A combination drill-reamer drawing fails in a different place from a reamer drawing. The drill stage, the reaming stage, the transition between them, the coolant exits and the relief all sit on one tool, so a missing item is rarely a note added later. It is a regrind, or a second tool.
We quote these tools from customer drawings every week, and the files that come back for rework share the same gaps. This page is the checklist we use before a drawing goes to grinding.
Most drawings we receive describe the reaming stage well and the drill stage by diameter only. That is enough for a single-purpose reamer and not enough for a tool that has to drill and ream in one pass.
Twelve items decide whether the tool can be ground once and repeated. Three of them, the stage transition, the coolant exits and the relief, cause most of the rework when they are absent.
If the drawing is already in front of you, read it against the table below before sending it. The commercial inputs around the same file, material, batch size, machine and acceptance gauge, are covered in the PCD reamer RFQ checklist.
The time side of the same decision, including the tool change and the gauging that the drawing affects, is worked in cycle time calculation for combination tools.
Whether to combine the two operations at all is a separate decision, screened in when to use a combination drill-reamer before any drawing is issued.
Why a combination tool drawing fails differently
A reamer drawing describes one cutting zone. A combination tool drawing describes two, plus the body that carries both, plus the path the coolant takes through it.
The tolerance chain is what changes: the drill stage decides where the hole starts, the transition decides how the two stages relate, and the reaming stage decides the final size and form.
Each link in that chain is ground in a different setup. If the transition is drawn as a step with no axial position, the grinder has to choose one, and the next tool gets a different choice. The parts may pass the first article and drift on the third tool, which is the most expensive way to discover a drawing gap.
We also see the opposite failure: a drawing toleranced so tightly that no grinding sequence can hold it without a special fixture. A 0.005 mm concentricity callout between two stages on a 250 mm tool is not a quality statement; it is a request for a different process.
What we look at first is not the tightest tolerance on the sheet. It is whether the two stages are described with the same level of care. When the drill stage carries a nominal diameter and the reaming stage carries limits, the stock left for reaming is undefined, and the tool geometry cannot be completed.
Where we are careful: a drawing can be complete and still produce a tool that does not suit the machine. Spindle runout, holder type and coolant pressure belong to the machine-side questionnaire, and we keep the two documents separate so neither hides the other.
What we cannot do from a drawing alone is confirm the coaxiality of the two stages in the part. That is measured on the part, not on the sheet, and it is the number the drawing should be asking for.
The twelve items
The table is the list we tick before a quotation. It is an engineering convention for combination tools, not a standard, and it does not replace your own drawing system. An item can be satisfied by a note, a detail view or a table on the sheet; the point is that the information exists once and only once.
| Item | What to write on the drawing | What happens when it is missing |
|---|---|---|
| 1. Drill stage diameter and tolerance | Nominal diameter, tolerance class or limits, and the gauge used to accept it | The stage is ground to a nominal; the stock left for reaming becomes an assumption |
| 2. Reaming stage diameter and tolerance | Limits with the class named (for example H7), plus the acceptance gauge | A shop default is applied and the bore lands at the wrong end of the band |
| 3. Stage transition | Cone angle or radius, axial position, step height if the tool is stepped | The transition is blended by hand at the grinder; coaxiality differs between tools |
| 4. Overall and stage lengths | Overall length, drill stage length, reaming stage length, usable flute length | The reaming stage is shorter than the bore and the entry is tapered |
| 5. Flute count and spacing | Count per stage and spacing, equal or unequal | Chip space and guidance are chosen at the grinder; tool-to-tool repeatability drops |
| 6. Helix angle per stage | Drill stage and reaming stage separately when they differ | One angle is copied across both stages and the reaming stage pulls or pushes axially |
| 7. Margin and relief | Margin width, primary clearance, secondary clearance, and which stage carries which | The reaming stage rubs; size and finish move with coolant and spindle speed |
| 8. Coolant inlet and exit positions | Inlet thread and size, exit location per stage, number of exits and exit angle | Exits are placed by convenience, chips pack in the transition and pressure drops |
| 9. Shank interface and clamping length | Interface type, shank diameter, minimum clamp length | The tool is held shorter than designed and runout at the cutting end multiplies |
| 10. Relief and undercut | Where the relief starts, its diameter and length, corner radius | The reaming stage copies the drill stage burr and the corners chip at the step |
| 11. Burr requirement and exit form | Permitted burr height, exit chamfer or breakout allowance, inspection method | No acceptance criterion exists; the argument starts at first article |
| 12. Regrind allowance and datum | Stock left for regrind, the datum face, the surface that must not be touched | The tool cannot be reconditioned without changing the stage lengths |
Two rows are usually filled in well: item 2, because the bore tolerance comes from the part drawing, and item 9, because the machine interface is already known. The rest are the ones we ask about.
