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I built a simple bookcase out of 3/4" ply. I set the depth collar using the gauge on the case. I used 1 1/4" coarse screws. On some holes, it was right on. On some the screw went through the mating piece. Then, on some the screw didn't even make it to the mating piece. 

I checked the setting on the depth collar after each misfire and it was still in the same place. 

I made sure the jig was lined up correctly on the piece. 

What could I have done wrong?

Thanks

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sounds like the depth collar may be moving, or you are not drilling till the collar touches the jig. you have the right length screws for the material. are you dadoing any of the pieces? i cant figure out why the screws were too short sometimes. again, i think the depth collar is moving on you, check the set screw and make sure it is tight. hope this helps.

With the ''drill stop'' set properly---

a. the screws driven too deep and the screw points protruding out the other piece---

is generally a result of too long a screw or too much torque on the driver.

Over torquing will cause the screw to go in further than required.

 

b. Screws not driven deep enough---generally the result of sawdust or wood chips are at the bottom of the drilled hole.

All the wood chips should be removed from the base of the drilled hole.

After drilling, turn the piece upside-down, tapping on the piece to remove the sawdust from the bottom of the drilled hole, or use a vac to suck-up the sawdust/chip particles.

 

TIP: When drilling the holes, lift the drill up a short distance, while keeping the drill running---

this allows the flutes on the drill bit to carry the chips/sawdust, up and out.

Once the chips are removed, reinsert the drill bit, drill a short distance, and repeat the process, until the drill bottoms out to the drill-stop.

(This may take 1, 2 or 3 times---depending on the depth of the drilled hole).

Also, keep the drill running when removing the drill bit---allowing the flutes to carry the chips out of the drilled hole.

Failure to do this procedure, will result in wood chips in the bottom of the drilled hole.

(The small wood chips/sawdust particles will slide down the drill bit flutes, when the drill bit is not rotating, and remain at the bottom of the drilled hole).

 

After drilling the holes, drive the screws in, without the piece to be fastened---

the screw point will protrude out the surface---all screw points should protrude out the same distance.

Perform this procedure to check/recheck each drilled hole for proper drilling depth.

Each screw should protrude out the same distance.

At this time, place a piece of wood next to the side of the screw , the same thickness you are fastening the member to,

and check/recheck the screw length that it has adequate thread engagement/length into the mating piece, and, so that the screw point does not protrude out to far, so that it does not exit the piece you're fastening.

 

On occasions, when a screw exits the piece to be fastened, particles of wood will bulge out the back side---a common occurrence with plywoods.

 

TIP: Make sample test piece(s), to insure you're getting acceptable results, before final assembly/fastening.

Ken, that makes sense on both counts. 

I think using cheap a$$ plywood contributed also.

I know plywood is normally undersized, but I think this was way undersized. Maybe 5/8"

Should the kreg jig be set for 3/4" if the plywood is not really 3/4"?
Thanks 

That's a good point. Plywood is generally undersized and it would make good since to adjust the setting accordingly. The question would be how will this effect the screw size?
Adjusting the drill guide adjusts the point in the edge that the screw exits the stock. Screw size is primarily dependent upon the stock recieving the threaded part of the screw. If both jpieces are undersized, as appears to be the case, I would adjust the drill guide some and go a 1/4" shorter on the screw.

will delaney said:
That's a good point. Plywood is generally undersized and it would make good since to adjust the setting accordingly. The question would be how will this effect the screw size?
If the screw is not started straight it may break through the other piece

While searching thru stacks of ''fir'' plywood, at various suppliers, I've found 5/8" stock in the location, marked ''3/4'' stock---go figure.

 

Some 3/4" plywoods, vary from 5/8 to 11/16''.

Perhaps, someone is putting 5/8" stock in locations, marked 3/4".

 

There are some 5/8" thick plywood materials on the market, 

as well as 7/16" stock.

 

Some suppliers carry 2nd grade materials.

 

Good quality 3/4" thick plywood, should measure 23/32".

 

Router bits designed for undersize material:

23/32" diameter for 3/4" plywood,

31/64'' Diameter for 1/2" plywood, and

15/64'' Diameter for 1/4'' Plywood.

This is the ''standard'' in the woodworking industry.

 

When perusing plywood stock, I check the material thickness myself, 

using an accurate scale/rule.

 

Accurate measuring is a critical issue with me,

unless I'm cutting wood with a chain saw.

 

I've seen some guys ''measure once---cut twice, and it's still to short''.

 

 

It sounds like your board may not have been sitting level when you drilled the holes.  Try putting a board the thickness of the Kregg tool under each side on your lumber, this should allow for your holes to be drilled level.
So is there any kind of guidelines for setting up the jig on undersized plywood? Or is it just trial and error?
I would thing that it is  a good idea to try a test when working  with plywood. Starting out with the setting a little short of the plywood size. If the plywood is of good quality that would be a 1/16 inch difference between two boards.  That might not sound like a lot but a screw popping out 1/16 inch in the face of you project is not very desirable.

antkn33,

What jig are you using? There are three or four type Kreg Jigs out there, and I believe they are used differently. I have the table top jig, the mini, and the "portable base" that came with the Master kit. Then there is the Jr too. I think the Jr and the Mini are used pretty much the same (I havent used either, but they might not be) and the Portable base works just like the table top base but is clamped to the piece being drilled instead of the piece being clamped into it.

You have to make sure the jig is set on the piece you are drilling at the proper height from the bottom edge for the thickness of the piece you are drilling into. If you are drilling into the wide side of a 2x4 (actual size is 1.5x3.5in) and fastening the 2x4 into a 1 inch (actual size 3/4 inch) piece, you would set the jig for 3/4 stock, and use 1 1/4 inch screws for the 3/4 inch stock. If you are going through 1/2 inch into 3/4 inch, you would still set the jig for 3/4 stock (correct me if I am wrong here guys - I will have to do a test piece tomorrow when i shovel out my shop).

You said you set the stop collar on the case.... so I am suspecting you are using the Jr. If this is correct, the proper way to set the depth is here:

 

Material Thickness Setting

Material Thickness Setting
From thin drawer boxes and craft projects to stout 2x4’s, this jig can handle them all. Independent Easy-Press™ Positioning Sliders feature 9 separate depth settings, allowing you to easily join materials from 1/2” to 1-1/2” thick in 1/8” increments.

 


Easy-Press Positioning Sliders Easy-Press™ Positioning Sliders
To set the jig properly, simply depress the button on the side of the jig and move the sliders so your material thickness is aligned with the arrow on the jig body. It’s that easy

 

 

there is a lip at the bottom of the jig that you mate to the bottom edge of your stock, and then clamp the jig to the stock after you set the correct depth on the jig. Use the depth gauge on the case to set the stop collar. MAKE SURE you use the end of the thicker part of the drill bit to set the depth collar, NOT the very end of the bit (the thinner point at the end that is the pilot cutter for the shank of the screw).

 

And as stated by almost everyone above, measure your thickness. DO NOT rely on the marked or listed sizes.

One consideration I haven't seen mentioned yet is the fact that common plywood sheathing material often has voids in the inner plys. This could account for the screws that broke through as though the holes were drilled too deep. The only plywood that doesn't have this hidden defect is the Baltic birch ply you pay dearly for.

I have found that the other advice given above is valuable as well, but the difference 1/32" in material thickness doesn't translate into significant problems as described by the original question. One must work carefully to have consistent results. Sloppiness will produce the problems described. If you don't clear the sawdust thoroughly, you can't get your workpieces positioned properly, your holes to the proper depth, or your screws properly seated.

 

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