Build - Comb Clamp

[September 20180

This is a stub page for a potential future build of a set of comb clamps.

Comb clamps are used on a magnetic chuck to clamp down non-magnetic items. The comb clamps are made from from two steel plates joined by a spring steel "hinge". The plate with the metal "fingers" is angled slightly upwards; when the magnetic chuck is switched on, the fingers are pulled downward and apply a clamping force sideways and down.

The clamps are still made today by Magna-Lock USA, but they are quite expensive (around $500 in 2018) for a small set. They are referred to variously as Magna Vise, Magna-Grips, Magna-clamps, Perma-clamps, and Perma-Grips.

Dimensioned drawing for comb clamp and related jigs: (in progress)

Materials

As raw material for the clamps, I considered tool steel, but aside from the price I did not think this was necessary for several reasons:

• I plan to make the clamps with an integral hinge - hardened steel will likely not be flexible enough, and there is also a risk of cracking
• If the clamp comb teeth deform slightly when "digging in" to the work piece, that should only serve to improve the grip
• These clamps will be used in my home shop, and likely not very often, so wear and tear from heavy use is less of a concern

I decided to go with steel from a local farm supply store: 1/8" x 2" x 48" hot rolled steel bar (A36 steel, according to the vendor). The 1/8" thickness is nominal, the actual thickness is around 0.116".

While this bar stock has a nominally rectangular profile, the edges are slightly rounded, and there is a coating of mill scale. In addition, I later found that the piece I purchase was slightly cupped. However, I anticipated that there would be significant loss of material in cleaning up and squaring the pieces I needed, so I bought over-size to begin with.

Making a test piece

Order of operations:

1. Cut piece to rough dimensions
2. Surface grind both sides to provide flat parallel surfaces
3. Mill to required dimensions
4. Mill on edge to create tooth angle
5. Mill semi-circular cut-out
6. Cut comb teeth with a slitting saw
7. Mill face angle on one side to rough dimension
8. Surface grind face angle to final dimension
9. Mill slot for hinge

For an initial test piece, I cut off a ~4.1" length and surface ground both sides (see picture to the left). As can be seen in the picture, while the majority of the surface has been cleaned up, there is still some mill scale on the rolled edges.

This surface clean-up reduced the thickness from a nominal 1/8" (0.125") to 0.112".

I used a surface grinder for this because I happen to have one, but I expect that acceptable results could be achieved by milling as well.

The edges were cleaned up and squared in the mill, for final dimensions of 0.112" x 1.7" x ~4.0". The picture to the left shows the piece before final dimensioning.

The edge angle was cut by clamping the piece in the mill vise with a purpose-made angled wedge (explained in the drawings). The top edge walled down just far enough to make a full side-to-side cut (0.012 in this case).

Slitting saw cutting second set of teeth.

All teeth are cut. The slitting saw cuts were deburred using a small round needle file. Again, not the damaged area just below the semicircular cut-out.

In order to mill one face at an angle, I made a "step jig"to hold one edge when the work-piece is clamped to a fixture plate. The picture to the left shows the work-piece clamped on the fixture plate for rough milling (I put blue Dykem on the plate so I could better observe the milling).

After rough milling, the fixture plate (along with the work-piece) is transferred to the surface grinder to complete the angled face.

Here's the work-piece following surface grinding. Note there is a slight curve showing in the Dykem; I believe this is because the center clamp holding the work-piece was bending it down slightly. In future I will use a flat bar under the clamps to even out the clamping force.

I had originally intended to mill the slot for the hinge using the fixture plate to hold the work-piece, but the clamps did not leave sufficient clearance for the end mill. Instead I clamped it in the mill vise, using two regular parallels and an adjustable parallel for support. As an added precaution, I added a vise-stop on top to help keep the piece from lifting up.

This is really not a good work-holding solution, because as the hinge gets thinner there is a danger of the work-piece buckling in the vise. I'll have to come up with a better solution.

My target for the hinge thickness was 0.010" to 0.012". Initially I tried measurement with a depth gauge, but it was difficult to get accurate readings. I ended up having to remove the work-piece to check the thickness, and then restoring it to the mill vise for fial milling. I managed to achieve a hinge thickness of 0.012", but I need to devise a less time consuming method.

Here is the finished comb clamp.

Here is a test of the comb clamp, showing it being used (successfully!) to clamp a bench stone to the magnetic chuck.

At first I thought the comb clamp was not working, but once I used the proper technique, I got very good clamping.

Making Multiple Comb Clamps

Making multiple clamps at the same time opens up the possibility of performing some operations simultaneously.

Steps 1 & 2:

1. Cut piece to rough dimensions
2. Surface grind both sides to provide flat parallel surfaces

Here, 10 blanks have been prepared and are placed on the surface grinder to be all ground at the same time. My surface grinder chuck was not large enough to hold all ten pieces at once, so I asked a friend to help me out with this step.

Step 3 : Mill to required dimensions

This was accomplished by stacking the pieces together and milling all of the ends to length at the same time.

Step 4 : Mill on edge to create tooth angle

Again, all of the plates were stacked together and then clamped in the mill vise using a purpose-made "wedge" to hold the pieces at the correct angle. The bottom left ends supported in the vise by using two drill bits of the appropriate diameter in the triangular "hole" made by the angle.

It was then a simple matter to mill the top edges of all of the pieces at the same time.

Step 6: Cut comb teeth with a slitting saw

Here the stack (again held together with kant-twist clamps) is set into the mill vise at a 45 degree angle. Note the felt tip marker "X" on the forward face. After cutting the tooth angle on one edge, all of the pieces now have a front and back face, so I marked each of them to prevent any mix-up in the following steps.

Here is the slitting saw blade I used to cut the slots: HSS 4" diameter x 0.032" thickness. I'm not sure of the tooth count (picked it up at an auction), but I believe it is 18 TPI.

I tried a couple of other slitting saw blades before I settled on this one. The other had a much lower TPI count and did not cut well; ther was also a lot of vibration.

Here the first set of slots (and the corner) have been cut. The small clamps I used worked very well, and held the stack together nicely.

All slots completed and cleaned up. A bonus from the stack cutting was that only the front and back pieces had any burrs, so de-burring was much easier.

Step 7: Mill face angle on one side to rough dimension

Step 8: Surface grind face angle to final dimension

The face angles for the 10 pieces were done one at a time in the same way as the initial test piece, except that I made a clamping bar to make it easier to hold the pieces for milling. The clamp bar was a simple piece of hot-rolled steel with holes in each end to match the fixture plate. I bent a slight bow in the bar to improve the clamping force at the middle of the piece.

The same fixture and clamp bar was used in the surface grinder to complete the face angle.

By milling all of the pieces first (one at a time), and then surface grinding (one at a time) I was able to reduce set-up time to a minimum.