Precision Jigs for a Modest Outlay archive
Wednesday 23 March 2011
Dave Rigler takes us through his first experience of using CNC machined jigs fir use in his spindle
The project I'm working on is not a piece of furniture but the quality and precision of construction that I want is of the same order. Having made a prototype I changed the design and construction enough that a new set of jigs and templates were required. The primary reasons that made me consider the CNC route were: Time – In total 24 jigs and templates were required which would take a significant amount of time to produce when I wanted to concentrate my efforts in other areas. Size - Large templates were required for a 2m wide end frame, which consists of a variety of very large and small radii blended together. Physically laying out and cutting with precision templates of this size in my workshop was problematic. Precision – A range of smaller frames are constructed from radiused 'T' and 'L' shaped pieces at the intersections, connected by rails and stiles. The different radii and rail widths require 10 jigs to produce the 15 different parts used to construct all the variants. Precision is required to minimise size mismatches between interfacing parts off different jigs. Excessive mismatch at a join requires time, hand finishing after assembly and potentially some degradation of the true form. The precision I was looking for was of the order of 0.1 – 0.2mm.
Design of jigsThe part of the jig that is CNC machined is the base which controls the profile of the part. The bases were designed with space to accommodate clamps and position the hands a safe distance from the cutting tool. To take full advantage of the precision of CNC machining I also incorporated 6mm holes for pins to accurately position the work piece in the jig. Some jigs accommodate up to 3 different sized parts by incorporating multiple pairs of holes.
Creating the basesI created my drawings using CAD which suits CNC manufacture as the data can be readily translated to cutting paths and features. This company required the CAD in DRG format and preferably with profile information on a separate layer to the holes. I provided all 24 parts on one 2D drawing file. They carried out the nesting of the parts to make most efficient use of the board(s). Having CAD is not a pre-requisite. If you do your drawings manually then the company I used will create the CNC file from this.
I was pleased with the quality and precision of the parts, the edges were crisp and the curves smooth. I checked the centre arm of the 'T' pieces with a vernier and found them to be within the +/- 0.1mm quoted profile tolerance, tending to the lower limit. Laying the mating parts side by side the surfaces mate perfectly to the eye as does the scribed line from the gauge pin holes in the vertical direction.
Construction of jigsCompletion of the jigs is straightforward. Cheek pieces are added which locate the work piece in one direction and provide the run-on and run-off for the spindle tool guide ring. Extending them to the back of the jig also provides a handgrip when machining. A mark is placed on the cheek that the work piece should be pushed against to ensure consistency between parts. Dowel pins are inserted in the appropriate holes to locate the work piece in the other direction. A pad is added to take a toggle clamp and to hold the work piece firmly in place during machining. An adjustable wooden block is positioned where the tool path exits the work piece perpendicular to the grain, to prevent break out – a lesson learned from prototype jigs.
Producing partsHaving spent good money to get precision jigs, equal attention needs to be paid to producing accurate material to put in them. I plane and thickness enough stock of the required widths and thickness for a complete assembly in one session. This eliminates variation introduced by multiple thicknesser set ups.
At the first set up to size blanks to fit snugly between the cheeks of the jigs I produce a birch ply master. This master is kept with the jig to aid quick setting for future runs.
I use the jig initially as a template, holding the blank in place by hand and pencilling around the profile.
I cut 1 to 3mm from the line on the bandsaw to minimise material removal on the spindle moulder. This helps reduce the risk of tear out when cutting against the grain.
I set the cutter height to overlap the jig profile by no more than 1mm to minimise the risk of damaging more of the profile than necessary. With the bearing guide I find that its diameter is pretty much line on line with the cutter diameter, however, the risk is higher with a manually set ring fence.
The rough shaped part is placed back in the jig, clamped and finished on the spindle moulder. This may sound laborious but by working on a batch basis progress is rapid. Five of the jigs produced parts that achieved mismatches at the interfaces of between 0.05 – 0.16mm. The sixth jig produced a part with an error of 0.4mm, which was not acceptable. On close inspection I found the reference cheek was slightly off perpendicular, leaning into the jig.
A few wipes with a block plane quickly corrected this, but demonstrated the attention to detail required at all stages.
Assembly of the frame is carried out with Festool Domino loose tenons, a superb production machine which suit the narrow rails. The large end frame templates were equally successful and the critical precision for the stile interfaces were of the same order of precision as the small jigs.
CostOutsourcing work and paying someone else to do it for you is always hard, so there has to be a good justification. The cost of my project should be taken as a rough guide as I only have experience with one supplier and costs vary over time, size and complexity of the order.
For this project 24 jigs were specified, ranging from approximately 300 x 150mm to 2000 x 600mm, at a cost of £210 before VAT. On the CNC machine 2.5 sheets of 2400 x 1200mm x 12mm birch ply were used. Doing this myself would have cost around £80 in material so the net additional cost for the CNC work was £130. On average £5.40 per part, which for me represented good value. To get an indication of the cost for a smaller order the company gave me a first sight cost for just six of the T shaped pieces of between £40 and £60 before VAT.