In CAD Design of Jigs and Fixtures, Do you touch the product line to line?

A reader of this blog presented an interesting question the other day regarding weld fixture design using CAD. Maybe YOU too, have the same question.

I am the fixture designer for a small agricultural equipment manufacturer, I am new at this job and I am still learning. I am not an engineer but they had enough faith in me to promote me to this position.

My question is when designing a weld fixture in the CAD program should I allow any room between the holding pieces and the parts? One example would be a plate notched to hold a square tube on three sides.

Generally our tolerances are to be held to 1/32.

S. R.”

Thanks for the question regarding weld jig and assembly fixture design.

At Rentapen, we do not allow room between the locating parts in the fixture and the product parts. Most of the time we design so that the fixture locators are actually touching the product parts in the CAD design.

In the picture below the yellow Round Tube, the “product part”, is held in place by two RAPid Plates (RP03).  As you can see, the product part and the locators of the weld fixture touch in the CAD model.

RAPid Locating Plates (RP03) are placed line-to line with CAD model of product.


With 1/32 variation in tolerance, how do you know if your product is going to come in 1/32 too big or 1/32 too small?  In the real world if a product part is designated 2.00″, it will not be manufactured perfectly.  It might be 2.05″.  Then if the “notch” you cut for it is not perfect…

For example, If you put your locator 1/32″ away in your CAD design, the center line of the part could be over 1/32″ off from center if your product varies 1/32″.



At Rentapen we USUALLY design in thin metal adjustment shims to finely tune things once we get hold of the product parts and can check their fit in the fixture.

Using RAPid Blocks, Plates, and Shims to hold a round tube.

In the picture the yellow product tube is held with Rentapen’s RAPid Plates (RP03) and those plates are separated by two shim packs and one of Rentapen’s RAPid Blocks (RB03). 

The block has been altered with cuts, and holes for the plates.  And a hole for mounting the Jergens Flat Foot.  (Note that if you order an altered Rentapen RAPid Block(TM) , Rentapen can do the alterations for you, why have the shop touch the RAPid Block(TM) twice?)

The plates are mounted to each other with the RB03 and the shims in between.  Some companies use a variety of thicknesses in their shim packs.  But with a full 1/32 tolerance, you could probably use 3 or 4 “J” shims which are .015″ thick each.

When DON’T we use shims? If it is fixed metal to fixed metal we use shims unless…
1) the customer doesn’t want shims; 2) it is a rough locator;  in that case you design with a gap (usually .062″ but could be more or less depending on the application) between the locating plate and the product part. 

We don’t use shims when the parts are not in a fixed location, such as a SPRING STOP, or an ajustable clamping situation.

Regarding Shim Packs
I would like to note here that each shim pack of shims contains several thin metal shims. Sometimes the shims are all one thickness, sometimes there are thin and thick shims. But the “PACK” is one line item in the CAD BOM and the pack is also usually represented by one CAD part.

A shim pack consists of several thin metal shims of the same shape but can have the same or different thicknesses.

If you look at our parts download web site, you can order a pack. After you tell the program the dimensions of your shim, you can choose how many shims of what thickness you want in your pack.

The download program will tell you your part number for the whole shim pack.  When you receive your CAD model it will be the thickness of the whole pack.


These larger custom shim packs are made of shims all the same thickness.







In your example you have locators touching the product part on three sides by just notching one block.  If you put a fixed metal locator in all three locations (like a rectagular notch) , there will be no adjustment for tolerance variations in your fixture or the product from the “ideal” that you designed to in CAD.

This “notch” design COULD work though if a) the notch were a v block style AND b) this was the “fixed part”, then make all the other product parts that will be welded in this fixture adjustable.

A V-Block will allow for tolerance variation in the product tube and is easier to remove the product from the fixture after welding.









Add a RAPid Plate (RP03) as a stop for your third side locator.



A 3-sided "notch" doesn't allow for tolerance variation in the tube or fixture.

 If the “notch” is a rectangle and hugging the sides of the rectangular tube…  there is a good chance your “notch” will be too large or too small due to an imperfect world and the tube won’t fit or there will be room to spare making your welded products inconsistant

So, I hope you can see the advantages of designing in thin metal adjustment shims. 

Without designing for an imperfect world, your fixture may either be too tight a fit to get the product parts into and out of the fixture with ease, or it could be too loose causing the final welded product to be inconsistent and possibly out of the acceptable tolerance range.


Welding can cause metals to expand and distort and make the welded product difficult to remove from the fixture. The exact amount of the expansion is very difficult to anticipate due to the heat-sink action of the fixture and other product parts, and the possible inconsistency of the welding action. It is best to be able to adjust the fixture during the initial testing of the fixture.  The product can be made close to perfect and  the shop team won’t be cussing you out!

The Queen

Rentapen is growing!  We are seeking a talented weld fixture designer interested in growing their career in a small fast-growing company in Waukesha, WI.  See Rentapen’s Career page.

Send your resume to hr-rentapen at

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