 1/2” long qty 2 – for the rails
5/8” plywood, 7” x 3 5/16” and 7” x 4 1/2” – for end caps

#6×1 5/8 screws qty 8

Moveable stop

ash or maple 1” thick 2 1/4” by 11 1/4” – I used a hardwood scrap unknown species
1/4-20×1” long thumb screw
1/4-20 furniture connector nut (aka binding barrel) – flanged nut for squeezing against rail
1/4-20 threaded insert

Supporting Structure
2×4 47” long qty 2 – for horizontal members
2×4 11” long qty 4 – for vertical members
2×4 84” long qty 2 – X-axis rails
F clamps – 4” or 6” qty 4 – I used 12” F clamps because I had them on hand
1/4-20 double wedge anchors qty 4 – for creating threaded holes in concrete pillars
1/4-20 3” long hex bolt qty 4

Tools used
Radial Arm saw – for all cutting and dadoing
Jointer – to edge joint the rail boards
drill press
various hand tools – socket, open end wrench, file, clamps, tape measure, square, sandpaper

Operation:
Since I don’t make videos myself, I suggest you find a few videos that show someone using a router based planing sled. The operation of this mechanism is very similar. The carriage is pushed/pulled along the Y-Axis and the box upon which the carriage rides is moved along the X-Axis. The biggest difference is this… With a router the bit is parallel to the workpiece so it doesn’t matter much which direction you start the stroke. With the angle grinder the cutting surface of the diamond wheel is at a small angle to the work surface. Therefore, it’s much better to begin a pass by pulling the carriage such that the higher edge of the wheel passes above (and doesn’t make contact) the work surface first. If you do it the other way, you’ll be ramming the blunt edge of the cutting wheel into the edge of the work surface. Once you’ve taken off most of the material you can move the carriage either way to finish. I found it takes several strokes along the Y-axis for each X-axis setting. Once the grinding noise diminishes you know you can move on. Use a firm but light touch on the handle. If you press down hard, it will overburden the cutter and probably deflect the rails and cause you to cut too deep.

Construction:

First let’s have a walkaround the entire machine.

In the picture below you can see the concrete workbench is supported on pillars made of concrete block. The horizontal members of the supporting structure are attached to the pillars by two hex bolts. There are two threaded anchors in the concrete pillars to receive those bolts. The vertical members are attached to the horizontal members with hex bolts/nuts. The X-Axis rails are clamped to the vertical members with 12” F clamps but it would be better to use 4” or 6” F Clamps. It’s critical that top edges of these rails be straight and coplanar. Both the horizontal and vertical members have several sets of matching holes so that workpieces of various width and thickness can be accommodated. In these pictures it’s set up to mill the concrete surface itself but by selecting different holes it can be used to work on slabs up to 4” thick. I anticipate using the same supporting structure for a router planing sled in the future – for wood slabs.

Below is a view from the back side. The black/yellow clamp on the top of the rail is placed so the carriage will not go too far and gouge the X-Axis rail.

Below is a view down length of the Y-Axis box. You can see how the glide rails sit on and overhang the Y-Axis rails. The angle grinder is suspended from the grinder base plate.

Now let’s look at some details.
In the picture below is the box that serves as the Y-Axis. It’s made from 1×4s. The bottom edges are jointed and they are both ripped to the same dimension. The end caps are not identical. One end hangs down below the bottom of the rails and serves as a fixed stop. The other end (shown in the picture) is shorter than the rails. On that end a moveable stop is clamped. The moveable stop allows for changing the distance between the two X-axis rails.

The 1×4s I used are not perfect: there is a little twist/bow. When I assembled the box, I found that if I forced the rails to match up with two perfectly square end caps, the entire box twisted. A twisted box does not make a plane so that was unacceptable. My solution was to force it into alignment with one perfectly square end cap and then make a custom end cap that would fill the non-square space in between the rails on the other end. This worked but may not be the best technique. Perhaps it would have been better to start with a 2×4 and face joint with a jointer and then plane with a thickness planer.

Below are some pictures of the moveable stop. It’s made from a scrap of hardwood. There is a through hole with a 1/4-20 threaded insert. The furniture connector nut threads onto the thumb screw and presses against the side of the Y-Axis box

Below is a picture of the carriage and glide rails. The angle grinder is rigidly attached to the grinder base plate using L brackets attached to the threaded holes in the angle grinder body. The L brackets came with holes in the right place, but I had to drill two of them larger for the bolts that go into the grinder body. On the cord end of the grinder a 2” pipe strap holds the handle down. It’s double nutted since I wanted it to be snug but not so tight as to crack the plastic housing. For the glide rails, I cut strips from an old cutting board and used a dado blade on a radial arm saw to remove material. It’s important that these surfaces be flat: they ride on the Y-Axis rails. I waxed all the sliding surfaces with Johnson Paste Wax.

The picture below shows the other side of the glide blocks. Note the 1/4-20 threaded inserts.

Below is a side view of the carriage and glide blocks. Note the gap. The gap is filled with 1/4 flat washers. The number of washers is used to set the cutting depth. More washers raises the cutting wheel and less lowers it.

Here’s a close up of the washers

When I started planing the workbench I had 8 washers under each of the four posts and the cutting wheel only touched down in a few places. On pass two, I removed one washer from each post and the diamond cup wheel touched everywhere on the surface. For pass three, I didn’t remove or add any washers, I just went over the entire surface using several strokes at each X-Axis setting until the grinding noise subsided.

The picture below shows the handle. The two plywood blocks have 1 1/8” blind holes drilled most of the way through: I used a forsner bit. The blocks are screwed (from underneath) to the grinder plate. The round handle came from an old rake handle. I put one screw through the rectangular block and into the round handle so it wouldn’t turn.

Below are my hand drawn design sketches.



4 comments so far

View Redoak49's profile

Redoak49

5387 posts in 3229 days


#1 posted 07-27-2021 12:58 AM

Interesting build….

Why a concrete workbench?

View KTNC's profile

KTNC

187 posts in 1496 days


#2 posted 07-27-2021 01:03 AM



Interesting build….

Why a concrete workbench?

- Redoak49

Hi Redoak49:
I wanted something that could be outdoors (weatherproof) and stay flat. I didn’t know it would be so hard to make it flat though!
regards, Kerry

View DevinT's profile (online now)

DevinT

1889 posts in 206 days


#3 posted 07-27-2021 01:44 AM

This might be just the thing I’ve been looking for!

-- Devin, SF, CA

View KTNC's profile

KTNC

187 posts in 1496 days


#4 posted 07-27-2021 02:07 AM



This might be just the thing I’ve been looking for!

- DevinT

Hi Devin:
Glad to hear it. Let me know how your project goes!
regards, Kerry

Have your say...

You must be signed in to post the comments.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com