Hold Downs and Sacrificial Board.

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woodworkerbob
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Hold Downs and Sacrificial Board.

Post by woodworkerbob » Mon Jul 13, 2015 10:00 pm

For me, hold downs should be: low profile; cheap; customizable; and quick to produce. Hold downs must also work with the sacrificial board. Combining those design elements led me to a sacrificial board of 1/2" MDF with 1/4x20 5/16" T-nuts placed every four inches and hold downs made of 1/2" Baltic Birch plywood. The hold downs vary from 2" to 5" in length.
IMG_0194.JPG
Hold downs
IMG_0194.JPG (216.28 KiB) Viewed 2317 times
The notches in the hold downs are different on each end. One side is .1" deep the other side is .2" deep. I don't see these parts to hold things down but to really hold things from moving side to side. The notches are kept small so the SO3 can get almost to the edge of the material being cut.

I use three different lengths of 1/4x20 bolts: 3/4"; 1"; 1 1/4"; that I spray paint so I can quickly grab the right length. The fender washer spreads out the pressure of the bolt and the star lock washer, placed under the fender washer, digs into the plywood holding everything in place. The star washer really does a great job holding hold downs in place without having to tighten the bolts too tight.

Hold downs were cut with a 1/4" up cut end mill from Carbide3D. Worked great, but the feed rate of 30 inches per minute was way too slow. Probably could have gone to 50 or even 80.

Bob
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halfnormal
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Re: Hold Downs and Sacrificial Board.

Post by halfnormal » Tue Jul 14, 2015 3:56 pm

Very nice! Any chance posting drawings? :D

xfredericox
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Re: Hold Downs and Sacrificial Board.

Post by xfredericox » Tue Jul 14, 2015 5:09 pm

you probably could draw them yourself in the time it took to write your post 8-)
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CastIrony
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Re: Hold Downs and Sacrificial Board.

Post by CastIrony » Tue Jul 14, 2015 6:52 pm

I bought 100 of these for $12.50 shipped, which is cheaper than I could find them most places. I like keeping everything metric for now.

I just tried installing one last night, and the M5 insert easily fit in a 1/4 inch hole. The flange is thin enough that you can get it to sit flush simply by overtightening, no countersinking necessary.

I'm using a 3/4" thick MDF panel. Could have gone 1/2" but these inserts are 10mm long, and I wanted enough clearance that I wouldn't accidentally hit one. I haven't finished the board yet, but if I'm measuring correctly I should have plenty of vertical room even with the extra thickness.
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woodworkerbob
Posts: 244
Joined: Tue Apr 15, 2014 6:31 pm
Location: SVE - Mesa, AZ

Re: Hold Downs and Sacrificial Board.

Post by woodworkerbob » Tue Jul 14, 2015 10:51 pm

CastIrony wrote:I bought 100 of these for $12.50 shipped, which is cheaper than I could find them most places. I like keeping everything metric for now.
You're using threaded inserts. I used Tnuts I purchased locally, Mesa AZ. I had tried threaded inserts at another CNC site and found the users, high school students, would tighten the bolts too tight and pull the inserts out of the MDF.

Bob
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CastIrony
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Re: Hold Downs and Sacrificial Board.

Post by CastIrony » Tue Jul 14, 2015 11:39 pm

Ahh, yeah, I probably would have different criteria if I were making a machine for public use by teenagers :D

EDIT: Though I would be seriously impressed if they managed to pull that flange through 9mm of MDF accidentally.
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woodworkerbob
Posts: 244
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Re: Hold Downs and Sacrificial Board.

Post by woodworkerbob » Tue Jul 14, 2015 11:46 pm

halfnormal wrote:Very nice! Any chance posting drawings? :D
Author Disclaimer. These designs are free of charge and definitely worth that price. No warranty is given to accuracy or completeness. Possibilities range from having no problems, to the design causing the Earth to explode and all humanity vanishing for eternity. Use at your own risk. That should cover it. :D

I attached the dxf files. I think I can do svg as well.

Hold downs. If you have access to a table saw, make a shallow cut around the outline of the hold downs on the SO3. Using that shallow cut as a guide, then use the table saw to cut the parts out. No table saw, cut all the way thru, don't forget "tabs" or "bridges" between the part and the larger material.

Sacrificial Board. This is a little more complicated.
The board to be cut is 22.872 x 22.872 inches square, or the Y dimension of the SO3 "waste board". I placed the zero for X in the middle of the board to be cut, and now I've forgotten why. Must be age. :D I think it had to do with zeroing the bit straight above where the two waste board meet and at that front edge.

For my Tnuts the cut depth of the outer circle, aka the Tnut plate, was cut to 0.08 inches. You could probably go to .9 or .1 inch so you don't have to pound the Tnuts in. The T-nut cylinder was 5/16" going into a 1/2" MDF sac board.

To cut a sacrificial board:
Center the sac board aligned to the front edge of the waste board and centered between gantry supports. Zero the bit, in X, to where the two waste boards meet. Zero Y to that front edge. I zero Z to the top of the material to be cut.

I created two G-code files: one with the 12 T-nut locations, and one with only the first, lowest in Y, row of T-nuts.

Cut the 12 T-nut locations directly from the design. 4 columns and 3 rows.
Rotate the board 180 degrees, cut the 12 T-nut locations again.
Rotate the board 90 degrees, cut only the first row, 4 locations.
Rotate the board 180 degrees, cut the first row, 4 locations again.

Hope this helps.

Bob
Attachments
SO3SacBoardV7.dxf
Sacrificial Board
(18.81 KiB) Downloaded 118 times
SO3-HoldDownV4.dxf
Hold Downs
(87.06 KiB) Downloaded 111 times
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woodworkerbob
Posts: 244
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Re: Hold Downs and Sacrificial Board.

Post by woodworkerbob » Tue Jul 14, 2015 11:49 pm

CastIrony wrote:Ahh, yeah, I probably would have different criteria if I were making a machine for public use by teenagers :D

EDIT: Though I would be seriously impressed if they managed to pull that flange through 9mm of MDF accidentally.
Our inserts didn't have a flange, so... But remember these are teenagers running on Cheetos and Red Bull! :shock:

Bob
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woodworkerbob
Posts: 244
Joined: Tue Apr 15, 2014 6:31 pm
Location: SVE - Mesa, AZ

Re: Hold Downs and Sacrificial Board.

Post by woodworkerbob » Wed Jul 15, 2015 4:48 pm

CastIrony wrote:I bought 100 of these for $12.50 shipped, which is cheaper than I could find them most places. I like keeping everything metric for now.

I just tried installing one last night, and the M5 insert easily fit in a 1/4 inch hole. The flange is thin enough that you can get it to sit flush simply by overtightening, no countersinking necessary.
I like the idea of not having to counter sink. I'll suggest getting the flanged threaded inserts for next fall. Thanks.

The T-nuts, qty 100, cost less than $9, yea Copper State Nuts and Bolts here in Mesa AZ.

Bob
Last edited by woodworkerbob on Thu Jul 16, 2015 12:17 am, edited 1 time in total.
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WillAdams
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Re: Hold Downs and Sacrificial Board.

Post by WillAdams » Wed Jul 15, 2015 5:01 pm

A thought on installing such inserts --- while one could get a special-purpose tool such as:

http://www.japanwoodworker.com/Product/ ... serts.aspx

one could just stick a SHCS into the insert, then use a hex key to drive it in, then back out the SHCS....
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