Work holding for non-vacuum tables

[rachelatrandolph]

I have a 36X24 desktop and work in an Innovation Center in a rural school in Vermont. I'm trying to come up with an easy and safe way for holding down material (wood exclusively at this point). The Shopbot came with some bolts and bits that fit in the t-rail but they seem too short to hold anything over 1/2 inch thick! Or I'm just really confused.

From your pictures it looks like you are using some kind of hold down jig. How does that all work? Where does the carpet tape/no residue tape go? I'm a real newbie and don't even understand most of the acronyms people are using on this site. HDO? MDO?

I've tried the super glue thing and it worked but seems expensive to go through that much glue. Any ideas/further explanation would be greatly appreciated!
Thanks!
Rachel

G.
Had VERY good luck using the carpet tape shown(1.88" width essential)/ and the "No Residue" Duck tape on a surface like HDO(waxed) or lacquered/waxed wood.
"Bonnie" buttons were .75"D, and the Bloodwood "Trees" were cut Monday and no residue even though that Bloodwood has been stuck down 11 months(AND STAYED stuck FLAT) and were cut out using a .5" (.25"R)pointed round over bit.
The trick is to cut only into the no residue tape, and to make last pass less than .01".
I don't know, but works great for me.
scott

[coryatjohn]

The art of hold down is a lot like the art of setting concrete forms. Too little and the project is ruined. Too much and the setup/teardown process is too arduous and time consuming. What you want is the goldilocks zone of hold down.

Before deciding on your hold down, you should understand the forces that the machine will exert on the workpiece. A good rule of thumb is that the bigger the end mill, the more forces are involved. So if you're using a 1/8" end mill, then your hold down is minimized.

Feeds and speeds impact the forces too, so if you're not in a hurry, using slower speeds will reduce the side loads and allow for less hold down.

The type of bit also influences the hold down requirements. An upcut end mill will try to pull the workpiece off the machine, while a downcut will push down on the piece. Cutting thin material without a vacuum would be a trick with an upcut. V-bit's have no pull or push on the piece so if you're using one of these, all you need to do is stop sideways movement.

For thick pieces, I typically will use an edge clamp on at least two places and then block the sides with scraps of wood. Block all sides. The key thing is to stop any movement from starting so if your hold down is weak in one direction and the piece starts moving, it won't stop until it's thrown off the table or the bit breaks. So keep your hold down jigs tight so there's not the slightest wiggle.

To tell if your piece is down tight, tap on the corners of the piece and listen for a hollow sound. That means the piece isn't firmly attached to the machine and will likely wiggle and break free.

Be wary of steel clamps and bolts that may be in the path of the endmill. While the bit can probably chew through the edge of your aluminum clamp, it will break the millisecond it touches steel. So steel is bad. Wood, plastic, aluminum good.

Hold down is a topic that's both wide and deep and the best way to learn is to try different methods.

[scottp55]

Sorry so long Rachel, but had a deadline for my Mom's 90th Bday party tomorrow and have been out Straight Making Tiny Harleys with that same rig.
Each 3D cut was a.25mm TBN followed by a 30* engraving bit for a total of over 4 hrs a Pair. So thin stock was on a minimum of 5 days. Worked Great for 30 pairs!
NOT sure it's a good setup for a school though(?), as I made it for thin stock cutting(buttons and key fobs .23" thick).
MDO(Medium Density Overlay) is resin coated ply, and most of our highway signs up here are made out of it.
HDO(High Density Overlay) is just ply with a thicker resin coat so I could wax it for easy removal of carpet tape that goes down first.
Then sanded to finish grit(600G) thin stock, is cleaned and Duck brand "No residue" tape applied to stock and both tapes are rolled down Tight.
Then Paper is pulled off carpet tape, stock carefully placed, and then firmly whacked down with a spacer block and a deadblow.
I think it's overkill for you, but we were making thousands of buttons with 7 HDO blanks, and bit was ZZeroed to spoilboard from surfacing to cutout,
so just change blanks, not bits...and sped things up a lot.
Worked great this last month on the project shown in pics.

The spoilboard is a combo of Bill Young's design on this thread;
http://www.talkshopbot.com/forum/sho...p-Jigging-Pics
and the addition of threaded inserts has worked Great for 7 years!
It is easy with cam clamps to change out blanks and set up for a new material!

Lots of other members hold down methods are on this thread;
http://www.talkshopbot.com/forum/sho...-please-share!
As John mentioned, there is not one "Perfect" hold down for all projects.

Bullet points from Ryan are worth looking at;
https://www.youtube.com/watch?v=o_KWFVefsQg

as are the Shopbot document;
https://www.shopbottools.com/ShopBot...oldingdown.pdf

Read somewhere that some teachers and Makerspaces make their students put their material on ply or MDF, so mistakes don't screw up the Actual spoilboard,
but I've never been in a teaching/school environment.
Kyle Stapleton is a teacher who had a full size machine and a Desktop in his classroom...Hoping he'll chime in.

After tomorrow, ask any questions that stump you please....and you'll get faster responses
Have to go pack up the last of the BDay stuff now!
scott