Item 12 deserves a sentence of its own. A combination tool that cannot be reground is a consumable, and it should be priced as one. If the drawing says nothing about regrind stock, we assume none and quote a tool that is used once.
What each missing item costs
The three expensive gaps are the transition, the coolant exits and the relief. They are expensive because each one is discovered on the part rather than at the drawing review.
An undefined transition shows up as coaxiality that moves from tool to tool. The drill stage drills where the machine and the pre-hole put it; the reaming stage follows the transition.
If the transition is a cone with no axial position, two tools ground to the same sheet can sit at different axial points, and the reaming stage then removes a different amount of stock on each side. The bore diameter may still be inside limits. The position is not.
An undefined coolant exit shows up as chip packing. In a tool with internal coolant, the drill stage generates most of the volume and the reaming stage generates very little.
If the exit is placed where the transition is, chips have nowhere to go except back up the flutes. The usual response is to raise pressure, which helps until the weakest joint in the system leaks. Every regrind then moves the exit a little, and the behaviour changes again.
An undefined relief shows up as a burr copied into the finished bore. The drill stage exits into the region the reaming stage will later finish. If there is no relief or undercut between them, the reaming stage margins run over that exit and press the burr into the wall instead of cutting past it.
What we do about it is refuse to treat the first tool as production. When one of the three is missing we quote a first article with a drawing mark-up, and we ask for the mark-up to be returned before the second tool is ground. That is slower on paper and faster in total.
Where we are not certain: we cannot put a number on the cost of a missing item without the part. The same gap costs one regrind on a short through-hole and several tools on a deep blind hole, because the reaming stage has less room to correct what the drill stage left.
Tolerancing the two stages
The drill stage does not need the same tolerance as the reaming stage, and giving it one usually makes the tool more expensive without making the bore better. The drill stage exists to remove volume and to establish direction. The reaming stage exists to size and finish.
As an example, a 20 mm bore called out as H7 allows +0.021/0 mm under ISO 286-2. The drill stage on the same tool might reasonably carry a nominal 19.6 mm with a general tolerance of plus or minus 0.05 mm, because the reaming stage removes the difference. The two numbers come from different jobs and should be written as such.
The datum is the part that matters most. We ask for the reaming stage to be the datum for the transition, and for the shank to be the datum for runout at the reaming stage. That gives the grinder one functional surface and one mounting surface, which is the minimum for a repeatable regrind.
A drawing that datums the transition to the drill stage inverts the chain and makes the finished bore the dependent variable.
Runout is worth writing once, at the cutting end, with the measurement plane named. A callout at the shank tells us about the holder; a callout 5 mm behind the reaming stage tells us about the tool. When both appear without planes, we have to choose one and say so in the quotation.
A drawing can also over-specify the transition. If the cone angle is toleranced to a few minutes of arc and the axial position to 0.01 mm, the grinding sequence needs a setup that most tool shops do not keep on the floor.
We would rather see a functional callout, such as a maximum runout between the two stage axes measured at the reaming stage, than a set of numbers that only describe the grinder's setup.
For the geometry itself, the toolmaker references describe how the stages are built: flutes, chamfer lead and guide part in the Guhring reaming tools overview, and guide-pad and floating-holder designs in the MAPAL reaming and fine boring pages. The datum and tolerance framework we ask for follows ASME Y14.5 dimensioning and tolerancing.
How to review a drawing in five minutes
Read it backwards, starting at the reaming stage. The forward reading follows the tool from the shank to the point and tends to stop at the diameter. The backward reading starts at the surface that has to be right.
- Minute one: find the reaming stage limits and the gauge. If either is missing, stop; nothing else on the sheet can be checked against them.
- Minute two: find the datum for the transition and the datum for runout. Two datums, two surfaces, both named.
- Minute three: find the transition. Angle or radius, axial position, and what happens at the step.
- Minute four: trace the coolant from inlet to each exit. If an exit is not dimensioned, it will be chosen at the grinder.
- Minute five: find the relief, the burr acceptance and the regrind note. These are the three items that decide whether the tool can be used twice.
The read-aloud test from single-purpose reamer drawings still applies: if you cannot read the sheet out loud without inventing a number, the sheet is not finished. On a combination tool the invented number is usually an axial position, which is exactly the one that changes the bore.
