Blank to Mandrel Fitment

We get a lot of emails from customers that have issues with blank to mandrel fit.  Usually the first assumption is that the mandrel is bad.  Unfortunately that is seldom the case.  It is usually and issue with the hole in the blank.  Whether it be under-sized, over-sized, or not straight... its an issue that will end up needing to be addressed one way or another.

This article was created to help break down the 'problem' into identifiable 'pieces' and what can be done about each piece of the puzzle. 

Checking the mandrel is simple, provided you have an anvil micrometer.  Measuring along the diameter of the mandrel body in various places you want to look for any measurements that are larger than the specified diameter of the mandrel.  For example: they typical specification for a 5/8" expanding mandrel is .624" +.001" / - .002" for the majority of the body of the mandrel, with the exception of the end of the petals where the mandrel expands.  At the end of the mandrel, it depends a bit on how far the expansion screw or plug is backed out.  If it is completely removed, it could be as much as .010-.015" smaller than nominal, or with the expansion screw or plug installed you should be able to easily adjust the screw or plug to make the end of the mandrel measure .625" or a touch under.  Since this end of the mandrel is designed to move, its less critical that it measures exactly .625" - what is more important is that it's range of motion at least goes at least down to .625 and expands to larger than .625" by approximately .015" (.640").  Odds are good that if the mandrel body, where it does not expand, is not .625" +/-.002" - that there is likely issues for the entire length of the expanding mandrel.  If you are dealing with a Pin-Lok or PinSTUB mandrel, the mandrel body should be .625" +/-.001" for the entire length of the mandrel body.  Since the PinLok and PinSTUB mandrels do not expand, they have tighter tolerances.

If you mandrel falls outside those guidelines, it may well be the mandrel that is causing the issue.  Email us, and we can start the process of getting the mandrel back here for us to inspect, and potentially exchange for you.

Now comes the more complicated side of things...  if the mandrel is fine, it's the bore of the blank, or more accurately, what made, or how the hole was made.  One thing most people do nor realize is that drill bits are NOT precision hole making tools - they are roughing tools.  They are not designed to make an accurate hole, they are designed to make a rough hole - quick and easy.  If accuracy is required, then usually an additional process is required.  So if you are expecting a 5/8" drill to give you a 5/8" +/-.001" hole - you're expectations are WAY TOO HIGH!  Does it happen?  It can, but expecting it from a drill is not a good plan - it is usually the exception, not the rule.  The machinists hand book suggests that a hole that is within 8% of the drills specified size is "within spec".  8% is a lot on a 5/8" drill - .050" to be exact.  Add in the variable of changing material from hole to hole, like one blank of hedge, and one of cocobolo, another in cedar, the bore left by that drill bit could be all over the place and different on each blank. 

Bore accuracy is a bit less important with an expanding mandrel than it is with a Pin-Lok, PinSTUB, or the various other non-expanding mandrels out there (such as a tapered mandrel) because the expanding mandrel is designed to expand and hold, where as the others rely on the bore of the blank to align the blank and affect the locking method for that mandrel - like the pin in the case of a Pin-Lok.  If you are having issue with getting your pin style mandrel to lock up on the blank, you will need to focus on getting your hole sizes as accurate as possible.  If you are using an expanding mandrel, you dont have to be as critical on the bore diameter, BUT the better the fit, the better the mandrel will hold and less issues you will have should you remove the barrel from the mandrel and then have to put it back on for additional turning.  Remember with an expanding mandrel, only the very front portion of the mandrel expands, the rest of the body does not.  So if the blank doesn't fit well along the entire mandrel, it can wiggle and move when pressure is applied.

Some of the more common causes of OVER-sized holes:

• Poorly ground drill bit
• Drill bit run out
• Improper feed and/or speed (speed drill bit is advanced and/or RPM)
• Clogged flutes
• Equipment Misaligned/Run-out

Despite bore oversize being a bit less imperative for proper fit with expanding mandrels, keep in mind, any oversize of the bore will allow for misalignment on the mandrel.  If perfect centering is of utmost concern (this shows up when doing inlays, and other "fancy" aspects), then you will want to pay just as much, if not more attention to the proper bore to mandrel fit.   Also remember, the better the bore to mandrel fit, the more holding power you will have.   If your mandrel wont hold up to the turning forces, you can almost guarantee that your bore is oversized.

Diagnosing misalignment will take some attention to detail and likely a few measuring devices that are readily available but not likely already on hand for the average call maker.   A high quality caliper, more micrometer, and a 1" travel dial indicator and stand (at the least - and a Dial Test Indicator that reads .0005 or .0001" for the utmost in precision).   You will need to methodically check each connection point starting from the bare spindle, and then continue moving down the line.  It is fairly simple, but it is involved and will take time and attention to detail.