Another way to skin the work-holding cat

An acrylic pointer with most of the pertinent dimensional data

The material for this part is clear polymethyl methacrylate, also known as “acrylic”, “plexiglass” and a host of other trade names. Our client, somewhat short-sighted in their ordering plans, can not wait out the lead time from the supplier as they need these items – 20 of them – by end of the business day tomorrow. That said, the method of manufacture falls to the machine shop.

Sure, we could build a mold but what about CNC machining the part from acrylic sheet? That’s exactly how we plan to make these and in the long run, it is the fastest method of manufacture when starting with a clean slate in such small quantities.  You’ve undoubtedly noticed the scale… they are small, delicate and considering the material – susceptible to chipping and breakage. Work holding and machining technique become critical in this type of application.

With such a short run, we can not amortize the additional cost of tooling nor is it needed IF we machine them with a “break-out” technique. Not only that but, machining these without the additional cost of tooling allows us to price and win the job from those who cannot see a way around tooling for the part.

Many of you might be familiar with the break-out. It’s similar to tabbing a part, except that in this case, the floor web itself holds the finished workpiece in place. This may sound crazy to those who have very little experience holding parts this way but I’ve been amazed at the strengths of this method and the parts it’s capable of producing, with a little ingenuity. A basic understanding of the technique puts another workholding tool in your arsenal should the right candidate come along… and that’s what experience is all about.

Tooling needed:

• Tooling Plate: preferably with a grid pattern; tapped and reamed/bored locations for pull-pins to use in workpiece rotations
• Pull pins from Jergens, Carr-Lane, etc..

You can build a tooling plate with any number of systems but employing two plates with Jergen’s “Ball-lock” (locating & locking system) allows you to load parts off the machine while others are in process, should that need arise. This is a good piece of extremely versatile workholding to have available in your shop.

Short of a tooling plate, there’s not much needed to pull this off beyond an understanding of the machining techniques involved: the amount of floor-stock to hold the part in place, the amount to leave on walls for the final finishing pass and much of this is dependent on the workpiece material and the size of tool used to perform the final peripheral milling.

Looking at this part print, left to right, we’ll call them views “A”, “B” and “C”. We will need to choose view “A” or “C” and due to the small radii (.02) we’ll go ahead and start with view “A”. Now, in most cases we’d want to finish our break-out with the side that puts the greatest surface area to the tooling plate but the use of a 1mm-3/64″ end mill complicates the situation. Not to mention, using such a small tool for the final peripheral cut is plenty feasible, as it allows very little floor-leave and can be accomplished at a healthy feedrate in this workpiece material. Not to mention, 1/8″ length of cut, is pretty much standard fare for this size tooling.

Side “A” features a .250 diameter boss, a countersunk .062 hole and a .500 floor dimension along the “pointer” geometry and the base of our boss. We need a tool that encompasses our major diameter while milling the .250 boss to size (.500-.250) so we have .125 on each side of the boss. A 5/32″-3/16″ end mill looks like a good candidate for this operation. Not only that, but it covers the pointer portion of the .500″ depth in one single-line cut.

We could very well have used a 1/4″ or 6mm end mill but it is best to use a tool that creates the least amount of over-cut due to the peripheral operations breaking-out on that floor surface.

Lining the processes out, this is what we come up with:

1. Use a 90º spot drill to establish the .220 dimension
2. Drill the .062 hole with a 1/16 drill
3. Mill the .250 boss with a 3/16 end mill
4. Mill the pointer (.043/.043) geometry down .500 to establish that floor thickness
5. Drill and ream two pull-pin holes for locating the second side (H7/g6)

We are starting with a piece of 3/4″ acrylic and for side “A”, setting our tool zeros from the top of the part. The easiest way to do this is to write a program where all tools are calibrated to the top of the tooling plate, then use a G54 which employs a Z equal to the average material thickness across the used portion of the stock for the first side. It is not imperative that we face the first side as the surface area of unfinished material is extremely small but a set of toe clamps would perform the job admirally, should you decide to face on a piece of material that varies greatly in thickness, or is heavily scratched.

How much material to leave between parts? Yes… did I mention that we’d be running these parts in-series? Think “loop-linear” or “linear array” and you have the basic idea. The amount of leave between parts – the un-machined ‘web’ –  should be .100″. We have a major diameter of .500 and a 3/16″ end mill cutting the .250 diameter portion of the shaft, which will extend .0625 beyond the .250 radius dimension. So we need .250+.250+.0625 +.0625+.100 (web between parts) or .725″ distance between parts. Crystal clear?!? Don’t worry, there’s a diagram in here somewhere!

Dimensions

With a little math: 10 parts to each side of center or 9 x .725 + (.725/2) (remember, we are splitting parts #10-11 across the middle) for 6.8875″ to the center of our first part in-series. By the way, 4-place decimals are only being shown to eliminate any ambiguity in the numbers. It is only necessary that we split the center-line accurately, because of repositioning for the opposite side. Once we have our 1st part’s location, we then add .125 (half the pointer shaft diameter) .19 (the 3/16″ end mill) and .200 for a little insurance (6.8875+.19+.2). Our total stock length is 7.4 x 2 or 14.8″. Well within the ‘X’ travel capacity of most small machining centers. Could we do this in two rows of 10? Absolutely, just remember to calculate the .100″ web between parts.You’ll also notice that I have offset the first part -.375 in ‘Y’ to better center it in our material, which also makes a little room for pull-pin locations.

That said, those locations are totally up to you and of course, dependent on your tooling plate design. I generally like to design a tooling plate that incorporates ‘X’ and ‘Y’ dowel locations to cover the gamut of stock needs. Let me also emphasize that should you decide to go more rectangular in your array, you will need to pay close attention to part flatness, as clamping will be entirely peripheral. This can cause problems in any material that is highly irregular across the finish surface, especially heavily concave sheets. You will want to place the convex side UP for the side “A” operations… and this is always easier and more predictable using more rectangular arrays.

We’ll cover side “B” operations in the next installment along with cut-away diagrams to show how the breakout works and variations upon this technique.