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pitbull clamps and getting tight to the table

Pattnmaker

Stainless
Joined
Nov 2, 2007
Location
Hamilton, Ontario
I built a fixture last week for machining matchplates. The outside length for the 2 sizes I make the most are the same. Because I typically have to machine the entire top I built pockets in the ends to accept pitbull clamps. I asked my local tooling supplier for pricing on the knife edge clamps. He quoted me the blunt ones and I didn't notice. When they came in I had a job I had to get out of the door so I used them rather than returning them.
The plates I need to hold down are 27.5"×23.5" and range from 5/8" thick to 6" thick. Unfortunately I found the 4 pit bulls didn't pull the stock down. They held it securely but all 4 corners were different heights. Varying by up to .02". Do the knife edge pitbulls do a better job of pulling the part down? I had a work around on this plate but won't always have that luxury. I will buy the knife edge clamps if they will solve the problem but if they won't I would rather save the money.
 
Even the blunt ones are supposed to pull down. That said, the stock/part should be flat and down before you start clamping. I wonder if something wasn't flat, or had a chip under it? I'm sure you checked, but...
 
The plate is either off(sawed crooked or tapered) or your pockets you milled for the pitbull is off. They should never be flush with the fixture. Knife edge wear faster if you are putting parts in and tapping them down. A possible solution would be shim the back side of the pitbulls.
 
I've never used pitbulls, but I've always wanted to.

I agree with Shawnrs though, they probably weren't placed in the right spot.

I was looking at the pictures on mitee-bite's site, and have another possible solution for the stock being off/small.

https://www.miteebite.com/wp-content/uploads/2014/02/Pitbull-Clamp-5.jpg
https://www.miteebite.com/wp-content/uploads/2014/02/Pitbull-Clamp-6.jpg

They put the pitbull on a carrier of sorts, which has slotted holes. So if the stock is bigger/smaller or crooked, you could adjust where it is by a bit, all around if needed.
 
I did use the Drawings from the Pitbull site. BUT I did make the outside a little bit oversized I think .015" overall between the opposite pockets as the stock is usually oversized often .030-.060 Maybe the stock on the couple of parts I did was not oversized. I don't remember the outside size as it is not critical and does not get machined. I will have to try again placing a shim at the back of the pocket. It was cast and ground tooling plate so it was flat and we did clean out any chips and then tried resetting checking again for chips.

I will have to double check how much extra clearance I added to the back of the pockets and try again next week with a similar plate. Checking the size of the plate and seeing if it is oversized by as much as my clearance.
 
As big as those plates are, can you use vacuum as well? As noted, the Pitbulls have a narrow throw so if either the pockets are off or the stock is off nominal size you'll run into issues.

We machine a lot of 12" x 12" plates of aluminum, phenolic, and plastic. Some of it can be held down with vacuum alone, some of it has space around the edges for hold down clamps.

There are stops around two sides (X- and Y+), so sometimes we'll slide stops up against the other two sides (X+ and Y-) to add lateral stability, with the vacuum holding it down. And for the parts that need the entire top machined and have deep slots that want to pull off the vaccum we'll add a M00 stop after the top is faced to add a hold down clamp. Those are the annoying ones because of the extra step, but it works.

Most the time we keep through features .015 or so off the table and chmfer the backside, but if you are doing the same part you can gasket off those places and cut clean through.
 
Unfortunately vacuum wouldn't work on these. The plates last week were tooling plate with a stepped hole through the plate. There was a Renshape insert screwed into the hole. Every plate done this way has a different size and shape though hole or holes. Then it will also be used to cut raised and or lowered features on one side of a plate and then corresponding features on the opposite.
If I can't get the pitbulls to work there are holes in the one style of plate for mounting to the moulding machine that I can use to mount for cutting But it means a 2 step setup which I was trying to avoid.
 
Unfortunately vacuum wouldn't work on these. The plates last week were tooling plate with a stepped hole through the plate. There was a Renshape insert screwed into the hole. Every plate done this way has a different size and shape though hole or holes. Then it will also be used to cut raised and or lowered features on one side of a plate and then corresponding features on the opposite.
If I can't get the pitbulls to work there are holes in the one style of plate for mounting to the moulding machine that I can use to mount for cutting But it means a 2 step setup which I was trying to avoid.

Bummer. The parts we do with through holes are for production, so it was worth it to make a custom vacuum top plate. The knife edge Pitbulls should give a bit more pull down, but your stock needs to be pretty uniform (maybe +/-.002?) for best results.
 
Pitbulls can be really finicky. I avoid using them on wide parts.

A few things to check-
1. Distance of pitbull to part (as others have said). They rotate as they're tightened - if they hit the part soon it's more lateral force and if they hit it late it's more vertical
2. Make sure it's flat before you clamp, because it kind of wedges in and can actually prop the part up
3. Apply downward pressure to the workpiece near the clamp while you clamp, and sometimes you might need to reclamp if it doesn't feel right
4. Consider whether the edge of the workpiece is square to the clamp or not (eg saw cut could be an issue)

Maybe try to indicate and see what happens when you clamp
 
A couple things, if this is a repeat job, you definitely want a hardened backer of some sort for the pitbull to push against or the pocket will wear out and you will lose holding force.

Also, may want to look at some of the other mitee bite stuff, such as -

Knife Edge Clamps | Mitee-Bite Products LLC.

OK-Vise(R) Clamps | Mitee-Bite Products LLC.

T-Slot Toe Clamps | Mitee-Bite Products LLC.

One of those may be better suited (in case you didn't know they were available)...


I agree that the backs can wear or bulge up but another quick solution is to mill a pocked for a dowel pin and use that as the back support for the pitbull.
 








 
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