Production Steps to Make the Acu-Math 401 Slide Rule

From An interview with Eugene Castiglione,  former manager of the Mt. Olive plant. 
Publication by  George Keane, © December, 2006


After making wood slide rules since 1938 and dealing with the raw material shortages and allocations during World War II,  the Acu-Rule Mfg. Co. was ready to develop new products.  Magnesium core slide rules were produced in 1946 and then vinyl slide rules around 1948.  After a few unsuccessful tries their final choice of a substrate was a material consisting of eight layers of a special, 0.015 inch thick, vinyl film laminated with an adhesive under heat and pressure made by the Bakelite Company of Ottawa, Illinois.  It was sold in a 30 inch by 20 inch piece that was 0.12 inches thick. 

For this example the steps needed to make the all-vinyl model #401 will be described. The #401 was a late development intended to replace the #400.  The company believed the scale arrangement made the 401 easier to use. This was one of their production rules as opposed to the professional grade magnesium or pocket rules. The first  step was to quarter the 20 X 30 stock and then cut each quarter  into blanks about 1.8 inches wide by 12.25 inches long. This was done using a band saw.  The blank was wider than the finished slide rule because it was to be cut and grooved and edge polished.  The blanks were then taken to a hot stamp printing machine. The operator washed and cleaned the blanks and placed one at a time in an air operated stamping machine which was heated 180 to 200 degrees. A blank was placed on a platen. The die lowers by air pressure and a final hydraulic  push drives the die into the vinyl transferring the ink from  a Mylar stamping foil.  This process was less than a second.  Each blank was then washed in warm, soapy water and cleaned with very fine steel wool to remove any residual foil. It was carefully inspected for complete stamping.  All the printing must be there.  A void or low spot in the stock would cause incomplete stamping and the blank would be scraped. Twelve blanks are stamped and inspected.   If  the inspectors are satisfied the printer was allowed to start a production run.   This step was the major source for damage and loss.   It was impossible to correct a blank by trying  to stamp it a second time.   The company had 16 stamping machines and some were dedicated to a single high volume model to save set up time and eliminated scrap that would be created in set up.

At this point the square corners of the blanks were rounded to match the curve of the bracket.  A low speed router mounted in a jig was used to  produced the correct radius to match the bracket.  Next  the stamped blank was cut into three strips that will become the slide and the two stators.  This was done with a Delta band saw.  The three pieces must be kept together as a set because of variations of the stock. The pieces were placed in their own tray.  A Craftsman high speed shaper or the even higher speed routers are used to shape the tongue and grove.  The Craftsman motor turned at 1700 rpm and a belt and pulley system increased the rpm by a factor of three or 5100 rpm.  The cutting head on the spindle had three blades and the result was over 15,000 hits per minute.  Two shapers were used.  One was used to make the grove and another was used to form the tongue on the slide.  That cutter had two cutting heads with a spacer between to form the tongue.  After a few were made the machine was stopped and the parts checked.  If the slide could be felt above the stators it meant the machine needed adjusting.    The tolerance was to be within a half of a thousands of an inch. This was a high precision step and was the second source for damage and loss.  Forcing the stock into the milling machines to fast resulted in chipping the edges. The company had 20 or more Shaper stations. Next was the gluing station.   Another complete inspection was done  for quality, fit and completeness. The parts were placed into a jig to align them to receive the brackets.  At this point the rule must be exactly 1.65 inches in width or the brackets and the cursor will not fit correctly.  Two droplets of a special solvent  were applied to the stator’s ends and the plastic brackets positioned. There was no second chance to correct a misplaced bracket.  This solvent produced heat and after a few minutes a bond resulted that could not be broken.  Two gluing jigs were used and each held 12 rules but this operation was a bottle neck.

For the  model 401 a one piece molded cursor was purchased from American Molding Co. in St. Louis. The center hairline had to be inscribed using  a Kingsly machine at 200 degrees. A blade was used with stamping foil to make the hair line.   A tension spring was purchased from Midwest Spring Co. in St. Louis and  inserted in the cursor. Then the slide rule and the cursor were sent to the packing station. The cursor was installed followed by another inspection. If this inspection passed the shaper operator was told to make a production run. If it failed the machine was adjusted.  In either case an inspection was made after every 100 rules in case the machine got out of alignment.   A shaping station could make about 450 rules a day so multiple shaping station were used.  The slide rule, a sheath and an instruction sheet were placed into a box and 12 boxes are packed per carton.  Depending on the order, either six or twelve cartons are put into a shipping case for a gross or half gross order.

At each step there is a chance for damage that will result in a reject. The model 401 averaged a 13% loss with most of it from the stamping and milling operations.  Some of these rejects may have been almost finished and both the raw materials and the labor were a loss.   Each model of slide rule had a stamping die.  These were obtained from the Western Die Co. in St. Louis and were expensive.

The Economics of the Business.

The production models, 400 and 401, had a retail price of $2.00.  They were sold to vendors and resellers on terms of 50 / 10 / 5 .  This meant a 50 % reduction which reduced the wholesale price to $1.00 and the 10 % reduced it to 90 cents followed by the 5 % left about 85 cents.  Also sales terms of 2%10 net 30  gave another  2% if paid for in 10 days - the final bill was due in 30 days.  The company would get about 83 cents for a rule.  Labor in 1960 was $1.48.  The labor force was about 18 people. The cost estimate for this rule was 67 cents so the profit was low.  This did not include general overhead of running the business.  
The  overall profit of the company was about 6%.  
Based on the same 1960 process a cost estimate to make a #401 today would be about $2.40 and would need to  sell for $8.00 to $9.00.   Modern technology probably will reduce the cost rather than increase it. Around 2,000 or more rules a day could be made depending on the model.  This would put the estimated yearly  output at around 600,000 slide rules.

With some professional models scrap was as high as 20%.  Some of these rejects may have been almost finished and both the raw materials and the labor were a loss. The more production steps the greater the chance for damage. A slide rule with two color printing and scales on both sides required four hot stamping steps and perfect register. Some higher grade professional models required processing Magnesium (the Magnesium dust being very flammable), assembly with screws and bolts, cursor assembly using 8 screws, two color printing and adjustment for alignment by hand.  The higher grade professional models used a thicker 10 layer substrate.  Seventeen models  were produced. The top of the line was the 1311 selling at retail for $13.95.  This was a magnesium core with a single sheet of 0.015 vinyl film laminated to it.   Other magnesium rules were the 511 and the 1211.  The rules were guaranteed but very few were returned. 


       The 21 steps to make the #401 slide rule

1.         Vinyl stock is cut into blanks.

2.         Each edge of the blank is machined.    

 3.         The scales are now applied 

4.         Inspect for complete printing.

5.         Remove excess printing foil

6.         Round each corner of the blank.

7.         Slice the blank into three strips.

8.         Machine the stators 

9.         Machine the tongue on the slide.

10.       Inspect quality, fit, completeness.


11.       Place in a gluing jig.

12.       Apply a chemical bonding material.

13.       Bond the plastic brackets.

14.       A twenty minute dry time is required.

15.       Apply  center line to the cursor.

16.       Fit the cursor with a tension spring.

17.       Move rule to the packing station.

18.       Install cursor and final inspection.

19.       Insert rule into a sheath.

20.       Pack rule, sheath, manual into box

21.       Pack for shipment