One more check that costs nothing: confirm that the twelve items do not contradict each other. A reaming stage length longer than the usable flute length, or a relief that starts inside the reaming stage, are contradictions that survive a checklist and fail at the grinder.
If the drawing passes these five minutes, the remaining questions are commercial and process-related, and the custom cutting tool RFQ is the right place to send it. If it does not pass, the gaps are cheaper to close on paper than on the second tool.
Send the drawing after this check
Send the drawing, the part material and temper, the machine and holder, the coolant type and pressure, the pre-hole method if there is one, the batch size and the acceptance gauge. With those we can mark up the twelve items and quote against the marked-up version instead of against assumptions.
The 12 item check is also the fastest way to compare two quotations. Two suppliers quoting the same sheet with different assumptions will produce different tools, and the difference is usually visible in items 3, 8 and 12.
Use the custom cutting tool RFQ for the drawing and process data, and the combination drill-reamer page for the tool family. If the bore is deep and blind, the risks that sit outside the drawing are collected in deep blind holes with combination tools. Position and guiding are covered in pilot hole and bushing design for combination tools.
What published sources say about this
These are the pages we used for the geometry, tolerance and measurement points. Each is cited for what that page states, under its own conditions.
- Reamers, machine reamers (toolmaker technical overview): reamer structure, the guide part, chamfer lead and the A, B and C flute shapes, including which shape suits blind holes and which suits through holes.
- Reaming and fine boring (toolmaker technical page): reaming and fine-boring tool designs and holders, including guide-pad and floating-holder variants used where bore alignment is the constraint.
- Dimensioning and Tolerancing (ASME Y14.5): the datum, position, profile and runout framework that a tool drawing borrows when it has to relate two cutting stages.
- What are the issues for characterization? (NIST/SEMATECH e-Handbook of Statistical Methods): why a tolerance has to be defined together with the measurement that decides it.
- How are bias and variability controlled? (NIST/SEMATECH e-Handbook): the difference between a gauge that reads consistently and one that reads correctly, which is the point behind naming an acceptance gauge on the drawing.
- What is Process Capability? (NIST/SEMATECH e-Handbook): comparing process spread with specification limits, the reason a tolerance that the grinding process cannot hold is not a quality requirement.
- Levene Test for Equality of Variances (NIST/SEMATECH e-Handbook): a test for whether two sets of measurements differ in spread, useful when two tools ground to the same sheet behave differently.
Outbound references on this page are limited to pages we opened on 13 November 2026 (Asia/Shanghai). The twelve items are an engineering convention for combination tools, not a standard, and not a HENTRY specification. No tolerance value on this page is a HENTRY capability promise; the ISO 286-2 example is a standard grade shown for illustration.
Frequently asked questions
Can a combination tool be quoted from a drawing that only gives the drill stage diameter?
It can be quoted, but the quotation will contain an assumption about the stock left for reaming. We prefer to write that assumption on the quotation and ask for confirmation, because it changes the transition and therefore the tool.
If the assumption is acceptable, the first tool becomes the reference and the drawing should be updated before the second is ordered.
Which stage should carry the tighter tolerance?
The reaming stage, because it produces the functional surface. The drill stage needs enough tolerance control to leave a consistent stock, not the bore tolerance itself.
Making the drill stage as tight as the reaming stage adds grinding cost and does not improve the bore unless the pre-hole is also controlled.
Do coolant exit positions matter if we machine with external coolant?
They matter less, but they still matter for chip space. A tool designed around internal coolant has exits that also act as chip paths; running it dry or with external coolant leaves those paths unused.
Tell us the coolant method before the tool is designed. It changes the flute space and sometimes the stage lengths.
What is the cheapest item to add to an existing drawing?
The regrind datum and the regrind allowance, item 12. It costs one note and it decides whether the tool can be reconditioned at all.
The second cheapest is the burr acceptance, item 11, because it prevents an argument at first article that no drawing information can settle later.
Will a hand-marked PDF be accepted?
Yes, if the marks are unambiguous and dated. We return a clean mark-up with the quotation so both sides are working from the same sheet.
What we cannot accept is a verbal change to the transition or the coolant exits, because those two are the ones most likely to be remembered differently by the two people involved.
Updated 13 November 2026: first publication. Scope: the twelve items a combination drill-reamer drawing needs, the three gaps that cause most rework, how the two stages are toleranced and how to review the sheet in five minutes. The checklist is an engineering convention, not a standard. No HENTRY capability value is stated on this page.