Carba-Tec Height Guage - Done
Product Review Carba-Tec Height Gauge.
I recently visited Carba-Tec in Melbourne in response to their ad in the local woodworking magazines. While my primary interest was in a dovetail jig, I was aware they were also discounting a vernier height gauge. Unfortunately my initial quest was unsuccessful as both were out of stock, and the dovetail jig had been a run out item so couldn't be ordered in. However I did manage to leave with an alternate jig and the height gauge on back order. I collected both items a couple of days ago.
The height gauge is of unspecified Asian manufacture and comes in a blister pack with a card insert claiming that the contents are a Height Square. There are no instructions or information on how to read the scales on the unit. The unit would normally be used for setting up table saw blade height, router bit depth, or similar items where moderate accuracy is required. It should be able to perform well enough for this type of task, but I would suggest checking and adjusting a couple of times a year to avoid accumulating errors over time. I would not consider it a primary measurement device due to the metals and scales employed and the overall construction.
The unit consists of a cast iron base, with extruded aluminium column, elevating housing, and extension piece, a plastic adhesive scale fixed to the column, and two printed vernier scales which screw to the elevating housing, and have slotted mounting holes for adjustment and zeroing.
Once out of the packaging, it was obvious that the unit had potential to be developed into a reasonable unit, but it needed some attention to achieve its maximum potential. Issues immediately identified were;
· Neither imperial nor metric scales had been zeroed properly, but were adjustable.
· The elevating housing was an extremely loose fit on the column, barely able to be clamped in place by the locking screw.
· The extension piece was not properly attached to the housing, with a gap of approximately 0.5mm between the top of the extension piece and the elevator housing, so that gauge face was not parallel to the surface.
· The bottom surface of the cast iron base block had been ground, and was fairly smooth, but not flat. The slot for the column mounting had obviously been cut after the base was ground, because the cutting process had left a burr about 0.5 mm deep protruding from the base.
I spent a short while considering whether to return the unit or to attempt to sort the issues myself. I opted to sort it myself as there was nothing insurmountable or difficult involved.
Corrective procedure
The first thing I did was partly dismantle the unit. This can be done in a couple of minutes with a miniature screwdriver set. First, locate and remove the three Phillips head screws which attach the extension piece to the elevator housing. This gives access to the brass screws that adjust the free play between the elevator housing and the column. Then remove the two vernier scales from the elevator housing, and the limiting screw at the top of the column.
With the elevating housing locking screw released a couple of turns, the elevating housing should now be able to slide right off the column. Beware when it comes off as it has a single nylon rub strip that sits on the locking screw side. Remove the rub strip and the two brass adjusting screws. This now leaves the base and column as an assembly, and the rest of the unit as components.
Reface the lower surface of the base by lapping it on some wet and dry paper on a quality flat surface. I used the top of my ancient cast iron table saw, but a thick piece of glass may also suffice. I managed to remove the burrs on the base of mine in about a minute of light rubbing, but continued rubbing for a while longer until I achieved a more even surface all around the edges of the base. The base is cast with a recess in the centre, and the factory surface was found to dish in towards the recess slightly, but I did not try to completely eliminate the dishing.
I then examined the elevating housing and found burrs inside on the contact areas near the screw holes for attaching the extension piece. I cleaned these out by scraping with hobby knife blade and small screwdriver. When I examined the mounting holes for the two brass adjusting screw, I noted that they were only tapped part way through. With a little coaxing, I was able to get screws to extend the thread, but would recommend retapping if a suitable tap was available.
Reassembly procedure
When satisfied that the surfaces inside the elevating housing are free of burrs, insert the two brass adjusting screws and run them in a couple of turns only.
Insert the nylon rubbing strip into the locking screw side of the elevating housing. It has a moulded pimple on one face which fits inside a hole at the lower end of the housing. This ensures that the strip cannot move out of position when the housing is installed on the column. Make sure that the locking screw is retracted and clear of the rubbing strip for now.
Fit the assembled housing part way onto the column. Stop when the first brass adjusting screw is approximately level with the top end of the column graduations and set the adjusting screw so that the housing slides smoothly on the column, without allowing side play. Sneak up on this setting, rather than tightening the screw and sneaking back, in case the screw bruises the column slide. Then slide the housing further onto the column until the second adjuster is at the same spot and adjust it in the same way. The housing should now slide up and down the column with light resistance, and stay in place with the locking screw released. Turn the locking screw until it is firm, this will lock the housing, and release about half a turn to regain restrained movement. Refit the limit screw at the back of the column and tighten fully. This stops the housing from being able to slide off the column.
Place the assembled gauge on a flat surface and slide the housing and extension down until they also rest on the surface. Lock the housing to the column with the lock screw. This has zeroed the unit, now lay it on its side and install the two vernier strips, ensuring that the 0 markings on the column and vernier coincide.
Reading a vernier
To read a vernier gauge, note the measurement of the column graduation at or just below the 0 graduation on the vernier scale. This is the primary measurement. Then look along the length of the vernier scale and locate the point where a vernier and a column graduation coincide. Ignore the column measurement, but note the vernier graduation measurement and add this to the primary measurement to obtain a final measurement. There are a number of detailed explanations on the internet, easily found by searching "vernier calliper".
Accuracy
Once corrected and properly setup I made measurements of a number of items with the height gauge, an Aldi digital micrometer, and a Mitutoyo vernier calliper. In general, I found that all three readings for each item were within 0.05mm of each other at approximately 50 and 100mm. Since this is the resolution of all three devices this is an acceptable accuracy. Because the gauge uses an adhesive scale, I would be moderately cautious and make comparative measurements on a monthly basis just in case the scale did creep, expand or contract.
