Tuesday, April 11, 2017

A Ming-Inspired Cabinet (90)

In the past couple of days I picked up a 5/8" (15.875mm) brad point drill, made by Fisch, and a 7/8" (22.22mm) Forstner bit out of Famag’s ‘Bormax’ line. These were the tools needed to complete the upper hinge receivers in the cabinet’s upper sill/cornice pieces.

The Bormax, designed for hardwood, was just the ticket of cutting the counterbore for the flange on the bronze plain bearing. Of course, the natural tool for (quasi-) precision drilling was the Zimmermann mill, once again:

Target depth for the counterbore is 0.0625":

I like the Bormax bit as it cut cleanly and without burning- I think I’ll spring for a boxed set of this at some point later on.

One significant advantage to using a mill rather than a drill press, besides the obvious greater rigidity and accuracy is the tool holding. This might not be obvious to folks unfamiliar with milling machines, so I thought it worth a mention.

In a regular drill press the Forstner would have been held in a 3-jaw drill chuck. This is generally adequate to be sure, especially with higher quality chucks, but I have found that, especially with larger Forstners, the torque load while cutting can overwhelm the chuck holding capacity and this leads to the shank of the bit spinning slightly in the jaws of the chuck, marring the surface of the shank. This in turn can lead to the shank becoming sufficiently damaged that it no longer allows for concentric spinning of the bit. One solution to this is to have a bit with a hex shank, however this is not available, it seems, for every type of large bit.

In the mill, while I could use a chuck, it is generally preferable to use a collet holder. In the case of the Bormax bit, the shank is 10mm, and I happen to have a ER27 collet in that size. With a collet, which is normally encountered in woodworking only in a router, the shank of the bit is gripped much more completely on its surface, instead of only at 3 points, and thus there is far less chance of shank slippage. Total indicated bit runout (TIR) is also significantly less with a collet as compared to a chuck.

In truth, if you want a truly accurate hole, besides dispensing with the 3-jaw chuck and moving to collets, one would also not use a drill of any kind to make the final hole. Rather, one would drill and undersize hole and then use a boring bar to clean the hole to the required size. In this case, going to that extent is unnecessary as the interface between the bushing and the wood is simply a decent slip fit, and I can get there with the drills I have on hand.

One disadvantage to collets is that the gripping range of an individual collet is limited, and thus to cover a wide range of shank sizes you need many collets. So, in my case, I have but a modest collection of collets at this point - and two different collet chucks with different collet standards (ER27 and 444E) so in cases where I do not have the right size collet for a drill, I need to use the 3-jaw chuck. I would prefer the collet though, and am gradually acquiring a full range of sizes. The chuck I have goes to 5/8" shank size maximum, while I have collets in the 444E type which go up to 25mm in diameter.

One final note is that collets are a wear item, and have only a finite lifespan as an accurate tool holder. I haven’t run into that limit yet, probably because I am not doing work at an accuracy level where collet wear becomes apparent.

Next, I’m checking that the counterbore is a good fit for the flange of the bushing:

The flanged bushings themselves are extremely accurately made, pretty much bang on dimension down to a thousandth of an inch (about 0.025mm), so the above was really just a check of the result from the drilling with that Bormax.

Onto the through hole with the 5/8" brad point:

While the hole was bored cleanly enough, though not without some complaint from the bit, the hole itself proved to be a hair too small for the bushing. What the…?

I checked the drill with a caliper:

5/8" is 0.6250".

So, a brand new brad point from Fisch was about 0.005" undersize. Fail. Won’t be buying any more from that company, and will return the bit to the Woodcraft store. Once bitten, twice shy. Fisch’s quality control seems to be lacking to say the least - I guess I thought that an Austrian company might have better products than that.  Mind you, I don’t know for a fact that the bit is in fact made in Austria - it might come out of China.

I wonder who does make a decent, accurately-dimensioned, large brad point through? I’m guessing there will be something in Germany that would fit the bill - any readers who know a good brand to recommend for future reference?

Anyway, I happened to have a decent 5/8" Forstner kicking about, and by running this through the bore hole I managed to enlarge the hole to the required size:

As it turned out, drilling with the 5/8" Forstner alone on subsequent holes made for a bit too much work for the bit in such a hard wood as bubinga - it tended to burn in the cut - so I ended up using the undersize brad point followed by the Forstner for all the holes. A bit of added faffing about, but the results were what counted in the end.

All the way down into the counterbore and things shaped up as I hoped they might:

On the upper face the bushing came nearly to the surface, as desired:

With both mounting holes for the plain bearings completed on one frame, I proceeded to final assembly. I planed up some ¼" (6.35mm) pegs, tapered and chamfered their ends, and then placed them in preparation to be driven:

Once the pegs were down, and trimmed off both sides, using a crosscut saw followed by a small router, a first coat of finish could be applied:

By the end of the afternoon I had the frame for the second cabinet through to the same stage. Time now to tackle the hinge mounting holes on the lower sills. I’ll save that for the next post in this series.

I hope you enjoyed your visit to the Carpentry Way.

via Tumblr https://davidpires578.tumblr.com/post/159444916209

No comments:

Post a Comment