Cam Lock - cheap, effective way to hold your work

[bajaru]

I've been trying a couple different clamping methods and in addition to PinchBlox ( http://www.pinchblox.com/ ) I thought I'd try making a cam lock.  I made this one based on the Golden Ratio info found at Wikipedia ( http://en.wikipedia.org/wiki/Golden_ratio ). 

Its maximum holding distance is approximately 2" (51mm) and its minimum is about .84" (21mm).  When clamping, I use two cams per side, pointing in opposite directions.  I tap on one a bit, then tap on the other a bit, alternating until the piece is sufficiently secure.  I do it this way to prevent the piece from walking around and if they're facing the right way, the one you're not tapping on, will get tighter as the piece shifts a little.  If you have them pointing the wrong way it'll be obvious right away, because the one you're not tapping on will keep getting loose, so just flip them over.

The pattern here is the one I use for .25" (6mm) MDF, but of course it can be adjusted to whatever thickness of scrap you have lying around.  I took a guess as to where the center hole should be, and it's not quite perfect but in the end, it works fine in my dusty shop.  If you have suggestions for this little fella, let me know.

This is probably basic stuff for folks here, but perhaps it could be useful to someone looking for a cheap clamping method for a CNC router.  Thanks for looking.

-Brian

[GeorgeRace]

My mill has a "T" slot top.  I found a really nice work hold down clamp that really works well for me.  They are available on Amazon for a very good price.  You can  see it at the link below.  I have 6 of them and they are excellent at work holding.  I do a lot of milling on thin aluminum and work on a 3/4 in disposal board under my work.  They really hold down tightly and I have never had one let my work slip, even do 1/4 aluminum machining.

http://www.amazon.com/dp/B000W07PEM/ref=asc_df_B000W07PEM1029457?smid=ATVPDKIKX0DER&tag=googlecom09c9-20&linkCode=asn&creative=380341&creativeASIN=B000W07PEM

George

[EL34]

BTW,
I looked into the Incra T-tracks

http://www.amazon.com/Incra-TTRACKREG36-36-Inch-T-Track-Regular/dp/B0000DD5EY/ref=pd_bxgy_hi_img_c

I ended up not getting them because the Peachtree T-tracks are built way more solid and cost the same price.
http://www.amazon.com/48-Aluminum-Mini-T-Track-Peachtree-Woodworking/dp/B000H4Z040/ref=pd_bxgy_hi_img_b

I got a few 48" lengths and sawed them up into shorter sections.
I also grabbed a few other items at Peachtree.
The threaded knobs are cheap

Peachtree has a lot of cool clamping products.
They have Two different T-Track types
I got the mini T-Track which is good enough for my CNC machine

Here is the link to their T-Track clamp page
http://www.ptreeusa.com/ttrack_list.htm

[garym1957]

I realize this is an old post, but kudos where its due. These holding cams are going to same me a lot of time.

[EddyCurrent]

Here's another similar thread; http://forum.vectric.com/viewtopic.php?p=18096#p18096
I particularly like the idea of the peg board with threaded inserts, this could be used as a removeable 'bed' that is itself clamped to the machine's main bed. The clamps are low level and in some cases would be lower than the stock being held which is great if you are in the habit of crashing into clamps. In fact I've just convinced myself to go out and make one today seeing as I've a bag full of these; http://www.toolstation.com/shop/Screws+Fixings/d90/Nuts+Washers/sd920/Pronged+Tee+Nut+M6/p41724
Someone did suggest these vibrate loose but I intend to counterbore the underside of my plywood base to recess the heads then glue them in with epoxy.
Also similar here; http://www.talkshopbot.com/forum/showthread.php?t=5508

[Garyhlucas]

In looking at the cams shown in the vectrix thread they appear to be made wrong. You really want a spiral shape such that the surface at the tangent point where the clamp touches the material is perpendicular to a radial from center to that point.  That shape will be self-locking all the way around the cam.  It just happens that I cut three different sizes of these using the Flat Spiral function in CamBam. By simply changing the step over distance you change the effective mechanical advantage of the cam.

[atwooddon]

Gary

Maybe I am not understanding your concern about the Vectric style cams, but the pivot point is not in the center of the 'circle' which provides the cam action when you rotate the handle.  I have used these for a couple years and really like them, they hold the material (woods, plastics) well and can be easily replaced should you cut thru them in the machining process.  Not that I have ever done that....  ;-)  I usually just use a wood screw and washer to attach them to my spoil board then tighten the screw down when you have the part clamped.  

I usually move the pivot point about 1/8" toward the handle which provides a nice amount of adjustable pressure as you rotate the handle away from the piece being clamped.

Don

[EddyCurrent]

It's true that maybe the spirals shown in all the examples are not ideal. For example in the OP's case I feel the spiral is increasing in diameter too quickly and the clamp is in danger of slackening off. I found on the internet some 2D CAD drawings of commercial clamps and these might serve as a better starting point. I will post some dxf files shortly.
Another aspect is, where do you want the handle to be in relation to the work piece once it is clamped ? or maybe in practice this does not matter.

[Garyhlucas]

Don,
I understood how yours were made.  Pushing hard against yours causes them to rotate except if you are very close to top dead center.  If you try them the way I suggested I believe you will find they really are quite  a bit better.  I just made a set to adjust a part on a machine and every one was impressed that they hold position under even high forces without the pivot bolt even being tightened.  I also made one as a belt tightener for my CNC machine, with a center hole for the motor and the outer spiral pushing against the spindle body.

[lloydsp]

You really want a spiral shape such that the surface at the tangent point where the clamp touches the material is perpendicular to a radial from center to that point.  That shape will be self-locking all the way around the cam.

-0-

Gary, I question how that's even possible.  As far as I know, no point on any spiral forms a tangent perpendicular to the center of the spiral.  By the very definitions, only a perfect circle can have that characteristic.

Did yours have "flats" rather than a continuous curve?

Lloyd

[Garyhlucas]

Lloyd,
You are correct, however the angle for a shallow slope approaches tangency close enough that the small side force that is generated is so small it effectively self locks with only a little friction.  Very much like ratios above about 20 to 1 on a worm gear are essentially self-locking.  The eccentric type cam though only approaches self-locking within a couple of degrees of top dead center.  Try it, I think you'll like how it works.

[lloydsp]

Gary, yes.  The longer the radius, the smaller the deviation of the tangent from perpendicular, no matter the pitch of the spiral.

But it does reach that "practical limit" where such a clamp can only have at most one volute of the spiral, and the pitch must be very small to keep the tangents close to perpendicular.

I was mostly just 'needling' you on the geometry.  I have some cam clamps, also, and they work quite well except under severe vibration.  Steady-state thrust doesn't seem to cause any problems.

Lloyd

[EddyCurrent]

Please see if the attached shape is any use (CamClamp.dxf). It's about 6" long and I've got some 5mm thick perspex to make them out of. My intention would be to have a bag of various size wood offcuts to use as packing between the clamp and workpiece.
I've added another (CamClamp2.dxf) that uses Gary's method of Flat Spiral, it has a stepover of 20
Another, based on a commercial lever, (CamClamp3.dxf)